Ancient and Historical Ceramics: Materials, Technology, Art and Culinary Traditions 3510652908, 9783510652907

Table of contents :
Cover
Title
Copyright
Table of Contents
Preface
Acknowledgements
Exordium
Part I Fundamentals
1 The nature of ceramics
1.1 Materials and technological evolution of societies
1.2 Ancient roots
1.3 Holistic and prescriptive technologies
1.4 Ceramics and their production environment
1.5 Ceramics and cooking
1.6 Ceramics as subject of archaeometry
2 Classification and properties of ceramics
2.1 Classification and types of ceramics
2.2 Definitions of common ceramic types
2.3 Properties and functions of ceramic cooking pots
3 Clay raw materials: origin, composition, and properties
3.1 Types of raw materials
3.2 The formation of clay minerals
3.3 Nomenclature and structure of clay minerals
3.4 Mineralogy of clay minerals relevant for pottery
3.5 Clay-water interactions
4 Processing of clay, and forming and finishing of pottery
4.1 The operational sequence of making ceramics
4.2 Preparation of clay
4.3 Forming of ceramic green bodies
4.4 Drying of green pottery
4.5 Glazes and glazing
4.6 Post-firing painting
5 Ceramic phase diagrams
5.1 Introduction
5.2 Anatomy of three-component (ternary) phase diagrams
5.3 Selected model ceramic phase diagrams
6 Materials science of ceramics
6.1 Ceramics as man-made ‘rocks’
6.2 Firing temperature vs. state of sintering
6.3 Thermal transformations in kaolinitic clays
6.4 Thermal transformations in illitic clays
6.5 Thermal transformations in phosphatic ceramics
6.6 Densification during firing
6.7 Determination of firing temperatures
7 Pottery kilns and firing technology
7.1 Pottery firing structures and devices
7.2 Fuel consumption and production economy
Part II Selected ceramics and culinary traditions
8 Ancient Near Eastern wares
8.1 Neolithic cultures in the Near East
8.2 Mesopotamia
8.3 Anatolia
8.4 Egypt
8.5 Iran
8.6 Hidden messages from Neolithic cooking pots
9 Aegean Neolithic, Bronze and Iron Age pottery
9.1 Setting the stage
9.2 Neolithic to Bronze Age Thessalian pottery
9.3 Cretan pottery
9.4 Bronze Age (Helladic) pottery
9.5 Iron Age Greek wares
9.6 Culinary traditions: Greek delicacies revealed
10 Roman earthenware
10.1 Historical development
10.2 Italian and Provincial Roman Terra Sigillata
10.3 Manufacturing technique
10.4 Materials science of Terra Sigillata
10.5 A Roman Terra Sigillata workshop in Tabernae, 2nd century CE
10.6 What distinguishes a mould from the Terra Sigillata pottery?
10.7 The Roman gourmet Apicius and his legacy
11 Medieval and early modern German stoneware
11.1 Unglazed Carolingian earthenware: Badorf, Mayen, Pingsdorf
11.2 Rhenish stoneware: Siegburg, Frechen, Cologne, Westerwald, Raeren
11.3 Saxon stoneware
11.4 Bunzlau stoneware
11.5 Of late medieval broth and mush
12 English and French white earthenware (creamware, faïence fine)
12.1 French Renaissance precursors
12.2 English white earthenware (creamware)
12.3 French white earthenware (faïence fine)
12.4 Scientific analyses of English and French white earthenware
12.5 Fast food and sweet cake
13 Tin-glazed ceramics from the Near East and Italy
13.1 Technological background
13.2 The beginnings of the tin-glaze technique
13.3 The spreading of tin-glaze technology in Europe
13.4 Italian maiolica
13.5 Renaissance gastronomy
14 French soft-paste porcelain
14.1 A short history of selected French manufactures
14.2 Technology of French soft-paste porcelain
14.3 Conclusion
14.4 The ‘plaisirs de table’ of Louis XV and his favourite, Marquise de Pompadour
15 The first European hard-paste porcelain: Meissen
15.1 Historical beginnings
15.2 The invention of European porcelain at Meissen
15.3 Material basis and technology of Böttger stoneware
15.4 Development of porcelain microstructure
15.5 From the royal table of Augustus the Strong
16 English bone china
16.1 Early developments
16.2 Forerunners of bone china
16.3 The invention of bone china
16.4 Microstructure of bone china
16.5 Staffordshire potter’s favourite dishes
17 Prehistoric New World pottery
17.1 South American pottery
17.2 Central American pottery
17.3 South-western United States
17.4 Mississippian culture
17.5 Native cuisine of the Americas
18 Chinese pottery: From earthenware to stoneware to porcelain
18.1 The European perspective
18.2 Chinese history and pottery
18.3 Neolithic earthenware ceramics
18.4 Earthenware and stoneware of the Xia and Shang dynasties
18.5 Chinese proto-porcelain
18.6 True Chinese porcelain
18.7 Ancient Chinese cookery: a feast of plenty, perfectly balanced
19 Thai ceramics
19.1 Historical account
19.2 Neolithic pottery
19.3 High-fired glazed stoneware
19.4 Northern Thai (Lan Na) kilns
19.5 Ancient Thai cuisine
20 Japanese ceramics
20.1 A philosophy of natural aesthetics
20.2 Jōmōn, Yayoi and Kofun (Yamato) pottery
20.3 Asuka, Nara and Heian periods
20.4 Kamakura and Muromachi period
20.5 Momoyama wares
20.6 Edo period
20.7 Ancient Japanese cooking: what Samurai and Sumōtori enjoyed
References
Ceramic index
Location index
Names index
Recipe index

Citation preview

Ancient and Historical Ceramics Materials, Technology, Art, and Culinary Traditions

Readers are being acquainted with the science of ceramics and their technology, and with the artistry of ceramic masterpieces fashioned by ancient master potters. Ceramics treated in this book range from Near Eastern pottery to the Meissen porcelain wonders, from the Greek black-on-red and the Minoan Crete masterpieces to British bone china, and from Roman Terra Sigillata to the celadon stoneware and porcelain produced in the kilns of China, Japan and ancient Siam. Ancient and historical ceramic plates, pots, beakers and cups are juxtaposed with food preparations that likely may have been cooked in and served on these ceramic objects in the distant past. As it also presents ancient recipes, this book will also serve as a unique cookbook. This generously illustrated book with hundreds of colour photographs and figures not only addresses professionals and students of archaeology, art history, and archaeometry working at all levels but anybody fascinated by historical ceramics, ceramic materials and production techniques of ancient ceramics.

ISBN 978-3-510-65290-7 www.schweizerbart.de

9

783510

652907

Ancient and Historical Ceramics Materials, Technology, Art, and Culinary Traditions

Ancient and Historical Ceramics

By stressing the congruence between cooking ceramics and tableware, and food and its consumption, this book offers a completely new view on ceramic science. It provides an interdisciplinary approach by linking ceramic science and engineering, archaeology, art history, and lifestyle. The selection of ceramic objects by the authors has been guided by historical significance, technological interest, aesthetic appeal, and mastery of craftsmanship.

Robert B. Heimann Marino Maggetti

Heimann · Maggetti

Robert B. Heimann • Marino Maggetti

E

E

Schweizerbart Science Publishers

R. B. Heimann and M. Maggetti Ancient and Historical Ceramics: Materials, Technology, Art, and Culinary Traditions

Ancient and Historical Ceramics: Materials, Technology, Art, and Culinary Traditions Robert B. Heimann and Marino Maggetti With contributions by Gabriele Heimann and Jasmin Maggetti

With 303 figures and 47 tables

Schweizerbart Science Publishers Stuttgart 2014

R. B. Heimann and M. Maggetti: Ancient and Historical Ceramics: Materials, Technology, Art, and Culinary Traditions Authors: Prof. Dr. Robert B. Heimann, Am Stadtpark 2A, 02826 Goerlitz, Germany. E-mail: [email protected] Prof. Dr. Marino Maggetti, University of Fribourg, Dept. of Geosciences, Earth Sciences, Chemin du Musée 6, CH-1700 Fribourg, Switzerland. E-mail: [email protected] We would be pleased to receive your comments on the content of this book: [email protected] Front cover: See this volume, page 406: Figure 18.4. White pottery bu with carved geometric pattern emulating cast bronze. Shang dynasty, Anyang. 16th–11th centuries BCE. Height 25 cm. © Collection of the Imperial Palace Museum, Beijing, China. The use of this image is licensed under the Creative Commons Attribution 2.0 Generic license (www.creativecommons.org/ licenses/by/2.0)and attributed to user Rosemania (en.wikipedia.org/wiki/File:China_shang_ white_pottery_pot.jpg; accessed Jan 21, 2012).

This publication has been supported by

CERAMICA-STIFTUNG BASEL  

ISBN ebook (pdf) 978-3-510-65482-6  ISBN 978-3-510-65290-7 Information on this title: www.schweizerbart.com/9783510652907

© 2014 by E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller), Stuttgart, Germany All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical photocopying, recording, or otherwise, without the prior written permission of E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart Publisher: E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller) Johannesstr. 3A, 70176 Stuttgart, Germany [email protected] www.schweizerbart.de ∞ Printed on permanent paper conforming to ISO 9706-1994 Typesetting: Satzpunkt Ursula Ewert GmbH, Bayreuth Printed in Germany by DZA Druckerei zu Altenburg GmbH, Germany

Preface Ceramics play a major role in the understanding of ancient societies, both because they were the first man-made material and because, if only in the form of pottery shards, they have a very high survival rate in archaeological contexts. A starting point for the study of ancient ceramics is the reconstruction of their life cycle from the procurement and processing of the raw materials, through their forming, decoration and firing, to their distribution, use and reuse. Reconstruction of the life cycle is then followed by its interpretation in order to obtain a better understanding of the people associated with the ceramics. Such a study requires a holistic approach, taking account of the fact that production, distribution and use are firmly embedded within the wider environmental, technological, economic, social, political and ideological context. Thus, close collaboration among archaeologists, historians and physical scientists is essential for success in such studies. The present book, because of the very wide range of topics, both scientific and cultural, that are covered, represents an extremely valuable contribution to our understanding of the role that ceramics have played in ancient societies. The book starts with a comprehensive introduction to the basic science and technology associated with ceramic production. Of particular importance are the inclusion of a brief description of ceramic phase diagrams and their role in interpreting the mineralogical changes occurring during the firing of ceramics, together with a discussion of the mechanical and thermal properties of ceramics particularly when in use as cooking pots. The reader is then taken through the historical developments, production technologies, physical properties, and stylistic attributes associated with individual groups of ceramics used in preparation, serving and storage of food. Although, as the authors admit, the coverage cannot be exhaustive, it is unusually wide ranging both geographically, covering much of Europe, the Near East, the Far East and the Americas, and chronologically, spanning the period from more than 10,000 years ago up to the 18th century AD. In considering production technology, the authors include information provided by contemporary treatises such as those by Abu ‘l Qasim at the beginning of 14th century AD and Cipriano Piccolpasso in 16th century AD, reports by contemporary travellers such as Marco Polo in 13th century AD and Père d’Entrecolles in 18th century AD, and in the case of the production of European porcelains, surviving contemporary documentation. In addition, full use is made of phase diagrams in explaining the mineralogical changes occurring during firing of the different types of porcelain. Finally, a unique feature of the book is that the last section of each of the thirteen chapters on specific ceramic types provides a description of the culinary traditions associated with the region and period. A selection of ancient recipes is included for some of which modern versions are provided and tested, with the finished product being photographed and presumably consumed. In terms of readership, I believe that this book will be valued and enjoyed by both the general reader with at least some scientific knowledge, and by students of archaeology, art history and archaeometry working at all levels. For the former, by including information on production technology and the potential culinary uses of the ceramics, the book will supplement the standard histories of ceramics, such as World Ceramics edited by Robert

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Preface

Charleston (1981). For the latter, it will be invaluable because its range goes far beyond that of the, in some ways, comparable volume on Ceramic Masterpieces by David Kingery and Pamela Vandiver (1986). Furthermore the range and depth of information provided is such that many chapters will be read with interest by scientists who themselves have researched extensively into the production technology of ancient ceramics. On a personal note, three chapters that I found particularly interesting are those on Roman earthenware (Chapter 10), Medieval and early German stoneware (Chapter 11), and Prehistoric New World pottery (Chapter 17). The Roman earthenware chapter concentrates on the production of the high class Roman tableware, Terra Sigillata, and provides valuable discussions on the properties of the moulds into which the vessels were thrown, the factors determining the reflectivity of the high gloss surfaces, the operation of the kilns in which the vessels were fired, and the logistics and scale of production and distribution. The German stoneware chapter describes both the products of the Rhineland region from their beginnings with unglazed stoneware in 8th century AD through to the salt-glazed wares starting in the 13th century AD and reaching their peak during the second half of the 16th century AD, as well as stonewares from Saxony and Silesia in the east of Germany. A highlight for me of the New World chapter is the section on Mississippian culture pottery, tempered with mussel shells, that was produced along the Mississippi valley from about 800-1500 AD. In view of the potential problem of the destructive power of lime blowing that occurs with shell tempered pottery as a result of volume expansion during the post-firing reformation of calcium carbonate, the reasons for the use of shell temper in terms of the resulting improved workability of the clay and mechanical properties of the pottery are first discussed. The mechanism by which lime blowing can be avoided through the addition of common salt (NaCl) to the clay is then fully explained, and whether or not the problem of lime blowing was avoided in the case of Mississippian pottery by the intentional addition of small quantities of salt is considered. Both authors are mineralogists by training. One (RBH) has researched into both ancient and modern ceramics, with his interest in and understanding of ancient ceramics undoubtedly gaining significantly as a result of his long-time collaboration with the Canadian guru of technological studies of ancient materials, Ursula Martius Franklin. In contrast, the other (MM) has spent a major part of his career undertaking research and supervising PhD students in the field of ancient ceramic technology and provenance. Thus, the authors are very well qualified to produce a book that makes an extremely valuable and, through its inclusion of history, technology and culinary practice, a unique addition to the currently available literature on ancient ceramics. Michael Tite Oxford, UK

References Charleston, R.J. (ed.) (1981). World Ceramics – An illustrated history from earliest times. London: Hamlyn. ISBN 0-600-34261-1. Kingery, W.D. and Vandiver, P.B. (1986). Ceramic Masterpieces – Art, Structure, and Technology. New York: The Free Press. ISBN 0-02-91848-0-0.

Acknowledgments Many colleagues, research organisations and museums generously provided information and expertise, digital images, SEM micrographs, graphics, analytical data, and advice on ancient and historical pottery. We are gratefully acknowledging this invaluable support. We are also most thankful for the time many colleagues devoted to critically reviewing individual chapters of this volume. We would like to acknowledge able assistance by Dr Barbara Helwing, Deutsches Archäologisches Institut, Berlin (Arismān pottery); Dr Lutz Martin, Vorderasiatisches Museum Berlin (Tell Halaf pottery); Prof. Dr Pieter ter Keurs, Rijkmuseum van Oudheden, Leiden, The Netherlands (Tepe Sialk pottery); Prof. Dr Walter Noll †, Leverkusen, Germany (Mesopotamian and Minoan pottery); Prof. Dr Herbert Kroll and Dr Martin Görres, Westfälische Wilhelms-Universität Münster, Germany (Grey Minyan pottery); Dr Alexandra Christopoulou, National Archaeological Museum, Athens, Greece (Sesklo and Dimini pottery); Dr Yannis Maniatis, N.C.S.R. Democritos, Athens, Greece (Neolithic Greek pottery); Dr Michael Lindblom, Uppsala University, Sweden (Helladic pottery); Prof. Eleni Hasaki, University of Arizona, Tucson, AZ, USA (Greek pottery kilns); Dr Rüdiger Schmidt, Landesarchäologie Rheinland-Pfalz, Speyer, Germany (Roman Terra sigillata mould); Ms Martine Beck-Coppola, Réunion des Musées Nationaux Sèvres, France (White earthenware), Dr Thorsten Schifer, Reiss-Engelhorn Museum, Mannheim, Germany (Saxon stoneware); Dr Sally Schöne, Hetjens-Museum, Düsseldorf, Germany (Rhenish stoneware); Prof. Dr Ulrich Pietsch, Cora Würmell and Annette Loesch, Staatliche Kunstsammlungen Dresden, Dresden, Germany (Meissen and Chinese porcelains); Dr Bernd Ullrich, TU Bergakademie Freiberg, Freiberg, Germany (Böttger stoneware and Meissen porcelain); Prof. J. Victor Owen, Saint Mary’s University, Halifax, Nova Scotia, Canada (phosphatic stoneware); Prof. Ian Freestone, University College, London, UK (bone ash porcelain); Mr Sam Richardson, The Potteries Museum & Art Gallery, Stoke-on-Trent, UK (Spode bone china); Dr Alpagut Kara, Anadolu University, Turkey (modern bone china); Dr Daniela Triadan, University of Arizona, Tucson, AZ, USA (White Mountain red ware); Mr William R. Iseminger, Collinsville, IL, USA (Mississippian effigy bowls); Ms Heather A. Shannon, National Museum of the American Indian, Smithsonian Institution, Washington, D.C., USA (Mississippian engraved pottery); Prof. Prudence M. Rice, Southern Illinois University, Carbondale, IL, USA (Maya pottery); Prof. Thilo Rehren, University College, London, UK (Chinese proto-porcelain); Prof. Michael Tite, Oxford, UK (Longquan celadon ware); Minneapolis Institute of Arts, Minneapolis, MN, USA (Song Longquan celadon ware); Mr John C. Shaw, Chiang Mai, Thailand (Sukhothai, Si Satchanalai and Northern Thai pottery); Freer Gallery of Art and Arthur M. Sackler Gallery, Smithsonian Institution, Washington, D.C., USA (Northern Thai pottery); Prof. Yoshihiro Kusano, Kurashiki University, Okayama, Japan (Bizen stoneware); The Trustees of the British Museum, London, UK (Jōmōn, Egyptian, Anatolian, Near East, Mycenaean, Attic, Corinthian and Roman pottery; German stoneware, bone china); The Victoria and Albert Museum, London, UK (Italian maiolica, French soft-paste porcelain, Japanese pottery, stoneware and porcelain); Tokyo National Museum, Japan (Imari porcelain); Ms Anne-Claire Schumacher,

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Acknowledgments

Musée Ariana, Geneva, Switzerland (Queen’s ware, Mesopotamian tin-glazed ware, Italian maiolica, French soft-paste porcelain); The American School of Classic Studies at Athens (Greek mainland polychrome ware); The Gardiner Museum of Ceramic Art, Toronto, ON, Canada (steatitic English porcelain); Citylife Magazine, Chiang Mai, Thailand; Japanese Photo Library (Tokyo); and Bibliotheca Gastronomica, SLUB, Dresden, Germany. Special thanks for critically reviewing individual chapters of this treatise are due to Prof. Andrew Shortland (Cranfield, UK; Chapter 8), Prof. Hans Mommsen (Bonn, Germany; Chapter 9), Dr Gerwulf Schneider (Berlin, Germany; Chapter 10), Prof. David Gaimster (Glasgow, UK; Chapter 11), Prof. Trinitat Pradell (Castelldefels, Spain; Chapter 13), Dr Antoine d’Albis (Sèvres, France; Chapter 14); Prof. Victor Owen (Halifax, Canada; Chapter 16), Prof. James Feathers (Seattle, USA; Chapter 17), Prof. Nigel Wood (London, UK; Chapter 18), Mr John Shaw (Chiang Mai, Thailand; Chapter 19), and Prof. Yoshihiko Kusano and Dr Minoru Fukuhara (Okayama, Japan; Chapter 20). Ms Nicole Bruegger, Fribourg, Switzerland deserves a special Thank You for drawing the maps of the archaeological sites mentioned in the individual chapters. While we considered the valuable comments and suggestions for improvement freely given by the reviewers, remaining factual errors, misconceptions, ambiguities and omissions are entirely ours. The culinary part of the book owes everything to the dedication of our spouses Gabriele and Jasmin who diligently searched ancient culinary texts, selected appropriate and manageable recipes, experimented with numerous ingredients, tried, tasted, dismissed, retried and finally approved the fruits of their labour of love. Publication of this work would not have been possible without generous financial assistance by CERAMICA-STIFTUNG BASEL, Switzerland, represented by its president, Dr Thomas Staehelin. We are very grateful for this much needed support. Dr Andreas Nägele and Ms Angela Pfeifer of Schweizerbart Science Publishers, Stuttgart, Germany are acknowledged for their expert advice, and constant encouragement and technical support during preparation of this text.

Table of Contents Preface V Acknowledgements VII Table of Contents Exordium

Part I

IX

XV

Fundamentals

1

The nature of ceramics 1

1.1 1.2 1.3 1.4 1.5 1.6

Materials and technological evolution of societies 1 Ancient roots 4 Holistic and prescriptive technologies 5 Ceramics and their production environment 8 Ceramics and cooking 10 Ceramics as subject of archaeometry 11

2

Classification and properties of ceramics 12

2.1 2.2 2.3

Classification and types of ceramics 12 Definitions of common ceramic types 13 Properties and functions of ceramic cooking pots 18

3

Clay raw materials: origin, composition, and properties 22

3.1 3.2 3.3 3.4 3.5

Types of raw materials 22 The formation of clay minerals 23 Nomenclature and structure of clay minerals 25 Mineralogy of clay minerals relevant for pottery 28 Clay-water interactions 31

4

Processing of clay, and forming and finishing of pottery 37

4.1 4.2 4.3 4.4 4.5 4.6

The operational sequence of making ceramics Preparation of clay 37 Forming of ceramic green bodies 39 Drying of green pottery 47 Glazes and glazing 48 Post-firing painting 55

37

Table of Contents

X 5

Ceramic phase diagrams 59

5.1 5.2 5.3

Introduction 59 Anatomy of three-component (ternary) phase diagrams Selected model ceramic phase diagrams 65

6

Materials science of ceramics 70

6.1 6.2 6.3 6.4 6.5 6.6 6.7

Ceramics as man-made ‘rocks’ 70 Firing temperature vs. state of sintering 71 Thermal transformations in kaolinitic clays 73 Thermal transformations in illitic clays 76 Thermal transformations in phosphatic ceramics Densification during firing 97 Determination of firing temperatures 99

7

Pottery kilns and firing technology 103

7.1 7.2

Pottery firing structures and devices 103 Fuel consumption and production economy

Part II

60

95

126

Selected ceramics and culinary traditions

8

Ancient Near Eastern wares 129

8.1 8.2 8.3 8.4 8.5 8.6

Neolithic cultures in the Near East 129 Mesopotamia 131 Anatolia 135 Egypt 137 Iran 144 Hidden messages from Neolithic cooking pots 149

9

Aegean Neolithic, Bronze and Iron Age pottery 157

9.1 9.2 9.3 9.4 9.5 9.6

Setting the stage 157 Neolithic to Bronze Age Thessalian pottery 159 Cretan pottery 164 Bronze Age (Helladic) pottery 170 Iron Age Greek wares 176 Culinary traditions: Greek delicacies revealed 184

Table of Contents

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10

Roman earthenware 192

10.1 10.2 10.3 10.4 10.5 10.6 10.7

Historical development 192 Italian and Provincial Roman Terra Sigillata 194 Manufacturing technique 198 Materials science of Terra Sigillata 203 A Roman Terra Sigillata workshop in Tabernae, 2nd century CE 206 What distinguishes a mould from the Terra Sigillata pottery? 209 The Roman gourmet Apicius and his legacy 213

11

Medieval and early modern German stoneware 227

11.1 11.2 11.3 11.4 11.5

Unglazed Carolingian earthenware: Badorf, Mayen, Pingsdorf 227 Rhenish stoneware: Siegburg, Frechen, Cologne, Westerwald, Raeren Saxon stoneware 236 Bunzlau stoneware 244 Of late medieval broth and mush 245

12

English and French white earthenware (creamware, faïence fine) 255

12.1 12.2 12.3 12.4 12.5

French Renaissance precursors 255 English white earthenware (creamware) 259 French white earthenware (faïence fine) 265 Scientific analyses of English and French white earthenware Fast food and sweet cake 275

13

Tin-glazed ceramics from the Near East and Italy 279

13.1 13.2 13.3 13.4 13.5

Technological background 279 The beginnings of the tin-glaze technique 282 The spreading of tin-glaze technology in Europe Italian maiolica 289 Renaissance gastronomy 300

14

French soft-paste porcelain 309

14.1 14.2 14.3 14.4

A short history of selected French manufactures 309 Technology of French soft-paste porcelain 318 Conclusion 326 The ‘plaisirs de table’ of Louis XV and his favourite, Marquise de Pompadour

229

270

288

326

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Table of Contents

15

The first European hard-paste porcelain: Meissen 333

15.1 15.2 15.3 15.4 15.5

Historical beginnings 333 The invention of European porcelain at Meissen 336 Material basis and technology of Böttger stoneware 341 Development of porcelain microstructure 348 From the royal table of Augustus the Strong 351

16

English bone china 354

16.1 16.2 16.3 16.4 16.5

Early developments 354 Forerunners of bone china 357 The invention of bone china 359 Microstructure of bone china 362 Staffordshire potter’s favourite dishes 365

17

Prehistoric New World pottery 371

17.1 17.2 17.3 17.4 17.5

South American pottery 371 Central American pottery 374 South-western United States 377 Mississippian culture 379 Native cuisine of the Americas 393

18

Chinese pottery: From earthenware to stoneware to porcelain 395

18.1 18.2 18.3 18.4 18.5 18.6 18.7

The European perspective 395 Chinese history and pottery 398 Neolithic earthenware ceramics 401 Earthenware and stoneware of the Xia and Shang dynasties 403 Chinese proto-porcelain 407 True Chinese porcelain 411 Ancient Chinese cookery: a feast of plenty, perfectly balanced 433

19

Thai ceramics 439

19.1 19.2 19.3 19.4 19.5

Historical account 440 Neolithic pottery 441 High-fired glazed stoneware 442 Northern Thai (Lan Na) kilns 449 Ancient Thai cuisine 452

Table of Contents

20

Japanese ceramics

20.1 20.2 20.3 20.4 20.5 20.6 20.7

A philosophy of natural aesthetics 457 Jōmōn, Yayoi and Kofun (Yamato) pottery 460 Asuka, Nara and Heian periods 463 Kamakura and Muromachi period 464 Momoyama wares 466 Edo period 468 Ancient Japanese cooking: what Samurai and Sumōtori enjoyed

References 481 Ceramic index 537 Location index 542 Names index 547 Recipe index 550

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XIII

Exordium

Ceramics are known to be the most ancient man-made materials. At the dawn of civilisation, man has seen more in ceramics than a mere material stuff from which useful things could be fashioned. There are many creation myths worldwide that link the very life, in particular human life, to clay and as such, pottery. In Assyro-Babylonian mythology, Nammu, the Mother goddess, assisted by Enki, the God of Crafts, created man from clay mixed with the blood of the God of Intelligence, Geshtu-e (Kramer & Maier 1989, Leick 1998). The ancient Egyptian believed that the ram-headed potter god Chnum formed the body of the first human on a potter’s wheel while his consort Heket offered the ‘breath of life’ by touching the mouth of their creation with the ank’h cross, the hieroglyphic symbol of life. The Greeks of the Classic Age told the story of Prometheus, scion of the Titans who were overthrown by Zeus in the Titanomachy (War of the Titans). Prometheus took clay from the earth, wetted it and shaped humans after the image of the immortal gods (P. Ovidius Naso 1922). Into these forms he implanted good and bad character traits taken from the animal world. The goddess Athena, admiring the creation of the son of the Titans breathed mind, reason and wisdom into the half-animated creature to bring it to full human life. In the Bible reference is made to the origin of man: ‘And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul’ (Genesis, 1. Mose; 2, 7). According to the Qur’an the first human being was created by Allah from clay shaped into human form: ‚[So mention] when your Lord said to the angels, “Indeed, I am going to create a human being from clay. So when I have proportioned

Creation of the first human from clay as depicted at the walls of Queen Hatshepsut’s temple of Deir el Bahari, Egypt (Noll 1991).

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Exordium

him and breathed into him of My [created] soul, then fall down to him in prostration.” (Qur’an, Surat Şād, 38: 71–72). The Shilluk (Chollo) living along the Nile in southern Sudan were more specific when it came to the origin of different human races. Juok (God) created men out of clay. He traveled north and found some white clay, out of which he fashioned the Caucasian race. The Arabs were made of reddish-brown clay, and the Africans from black earth (Cavendish 1980). Spurred by these ancient mythologies there was reason to believe that the creator gods were potters and, by inference, making pottery may be considered not only a creative but also a sacred if not divine business1. The very process of ceramic firing, in particular firing of stoneware and porcelain in huge Chinese dragon-type kilns (see Chapter 7.1.2), is an awe inspiring, highly spectacular sight as succinctly described by Kerr & Wood (2004; p. 52): ‘... the interior of a large stoneware or porcelain kiln at the height of a firing can be an awesome sight. A few inches of brickwork separate the viewer from the torrent of flame and the incandescent wares within. The ground the viewer stands on is hot, and resonating with the pulse of the fire, and the eyes have to be shielded from the intense glare as the ceramics or saggars shimmer in the whiteness. The firing of porcelain is a dramatic event, but the feeling that weeks or months of effort have been committed to an extreme and difficult finishing process is not restricted to porcelain. It must have been common sensation to potters through history, not least because firing is as capable of destroying wares as it is of transforming them. This is surely one of the reasons that many potters invoked the spirit of a Pottery Deity to aid them’. Several stone stelae erected to worship a Patron Deity of Kilns at the Yao-chou kiln site, Shaanxi province, China attest to this custom thought to ensure a lucky outcome of a very difficult to control technological process: the firing of stoneware and porcelain. Indeed, the dull, soft and unassuming clay, miraculously transformed by fire to shiny, translucent and hard porcelain, instilled among potters the impression of wonderment and divine intervention. Amazingly, 5000 years later there is the intriguing notion that certain expanding clays of the smectite family, for example montmorillonite, may have acted as catalyst for the origin of life on Earth by providing a template for self-organisation of primitive organic molecular compounds that later served to form the four nucleobases adenine, cytosine, guanine and thymine (uracil), the building blocks of life (Cairns-Smith 1986, Güven 2009; also Southam 2012). Experimental evidence suggests that montmorillonites assist fatty acid micelles to self-organise into membrane vesicles that are able to growth and divide in response to changes of the pH and the ion concentration of the surrounding liquid, bringing together the key components of early life in a proposed ‘RNA world’ (Ertem & Ferris 2000, Hanczyc et al. 2003). Hence, in a very subtle and unexpected way the ancient stories of the origin of life and thus man from a clay matrix may have a kernel of truth.

1

This is in stark contrast to the generally squalid conditions potters in most societies had to endure. For example, making domestic pottery was regarded a lowly profession in ancient China and potters like other artisans ranked together with merchants in the lower half of the Chinese hierarchy (Kerr & Wood 2004, p. 18/19). Similar views were held by Platonic philosophers in ancient Greece who considered craftspeople incapable of origination. Hence the latter had to wait for divine intervention by the gods to create both the idea and form of the pots they made (Farrington 1969).

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The onset of ceramic technology is difficult to assess in both geographical and temporal contexts since it is shrouded by an impenetrable veil of time. The history of ceramics as part of everyday’s life of man presumably has its origin at a time when late Neolithic huntergatherer bands turned into agrarian societies, some 12,000 years ago. However, it is likely that much earlier some late Palaeolithic or early Mesolithic genius observed a clay lump hardening in a camp fire and realised that this property may be exploited to fashion cooking pots and storage containers to hold liquids as exemplified by the Incipient Jōmōn culture of Japan (MacGregor 2010) and later, in Northern and Central Europe, by the succession of Neolithic funnel beaker, corded ware, and bell beaker cultures. Thus during the past decades our knowledge of the origin of ceramics technology has been pushed back by archaeological research well into the Upper Palaeolithic/Mesolithic period. Arguably among the first objects fashioned from clay were maternal goddess images such as the famous Upper Palaeolithic ‘Venus of Dolni Vĕ ĕstonice’, Moravia and fragments of animal and human figurines dating between 28,000 and 27,000 BP (Klima 1962). Near the end of the Mesolithic (13,000–12,000 BP) hunter-gatherers living in what today is Japan independently discovered ceramic technology (Nakamura et al. 2001), but this time applied it to manufacture ceramic vessels of the Jōmōn culture, among the world’s oldest pots (Chard 1974, Klein 1980). Very recently still earlier remnants of ceramic technology came to light in a cave in southern China and were dated to between 18,300 and 15,430 cal BP (Boaretto et al. 2009). Since ceramic shards are well preserved in all but the most acidic soils they are of overriding importance in archaeology to date and distinguish prehistoric cultures by the unique and enduring physical and stylistic features of their characteristic pottery. While at the dawn of human civilisation readily available natural raw materials such as hard rock and flint were utilised for tools, with the advent of fire soft and pliable clayey and loamy raw materials could eventually be converted by firing into hard, durable shapes that would hold liquids. Hence intentional firing is very much the hallmark of ceramics since etymologically the meaning of the word ‘ceramic’ relates to ‘burnt stuff’ (Greek: ˧˚ˮ˞ ˧˚ˮ˞µˬ˯ ˬ˯) (Oldfather 1920). Through the firing process clay minerals generated by weathering of rocks could be transformed back into something resembling an artificial ‘stone’ (Heimann & Franklin 1979, Maggetti 2001). This development is thought to have been triggered by the above mentioned Neolithic transition from hunter-gatherer to agrarian societies. Indeed, the manufacturing of pottery typically implies some form of sedentary life since pottery is inherently breakable and thus generally useless to hunter-gatherers who are constantly on the move. Presumably the first cooking vessels were not in intimate contact with the cooking fire per se as their crude fabric would not have tolerated the substantial differential stresses imposed by high temperature gradients. Instead, stones heated in a camp fire and dropped into claylined dug-outs filled with water, and later into liquid-filled hardened clay containers will have accomplished the task of heating its food ingredients. Nevertheless, the invention of ceramic containers to prepare, serve and store food was a revolutionary step in human development as it allowed controlled boiling, steaming, and simmering in portable ‘dug-outs’ as opposed to simple and crude roasting and frying of meat and vegetables in a camp fire, food preparation techniques that had been already invented by Homo erectus as early as 1.9 million years ago. Recently a research team at Harvard University has shown that Homo erectus, Homo neanderthalensis and Homo sapiens may have evolved smaller molars com-

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pared with other primates as a result of softening their food with tools and fire. This theory that extinct hominids may have cooked their food explains the comparatively small teeth, jaws, and guts of modern humans (Organ et al. 2011). Remains of pre-ceramic burned corncobs recently discovered in two ancient mound sites in Peru and dated to 4,700 BCE suggested that heated stones were used to soften the kernels not unlike modern pop corn. This evidence indicates that in many areas corn arrived before cooking pots did and that early experimentation with corn as a staple food was not dependent on the presence of pottery. Corn was first domesticated in Mexico nearly 9,000 years ago from a wild grass called teosinte (Grobman et al. 2012). Clay-based ceramic vessels also allowed storing safely perishable goods such as grain and dried fruits, and liquids such as oil, wine and beer. Moreover, with the invention of watertight utilitarian ceramic cooking pots, food could now be made tender and thus palatable by the process of boiling that effectively loosened the cellular network, released nutritional proteins, fats and carbohydrates, activated fragrant ingredients, and allowed to add aromatic essences, herbs, spices and other seasonings. Soups and gruels could also be prepared that constituted the major part of the nourishment of early Neolithic people. The denaturation products of lipids left in the fabric of the remnants of the cooking pots serve well to reconstruct the nutritional base of ancient societies (Dudd et al. 1999, Evershed et al. 2008, Gregg & Slater 2010). With the advent of ceramic cooking pots man turned into what has been called the ‘cooking animal’. To quote James Boswell (1773): ’My definition of Man is “a Cooking Animal”. The beasts have memory, judgement, and all the faculties and passions of our mind, in a certain degree; but no beast is a cook’. From the invention of cooking vessels by our unsung late Palaeolithic/Mesolithic hero stems the lasting and inseparable marriage between pottery, and food and drink: we all enjoy well-prepared, tasty food arranged in an optically pleasing manner on sparkling porcelain table ware possibly adorned with a colourful design palette. Most of us prefer being served our nourishment on ready to use, easy to clean and thus hygienically uncompromised ceramic plates as opposed to cheap plastic containers utilised by the fast food outlets of the modern ‘civilised’ world. Indeed, the connections between ceramics and food are manifold. First, storage containers for solids such as grain and dried vegetables, or liquids such as oil and wine were manufactured from clay by either coiling in the case of large pithoi, free-forming or, in the case of smaller amphorae, by wheel-turning. Second, cooking pots2 and strainers3 were among the

2

3

Analyses of the Neolithic ceramic assemblage from Franchthi Cave in southern Greece showed that cooking pots can be documented only close to the end of the Middle Neolithic. This raises questions about the close association of pottery and food preparation in the earlier Neolithic (Vitelli 1989). Abundant milk fat found in specialised perforated pottery strainers excavated at the Wolica Nowa, Stare Nakonowo and other sites near Wloclawek along the Vistula river in Poland and dated between 5200 and 4800 cal. BCE provides compelling evidence for the vessels having being used to separate fat-rich milk curds from the lactose-containing whey. At present this appears to be the earliest evidence for cheese making in northern Europe (Bogucki 1984, Salque et al. 2012).

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oldest functional kitchen products formed from clay. Third, ceramic shapes such as trays were utilized to serve food, and large jugs such as the Greek oinochoe and dinoi to serve wine. Lastly, dishes, plates, bowls, cups and beakers were used as tableware to display and savour food. The well-produced, aesthetically refined ceramic forms and shapes of tableware were largely reserved for the affluent portion of ancient societies. The poor had to do with crude plates and bowls made from wood. It is rather remarkable that this distinction by the quality of tableware between the well-to-do and the average people is still perpetuated today on our airplanes: compare the cheap plastic trays and containers in economy class with the noble china ware enjoyed by first and business class travellers! Ceramic is also the most preferred choice as far as kitchen tiles are concerned. Such tiles are extremely durable and can be cleaned by investing minimal time and effort. Today there is a wide variety of tiles in terms of sizes, shapes, textures and colours. More than that: recently modern advanced ceramics have conquered our kitchens as knifes, vegetable peelers and knife sharpeners fashioned from hard and tough partially-stabilised zirconia with unmatched and lasting acuity (Heimann 2010). In this treatise it is our aim to acquaint the reader with the science and technology as well as the artistry of selected ceramic traditions, in particular utilitarian wares fashioned by ancient master potters, and to juxtapose their plates, pots, beakers and cups with those food preparations that likely may have been prepared in and served on these ceramic objects in the distant past. The selection is, of course, by no means exhaustive in an encyclopaedic way but based on our personal preferences and whims. Thus we have been guided by historical significance, technological superiority, aesthetic appeal, and mastery of craftsmanship. Our subject matter reaches from early Near Eastern pottery of Mesopotamia, Anatolia, Egypt and Iran to the Meissen porcelain wonders, from the Greek Attic black-on-red and Minoan Crete masterpieces to English bone china, and from the highly standardized Roman Terra Sigillata to the celadon-glazed stoneware produced in the kilns of Sukhothai and Sawankhalok in ancient Siam of the 14th century CE. However, in a few cases we had to make allowance for the fact that some pottery, important as it may have been for the development of ceramics technology, was never directly used for food purposes but instead served to adorn the feasting tables of kings and princes. French soft-paste porcelain is a case in point. Also, we included several more recent ceramic wares from the 18th and 19th centuries CE to show the complete chain of technological accomplishments that allowed tracing the developments in an uninterrupted sequence. We like to emphasize that the art, structure and technology of some of these ceramic traditions have been magnificently researched and pleasingly displayed in the book ‘Ceramic Masterpieces’ written by David Kingery and Pamela Vandiver in 1986. While it is not our intention to compete with this seminal work, in our treatise we like to stress the congruence between functional cooking ceramic and food. Another important forerunner of our endeavour is Walter Noll’s ‘Alte Keramiken und ihre Pigmente’, published posthumously in 1991 (Noll 1991). It is the result of a true labour of love, providing a thorough analysis of the raw materials, decoration techniques and firing technologies of ceramic objects of the ancient cultural centres of Egypt, Greece, the Mediterranean islands Crete and Cyprus, Mesopotamia, Anatolia and Iran. Unfortunately this gem of a book was never translated into

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English and consequently did not reach the wide dissemination and acclaim it certainly deserves. Likewise arbitrary is our selection of food preparation recipes in a temporal and local perspective, thought to be contemporary to the creation of the ceramic objects. As the nature of the subject has it, much originality had to be sacrificed as many condiments, herbs and food stuff utilised during antiquity are not available anymore in our local supermarkets and greengrocer’s stores. Moreover, many of the ancient food preparations will not cater to our modern taste and nutritional requirements anymore. This makes adaptation of ancient recipes rather difficult and in some cases will falsify the original intent. Also translation from cuneiform or hieroglyphic texts may be ambiguous. For example, a recipe from ancient Mesopotamia reported in Chapter 8 mentions an ingredient called samîdu that could be either a kind of flour preparation or an (unknown) spice. An additional obstacle to transpose the ancient food recipes into modern reality relates to the fact that the former were generally only a shopping list of ingredients without any indication of the quantities used, preparatory steps taken, tools utilised, and cooking times applied. Likely, the authors of such recipes took for granted that their fellow chefs were well acquainted with the ways in which the ingredients should be prepared, combined, and grilled, fried, steamed, boiled, stewed or simmered. Hence the recipe became a mere reminder of what to use rather than a straightforward account to teach the apprentice cook how to prepare a wellcooked dish. In retracing the steps of these preparation procedures much creative ingenuity is required to achieve the desired savoury effect. Fortunately some guiding literature is available to aid in this endeavour (Garnsey 1999, Barham 2001, Dalby 2005, Grimm 2006). To give an impression of the content of this treatise, and the breadth of ceramic science and engineering, technology, and art of classic ceramics covered therein a short description of the individual book chapters is appended. Chapter 1 deals with the nature of ceramics, the role they have played during the cultural and technological development of ancient societies, and the transformation of ceramics as a product of holistic processes to the fruits of industrially determined prescriptive technologies. Depending on the composition of raw materials, the degree of clay processing, and the firing temperatures and atmospheres applied ceramics can be classified as shown in Chapter 2. In this context it should be emphasised that for classic ceramics frequently the term ‘pottery’ is used, defined as ‘all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products’ (ASTM 2012). Some archaeologists use a different understanding by excluding from ‘pottery’ ceramic objects such as figurines (see Chapter 14) which are made by similar processes, materials, and often the same people but that are not considered vessels per se. Traditional ceramics are based almost exclusively on naturally occurring raw materials, most commonly silicate minerals such as clays sensu lato, micaceous rocks, quartz, and feldspars. A smattering of other non-silicate oxide and carbonate minerals joins in such as calcite, dolomite, haematite, goethite, gibbsite, magnesite, and a few others. Origin, composition, structure, and properties of the typical clay minerals kaolinite, illite, montmorillonite, and palygorskite will be briefly discussed in Chapter 3.

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Processing of clays usually starts with cleaning and classifying by levigation (elutriation)4 of the raw materials, sometimes followed by mixing clays with different properties by blunging, wedging and kneeding, followed by shaping using a host of forming techniques, ranging from simple coiling and slabbing to wheel turning to slip casting, and, in modern times, to applying to special advanced ceramics cold (unidirectional) and hot pressing, or even hot isostatic pressing. After forming the green ceramic object must be dried to a leather-hard state that still allows final corrections, decoration by incising, scraping or other surface techniques, as well as painting and glazing. In the case of faience, that is, white tin-opacified glazed earthenware, and European white earthenware and porcelain a biscuit firing precedes the decoration and glazing. Some common forming techniques and the principles of drying as well as glazing and post-firing painting will be described in Chapter 4. The firing of ceramics includes high temperature transformations of the minerals contained in the raw materials that impart to the ceramic body typical mechanical, thermal, electrical, tribological and optical properties. The products of temperature-imposed transformation of clay can be identified and predicted using ceramic phase diagrams explained in Chapter 5. In this chapter a tutorial approach has been adopted to describe the construction of phase diagrams, to emphasize their significance and predictive power used to explain the occurrence of different mineral phases after firing of clays and, from this knowledge, to work back to define the nature and origin of clays and the technological processing steps applied including firing temperature and atmosphere, leading to the ceramic object. Firing of the ceramic body imparts rigidity and permanence to a ceramic vessel. While in the late Neolithic the firing temperature rarely exceeded 700–900 °C (Maggetti 2009, Maggetti et al. 2011a) and thus resulted in soft and abradable ware that during burial in acidic soil under humid climate conditions disintegrated with time, increasing control of the firing process in increasingly sophisticated kilns (Kingery & Vandiver 1986, Rehder 2000) led to admirable ceramic masterpieces, including high quality stoneware and porcelain fired to high temperatures. The materials science, mineralogy and technology of the ceramic firing process are being dealt with in Chapter 6. This is followed in Chapter 7 by a cursory review of basic types of pottery kilns and some examples of fuel consumption and economy in antiquity. Hence in the first part of this treatise (Chapters 1 to 7), basic facts of the science and technology of ceramics, their classification, natural raw materials, and properties, processing, forming, glazing, firing and uses will be briefly reviewed. In particular, the textural alterations will be described that clay minerals undergo in response to dehydration of the precursor ‘green’ masses as well as physico-chemical phase changes during oxidising and reducing ceramic firing. This knowledge will be used in the second part of the book (Chapters 8 to 20) to describe in more detail individual ancient ceramic wares, their tradition, historical roots and developments, technology, composition, properties, decoration, appearance, and artistic expression. We will show some typical examples without striving for encyclopaedic completeness and juxtapose them with likewise typical food preparations cooked accord4

The term levigation or elutriation describes a sedimentation process in which clay is being stirred up in water in order to remove non-clay impurities such as leaves and twigs, and to separate the finer from the coarser particles.

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ing to either original ancient recipes or ancient recipes adapted to modern cooking style and eating customs. In some cases, no written recipes were available, for example in the prehistoric civilisations of the New World. In other cases, for example in Chinese cuisine, the numbers of ancient recipes are so overwhelmingly large that we refrained from singling out a precious few. Apart from these restrictions, we invite the reader to marvel at the artisanship of master potters of times long gone and to undertake the sometimes quite arduous task of replicating the recipes provided. This may not always be successful at the first attempt. However, by trial and error triumph will be at hand and the effort will eventually pay off. Enjoy reading, cooking and savouring the results. Recently the production of ceramics as well as their provenance and use in the context of the intellectual environment of scientific archaeology have been excellently reviewed in the 50th anniversary issue of the prestigious journal ‘Archaeometry’ by Michael Tite, one of the pioneers of archaeometrical investigation of ancient ceramics (Tite 2008; see also Tite 2001, Tite 2009a). In his article the author calls for a much needed adoption of a holistic approach to the study of ancient ceramics in which production, from the winning and processing of raw materials to firing of the pottery, is integrated with the study of provenance and use. Another required effort calls for integration of the role functional ceramics are playing in subsistence systems and the identification of differences in settlement types. This integration has been attempted by modelling ceramic assemblage formation (Mills 1989). These views have recently been supported and strengthened by Artioli & Angelini (2011) who argued that mineralogy in particular is a key area of innovation for cultural heritage studies including investigation of ceramics. As the authors of the present book are mineralogists by training they feel qualified to enter the discourse. At the same time they want to emphasise that they have made any effort to describe correctly the archaeological and historical facts according to most recent research results. However, where they have failed in this endeavour they are asking the gentle reader to make allowances for any omission, misinterpretation, and factual errors. Görlitz, Germany Fribourg, Switzerland

Part I Fundamentals Chapter 1

The nature of ceramics Synopsis Ceramics are inorganic, non-metallic materials shaped at room temperature from various naturally occurring silicate-based minerals (clays) that obtain their typical physical and chemical properties by sintering at high temperature. Making of ceramics is a prime example of a generally observed trend that in all human societies with increasing control over the technological production environment a transition from holistic to prescriptive technologies occurs. Whereas at the formative stage of a society all technology is holistic by necessity, after accumulation of extensive practical knowledge and theoretical understanding generalisation and abstraction can take place. Only then can a prescriptive process emerge that in time achieves standardisation and organisation. In this regard the making of Roman Terra Sigillata or Chinese Ding wares were turning points in ceramic development as the holistic process of making pottery was replaced by a novel prescriptive technology that relied on process-determined division of labour, using the combined skills of many individuals as well as transfer of information by sets of self-normalizing ‘memes’, that is, ideas that appear to drive cultural including technological evolution. The chapter provides basic information on the development of materials technology and, in particular the commanding role pottery has played during evolution of ancient societies as well as the mutual interaction of the ceramic product and its production environment.

1.1 Materials and technological evolution of societies Of all man-made material things we depend on in our daily life ceramics are the most ancient ones. All material things possess either metallic, polymeric (plastic) or ceramic properties, distinguished by the nature of their chemical bonds. The particular bonding type imposes on them typical physical properties, for example high thermal and electric conductivities as well as ductility in metals, thermal stability, hardness and brittleness in ceramics including glasses, and low melting point and high elasticity in polymers. Information on bonding-property-application relationships of materials can be found in modern textbooks on materials science (see for example Smith 1996) and ceramics (see for example Kingery et al. 1976). Exploitation of existing, and development and use of novel materials are closely related to social development and technological progress of humankind. Ceramics are a case in point. Utilisation of natural ceramic materials such as rocks, flint and obsidian defined the earliest development stages of human societies. Fired clay products were to follow. In modern par-

2

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lance, traditional (classic) ceramics are inorganic, non-metallic and predominantly polycrystalline materials shaped at room temperature from various silicate-based raw materials. They obtain their typical properties by sintering at high temperatures and display an overwhelmingly wide variability in terms of origin, history, utilisation, and mechanical, thermal, and optical properties (Heimann 2010). Making traditional ceramics may be considered the result of an attempt to turn clays, the weathered remnants of natural rock, back into an artificial rock-like product by the action of heat (Heimann & Franklin 1979, Maggetti 2001). As indicated in Fig. 1.1 ceramics played a very important role during the early technological development period of mankind. The knowledge acquired during making of ceramics vastly exceeded that necessary to fashion simple tools from stone, wood or bone (Hench 1988). This knowledge, in particular mastering high temperature technology required to fire ceramic objects was the precondition of transforming ore into metals such as copper and iron, and its purification, alloying to form bronze or steel, and subsequent forging or casting (Heimann 2004). Fig. 1.2 shows the main differences in selection and processing among the material classes of stone, pottery and metals in a general schematic. From an archaeological perspective class I materials are naturally available ones such as rock, flint, obsidian or jade that were carefully chosen, separated from unwanted by-products, and fashioned by removing excess material. For class II materials, that is, ceramics per se the preparation of raw materials is more elaborate. By mixing with water and organic or

Figure 1.1. Historical timeline of development of materials (modified after Froes 1990, Heimann 2010). For discussion of Anthropocene see text.

1 The nature of ceramics

3

Identify

Separate

+

+

Select

Reduce in size

Class II: pottery Mix + Prepare

Finished Object

Class I: stone/flint/obsidian

Heat Mix + Heat + Cast Raw metal

Alloyed metal

Class III: metal

Figure 1.2. General schematic classification of materials selection and processing from an archaeological perspective (adapted from Franklin 1983b).

inorganic temper, natural clays are transformed into new composite materials, usually by addition rather than removal of material. Heating generates a new rock-like material, not existing in nature, as well as a new object. Class III materials such as metals require much more complex and sophisticated selection and processing steps. It is not sufficient anymore to select and mix the starting materials. Instead, as the raw material preparations are not longer directly related to the finished object they become a separate and separable production activity, involving mixing (alloying), heating, quenching and other steps, and finally casting, hammering or forging. Hence in contrast to stone or pottery most metals are derived materials that are produced from a suitable ore by a smelting process (Franklin 1983b, see also Tylecote 1992). As a result of these hierarchical technological development steps, around 1500 CE ceramics technology was quantitatively overtaken by metals technology. Then the large-scale production of cast iron and steel replaced bronze as base material to cast gun barrels, and thus gave rise to so-called ‘gunpowder empires’ such as Ottoman Turkey, Mughal India, Persia and Ming China, the territorial expansion of which depended on guns (Hodgson 1975, Pacey 1991). The predominance of metals as a choice construction material lasted until the 1970th when ubiquitous application of engineering polymers (‘plastics’ and elastomers) and their composites reduced the economic impact of metals. Moreover, in parallel a second ‘ceramic age’ emerged highlighted by the development and practical use of tough engineering, functional and other advanced ceramics (Heimann 2010). Recently the term Anthropocene (Fig. 1.1) has been popularised by the Nobelist Paul Crutzen (2002) to mark both the evidence and the lasting effect that human technological activities have on the state of the Earth. Human activities triggered those processes that are significantly and irreversibly changing many global ecosystems, including the biosphere,

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Part I

pedosphere, hydrosphere, and atmosphere. Among those who subscribe to this new concept of extending the geological timescale beyond the Holocene there is disagreement on the start of the Anthropocene. Whereas Crutzen (2002) takes the onset of the Industrial Revolution around 1800 CE as the starting point, others maintain that the acme of the Roman Empire is a plausible begin (Certini & Scalenghe 2011) as marked by the discovery of large-scale lead pollution by Roman industrial activities (Hong et al. 1994), or even much earlier by the transition from hunter-gatherer to sedentary agricultural societies during the later stages of the Neolithic Revolution (Ruddiman 2003) that led eventually to the extinction of large mammals and land birds, and alteration of the composition of soils (Amundson & Jenny 1991).

1.2 Ancient roots The quest of discovering, defining and explaining the nature of our material world has deep historic roots. According to the teachings of the Greek natural philosopher Empedocles (495–435 BCE), laid down in his text ‘On Nature’ (ˎˢˮఖ˳ఛ˰ˢ˶˯ ˎˢˮఖ˳ఛ˰ˢ˶˯, Peri phýseōs), all matter is thought to be composed of only four immutable elements he referred to as ‘roots’: earth, water, fire, and air5. Based on the different proportions in which these four eternal, noncreated, indestructible and unchangeable ‘roots’ are combined with each other, structural differences of matter are generated. It is in this interplay between aggregation and segregation that Empedocles, like the more or less contemporaneous Greek atomists Leukippos and Democritos (460–390 BCE), saw the real process which corresponds to what we call growth, increase or decrease of things and actions. Nothing new can ever come into being; the only changes that can occur are variations in the juxtaposition of one root with the other roots, and their relative proportions. This hypothesis commanded remarkable power and longevity as it became the standard dogma for the next two thousand years until the Irish natural scientist Robert Boyle (1627–1692), called the ‘Father of Chemistry’, considered the Empedoclesian view nonsensical. However, in a leap of imagination, ceramics may be seen as the material embodiment of this Empedoclesian paradigm, being made by artfully combining the four ‘roots’ or ‘elements’: earth-like clay soaked in water to gain workability is subsequently exposed to fire in either oxidising or reducing atmosphere, that is, air. Hence the product of this creative process aggregates synergistically all four ancient ‘elements’ and thus is a powerful symbol of the eternal harmony of all things and beings in the antique world view. In modern scientific parlance we are indeed dealing with a juxtaposition of the two universal empowering forces, energy and entropy (information). To quote the quantum physicist Seth Lloyd: ‘Earth, air, fire, and water …are all made of energy, but the different forms they take are determined by information. To do anything requires energy. To specify what is done requires informa5

˱ఓ˰˰˞ˮ˞ ˠఐˮ ˭఑˪˱˶˪ ౾˦ˣఝ˩˞˱˞ ˭ˮಌ˱ˬ˪ ர˧ˬ˲ˢʷ ˄ˢచ˯ ஬ˮˠఔ˯ ௕ˮˤ ˱ˢ ˳ˢˮఓ˰˟˦ˬ˯ ைˡಬ ழ˦ˡ˶˪ˢఛ˯ ˋౢ˰˱గ˯˥ಬோˡ˞˧ˮఛˬ˦˯˱ఓˠˠˢ˦˧ˮˬఛ˪˶˩˞˟ˮఙ˱ˢ˦ˬ˪ (Sext.10,315). Translation: At first hear the four roots of all things: Zeus the Radiant [the etheral fire], and Hera, the Life giving [earth] as well as Aidoneus [the invisible air] and Nestis [the water], who lets flow through her tears the earthly springs (Sextus Empiricus 1997).

1 The nature of ceramics

5

tion’ (Lloyd 2006). Thus the art and craft of the potter involve not only energy and, respectively its equivalent mass, but also information, knowledge, experience and judgement. In terms of modern physics this boils down to the universal interplay between energy and entropy. Since times immemorial people transformed matter – in metallurgy, in preparing of medicines, potions and cosmetics, in dyeing fabrics, in cooking and, yes, in making ceramics. Some of these transformation processes were shrouded in mystery: alchemists searched for the Philosopher’s Stone able to transmute humble lead or mercury in shiny gold. At the outset of the 18th century European effort to recreate Chinese porcelain was initially based on alchemy. Johann Friedrich Böttger was pressed by his king, Augustus the Strong, to make gold by way of alchemy but what he found instead was the ‘white gold’, porcelain. Porcelain as the product of a very special kind of transformation process appeals to all human senses as succinctly and lovingly expressed by Roald Hoffmann, winner of the 1981 Nobel Prize in chemistry, in his essay on ‘Meissen chymistry’ (Hoffmann 2004): ‘Alchemy [was] a unique cultural experiment, which adopted chemical change (as we now know it) as a symbol, a kind of logo, for its philosophy of transformation. ... So the philosophy of change took on a chemical face. And then, I imagine, was co-opted by it. Alchemists became chemists.... One could make stoneware and glass, use them in everyday life. But anyone who has held a fine Song or Koryo vessel in one’s hands, rotated it, followed the fine crackle, I think feels that porcelain is something more. It is sublime. To aspire to transform mere clay into that refined essence that catches light and begs to be held as no other ceramic does—that vision takes more than laboratory skill. The synthesis … of porcelain demands faith in the possibility of transformation and a conviction that nature can be improved’.

1.3 Holistic and prescriptive technologies Throughout this book, our main task will be to analyse technological processes leading to pottery in a historical perspective. For this it is helpful to classify any technological development process by two terms elaborated on by the eminent Canadian scholar of ancient materials, Ursula Martius Franklin (1977, 1983a, 1999): holistic and prescriptive. Holistic processes may be associated with crafts and artistry, prescriptive ones with industry. During much of its history the process of making household pottery was, with a few notable exceptions, a typical holistic process involving a single, step-wise approximation towards the final object whereby the potter, starting with a conscious selection of appropriate raw materials, must master the whole succession of steps required to produce the pot (Fig. 1.2). The salient questions of who chooses and why such choices need to be made have been addressed recently in a series of papers presented to the World Archaeological Congress 4 (1999) that attempted to put the process of making pottery into a socio-cultural context, linking the producer to the consumer of pottery (Sillar & Tite 2000, Livingstone Smith 2000, Sillar 2000, Pool 2000). A lively discussion around these papers has added much additional background and compelling insights into the paradigm of technological choices in ceramic production and dispersion (for example Cumberpatch 2001, Griffiths 2001, Kolb 2001). As it turns out the holistic process is a sequential, linear development as each small step depends on, and is determined by the successful outcome of the preceding step. The potter

6

Part I

is always in control, and his or her knowledge, experience and judgement determine the sequence of the process as well as its end (Franklin 1980). This actually comes with a penalty. Owing to the nature of ceramics, the raw materials used in their production have a much larger impact on their final properties than is the case of metals and also polymers. This is because, beyond the initial basic cleaning and aging treatment of clays, there are no intermediate refinement/purification steps for ceramics as there are for metals that may include melting, solidification, refining and plastic deformation designed to improve the properties of the end product. Not so in the case of ceramics. All imperfections inherent in the ceramic paste and the resulting green body propagate into magnified imperfections of the fired product. This has been dubbed the ‘domino effect’ that emphasises the dependence of the final properties of the ceramic product on the characteristics, and error and failure range of every processing step, and in particular on the characteristics of the raw materials such as clay, quartz, and feldspars. None of these minerals used in processing traditional ceramics can be treated as well-defined compositions. This means that they do not have the compositions given by their (idealised) chemical formulae and consequently impart a large variability on the pottery produced therewith. Adding the variability inherent in the technological process such as type of forming, extent of drying, firing time, firing temperature, firing atmosphere and several other factors including applying decorations by painting and glazing it is evident that the end product of this chain of production steps is uniquely dependent on a myriad of factors and their interactions. On the other hand, prescriptive technologies involve what anthropologists call the division of labour: the total work process is subdivided into rather simple unit processes that represent autonomous skills and thus draws on different groups of workers. Hence a considerable degree of abstraction and a solid technical understanding are necessary to integrate the individual unit processes by appropriate measures of work organisation. Often special skills became exclusive prerogative domains of specific clans, groups or families to arrive at specialisation according to the type of product, for example one potter may produce utilitarian vessels such as pots, jars and pans for everyday use while others may produce exclusively pottery for religious rites such as vessels and figurines for divination (Herskovits 1952). There is also a notion that in some instances geologically similar clays from individual sedimentary strata of a common deposit were utilised to produce different ceramic wares with different functional characteristics caused by variable chemistries and grain size distributions (Michelaki & Hancock 2011). However, despite these specialisations the production process of pottery in essence was still holistic. The development of ceramics technology from holistic towards prescriptive may be recast in terms of the modern concept of ‘strange attractors’ (Ruelle 1980) that was introduced to describe the complex, in fact chaotic and thus non-deterministic interaction of technology and society (Kafka 1994). Changes in societal structures, technologies, and also materials utilisation require a paradigm shift (Kuhn 1996) that corresponds to a jump from one attractor to the next probable one, that is, a neighbouring attractor6. This next-nearest attractor

6

An attractor is a pattern in some sub-space of possibilities that interacts (attracts) with the development path of a neighbouring subsystem. ‘We actually might call it the “idea” of this pattern. An attractor that has proven its viability is likely to be used as a building block in the evolution of still

1 The nature of ceramics

7

can be reached by the stochastic process of random oscillations within the phase space of possibilities. During history all technologically oriented societies selected only those attractors, that is, thinking (‘ideas’) and action (‘skills’) patterns that had proved to be viable and hence successful, and stable over long times, providing the misleading impression of everlasting, eternal continuity in the way societies and their adopted technologies worked. In the case of ceramics technology, making pottery was determined by a succession of identical ideas, materials, tools, production steps, and working organisation: in potter dynasties the lives of great-grandfathers, grandfathers, fathers and sons differed little since their specific thought and action patterns were preferred and sustained over long times (Heimann 2004, see also Nicklin 1971). Only a paradigmatic shift in work organisation broke this equilibrium by moving from a holistic to a prescriptive attractor, involving differentiation of work along unit processing steps. An important early example of this tendency is the manufacturing of Roman Terra Sigillata (see Chapter 10) during which labour was organised and subdivided (Roberts 1997): different groups of workers mined clay, prepared the ceramic masses, made moulds, formed the vessels using these moulds, gathered fire wood, stoked the kilns, tested, sorted, crated and shipped the finished pots. A similar trend towards industrialisation occurred later in China. Mass production of Ding whiteware in northern China commencing during the Tang dynasty (Harrison-Hall 1997a) and blue-and-white porcelain in southern China starting in the Yuan dynasty and peaking during the Ming and Qin eras (Harrison-Hall 1997b) relied on establishing working groups within pottery factories, each performing different and highly specialised tasks. This is seen as the onset of the transition from holistic to prescriptive pottery production that in the end resulted in industrial technologies that are commonplace in ceramics industry today. However, ceramic industry is unique in that even today prescriptive technologies never completely replaced the holistic processes by which ceramic artisans create their objects. From the above it follows that making ceramics provides a prime example of a generally observed trend in technology development. At the formative stage of a society all technology is holistic. Only after extensive practical knowledge and theoretical understanding has been accumulated generalisation and abstraction can take place. Only then can a prescriptive process emerge that in time achieves the necessary standardisation and organisation. In this regard Roman Terra Sigillata production (see Chapter 10) was arguably a turning point in ceramic development as the holistic process of making pottery was replaced by a novel prescriptive technology that relied on process-determined division of labour, using the combined skills of many individuals. It should be emphasised that the presence and utilisation of machinery is not a prerequisite of this development. Instead the essential characteristic and requirement for the prescriptive process is control that includes being in command of material resources including money, information, and people. Information does not only include technical knowledge but also the tools of control such as regulations, co-ordination, supervision, and not in the least, enforcement.

higher and more complex structures. These new structures derive their own viability from the fact that they organise the fluctuations of their constituents even better than the old ones, protecting them from all stronger, disruptive interactions, and thus stabilising their “attractivity”’ (Kafka 1993). In this way ‘strange attractors’ are corollaries of self-normalizing ‘memes’ (Dawkins 1982, 1989) or ‘memeplexes’ (Blackmore 2000).

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Part I

In contemporary theoretical anthropology attempts have been made to explain technological evolution through spreading of informational entities interacting with the environment to cause differential replication (Hull 1989, Feathers 2006). This process is more clearly operational in biology where the lower level information unit ‘gene’ controls the development of the higher level unit ‘organism’. Along these lines Dawkins (1982) compared cultural evolution loosely to biological evolution by coining the term ‘meme’ (Dawkins 1989) that is, the informational unit ‘idea’ that drives cultural including technological evolution in the same way as the concept of ‘strange attractor’ discussed above does. Indeed, memes are ideas, behaviours, or skills that can be transferred from one person to another by imitation7: stories, fashions, inventions, recipes, as well as ways of doing things such as throwing a pot, glazing a ceramic bowl, or making a ceramic figurine (Boyd & Richerson 1988). During transmission of the skills of making pottery from master to apprentice the latter imitates the process by watching without necessarily copying each discrete movement or production step demonstrated by the master potter but instead acquiring knowledge by inference. Hence there are thought to be two modes of memetic evolution, the Darwinian mode of copying instructions by imitation, and the Lamarckian mode of copying the product by inheritance (Blackmore 1998). ‘Self-normalizing’ memes or sets of memes (‘memeplexes’, Blackmore 2000) may evolve by ‘natural’ selection in a manner analogous to that of biological evolution through variation, mutation, competition and inheritance, each of which influence their reproductive success and hence their longevity and geographic diffusion.

1.4 Ceramics and their production environment The ‘Ceramic Age’ (Fig. 1.1) started at a time when Neolithic hunter-gatherer bands turned into agrarian societies, about 12,000 to 15,000 years ago. However, during the past decades the assumed onset of ceramic technology has been pushed back by archaeological research well into the Upper Palaeolithic/Mesolithic period. Traditionally the subdivision of the archaeological record of the development of human culture and civilisation uses materialrelated terms such as Stone, Bronze, and Iron Ages that were devised by 19th century European praehistorians. The term ‘Ceramic Age’, however, transcends this still popular division as ceramics were made and used during all stages of the classic ‘three-age’ system. Indeed, quantitatively the ceramic record exceeds vastly that of the name-giving objects at least in the Bronze and Iron Ages whereas in the ‘Stone Age’, that is, the Mesolithic/Neolithic only stone implements have survived the ravages of time whereas the majority of the (low fired) ceramic objects (‘Urkeramik’; Weiss 1979) have perished, leaving only a small fraction of objects protected intentionally by burial at gravesites or stored safely at palatial and religious buildings. Archaeologists have cast a still wider net to assess the ceramic record beyond its materials content in context with its production environment (Chen 2006). The concept of ceramic ecology connects archaeology, ceramic technology in its chemical and physical expressions, and ethnology to arrive at a statement of cultural significance that was expressed by

7

Blackmore (1998) defines meme simply as ‘that which is passed on by imitation’.

1 The nature of ceramics

9

Ceramic Complex

Culture

Physical Environment

Human Biology Biological Environment Figure 1.3. Five-variable model of ceramic ecology (Kolb 1982).

Frederic Matson (1965) as follows: ‘Unless ceramics studies lead to a better understanding of the cultural context in which the objects were made and used, they form a sterile record of limited worth’. Many variables such as the biological and physical environments, the ceramic complex including clay, water, fuel, the potter, the work organisation, and the ceramic end product, human biological environment including food and its processing, and their many interactions provide a cultural framework within which the archaeological object ‘ceramic’ must be assessed. There exists a transitory equilibrium among these parameters that is easily altered by shifting emphasis from one to the other. Figure 1.3 shows schematically the interactions of the five primary variables in such transitory equilibrium (Kolb 1976, 1982). Within the cultural framework ceramics might be assessed, analysed and evaluated using the tools of material culture theory and methods based on the fact ‘that the existence of a man-made object is concrete evidence of the presence of a human intelligence operating at the time of fabrication. The underlying premise is that objects made or modified by man reflect …the beliefs of individuals who made, commissioned, purchased, or used them, and by extension the beliefs of the larger society to which they belonged. The term material culture thus refers quite directly, if not elegantly, both to the subject matter of the study, material, and to its purpose, the understanding of culture’ (Prown 1982). For the sake of completeness it should be mentioned that today we live in what may be called a second ‘Ceramic Age’ characterised by increasing uses of advanced ceramic materials (see Fig. 1.1). In contrast to traditional ceramics based on naturally occurring raw materials, advanced ceramics are produced from chemically synthesized micro- to nanoscaled pure alumina (Al2O3), titania (TiO2), zirconia (ZrO2), magnesia (MgO) and other oxide powders and their compounds as well as carbides and nitrides of silicon, boron, aluminium, and a host of transitional elements. Advanced processing technologies include high temperature transformation of the raw materials into the desired ceramic body with highly controlled mechanical, thermal, electrical, electronic, tribological and optical properties but also low temperature hydrolysis of calcium silicates and aluminates to synthesise chemically bonded ceramics (CBCs) such as advanced concrete. Hence modern application of ceramics sensu lato spans the chasm between traditional silicate-based structural

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Part I

materials such as bricks, earthenware, stoneware, porcelain and concrete, and high-tech structural and functional advanced ceramics as for example, thermal barrier coatings for aerospace gas turbine blades, electrolyte layers for high temperature solid oxide fuel cells, ferroic ceramics for sensor and actuator applications, diamond single crystals for future carbon-based integrated circuits, and bioconductive monolithic parts and coatings for bone reconstruction, and dental and endoprosthetic hip implants (Heimann 2010).

1.5 Ceramics and cooking In this treatise we will concentrate exclusively on traditional (classic) ceramics, the products of processing, forming and firing of natural clays. More specifically, our main subject will be utilitarian pottery such as cooking pots, plates, dishes, bowls and beakers ubiquitously used to prepare, display, serve, and store food. In pre-ceramic time basic preparation of food involved techniques that could be carried out in a campfire such as roasting, grilling, smoking and desiccating. Meat could also be boiled in holes dug into the ground and filled with water into which heated stones could be dropped. Temporary ‘cooking pots’ were made by wrapping food in layers of clay and heating it in the embers of a camp fire. However, true cooking by close control of temperature and time required refractory, that is, permanent fireproof containers that were invented during the late Neolithic in the Near East. Mass production of these early cooking pots in Mesopotamia truly revolutionised the art of cooking: not only food and accompanying sauces and gravies could now be processed but bread could also be baked, and cereals of all kind were turned into nourishing and easy to digest gruel. Most importantly, cooking pots allowed boiling food stuff and, in particular, meat in water. This was indeed an important paradigm shift in food processing that opened up new opportunities to make meat and vegetables more palatable by adding richer flavours and succulent taste, and also provided a gentle, easy to control treatment of heat-sensitive food ingredients (Bottéro 1995). Since most if not all ancient societies were economies of scarcity, cooking meat in water instead of frying it on a spit also retained much of energyproviding lipids that were otherwise lost (Hirschfelder 2005). Cooking pots allowed preparing food with minimum contamination from smoke, ash and soil hence providing not only cleaner food by also widening the range of taste. Nevertheless, as far as food hygiene, that is, bacterial contamination is concerned no improvement could be expected over food prepared in a camp fire since cleaning of the rough inner surfaces of a ceramic cooking pot would indeed have been a tough problem (Brothwell & Brothwell 1997). Moreover, larger ceramic jars were now available to safely store not only solids such as grain, herbs and desiccated fruits but also liquids such as oil and alcoholic beverages such as beer and wine. To reduce the inherent permeability of low-fired pottery, inner and outer surfaces were burnished, coated with resin, tar or vitrifiable clay slips, or covered with alkali silicate-based glazes. On the other hand, the high permeability of low temperaturefired ware provides the mean of evaporative cooling of the water content of pottery jars in hot climates (Schiffer 1988).

1 The nature of ceramics

11

1.6 Ceramics as subject of archaeometry Modern students of ancient ceramics can hugely benefit from the achievements of the scholarly discipline of archaeometry, invented fifty years ago that deals with the study of cultural objects by modern analytical methods. Archaeometry has greatly influenced and therefore enriched contemporary archaeology by going beyond the triad of sensory experience, intellectual engagement, and emotional response (Prown 1982) a ceramic object elicits in the eye of a classical archaeologist. Since archaeometry represents the interface between archaeology and the physical sciences its subject matter includes also coping with the dichotomy between deductive and inductive modes of thinking. This also requires integrating both functional information and assumptions, and unique historical trajectories to understand the prehistoric technological change in general, and ‘to get beyond the processural/post-processural debate that seems to dwell in either the functional or historical camp but not both’ (Feathers 2006). Regrettably this leads to another dichotomy. On the one hand, a strictly functional approach cannot account for the source of technological change but can determine only its direction. On the other hand, a strictly historical approach cannot account for direction but is only able to pinpoint the source of changes. Hence what is required is a synergistic interaction of both types of information of looking at the same object: the ceramic pot. This is where archaeometry comes in, the interdisciplinary nature of which requires close interaction and collaboration among archaeologists, art historians, museum curators, and physical scientists who apply modern instrumental techniques to extract structural and compositional information from ancient ceramic materials and objects. This collaboration of academics with diametrically opposed training and thought pattern requires the ability to overcome the frequently self-isolating terminology used by the partners by ‘learning’ the specific vocabulary, thesaurus, and thought pattern of the other partner like any foreign language. At best this results in a synergistic interaction and scientific success along the dictum expressed by the Nobelist Werner Heisenberg: ‘The most fruitful developments have always occurred in areas where two different modes of thinking have met’ (Heisenberg 1984).

Chapter 2

Classification and properties of ceramics Synopsis A wide spectrum of pottery types was created by ancient potters depending on the mineral content, grain size distribution, and beneficiation of the raw materials, the composition of glazing materials, the firing temperature, firing atmospheres, and firing schedule, the number of firing cycles, as well as post-firing decoration techniques. Modern classification of these types has been based on porosity and water absorption capacity, respectively that are functions of the firing temperature, as well as their fabric, being coarse or fine, depending on the grain size distribution of the non-plastic temper materials, and firing colour that for iron-rich clays is dependent on firing atmosphere, being either oxidising or reducing, or a mixture thereof. The chapter also discusses technological requirements for ceramic cooking vessels and attempts of ancient potters to solve the conundrum of disparate mechanical and thermophysical properties of calcareous and non-calcareous clays.

2.1 Classification and types of ceramics Historically, silicate-based ceramics have been classified in various ways. Very generally, all pottery can be subdivided, based on their water absorption capacity (WAC), into low-fired earthenware with WAC > 5 mass% and high-fired stoneware with WAC < 2 mass% (Hamer & Hamer 2004). According to this classification scheme earthenware includes red- and white-firing pottery, depending on the iron content of the raw clay materials, and includes such diverse pottery types as Greek slipware and Roman Terra Sigillata, Japanese Raku, Italian and French maiolica and faience, as well as British creamware and French calcareous white earthenware. On the other hand, white high-fired stoneware (‘whiteware’) includes stoneware sensu strictu and translucent porcelain. Here the distinction between ceramic types is essentially made based on firing temperature that controls the degree of sintering and thus dictates the degree of water absorption capacity. A somewhat different classification scheme (Hennicke 1967) divides ceramic wares according to the grain size of the macroscopically visible inhomogeneities in the final ceramic product (coarse: > 0.1…0.2 mm; fine: < 0.1…0.2 mm), porosity of the fired product as determined by water absorption capacity, and colour of the fired ceramic body (Fig. 2.1). Accordingly, coarse products include porous construction ceramics such as bricks, terracotta and chamotte with WAC > 6 mass% and dense ones such as clinker and tiles with WAC < 6 mass%. Fine products include the bulk of non-construction related pottery such as porous (WAC > 2 mass%) coloured-firing earthenware (in German: Irdenware, in Italian: terracotta, in French: terre cuite) and white-firing earthenware (Queen’s ware, creamware; in German: Steingut, in Italian: terraglia, in French: faïence fine; Maggetti et al. 2011a) as

2 Classification and properties of ceramics

Coarse porous > 6 mass%

dense < 6 mass%

coloured

coloured

13

Fine porous > 2 mass%

coloured

light to white

dense < 2 mass%

coloured to white

white

stoneware

porcelain

earthenware coloured

white

Examples:

bricks pipe terracotta chamotte refractories

clinker tiles construction ceramics

flower pots table ware pottery sanitary ware

sanitary ware tiles vitreous china

table ware insulators spark plugs dental porcelain

Figure 2.1. Classification of clay-based ceramics (after Hennicke 1967).

well as dense (WAC < 2 mass%) stoneware (in German: Steinzeug, in Italian and French: grès) and hard-paste (triaxial8) porcelain. The Hennicke classification includes as distinguishing parameter besides firing temperature also the fabric of the resulting ceramics, being coarse or fine. These somewhat different classification methods resulted in the past in considerable confusion when it came to assigning archaeological ceramic objects to different technological types mentioned above. Hence it will be useful to define the individual types as follows.

2.2 Definition of common ceramic types Coloured earthenware is the product of low temperature firing of predominately iron-rich, coloured-firing clays that produce many shades of red colour when fired under oxidising conditions but grey to black colours when fired under reducing conditions. The clay raw materials may be beneficiated by removal of coarse impurities such as twigs, leaves, pebbles and concretions. Larger quartz, pyrite or calcite grains are frequently separated out by washing and elutriation. The firing temperatures of modern earthenware are usually between 950 and 1100 °C. In antiquity firing temperatures were much lower, between 600 and 900 °C (Nelson & Burkett 2002), and thus have been accessibly to early potters in simple bonfires, later wood-fired 8

The term ‘triaxial’ refers to the way the three ingredients kaolinite, feldspar and quartz can be displayed in a triangular ‘phase’ diagram (see Chapter 5).

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and also coal-fired kilns. Special wares such as Roman Terra Sigillata were already fired in rather sophisticated kilns at temperatures reaching up to 1100 °C (Heimann & Franklin 1979). Nevertheless, densification through formation of copious amounts of vitrified materials was restricted and open porosity prevailed, making earthenware mechanically less strong9 and tough10 while more porous11 than stoneware (see below). Owing to its higher (open) porosity, earthenware must either be coated with an impervious slip (engobe) or be glazed to be watertight (see Chapter 4.5). The suitable firing temperature will be influenced by the mineralogy of the raw materials used, in particular the nature and amount of fluxing agents, and the desired characteristics of the finished ware. Application of higher firing temperatures to improve densification is likely to cause earthenware to bloat or warp. This is particularly obvious in highly calcareous clays that have a rather small range of suitable firing temperature (see Fig. 6.9). After oxidising firing, the body is porous and opaque with colours ranging from light orange to dark red depending on the iron oxide and lime contents of the clays used12. Virtually all prehistoric pottery as well as the Greek and Roman wares can be classified as earthenware. White earthenware was simultaneously invented in England and France (Maire 2008). It is reasonably dense and, with the exception of highly calcareous ware, hard enough to resist scratching by a steel point. It also differs from stoneware as it is only partially vitrified. It is usually coloured whitish because of impurities in the clay used for its manufacture, and is normally glazed to achieve impermeability (Dodd & Murfin 1994). White earthenware is produced by firing of iron-poor clays. Biscuit firing temperatures are c. 900 °C for calcareous white earthenware and c. 1250 °C for siliceous and mixed white earthenware, the latter having considerably higher temperatures than used for (coloured) earthenware pottery that also depend on the flux content. Owing to the rather low firing temperature densification is impeded and consequently glazing is required to provide water tightness. Traditionally salt glazing, wood ash glazing, and glazing with a sealing layer of high lead silicate content were used in antiquity whereas today feldspar-based glazes are applied by glost (glaze) firing at 900–1200 °C (see Chapter 4.5). Water absorption of white earthenware products exceeds 2 mass% by far, and is frequently between 10 and 14 mass% (Heuschkel & Muche 1974).

9 In the pertinent literature strength is reported as either compressive, bending (flexural) or tensile strength depending on the mode of testing and the direction of load. The unit is MPa. 10 Fracture toughness or stress intensity factor is the resistance of a solid body to propagation of a crack. The fracture toughness of a ceramic body can be improved by providing reinforcement through crack arrest, blunting of crack tips or crack deflection by tempering additions such as straw, sand, grog or shells. The unit is MPa·√ m or MN·m–3/2. 11 Porosity is the presence of voids in an otherwise solid body. The property of water tightness requires a high proportion of closed, that is, sealed porosity as opposed to open porosity that provides access paths to the surface and thus aids leakage and evaporation of water. In hot climates earthenware vessels with a high degree of open porosity are used to cool, by evaporation, its liquid content. 12 Whereas iron-rich clays attain a grey to black colour during reducing firing, Mn-rich clays yield black colours during oxidising firing (see Chapter 4.5). Variation of the oxygen partial pressure in the firing atmosphere of primitive kilns frequently lead to uneven colouration, with red on one and black on the other side of a pot.

2 Classification and properties of ceramics

15

Calcareous (soft) white earthenware is an 18th century CE French invention (Maire 2008, Maggetti et al. 2010). Suitable masses consist of 50–55 mass% clay, 35–45 mass% quartz, and 5–10 mass% calcite. The firing temperatures are around 1050 °C but may go up to 1150 °C. In contrast to this, feldspathic (hard) white earthenware (creamware, pearlware) was developed in Staffordshire, England around 1720 and later improved and widely sold by Josiah Wedgwood13. Typical compositions are 40–55 mass% clay, 35–55 mass% quartz and 3–12 mass% feldspar. This ware has been fired at somewhat higher temperatures between 1200 and 1250 °C. Mixed white earthenware is being produced from masses consisting of 50–55 mass% clay, 35–45 mass% quartz, 2–4 mass% feldspar and 2–6 mass% calcite at 1180 to 1200 °C (Heuschkel & Muche 1974). Stoneware is produced from well-processed fine, frequently aluminous refractory clay, carefully selected, prepared and only rarely blended to obtain ceramics that in contrast to earthenware are sintered to an almost dense, vitrified, non-translucent state, with water absorption normally much below 2 mass%. The Dictionary of Ceramics (Dodd & Murfin 1994) provides the following definition: ‘Stoneware, which, though dense, impermeable and hard enough to resist scratching by a steel point, differs from porcelain because it is more opaque, and normally only partially vitrified. It may be vitreous or semi-vitreous. It is usually coloured grey or brownish because of impurities in the clay used for its manufacture, and is normally glazed’. Stoneware pottery can be considered the historical precursor of true hard-paste porcelain from which it is distinguished by its lack of translucency. It was produced in copious amounts in China already during the late Shang dynasty (c. 1700–1028 BCE; He 1996) and reached its acme during the Han (202 BCE–220 CE) and Yuan (1280–1644; Medley 1974) dynasties (see Chapter 18). Many of the early stoneware products were coated with a greenish celadon glaze thought to imitate highly priced jade. Important stoneware traditions existed in Japan, Korea and the Indochinese realm including Thailand, Laos and Vietnam. At the beginning of the 14th century CE a special development occurred in the Rhenish part of Germany (Frechen, Cologne, and Siegburg) based on high-refractory clays that reached its zenith between about 1440 and 1620 (see Chapter 11). The products were blueish-grey plates, bowls and drinking vessels (so called Schnellen and Bellarmine (Bartmann) jugs), salt-glazed to improve their appeal and to provide added smoothness to aid easy cleaning. The typical grey body of Rhenish stoneware was caused by reducing firing combined with quick cooling of the fired product since during slow cooling the surface would have been reoxidized to form a brownish hue. During firing rock salt was added to the kiln from above that in the high temperature zone evaporates. The vapour reacts chemically with the hot ceramic surface and forms a thin glassy layer of sodium aluminium silicate that seals any remaining porosity (see Chapter 4.5). A quantitatively less important stoneware with nevertheless high artistic appeal was produced from the 14th century CE onwards in central (Bautzen, Waldenburg) and eastern

13 There is the notion that early cream-coloured earthenware made by Wedgwood did not contain any feldspar. Confirmation, however, has to await future archaeometrical work on this class of pottery.

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Part I

Germany (Bunzlau). The 18th and 19th century CE Silesian Bunzlau ware is salt-glazed and shows a peculiar bright and rich rust-brown colour (Fig. 11.15) that distinguishes it from the blueish-grey Rhenish ware. Porcelain is considered the golden standard of ceramic technology and artistry. It is a ceramic material with a dense, white, translucent body and coated with a vitreous, transparent glaze. Painted decoration can be added either as underglaze pigment on the dried and/or biscuit fired body or as overglaze enamel paint on the glost (glaze) fired body. Underglaze paint is very durable while protected by the transparent glaze. However, the colour palette to be used is restricted since only few ceramic colour pigments such as transition metal spinels are resistant against the corrosive attack by the liquid glaze during glost firing. Colouring metal oxides of copper, cobalt and manganese can also be stabilised by dissolution in the glaze. The name porcelain derives from the Italian porcellana, meaning cowrie shell (Cypraea nebrites) the whiteness and smoothness of which the ceramic ware appears to resemble. The first porcelain-like ceramic was arguably produced in China as early as the Tang dynasty (618–906 CE). While in the Chinese ceramic tradition fine-textured hard dusky and greyish wares were included in the designation of ‘porcelain’, European porcelain falls into two categories: true or hard-paste (pâte dure) kaolinite-based porcelain and artificial or soft-paste (pâte tendre) frit-based porcelain, the distinguishing criterion being the higher amount of fluxing constituents in the latter. Chinese soft-paste porcelain, fired between 1200 and 1300 °C, is produced from kaolinite (china clay), petuntse (from ⓑቛᏊ, in pinyin: bai dun zi) or china stone, the historic term for a wide range of micaceous or feldspathic rocks, and fine quartz sand. In contrast to this the classic hard-paste ‘triaxial’8 porcelain consist of 50 mass% kaolinite, 25 mass% feldspar and 25 mass% quartz whereas the content of fluxing, that is, alkali-containing feldspar is higher in soft-paste porcelain to yield ratios of approximately 30/50/20. However, in the ceramic praxis there are many deviations from these standard values. A special variant of porcelain has been invented in Britain in the mid-18th century, based on a mix of calcined animal bone (45 mass% tricalcium phosphate), 25 mass% kaolinite and 30 mass% Cornish stone, a feldspar-rich weathered granite akin to the Chinese petuntse. Porcelain is normally fired twice. In hard-paste porcelain the first (biscuit) firing occurs between 900 and 950 °C. After cooling, the white biscuit porcelain can be decorated by painting (underglaze decoration). After dipping in the glaze slurry and appropriate drying, the second firing (glost or glaze firing) is performed between 1200 and about 1500 °C (see Fig. 2.2). After cooling the ceramic can be painted with enamel colours including gold, and fired a third time at 700 to 850 °C to fix the enamel to the transparent glaze (overglaze decoration). In contrast, soft-paste porcelain is fired at considerably lower temperatures (biscuit firing c. 1100 °C, glaze firing c. 1000 °C). However, the decisive difference between hard and softpaste porcelain is not the higher firing temperature of the former but its different sintering behaviour. The feldspar compound in hard-paste porcelain forms with alumina and silica released from the kaolinite a ‘eutectic’ melt (see Chapter 5) that is quenched during cooling and thus remains in the vitreous state without crystallising (Zanelli et al. 2011). Hence hardpaste porcelain contains up to 70 mass% of glass as opposed to the more crystalline microstructure of soft-paste porcelain. This renders the latter more prone to mechanical destruction since it shows lower hardness, abrasion resistance, bending strength, and thermal shock resistance (see Chapter 14).

2 Classification and properties of ceramics

17

Figure 2.2 summarizes the four main groups of clay-based ceramics in terms of their firing temperatures, water uptake, and colour of the body. The temperature scale on the left refers to earthenware, that on the right to biscuit- and glaze-firing temperatures of stoneware and porcelain. Figure 2.3 displays the hierarchical tree of all clay-based ceramics divided into coarse (shaded) and fine wares. < 0.1 1100°C

White earthenware

Firing temperature (°C)

Porcelain

Firing temperature (°C)

buff/white body colored body

Fe2O3/Al2O3 ratio

1000-1480°C (glaze firing)

Colored earthenware

900-1100°C (biscuit firing)

1200°C

Stoneware

> 0.3 950°C

porous

2

dense

0.1

Water uptake (%)

Figure 2.2. Classification of the main groups of clay-based ceramics (see Fig. 2.1).

Figure 2.3. Hierarchical tree of clay-based ceramics.

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Part I

2.3 Properties and functions of ceramic cooking pots 2.3.1 A thermo-physical approach Utilitarian ceramics used in antiquity to prepare, serve, savour, and store food stuff and food cooked for human consumption include a large variety of ceramic objects including cooking pots, dishes, plates, trays, cups, beakers, strainers and others, manufactured to different specifications to fulfil their functions. Hence there is a close relationship between structure and properties of ceramics, and their intended use and functional advantages (Braun 1983, Woods 1986, Tite et al. 2001, Tite & Kilikoglou 2002). The thermal and mechanical properties of ceramic cooking pots require particular attention. They are generally manufactured from coarse raw materials fired at rather low temperatures to impart porosity and a certain degree of thermal shock resistance that would well tolerate the differential thermal stresses occurring during repeated heating and cooling of the pot. These advantageous properties notwithstanding the mechanical resilience of coarse ceramic ware, in particular cooking pots, is weak as aptly expressed by the term Brittle Ware associated with cooking pots characterised by thin, often ribbed walls, produced from iron-rich clays that confer to the pottery its typical red or black colour (Dyson 1968, Vokaer 2010). The behaviour of ceramic materials differs whether it is subject to a rapid thermal shock or to slow thermal excursion (Cannon et al. 2011). A rapid thermal shock yields the thermal shock resistance R = ˰F (1 – ˪)/˞ · E

[K],

(2.1)

a slow thermal excursion yields the thermal shock fracture toughness R’ = ˰F · ˨1 – ˪)/˞ · E

[W/m],

(2.2)

where ˰F is the average flexural strength [MPa], ˪ is the dimensionless Poisson number, ˞ is the linear coefficient of thermal expansion [K–1], E is the modulus of elasticity [GPa], and ˨ is the thermal conductivity [W/mK]. Cooking pots are considered to be subjected in service to a slow heating rate and thus a slow thermal shock (eq.2.2). Hence the critical performance parameters are the thermophysical parameters, ˞ and ˨, keeping the other parameters constant. Indeed the thermal shock fracture toughness increases with decreasing coefficient of thermal expansion and increasing thermal conductivity. Choosing non-calcareous clays with a rather low coefficient of thermal expansion (2.0·10–6 < ˞ < 3.5·10–6 °C–1; Tite et al. 1982b) over calcareous clays with a higher ˞ value (4.5·10–6 < ˞ < 7.0·10–6 °C–1; Tite et al. 1982b) tends to produce cooking pots with increased resilience against thermally imposed stresses (Paynter & Tite 2001). Choosing limestone or shell temper with ˨ = 1.26–1.33 W/mK over sand temper with ˨ = 0.15–0.25 W/mK (The Engineering Toolbox) is an alternative way to maximize R’ (Feathers and Scott 1989).

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19

Another way to decrease thermal stresses to which a cooking pot is subjected is to reduce its wall thickness. There is a downside to this, however. While by this measure the stresses driving crack propagation will be reduced, the mechanical strength of the vessel decreases. It has been argued that this reduction of strength could be compensated for by decreasing the amount and average size of temper grain, for example quartz that lead to increased flexural strength. However, Kilikoglou et al. (1995, 1998) have shown that above a minimum amount of temper grains around 20 vol% the contribution to the total fracture energy from crack propagation is independent of the temper content. Reduction of the amount of high thermal expansion quartz-rich temper grains leads to a decrease of the bulk thermal expansion and thus to reduction of stresses driving crack initiation and propagation during thermal shock (Tite et al. 2001). These technological considerations work well in theory. However, archaeological evidence reveals that both types of clay (non-calcareous and calcareous) and temper (limestone and quartz) were apparently indiscriminately utilised regardless of place and time of production even though it appears that more refractory, non-calcareous clays were fired at higher temperatures compared to calcareous clays with considerably lower temperatures of onset of sintering (Tite et al. 2001). Clearly, in most cases investigated the availability of raw materials was the governing factor rather than the quest for optimising pottery properties. Only if several options offered themselves simultaneously an informed technological choice could be made by the potter (see Chapter 1). Intimately related to the quest for high thermal shock resistance of a ceramic cooking pot is the achievement of high mechanical strength and fracture toughness that can be controlled by the use of tempering materials of different types, shapes, sizes, and volume proportions. For example, as discussed in detail in Chapter 17, the American Bottom clays used to make prehistoric native North American cooking pots during the Mississippian period were tempered with large amounts of mussel shell that provided the means of crack arrest, crack deviation, and increase of fracture toughness by particle pull-out from the clay matrix (Feathers & Scott 1989, West 1992). This advantageous increase in strength is counteracted by the high porosity of the resulting vessel that impedes thermal conductivity, reduces heating efficiency, and favours undesirable permeability of the vessel walls. Solutions to these disadvantages have been sought in coating the interior walls of the cooking pots with an impervious slip (Schiffer 1990). From the ethnographic literature it is also known that new cooking pots were ‘primed’ by cooking milk in them that penetrated the open pore network and partially sealed it by depositing insoluble lipids and proteins that were carbonised during use. Sealing of pots with birch or pine tar, bitumen (for example, Kato et al. 2008) and other resins had been practiced in Neolithic times. Waterproofing of Greek and Roman transport amphorae was done with resin of different origins (Beck et al. 1989) and pine resin was used to seal prehistoric New World pottery (Reber & Hart 2008). The problem of optimising the functional properties of ancient ceramic cooking pots is aggravated by the dichotomy between temper concentration and firing temperature. To achieve high thermal shock resistance and high fracture toughness of cooking pots large amounts of temper and low firing temperatures are required as evidenced by Mississippian pottery (Chapter 17). On the other hand, high mechanical resilience, in particular high flexural strength requires low temper concentrations of temper and high firing temperature,

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Part I

that is, low porosity. As these properties cannot be achieved simultaneously a compromise is necessary that relies on judicious technological choices of clay raw material, type and amount of temper, firing temperature, and firing atmosphere. For example, to avoid lime blowing in ceramics tempered with limestone or shells, firing in oxidising atmosphere essentially below 650 °C or under reducing conditions below 750 °C must be done to prevent thermal decomposition of calcite (Maggetti et al. 2011b). However, ceramics fired at these low temperatures suffer from the lack of mechanical strength and fracture toughness. The interplay of mechanical (bending) strength and fracture toughness of the ceramic fabric, and the amount and shape of temper (platy phyllitic vs. equiaxed granitic grains) has been studied recently by Müller et al. (2010). The authors reported that an increase of the amount of aplastic temper inclusions leads to a reduction of mechanical (bending) strength but to an increase in fracture toughness, in accord with earlier findings by Steponaitis (1984) and Kilikoglou et al. (1995). This decrease in strength is related to the shape of the temper grain in such a way that platy particles reduce the strength to a lesser degree than equiaxed (‘spherical’) grains. Hence for coarse utilitarian vessels such as cooking pots fracture toughness rather than bending strength is the more significant performance requirement, that is, avoidance of destructive crack propagation is more important than evasion of crack initiation. High fracture toughness is obtained by adding substantial amounts of temper material. In the case of granitic fabrics, toughness is not affected by higher firing temperatures, while pottery with phyllitic fabrics must not be fired at too high a temperature lest fracture toughness decreases again. Ceramic objects subjected to rapid thermal shock (eq. 2.1) behave differently. A process called ‘docking’ has been used in the brick-making industry to prevent lime blowing of bricks containing up to 10% limestone. It consists of complete immersing the ceramic while still red-hot in cold water (Laird & Worcester 1956, Avery 1983). The strengthening mechanism underlying such treatment is not quite clear but may be related to suppressing the growth rate of portlandite, Ca(OH)2 crystals and thus reducing the volume expansion during carbonisation of this mineral (Boxall, T.G.W, in Laird & Worcester 1956). Alternatively the process can be seen as a kind of proof testing, that is, changing the initial distribution of macro-sized flaws in the ceramic matrix that lead to spalling by truncating the distribution, that is, eliminating the Weibull failure probability over a wide stress range (Cannon et al. 2011). It would mean in practice that only those cooking pots fail during docking that show a high concentration of large, that is, critical-size cracks whereas those with sub-critical cracks survive the test thus increasing the probability that the pot subjected to a rapid thermal shock will also survive service-imposed slow thermal shocks experienced during cooking. There is some experimental evidence to support this conclusion (Avery 1983). As shown above many parameters interact to impose thermo-physically and mechanically advantageous properties on ceramic cooking pots. As emphasised in the excellent review paper by Tite et al. (2001) on the subject, ‘a knowledge of the extent to which…the thermal shock resistance of cooking pots has been optimized is clearly extremely helpful when trying to establish how the myriad other factors might have affected the technological choices made in the production (of) pots’ (see also Tite and Kilikoglou 2002).

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21

2.3.2 Ceramic vs. metal cooking pots While today in our modern kitchens cooking pots and frying pans are generally fashioned from metal owing to its superior heat conduction, homogeneous heat distribution without ‘hot spots’, mechanical stability, absence of porosity, and ease of cleaning, preparation of food in unglazed terracotta clay pots pre-soaked in water to release steam during the cooking process, invented in ancient Roman times, is still common in Europe (‘Roman pot’, caquelon), and serves well in the Chinese (shāguo), Japanese (donabe) and North African (tajine) ways of traditional food preparation. These clay pots allow stewing meat, fish and poultry in their own juices, retaining flavour, taste, nutrients and vitamins in their natural form, and in general preparing particularly savoury, palatable, digestible and wholesome dishes. As the cooking process dispenses with the addition of extra lard or oil the use of clay pots is well-suited to prepare food for special dietary and slimming purposes. Further advantages include the possibility of stewing food without supervision, preparing delicious sauces and gravies directly from the cooking juices collected in the clay pot by adding suitable flavouring ingredients, and, in particular, obtaining very crispy and golden-brown poultry after removing the cooking juices a few minutes before taking the pot out of the oven (see for example Fig. 10.18).

Chapter 3

Clay raw materials: origin, composition, and properties Synopsis Clays are weathering products of rocks and as such a mechanical mixture of very different minerals, each with a different and characteristic grain size distribution. The mineral content of clays varies widely depending on the composition of the parent rock and the details of the physical and chemical processes taking place during weathering. The chapter contains information on the formation and structure of important clay minerals such as kaolinite, illite. montmorillonite and palygorskite, and provides the basics of clay-water interaction and the role electrolyte ions and the pH play in ion exchange mechanisms governed by the Hofmeister series, the development of the electrokinetic (Zeta) potential, and their effect on the viscosity of clay-water slurries.

3.1 Types of raw materials Natural raw materials utilised to produce silicate-based ceramics can be divided into highly plastic materials such as clays comprising the minerals kaolinite, illite, or montmorillonite14 and non-plastic materials used as (i) tempering additives such as quartz but also organic material such as straw and chaff, and (ii) fluxes (feldspar, calcite, dolomite etc.) added to clays to alter the chemistry, workability15 and sintering behaviour of ceramic masses. The basis of our understanding of the processes that occur during firing is a clear understanding of the mineralogical nature of clays. Clay is a weathering product and as such a mechanical mixture of very different minerals, each with a different and characteristic grain size distribution. The mineral content of clays varies widely depending on the composition of the parent rock and the details of the physical and chemical processes that take place during weathering. Clay minerals have grain sizes smaller than 2 µm in diameter, whereas other fine-grained relics of the weathering process that one finds within the clay, such as quartz, feldspar and micas, have grain sizes up to 20 µm and beyond in diameter. Thus the mechanical mixture we call ‘clay’ consists of four main components: (i) finegrained weathering relics and rock fragments: quartz, feldspar, calcite, micas and mica-

14 Raw materials high in montmorillonite content were rarely used by ancient potters since the green body is prone to excessive shrinkage during drying. Examples to the contrary exist, for example, in the prehistoric Mississippian culture of the American Bottom (see Chapter 17.2.2). 15 The term ‚workability‘is preferred here over the term ‚plasticity‘ since in crystal physics plasticity has a different meaning.

3 Clay raw materials: origin, composition, and properties

23

ceous minerals such as sericite, chlorite, and hydromicas (interlayer-deficient micas), though the existence of these minerals as separate constituents are, today, in doubt; (ii) neo-formations, i.e. the clay minerals per se that are formed during weathering: kaolinite, halloysite, smectites16, illite and mixed-layer (ML) minerals17; (iii) remainders of organisms: calcium carbonate (shells), silica (chert), carbon; and (iv) neo-formations that occur after deposition: pyrite, dolomite, glauconite. For a summary of the mineral content of ordinary brick clay and of the method used to determine it, the reader may want to turn to Schmidt (1972) (cp. also Brownell 1976, Pawloski 1985, 1987, Davis 1987).

3.2 The formation of clay minerals The genetic heritage and origin of clay minerals have been described by many researchers, for example Millot (1970), Velde (1977), Eberl (1984), Galán (2006) and, most recently Velde & Meunier (2008). Three basic genetic mechanisms were found to be operational: inheritance, neoformation, and transformation. Inheritance means that clay minerals can originate from reactions occurring in a different environment during a previous stage of the rock cycle. Origin by neoformation means that clay minerals were formed by precipitation from dilute soil solutions or by reactions of amorphous materials. Transformation finally consists of reaction sequences involving alteration of the inherited structure by chemical reactions, either by ion exchange or layer transformation during diagenesis (Eberl 1984). As the results of layer transformation are preserved in the geological record investigation of their nature provides valuable information on the environmental conditions in the sediment source area as a function of time. While inheritance dominates in the sedimentary environment at generally ambient conditions characterised by slow reaction rates, layer transformation requires considerable input of activation energy and thus is found preferentially in the diagenetic and hydrothermal realms where higher temperatures prevail. In between these two environments the weathering environment exists in which all three mechanisms discussed above can be operational. Hence these three mechanisms occurring in three different geologic environments lead to nine possibilities of clay mineral formation in nature, attesting to the exceptional variability and complexity of clay mineral chemistries. Clay minerals are products of the interaction of rocks with aqueous solutions of the environment under more or less ambient conditions (Velde & Meunier 2008). All interactions are essentially a series of leaching and precipitation processes. However, the acidity of the solu-

16 Smectites are expandable three-layer clay minerals with partial substitution of Al3+ by Mg2+ in the octahedral sheet (montmorillonite) or partial substitution of Si4+ by Al3+ in the tetrahedral sheet (beidellite) (see Table 3.1, Fig. 3.2). In both structural types negative surface charges occur that will be compensated for by alkali or alkaline earth ions intercalated between the three-layer stacks. 17 Mixed layer (ML) minerals consist of randomly or regularly stacked layers of micas or chlorites or smectites. Regularly ordered structures are denoted by individual mineral names such as rectorite (dioctahedral mica/illite + dioctahedral smectite/montmorillonite ML), tosudite (dioctahedral chlorite/sudoite + dioctahedral smectite/montmorillonite ML) or corrensite (trioctahedral chlorite + trioctahedral smectite/saponite ML) (Moore & Reynolds 1997; Shimoda 1969).

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Part I

tions involved, that is, the pH value is of great importance for the details of these leaching and precipitation reactions. In addition to the nature of the rock and its mineral constituents, one of the most important variables is the climate. There is a basic distinction to be made between leaching reactions occurring in temperate climates or tropical climates, and precipitation reactions occurring in humid or in dry (arid) environments. The compositional and structural variety of clay minerals can be understood on the basis of the different modes of the environmental interaction schemes. In general, rain water contains considerable amounts of carbon dioxide and a certain amount of nitric acid, giving it an overall pH between 5 and 6. It is therefore slightly acidic. On the one hand, in the temperate-humid climate zone, the acidity is increased by the presence of humic acids in the soil overlying the bedrock. In the tropical-humid climate zone, on the other hand, the ground water can be almost neutral because of the interaction with soil solutions produced by intensified bacterial decomposition of plant material. While the environmental solutions readily dissolve soluble ions such as K+, Na+, Ca2+, and Mg2+, the degree to which Fe2+, Al3+ and even Si4+ are attacked depends largely on the pH of the leaching solutions. The environmental conditions also determine the degree of any precipitation into the surrounding soils, particularly of iron and manganese in its various valence states. By the same reasoning, freely moving ground waters can cause complete leaching of the soluble cations, while restricted water circulation can cause less soluble ions such as Mg2+ and Fe2+ to remain in the original mineral grain and react there to form new minerals. In arid climates with extremely restricted water circulation, conditions of precipitation result in the deposition of aluminium in the form of more or less pure, that is, iron-free Al(OH)3 or AlO(OH). If the original mineral was feldspar that weathered in a humid climate, and if complete leaching is achieved, then the clay minerals produced will belong to the kaolinite family. If, however, the water circulation is restricted and the less soluble ions such as Mg2+ and Fe2+ remain, they will form (together with the residual Al-Si-lattice of the feldspar) minerals of the smectite family. If the original mineral was mica rather than feldspar, the weathering process follows a somewhat different reaction scheme. The weakly bonded K+ ions are very quickly dissolved and the remaining lattice has to be stabilized by (H3O)+ and H+ ions. Thus, the so-called incomplete or interlayer-deficient micas are formed. They are broken up into small fragments by mechanical strain between the silicate layers; this strain is caused by the ion exchange mentioned above. This results in colloidal mica-type minerals known as illites. The majority of ordinary clays consist mainly of illites and this component is, to a large extent, responsible for the plastic behaviour of these clays. Clays with high illite content are referred to as ‘immature’ clays compared to the ‘mature’ clays with high concentration of kaolinite or smectites. Despite the fact that members of the illite family appear to be the most abundant clay minerals next to kaolinite, their state has not yet been defined unequivocally by the Association Internationale pour L’Etude des Argiles (AIPEA) (Jambor et al. 1998). Stubborn mysteries still

3 Clay raw materials: origin, composition, and properties

25

surround the formation and transformation of this most abundant clay mineral, mostly related to its widely varying chemical composition, small crystal size, degree of crystallinity or lack thereof, as well as complexity of transformation sequences in the geologic environment over time.

3.3 Nomenclature and structure of clay minerals The importance of clays as raw materials for traditional ceramics, their widespread occurrence, chemical and structural variability, and the dependence of processing and firing properties on the phase composition of the precursor materials of ceramic products has led, among much research into their physico-chemical properties, to several attempts to develop a comprehensive system of clay nomenclature. More recently, utilisation of clays as sealing and ion-exchange and sorption components in geological barriers of disposal facilities for domestic and nuclear wastes has added much to this quest (see for example Serne & Muller 1987, Ricci 1999). Brindley (1951) reported the earliest efforts to obtain international collaboration on nomenclature and classification of clay minerals. Since then, national clay groups were formed, and they proposed various changes in nomenclature at group meetings of the International Clay Conferences. Most national clay groups have representation on the Nomenclature Committee of the Association Internationale pour l’Etude des Argiles (AIPEA, International Association for the Study of Clays) established in 1966. It has worked closely with other international groups, including the Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association (IMA), which is responsible for the formal recognition of new minerals and mineral names, and the International Union of Crystallography (IUCr). In contrast to the other national clay groups, however, The Clay Minerals Society (CMS) Nomenclature Committee, established in 1963, remains in existence and occasionally produces recommendations. The precursor to this committee was the Nomenclature Subcommittee, which was organized in 1961 by the (US) National Research Council. From time to time AIPEA issues recommendations in close contact with the national organisations (Guggenheim et al. 2006). The structure of all silicates including clay minerals is best understood in terms of the geometric arrangement of atoms within each unit cell. The wide variety of silicate minerals in nature is essentially caused by the variety of geometrical combination of the basic elements of the constituting SiO4 tetrahedra. Since the changes that occur during the firing of pottery are essentially related to the rearrangement of the silicate into different structures, an understanding of the basic structures of clay minerals is essential for the understanding of the firing process. Clay minerals consist basically of hexagonal networks of SiO4 tetrahedra. The planes of all tetrahedra are in the plane of the network, and the tips of the tetrahedra point in the same direction. The oxygen atoms at these tetrahedra tips are bound to Al or Mg atoms; residual valencies are saturated through OH– ions. This means that the cations AI3+ or Mg2+ are in a six-fold coordinated (octahedral) position (Grim 1953).

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Part I

O,OH Si AI Mg, Fe

O,OH Si AI

Figure 3.1. Schematic structures of clay minerals. Left: Two-layer clay minerals (kaolinite) and tetrahedral SiO4 and octahedral AlO6 building units. Right: Three-layer clay minerals (talc, pyrophyllite). The arrangement of atoms refers to the six-rings of SiO4 tetrahedra typical for sheet silicates.

Such minerals consist, then, essentially of two layers. One is the basal plane of the SiO4 tetrahedra, the other the octahedral layer of the Al or Mg hydroxide which is normally called the gibbsite (with Al) or brucite (with Mg) layer. In the gibbsite layer there are always two aluminium atoms for each group of 6(OH)– ions (dioctahedral layer) while in the brucite layer three Mg cations combine with 6(OH)– ions (trioctahedral layer). Kaolinite, with the basic formula Al4[(OH)8/Si4O10], is typical of two-layer dioctahedral sheet silicates (Fig. 3.1, left). In that same structural category one also finds three-layer minerals in which the octahedral Al or Mg layer is sandwiched between two SiO4 tetrahedral layers (Fig. 3.1, right). Talc, with the formula Mg3[(OH)2/Si4O10], is one example of this type of trioctahedral three-layer mineral. Pyrophyllite is the dioctahedral three-layer equivalent of kaolinite with the formula Al2[(OH)2/Si4O10l. If, in the three-layer minerals, part of the Si is substituted by Al, negative surface charges occur. These are compensated for by alkali cations that are bound between the layers. In that way the basic group of mica is formed. Table 3.1 summarizes some more common di- and trioctahedral sheet silicates with mica-like structure. The sudoite-chlorite family can be described as mica-like 2:1 mixed-layer structures alternating with ordered

Table 3.1. Systematic of sheet silicates with mica-like structure. Dioctahedral with gibbsite layers

Trioctahedral with brucite layers

Two-layer structures Kaolinite Al4[(OH)8/Si4O10] Halloysite-10Å Al4[(OH)8/Si4O10]·2H2O

‘Serpentine’ (Antigorite) Mg6[(OH)8/Si4O10]

Three-layer structures Pyrophyllite Al2[(OH)2/Si4O10] Beidellite Al2[(OH)2/ (Si,Al)4O10]·(Ca,Na)0.3(H2O)4 Muscovite KAl2[(OH)2/AlSi3O10] Margarite CaAl2[(OH)2/(Si,Al)4O10] Sudoite (Al,Fe)2[(OH)2/AlSi3O10]·Mg2Al(OH)6

Talc Mg3[(OH)2/Si4O10] Vermiculite Mg3[(OH)2/ (Si,Al)4O10]·Mg0.35(H2O)4 Phlogopite KMg3[(OH)2/AlSi3O10] Clintonite CaMg2[(OH)2/(Si,Al)4O10] Clinochlore (Mg,Al,Fe)3[(OH)2/ AlSi3O10]·(Mg,Fe,Al)3(OH)6

3 Clay raw materials: origin, composition, and properties

27

Figure 3.2. Structure of smectite. In montmorillonite some of the Al3+ in the octahedral layer is replaced by Mg2+, in beidellite some of the Si4+ in the tetrahedral layer is replaced by Al3+, and in nontronite some of the Si4+ in the tetrahedral layer is replaced by Al3+ and (all) Al3+ in the octahedral layer is replaced by Fe3+.

gibbsite- (sudoite) or brucite (chlorite)-type interlayers. Margarite and clintonite belong to the brittle mica group. On the other hand the relation of montmorillonite to pyrophyllite can be understood when one considers the partial substitution of Al by Mg in the octahedral layers of the latter. Likewise the smectite group mineral beidellite is generated by partial substitution of Si by Al in the tetrahedral layer (Table 3.1). This again produces negative lattice charges (for determination of the layer charge of 2:1 sheet silicates see Mermut & Lagaly 2001) which are compensated for by monovalent or divalent atoms such as Na+ or Ca2+. This can, in addition, cause the disintegration of the layered crystals and in that manner enables the entrance of water between the layers. It is on this basis that the ability of the smectite mineral group to absorb large amounts of water can be understood. Figure 3.2 summarizes the structure of smectitic clays minerals.

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Part I

3.4 Mineralogy of clay minerals relevant for pottery 3.4.1 Illite Illite appears to be the most abundant clay mineral next to kaolinite. However, despite copious research performed stubborn mysteries still surround the formation and transformation of this clay mineral, mostly related to its widely varying chemical composition, small crystal size, degree of crystallinity or lack thereof, as well as complexity of transformation sequences in the geologic environment over time. It has been a long standing agreement that illite sensu lato can form basically by all three mechanisms discussed in section 3.2, in particular by inheritance, that is, through loss of potassium ions (degradation) during leaching of muscovite (dioctahedral illites) or biotite (trioctahedral illites), by transformation through addition of potassium ions (aggradation) to montmorillonite, and possibly also by neoformation involving precipitation from dilute colloidal weathering solutions. The loss of easily soluble potassium ions from the trioctahedral mica biotite will be compensated for by ion exchange with H3O+ ions, by oxidation of Fe2+ ions, and by replacement of Al3+ in the octahedral layer by Si4+ ions. On the other hand, the dioctahedral mica muscovite undergoes similar potassium loss by degradation and associated charge deficiency (Table 3.2). In the resulting dioctahedral illites the ideal Si/Al ratio of 3 in the tetrahedral layer of muscovite changes to between 5 and 40 (Hower & Mowatt 1966). In addition, some water is intercalated between the layer stacks. Starting from an ideal muscovite lattice three possible reaction paths have been suggested as shown in Table 3.2. Path 1 assumes that K+ ions in the interlayer space will be replaced by H3O+ ions, path 2 considers that one K+ ion together with one OH– group of the octahedral layer will be replaced by two H2O molecules, and path 3 suggests that one Si4+ ion in the tetrahedral layer will be replaced by 4 protons. It should be emphasized that illite with increasing degradation approaches the chemical composition of kaolinite but the structural state of montmorillonite. Hence numerous interstratified ordered and disordered structure variants exist. However, the higher proportion of aluminium in the tetrahedral layer compared to montmorillonite requires more potassium as interlayer cation. As a result very little intracrystalline expansion occurs in illite. Since the degree of crystallinity of illite in sediments increases with temperature it can be used as a marker to estimate the diagenetic-metamorphic zones (grades) of metasedimentary rocks of marine fine-clastic origin that are widespread in sedimentary basins and in the outer fold-and-thrust zones of the orogenic belts.

Table 3.2. Three feasible ways of illite formation from dioctahedral muscovites. Muscovite

Interlayer

Octahedral layer

Tetrahedral layer

K2

Al4(OH)4

Al2Si6 O20

Path 1

K(H3O)

Al4(OH)4

Al2Si6 O20

Path 2

K(H2O)

Al4(H2O)(OH)3

Al2Si6 O20

Path 3

K2

Al4(OH)4

Al2Si4H8O20

3 Clay raw materials: origin, composition, and properties

29

3.4.2 Kaolinite Kaolinite, Al4[(OH)8/Si4O10] is the typical weathering product of feldspar under temperatehumid climate conditions and in the presence of surplus water of slightly acidic pH that is sufficient to remove completely the alkali and alkali earth ions of the parent feldspars. In contrast to illite, most kaolinite minerals are formed in situ, that is, they remain where they were formed by weathering of granite or related rocks (autochthonous). Rocks rich in kaolinite are known as china clay, white clay, or kaolin. The name kaolinite derives from Chinese: 㧗㝠㧗ᕊ; pinyin: kao-ling (‘High Hill’) near Jingdezhen, Jiangxi province, China (see Chapter 18). The mineral exists in four main structural variants: triclinic kaolinite, monoclinic dickite, and monoclinic nacrite as well as b-axis distorted fireclay. While kaolinite is considered one of the products of deep weathering of feldspars occurring in granitic rocks, the mineral can also form by hydrothermal routes (Huertas et al. 1999) and as a product of diagenesis and low-grade metamorphosis in sandstones (Ruiz Cruz & Andreo 1996). There is also the notion that some kaolin deposits may have been formed under pneumatolytic conditions in the presence of fluorine ions that result in additional formation of fluorspar as found in Cornish stone. However, in this case the high temperature minerals dickite and/or nacrite should be present that, however, are absent in Cornish stone (Kerr 1952).

3.4.3 Montmorillonite Montmorillonite, a member of the smectite family (Fig. 3.2), has only limited importance for making pottery. This is based on the fact that ceramic green bodies formed from clays rich in expandable three-layer smectitic clay minerals such as montmorillonite show during drying a large degree of shrinkage and hence the appearance of many cracks in the leatherhard body. Indeed, clay containing montmorillonite in excess of 20 mass% shows reduced green body strength as well as reduced compressive strength of the fired ceramic object (Stegmüller 1956). Consequently such ‘fat’ clays must be rendered ‘lean’ by adding copious amounts of sand, rock, grog, bone or shell (see Chapter 17) temper. In the crystal structure of montmorillonite sensu strictu Al ions in the octahedral layer are partially replaced by Mg. For charge balance hydrated alkali ions such as Na+ or alkaline earth ions such as Ca2+ are fixed in the interlayer space. Intercalation of hydrated Ca2+ ions causes the crystallographic identity period in c-direction to swell from about 1 to 2 nm whereas Na+ ions, owing to their much lower crystal field strength and associated high zeta potential, lead to much larger c-axis values. Such Na-montmorillonites form gels due to lack of attractive forces within the electric double layer surrounding the clay mineral grains. On the other hand, addition of Ca2+ ions induce flocculation, limits the shrink-swell ratio and in general generate rheological properties that are conducive to good workability and green body strength. Since K+ and ammonium (NH4+) ions can also be intercalated smectites are important carriers of these fertilising ions. This may have been one of the reasons why in the past agricultural people settled preferentially in river valleys rich in such fertile clays. An example will be shown in Chapter 17 that describes American Indian pottery from the Mississippi valley.

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Part I

Smectites such as montmorillonite intercalate not just hydrated ions but also polar organic molecules such as fatty acids. This is why montmorillonite-rich clays, so-called bentonites were used since ancient times in the process of tanning animal hides to take up oils and fat. Also, they were used in fulling of felt and cloth, hence the old name Fuller’s Earth for bentonite rocks. For details the reader is referred to Heimann (2010).

3.4.4 Others Another mineral frequently associated with bentonite deposits is the clay-type mineral palygorskite (a.k.a. attapulgite), a magnesium aluminum phyllosilicate with the formula (Mg,Al)2[OH/Si4O10]·4H2O. The structure consists of sheets of six-membered rings of SiO4 tetrahedra parallel (100), linked by strips of edge-sharing MgO6 (and AlO6) octahedra aligned parallel to [001]. The four water molecules are accommodated in large channels parallel to the fibre axis [001]. These channels can also take up large organic molecule complexes such as indigo, a property exploited by the ancient Maya to synthesize the famous Maya Blue (see below; also Chapter 17.2). Palygorskite is presumed to have formed authigenically, either by conversion of detrital smectite or by direct precipitation in a dolomite-mixing environment. The Si, Mg, Al + Fe and Ca required for palygorskite formation were supplied in solution from ultrabasic rocks underneath such as ophiolitic rock series, smectitic clays and dolomitic carbonates (Kadir & Akbulut 2011). Palygorskite is the key constituent of Maya Blue used by the pre-Columbian Maya civilisation of Mesoamerica to colour ceramics, sculptures, murals and (most probably) textiles. It was produced by gentle heating of powdered palygorskite with the aqueous extract of añil leaves (Indigofera suffruticosa), with smaller amounts of other mineral additives. According to recent research (Chiari et al. 2003, Dejoie et al. 2010) the indigo molecule is partly accommodated internally in the structural channels of palygorskite thereby replacing water, and partly externally in grooves at the surface of the mineral fibres. In a remarkable development the ancient discovery of the pigment by the Maya was recently utilised to synthesise a modern environmentally stable blue pigment by incorporating the indigo molecule into MFI zeolite (high-silica silicalite), a spectacular example of reverse engineering by ‘archaeomimetism’ (Dejoie et al. 2010). Deposits of palygorskite in the Maya area were unknown for years, but archaeological research performed during the 1960s and more recently indicated two such sources at the cenote in the town of Sacalum and at a pre-Columbian mining site at Yo’ Sah Kab near Ticul, both in Yucatán (Arnold 2005). The Maya Blue pigment was also manufactured and used in other Mesoamerican regions and cultures, for example by the Aztecs of central Mexico to colour their codices and early Colonial-era manuscripts and maps. Human sacrificial victims in post-Classic Mesoamerica were frequently daubed with this blue pigment (Haude 1997).

3 Clay raw materials: origin, composition, and properties

31

3.5 Clay-water interactions In the process of making pottery the dry clay must be made workable by adding water and subsequent wedging, kneading or blunging (see Chapter 4). Hence the interacting of clay particles with water molecules plays a particular important role in preparing the ceramic mass. Clay particles have the tendency to strongly agglomerate under the influence of van der Waals forces. The small size of clay particles, typically 1

Shear stress τ

D = (dv/dt) = (dγ/dt)

η* = τ/D = τ/(dγ/dt)

Structural viscosity Dilatancy, n < 1

Newton liquid

Dilatancy

Shear stress τ

Figure 3.4. Dependency of apparent viscosity ˤ* (left) and rate of shear deformation D = (dv/dt) = (dˠ/dt) (right) on shear stress ˱ for viscoelastic liquids and suspensions, respectively. Heimann (2010).

18 Thixotropy is the property of a gel to become a liquefied sol by mechanical loading such as shaking but to stiffen again after resting to attain the viscosity of a gel. This reversible sol-gel phase transition is caused by particle interactions induced by van der Waals forces that lead to aggregation under occlusion of liquid to form a ‘house of card’ structure (Fig. 3.5). In terms of rheological parameters it is defined as the decrease of viscosity under constant shear stress or shear rate, followed by a time-dependent recovery after removal of the shear load. 19 The phenomenon opposite to thixotropy is called rheopexy (flow strengthening), the increase of (shear) viscosity with increasing shear stress.

3 Clay raw materials: origin, composition, and properties

33

Rate of shear deformation, dγ/dt (s-1)

Figure 3.5. ‘House of cards’ structure of a thixotropically solidified kaolinite suspension. 1000

3

4

2

6

5

1

500

0 0

40

80

120

Shear stress τ (N/m2)

Figure 3.6. Flow curves of H-loaded kaolinite (200g dry kaolinite/500 ml solution) (modified after Salmang et al. 2007). For explanation see text.

stress ˱. A suspension of 200g kaolinite in 500 ml distilled water displays so-called Bingham structural viscosity20 behaviour, that is, a yield stress of about 40 N/m2 is required to induce flow (curve 1), and the rate of shear deformation increases with increasing shear stress ˱.

20 Bingham structural viscosity relates to a plastic material that will not flow under applied stress until a certain value, the so-called yield stress ˥ is reached. Beyond this point the flow rate increases steadily with increasing shear stress.

34

Part I

Adding as little as 1 mmol21 NaOH/100 g dry kaolinite shifts the flow curve to a lower value of yield stress, that is, less stress is required to establish a steep flow velocity gradient (curve 2). Increasing the amount of monovalent electrolyte ions to 10 mmol NaOH/100g dry kaolinite alters dramatically the flow curve to display so-called quasi-Newtonian behaviour, that is, linear dependency of (dˠ ˠ/dt) on ˱ (curve 3). Even close to infinitesimal increase of shear stress induces easy flow. However, high amounts of NaOH, that is, high pH as encountered by addition of 1000 mmol NaOH/100 g dry kaolinite reverses this trend since the high density of negative OH– surface charges allows the particles to come in close contact again with an ensuing increase in viscosity and also required shear stress (curve 4). Cations with higher charge such as Ca2+ cause easy flocculation and hence lead to high shear stresses (curve 5: 0.5 mmol Ca(OH)2/100 g dry kaolinite; curve 6: 5 mmol Ca(OH)2/100 g dry kaolinite).

3.5.2 Additions of electrolytes to clay-water suspensions: Hofmeister series As shown in Fig. 3.6 the flow behaviour of clay-water suspensions changes in the presence of soluble electrolyte ions that can be exchanged with the ions adsorbed at the surface of clay particles. Natural clays yield predominately Ca2+ and Mg2+ ions in exchange sites and, in an acidic environment, also H3O+ ions. The kinetics, i.e. ease with which cations and, more pronounced, anions will be exchanged follows the so-called Hofmeister (lyotropic) series of preferential adsorption of ions on clay particles. The ions on the left will be easier exchanged by ions positioned further right than vice versa (Kunz et al. 2004): Li+ < Na+ < K+ < NH4+ < Mg2+ < Ca2+ < Sr2+ < Ba2+ < Al3+ < H+. This series is an expression of the increase of adsorption energies and a decrease of the thickness of the diffuse electrical double layer (Gouy-Chapman layer) surrounding the clay particles. This effect offers the principal possibility to influence the thickness of the double layer by adding to the clay-water suspension ions with variable thickness of hydration shells. This will in turn influence the workability (‘plasticity’) of the clays. For example, exchange of Ca2+ and Mg2+ ions by Na+ ions through the addition of NaCl, Na2SiO3 or Na6(PO3)6 (sodium hexametaphosphate, ‘Calgon’) results in a build-up of the double layer and thus a decreased interaction of individual clay particles. In Na-exchanged clays each negative surface charge is exactly compensated by one Na+ ion within the Stern layer. Hence there is maximum screening that reduces effectively particle-particle interaction. The viscosity of the clay slurry and degree of aggregation decrease in turn. On the other hand, clay suspensions can be flocculated by adding small and higher charged ions such as Ca2+, Mg2+ or Al3+. These ions tend to decrease the thickness of the electrical double layer and cause agglomeration (flocculation) of clay particles owing to increased van der Waals attraction.

21 The chemical amount of a known substance, in moles, is obtained by dividing the sample’s mass by the substance’s molar mass. Hence 1 mmol NaOH corresponds (roughly) to 56 mg.

3 Clay raw materials: origin, composition, and properties

35

3.5.3 Effect of pH on clay-water suspension Interaction of protons with the charged clay particles leads to several effects that are important for the rheological behaviour of clay-water suspensions and hence their workability15. At low pH < 5 the clay particles maintain strongly positive edge charges and negative surface charges owing to adsorption of protons at unsaturated O2– bonds as well as OH– bonds at edge sites. As a consequence, the positive edges and the negative surfaces of neighbouring particles attract each other and attain a mechanically rather stable configuration of a ‘house of cards’ structure (Fig. 3.5). This structure imparts a high viscosity to the highly flocculated clay suspension. At extremely low pH < 2 the negatively charged surfaces can become neutralized with the consequence that the ‘house of cards’ structure will collapse causing a decrease in viscosity. Such exceptionally low pH values have been observed by hydrolysis of exchangeable surface Al3+ ions involving the release of protons and the subsequent intercalation of gibbsite-like hydroxyl-Al complexes formed by this Brønsted acidification into the smectite lattice to form a non-expandable Al-montmorillonite (Heimann 1993). At intermediate pH (5 < pH < 8), protons will be gradually released from their edge positions. Hence the positive charge at the clay particle edges decreases, and the mutual attraction of the particles and the slurry viscosity likewise decrease. If the pH is further increased the edge charge decreases to zero, and deflocculation occurs forming a sol since the still negatively charged clay particle surfaces are sufficient to compensate the van der Waals attraction forces and thus to disperse the particles. At high pH (8 < pH < 12), significant proton release render both the edges and the surface negatively charged. The strong repulsion of the particles causes highly dispersed, low viscosity slurries to appear. At this high pH the slurry can be tailored to contain a very substantial proportion of solids (up to 50%) without a significant increase in viscosity. At very high pH > 12 flocculation occurs again since the high density of negative OH– surface charges allows the particles to come in close contact with an ensuing increase in viscosity (see Fig. 3.6, curve 4). It should be noted that a pH of exactly 7 is quite difficult to maintain for clay-water suspensions since any addition of water to clay immediately causes a drop in pH to about 5 since protons are rapidly released from the particle edges as described above.

3.5.4 Zeta (electrokinetic) potential Ion adsorption at clay particles is a dynamic process so that exchange of ions can take place readily in response to changing pH. These changes in pH influence the thickness of the Helmholtz-Gouy-Chapman electrical double layer and in turn the value of the so-called zeta (ˣˣ) potential that behaves inversely to the viscosity. The concept of the zeta potential ˣ is one of the most significant concepts in the science of colloidal processing of clays and ceramics. The functional dependency of this property on type and concentration of ions, including pH, in the suspension serves to characterise the suitability of clays for a variety of ceramic wares.

Shear plane

φ0

Stern layer

Part I

Potential φ

36

φ1

φS Φ 0* φ1

φ2 = ζ

φ sol r1

Distance r

Figure 3.7. Details of the potential distribution curve shown in Fig. 3.3. Heimann (2010).

The potential distribution around a clay particle with a negatively charged surface is shown in Figs. 3.3 and 3.7. The surface charge –Q0 will be compensated by an equal charge +Q0 in the surrounding solution. This counter charge is distributed between two shells. Immediately adjacent to the surface a tightly adhering layer of positive counter ions (Stern layer) exists at the distance r1 with charge Q1. Further outward a second charge Q2 extends as a diffuse layer that finally attains equilibrium with the polar liquid shell. Hence there are three potential: the Nernst surface potential ˳0 immediately at the clay particle surface, the Stern potential ˳1 at the transition between Stern layer and diffuse layer, and the ˣ-potential ˳2 that may be defined as the potential at the intersection between the hydrodynamic shear (slipping) plane and the ˳-r curve. The higher the (negative) zeta potential, the better is the dispersion of clay particles in claywater suspensions. Some typical values are: Ca-loaded clay –10 mV, H-loaded clay –20 mV, Mg-loaded clay –40 mV, Na-loaded clay –80 mV. The ranking of the exchangeable ions corresponds to the Hofmeister series discussed above. Clays treated with Na hexametaphosphate (‘Calgon’) reach zeta potential values as high as –135 mV. Hence this addition is ubiquitously used in modern processing of clay to achieve maximum particle separation as required for slurry preparation for slip casting of pottery. It should be mentioned that excellent dispersion of kaolinitic clays has been achieved by ancient Chinese and medieval European potters through a process called aging by adding urine and faeces to the raw clay. The urea contained acts as a peptization agent and penetrates the clay structure to be preferentially absorbed between layer stacks thereby allowing for easy sliding over each other of the clay mineral platelets. Whereas bacterial activity appears to assists in this process (Oberlies & Pohlmann 1958) microorganisms can also dissolve, precipitate, and transform clay minerals and thus change their physical and chemical properties (Dong 2012). Knowledge of the zeta potential of (dense) clay slurries and manipulating it by adding particular mono- or polyvalent ions, modifying the pH, and altering the viscosity by changing the proportion of solids is at the heart of achieving high-quality ceramic products such as Chinese porcelain, and also some advanced stoneware.

Chapter 4

Processing of clay, and forming and finishing of pottery Synopsis The chapter gives a brief account on the process steps required to transform the mined clay raw materials into a useful ceramic object. This endeavour includes beneficiation of clay by cleaning, levigation and/or mixing, shaping the green ceramic bodies by artisan (coiling, slabbing, free forming) or industrial (moulding, pressing, slip casting) techniques, drying, glazing, and post-firing painting. The physico-chemical transformation of the clay minerals during firing will be dealt with in a different context later. Emphasis is being put on forming techniques ubiquitously applied by ancient potters such as coiling and moulding, recipes of ancient and modern glazes, as well as colour pigments used in antiquity to paint the ceramic object after firing.

4.1 The operational sequence of making ceramics The logistics and organisation of labour in a pottery workshop has not changed much over time. Even today, despite the availability of labour-saving devices such as rotary mixers, pug mills, electrically powered potter’s wheels, fully or semi-automatic forming machines, drying cabinets, and electrically heated or gas-fired computer-controlled kilns the principal steps from the raw clay to the finished ware are still very similar to those used in antiquity. Figure 4.1 shows the processing scheme, divided into 1. making the ceramic mass, 2. making forms and decorations, 3. firing green bodies, and 4. surface treatment of the fired ceramics including decoration by painting and sometimes gilding. Technological operations are shown in rectangular boxes, utilised materials in elliptic spaces. Investigations related to step 1 provide information on the origin and selection of clay raw materials, investigations of the ceramic body and its surface related to step 2 provide information on production technology. Moreover, study of the ceramic reveals information on firing technology (step 3), and techniques of decoration, function of the ceramic object, and possibly its age (step 4).

4.2 Preparation of clay Following mining the raw clay must undergo a sequence of beneficiation steps to render it suitable for forming. After removing of coarse constituents such as stones, twigs and leaves

38

Part I 1. Making of the ceramic mass

2. Making of form and decoraon Clay slip, glaze material

Water

Raw clay Crushing, cleaning

Water

Applicaon of slip

Salt

Smoothing, burnishing

from 1. Aging

Washing

Blunging, mixing

Forming

to 2.

Crushing, cleaning

Predrying (leatherhard)

Surface treatment

Mounng, barbone technique

Painng

to 3. Final drying

Pigments

Temper, clay mix

Addives

4. Surface treatment and environmental effects

3. Firing of ceramics Firing atmosphere

Clay, milk, laquer

Smoking

Food remains, soot

Soil environment, abrasion

Use

Alteraons

Coang, sealing

from 2.

from 3. Cooling

Firing

Surface treatment

to 4. Overglaze painng, gilding, post-firing painng

Underglazing 2nd firing Addion of salt or wood ash

Underglaze pigments

Overglaze enamels, post-firing painng

Acid food, wine, vinegar Abrasive materials

Soil soluons, atmosphere

Figure 4.1. Operational technical steps and external factors involved in making pottery: the way of a ceramic object from raw material to its present state.

the clay will be dried, crushed or milled, sieved to remove finer non-clay ingredients, and soaked in soft water, ideally rainwater. Frequently several clays will be mixed to obtain a mass with the desired properties, for example maiolica/French faïence (see Chapter 13) or some Roman Terra Sigillata (see Chapter 10). These clay mixes will be separated by levigation into coarse, medium and fine fractions as shown in Fig. 4.2 So-called ‘fat’ clay, that is, very plastic clay containing sizeable amounts of e. g. montmorillonite must be tempered with sand, rock, grog (pre-fired and crushed ceramic material) or other (organic) materials such as mussel shells (see Chapter 17), chaff, straw or even bone to avoid deleterious shrinkage during drying. To produce particularly delicate fine pottery, thorough dispersion of the clay mineral platelets is required. Such dispersion of kaolinitic clays has been achieved by ancient Chinese and medieval European potters through a process called aging, often by adding urine and faeces to the raw clay (Weiss 1963). The urea contained therein acts as a peptisation agent and penetrates the clay structure to be preferentially absorbed between layer stacks thereby allowing clay mineral platelets to slide easily over each other. Presumably bacterial activity assists in this process (Oberlies & Pohlmann 1958). Owing to this extremely odorous way of clay processing and, more importantly, to the inherent fire hazard, potter’s quarters were usually confined to the outskirts of human settlements, in general outside the city walls.

4 Processing of clay, and forming and finishing of pottery

39

Cleaned clay

Settling

Coarse fraction

Medium fraction

Partial drying

Fine fraction

Urine Long-time storage

Slip

Homogenization

Bricks Utilitarian ware

Fine ware

Figure 4.2. Steps of clay processing for different uses.

Details of the individual steps of preparation of clay are shown in Fig. 4.1, step 1 (making of the ceramic mass). Important clues as to properly prepare and store clays are given by Gerard (1979, Chapter 3 therein).

4.3 Forming of ceramic green bodies Techniques of forming and shaping of ceramic bodies are manifold. They can be divided into artisan and industrial processes. Artisan processes are generally performed by hand and involve modelling and building vessels by coiling and slab-building, forming by pressing or throwing clay into a mould, and throwing vessels on a potter’s wheel. On the one hand, these techniques are characterised by low shearing force the clay body is subjected to during forming and the fact that the amount of liquid phase remains essentially constant during the shaping process. The latter also applies to several modern industrial processes such as roller forming, extrusion moulding, semidry and dry pressing, and isostatic pressing, with various shear forces apparent. On the other hand, there are industrial processes during which the liquid phase involved varies in terms of amount and viscosity, and the shear forces are absent or low such as in slip casting, RAM pressing, die casting, injection mould-

40

Part I

ing, tape casting, doctor blade technique, or making thin film ceramics for electronic applications. The amount of water needed to generate sufficient workability of the clay varies widely, depending on the mineralogical composition of the clays, their grain size, and the presence of so-called flocculants, electrolytes that interact with the surfaces of the clay particles by changing the distribution of electrical charges thus resulting in agglomeration (see Chapter 3.5). An experimentally simple but in its physico-chemical interpretation complex test to estimate the amount of water required to obtain workable clay-water mixtures is the Pfefferkorn workability test (Fig. 4.3; Ebert 2006). A clay cylinder of height h0 = 40 mm and diameter 35 mm will be compressed to a height h by the action of a free falling steel plate of 1.2 kg mass originally suspended at an elevation H = 186 mm. The plastic deformation ratio h0/h depends on the water content. The ratio of 3.3 defines the Pfefferkorn plasticity number P, the ratio of 2.5 defines the amount of make-up water required to make the clay workable. Moreover, tan ˞ is defined as the Bowmaker plasticity factor PB.

Figure 4.3. Pfefferkorn test to determine the workability of a clay-water system

The kinetic energy of the free falling steel plate can be equated to the work of deformation expended against the flow limit (yield stress) ˱f under the assumption of a uniaxial stress state: ˱f = (m · g · H)/[2V · ln ⎛⎝

h0 ⎞ ], h ⎠

(4.1)

4 Processing of clay, and forming and finishing of pottery

41

where m = 1.2 kg, g = 9.81 m·s–2, H = 186 mm – h, V = 38.5 cm3. For plastic deformation ratios h0/h = 2.5 and 3.3 the flow limits to overcome are ˱f = 2.84 and 2.18 N·cm–2, respectively. For details the reader is referred to Heimann (2010). In the following sections only processes used in the ancient past will be described in more detail.

4.3.1 Simple coiling Apart from pinching and slab-built pottery (Chavarria 1999), coiling is the most basic technique to form a pot without the help of a potter’s wheel. Hence the earliest hand-forming technique to make ceramic vessels was by construction with clay slabs or by coiling with hand-formed rolled clay strands, frequently around a solid core. The earliest Near Eastern pottery was built by coiling and often resembled vessels made from non-clay materials such as stone, wood or animal skins. Using clays with suitable workability this technique lends itself to making even very large free-formed containers like the Greek pithoi, large storage vessels for grain and liquids (Christakis 1999). Since the coiling technique required a flat rock or wood surface to build the body of the vessel onto, it was reasonable to make it rotatable by revolving around a round stone positioned under the flat working surface. Indeed it is known that from about 3500 BCE onward potters in Mesopotamia formed large storage pots and mortuary vases using a kind of ‘slow wheel’, that is, a platform turned around slowly by an assistant to ease the placing of the coils. Similar platforms (‘mats’) consisting of disks of fired clay with a slightly convex underside that may have aided in turning were discovered at Myrtos on the island of Crete and dated to the late 4th millennium BCE (Evely 1988, Williams 1997). In

Figure 4.4. Large storage vessels (pithoi) for grain produced by coiling. Minoan palace in Knossos, Crete. Photos: Heimann.

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Part I

Figure 4.5. Coiling technique using a banding wheel (adapted from Gerard 1979).

modern potteries where coiling is done for artistic reasons, a so-called banding wheel is used, a circular turntable moved by hand (see Fig. 4.5c, d). Coiling technique was used in Neolithic China around 2000 BCE to make large funerary jars with spiral decoration painted with red and black iron oxide pigments. Huge Minoan Crete (see Chapter 9) pithoi (storage vessels) fashioned by coiling can still be seen in the Minoan palace at Knossos, Crete (Fig. 4.4). Today coiling is still prevalent among indigenous African and South American Indian potters. The coiling technique is illustrated in Fig. 4.5 and the description of the individual steps follows the practice manual by Gerard (1979). The clay, well-mixed by blunging, wedging and kneading is made up with water into a mass that is firm enough to be workable but not too firm that a strip of about 2.5 cm width cracks during bending into a gentle curve. After making the base of the pot by pounding a clay lump into a circular disk of the same thickness as the intended walls of the vessel, a supply of coils of about 2 cm diameter will be rolled out as evenly as possible. The first coil will be joined to the base (Fig. 4.5a) in such a way that its end will be pinched off in a wedge shape so that they can overlap. Then the second coil and subsequent coils will be placed inside the first coil and squeezed into their correct position (Fig. 4.5b). It is important to join individual shorter coils instead of a long one wound in a spiral fashion as this will lead to the vessel leaning over to one side. Recently joined coils should be squeezed to the required thickness and shape while slowly turning the banding wheel as shown in Fig. 4.5c. As the final step the outside and inside of the pot will be smoothed by up and down movement of the smoothing tool (hacksaw blade, Fig. 4.5d; wooden stick or paddle etc.) as horizontal movement will lead to stretching of the coils and thus a possible collapse of the vessel.

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4.3.2 Free-forming by throwing on a potter’s wheel The birth of the potter’s wheel is thought to have occurred around 4000 BCE in the Near East. The invention of the potter’s wheel allowed producing large amounts of pottery to satisfy the demand of a steeply growing population: between the Mesolithic and the end of the Neolithic the human population increased from approximately 3 to 86 millions. The use of the potter’s wheel spread slowly from its birthplace in the Near East to Europe and, much later, to the Americas. It could be found in Sumer around 3250 BCE, in Egypt around 2750 BCE, and in Crete around 2000 BCE. In mainland Greece it appeared around 1800 BCE, in southern Italy in 750 BCE, in the Rhine region around 400 BCE, in southern England at 50 BCE, in Scotland around 400 CE, and in the Americas after 1550 CE. The invention of the potter’s wheel had also an important sociological component. It is presumed that prior to its invention each family produced its own pots, and that the makers were predominately women. This work was taken over by men after introduction of the wheel and the development of superior firing technologies using improved kilns (Boger 1971). This development can be followed through many societies. At the most basic level of making household ceramics women were the potters and their work was scheduled so as not to interfere with other subsistence activities considered more important. Stage two consisted in producing specialised pottery on a more industrial scale that employed both female and male potters, whereas stage three of ceramic development was characterised by workshop industry and thus the emergence of pottery making as a full-time occupation, carried out more or less exclusively by men (Arnold 1989). As with the coiling technique described in the preceding paragraph particular emphasis has to be devoted to proper preparation of clay. Clay used for throwing on a potter’s wheel

Figure 4.6. Essential stages in throwing a vessel on a potter’s wheel (modified after Gerard 1979). a, b Centring of clay; c Opening out the clay and forming a base; d Pulling up the clay into a cylindrical vessel; e Compressing the rim of the vessel; f Trimming and undercutting.

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should be plastic, soft and free of any coarse constituents. The mass has to be carefully wedged and kneaded, and ideally be extruded from a pug mill to assure homogeneity. The steps in preparation that must be carefully adhered to and simple methods to test consistency are well described by Gerard (1979). Figure 4.6 shows the basic stages of throwing a bowl on a potter’s wheel. A lump of clay of appropriate size is shaped by hand into a spherical ball and slammed onto the centre of an only slightly damp wheel head: if the wheel head is too wet the clay will not stick to it. While rotating the wheel at a rapid rate the clay is squeezed with one hand and the heel of the other hand so that it rises into a cone (Fig. 4.6a). Then the palm of a hand is pressed down onto the cone to flatten it again. The process of coning and balling is repeated several times to assure that the clay is centred properly and has reached the right consistency for the subsequent throwing process. Final centring is done with both hands around the clay and one thumb across it, applying even pressure (Fig. 4.6b). Then the clay is opened up by pressing index and middle finger of a hand into the centre of the clay until about 7 mm of clay is left at the base of the intended vessel. By pulling the two fingers inward a flat base is produced some 6 to 8 cm wide (Fig. 4.6c). With the fingers of one hand inside the pot and the index finger of the other hand as support on the outside the clay is evenly squeezed and slowly pulled up (Fig. 4.6d). This is repeated several times and finally the rim of the vessel is held between index and middle finger of one hand and compressed downward with the index finger of the other hand to keep it firm and prevent it from splitting (Fig. 4.6e). When the wall of the vessel has an even thickness of (in this example), 7 mm final shaping is achieved with both hand by gentle squeezing the body of the vessel. As the last step the base of the wall will be trimmed with an appropriate tool und undercut at a 45° angle (Fig. 4.6f), and any remaining water is removed from the inside of the vessel with a soft sponge. Then the vessel is removed from the wheel head by cutting it loose with a taut cutting wire while reducing the wheel speed to a low rate. Any other trimming or necessary corrections should be made after drying to a leather-hard state. For more detail and technical advice the reader is referred to Gerard (1979).

4.3.3 Pressing or throwing of clay using moulds Free-forming of ceramic objects on a potter’s wheel requires a considerable degree of skill and training. The process is relatively slow and hence the output limited. However, in preindustrial Roman times, potters were forced to boost their output to meet the increasing demand throughout the empire for pots, bowls, dishes, cup and other ceramic objects for daily use. Hence increasingly they relied on unskilled labour to perform the comparatively easy process of shaping a ceramic vessel by using prefabricated clay moulds (Fig. 4.7; see also Chapter 10). Clay could be either pressed by hand into a stationary mould or thrown in a mould centred on a rotating potter’s wheel. The sequence of two conceivable techniques is shown in Figs. 4.8 and 4.9 (Juranek 1976, Juranek & Hoffmann 1991). Method 1 (Fig. 4.8) starts with centring a hollow supporting pedestal on the wheel head (a), followed by centring the mould (see Fig. 4.10, left) on top of the pedestal (b), and also centring a clay lump inside the mould (c). Then the clay is thrown by pressing it towards the

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Figure 4.7. Terra Sigillata picture mould with ‘lion-and-hare’ motifs, and examples of stamps. © Generaldirektion Kulturelles Erbe Rheinland-Pfalz, Direktion Landesarchäologie.

wall of the mould (d), smoothing the inside of the intended bowl with a shaping tool (e), polishing with a strip of leather (not shown), and removing the bowl from the mould (f), followed by mechanical roughening of the outside bottom of the bowl, attaching a circular support ring, dipping in an illitic slip, drying to a leather-hard state close to the hot kiln, eventual final trimming, and firing in air at 950 to 1050 °C (see Chapters 5 and 10).

Figure 4.8. Method 1 of forming a Terra Sigillata picture bowl using a mould (Juranek 1976). For details see text.

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Figure 4.9. Method 2 of forming a Terra Sigillata picture bowl using a mould (Juranek 1976). For details see text.

Method 2 (Fig. 4.9) involves forming a thick-walled clay moulding blank (a), dusting the inside of the mould with lycopodium or dry clay powder to prevent the wet clay sticking to the initially dry mould (b), centring the blank inside the mould that has been previously centred on the wheel (c), and pressing the blank against the wall of the rotating mould (d). Figures 4.9e and f show a close-up of the finished bowl with ‘Amor and donkey’ design (see Fig. 4.10, left). It should be noted that the bowl up to the ornamental ‘egg-and-dart’ border was formed within the mould whereas the smooth top with the protruding rim was

Figure 4.10. Left: TS picture bowl mould (‘Amor-and-donkey’ motif). Right: Two ways of pressing and throwing a green clay blank into a collared mould as the one shown on the left (top) and a plain mould (bottom).

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drawn up freely on a fast turning potter’s wheel, presumably by a different specialist worker. Figure 4.10 shows on the left a picture mould with ‘Amor and donkey’ design as well as schematic rendering of the forming process on the right. Many moulds were found to have a hole at the bottom through which occluded air could escape during the process of pressing and throwing.

4.4 Drying of green pottery Before permanent densification of formed clay bodies by sintering at high temperature can be attained thorough drying is required. This must be done to avoid cracking of the ceramic during firing by the force exerted by occluded water that will build up considerable pressure inside the ceramic body. Hence great care must be taken to remove water without jeopardizing the microstructure of the green body. This is not an easy task as, depending on the forming process, up to 30 vol% of water can be contained in the green body. The graphical representation of the volume-time relation is the Bourry diagram (Fig. 4.11) (von Chiari & Hennicke 1986). According to this diagram the drying process for moist model clays with a solid content of 55 vol% and a water content of 45 vol% will be divided into three stages. Stage A, lasting about 12 to 18 hours, is characterised by the expulsion of water. The water loss corresponds to the volume loss, that is, the shrinkage is linearly proportional to the water loss. In this stage the clay is still highly plastic. During stage B, lasting up to 3 days, drying is accompa-

Figure 4.11. Typical drying (Bourry) diagram for clay masses. s. Heimann (2010).

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nied by shrinkage the volume percentage of which is smaller than that of the evaporated water owing to the formation of air-filled pores. The mechanical state during stage B corresponding to a water content of 35 to about 15 vol% is called ‘leather-hard’, that is, the clay body is very firm and only slightly pliable so that trimming, attachment of handles, and decoration by incising is still possible. In stage C, shrinkage ceases and stabilises at about 22 vol%. The initial volume of the expelled water is completely replaced by air, hence the clay body is said to be ‘air-dry’, with water content between 3 and 1 vol%. Elevated drying temperatures up to 250 °C can reduce the water content to virtually zero. At the end of the drying process, the clay body attains elastic-brittle fracture characteristics and has shrunk by about 22 vol%. The final dry clay body consists of about 70 vol% solid clay and 30 vol% pores. A very important property of dried ceramic bodies is their dry flexural strength, meaning the fracture strength of the green body at the end of the shrinkage phase. Lack of dry strength causes problems during handling the green bodies and in particular stacking them economically in the kiln. Dry strength is dependent in a complex way on many parameters as follows (Heimann 2010). • Drying temperature: increasing drying temperature increases the flexural strength • Raw density, that is, the packing density as a function of the forming process. The flexural strength increases from slip casting to throwing to pressing. • Particle size. Illitic clays with a high proportion of particles < 2 µm have a higher flexural strength than kaolinitic clays with larger average grain sizes. • Cation exchange capacity. The flexural strength of Na+-loaded clays is higher than that of Ca2+-loaded clays since the clay platelets of the former are easier to arrange in a parallel fashion than the latter. • Charge of cations. The flexural strength increases from La3+ to Ca2+ to Na+. • Drying rate. The slower the drying, the higher is the flexural strength. • Porosity of ceramic body. Decrease of porosity of the dried body through lowering the moisture content increases the flexural strength.

4.5 Glazes and glazing Within operational steps 3 and 4 (Fig. 4.1) glazing is frequently performed to obtain properties a fired clay surface usually cannot provide: a smooth, impervious, hygienic, and aesthetically pleasing appearance. Glazes are mixtures of oxides, in general glass-forming silica, refractory alumina, and fluxing sodium, calcium, potassium or lead oxides to obtain on firing a low melting glass of eutectic composition that sticks tightly to the clay surface without running off. This property is conferred to the glaze by the addition of alumina that acts to increase the viscosity of the ensuing glass. Several metal oxides can be added to the basic glaze formulation such as oxides of iron, copper or cobalt to provide colour, magnesia to provide additional mechanical strength, and tin oxide and, in modern glazes, titanium oxide to yield opacity.

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While low-fired red and white earthenwares with a high degree of open porosity need glazing to become impervious to liquids22, high-fired stoneware and porcelain are not glazed for functional but for aesthetic and hygienic reasons. There are five principal types of glazes: alkaline, lead and lead opacified with tin oxide, ash, salt, and feldspathic (mud) glazes. Alkaline siliceous glazes, essentially the product of fusing together sand, and soda or potash obtained from halophytic plant ashes (Tite et al. 2006), were copiously used on pottery of ancient Egypt, Mesopotamia, Syria and Iran (see also Chapter 8). Such highly transparent glazes were coloured by adding copper to obtain a rich palette of turquoise and blue. An ancient recipe written in cuneiform letters and stored in the library of Ninive has the Seger formula23 (see below) [0.57 Na2O, 0.06 K2O, 0.17 CaO, 0.20 MgO], [0.03 Al2O3, 0.01 Fe2O3], 1.28 SiO2 (Weiss & Wihr 2010). A pale greenish blue Cu-containing glaze on tiles from the 19th dynasty of the New Kingdom consists of [0.32 Na2O, 0.68 K2O], [0.13 Al2O3], 3.41 SiO2 (Hayes 1937). Other ancient Egyptian glazes were reported by Bakr (1956) to consist of [0.71 K2O, 0.29 CaO], [0.04 Al2O3], 4.3 SiO2 and [0.32 Na2O, 0.68 CaO], (0.02 Al2O3], 2.5 SiO2. Lead glazes were produced in antiquity by melting together sand and either lead sulphide (galena) or lead oxide (litharge) obtained by oxidising roasting of galena as a by-product of silver smelting. Basically two ways exist to apply the glazing components. Either lead sulphide or a lead oxide powder is added to the ceramic surface in the leather-hard state by dusting (type 1) or a mixture of a lead compound and silica is applied, either in form of a slurry of lead sulphide or lead oxide and silica, or as a frit containing lead and silica (type 2).

22 A detailed description of glazing of medieval earthenware has been provided by Theophilus Presbyter (c. 1122) (De diversis artibus, Book II, Chapter 16: Earthenware vessels, painted with different colored glazes) as follows: “They [the Byzantines, i.e. Greeks] also make earthenware platters, small boat-shaped vessels [incense containers?, Braun 1932] and other earthenware pots and paint them in this way. They take pigments of all kinds and grind each one separately with water. To each pigment they mix a fifth part of glass of the same color that has itself been very finely ground with water. With this they paint circles, semicircles or rectangles and in them animals or birds or leaves or whatever they want. After the vessels have been painted in this way, they put them in the window-glass kilns and light a fire of dry beechwood underneath, until the vessels are surrounded by flames and become red-hot. Then they take out the wood and block up the kiln.” 23 The Seger formula used in modern ceramic technology is a short-hand notation to describe the composition of glazes for white earthenware, high-fired stoneware and hard-paste porcelain. In these formulae the oxide compounds of a glaze are divided into three groups: (i) basic oxides (fluxes) such as K2O, Na2O, CaO, SrO, PbO, ZnO, MgO but also colouring oxides such as FeO, CuO, CoO and NiO; (ii) refractory (amphoteric, glass modifying) oxides such as Al2O3, Fe2O3 and Cr2O3; and (iii) acidic (glass forming) oxides such as SiO2 and B2O3 but also non-glass forming oxides acting as opacifiers such as SnO2, TiO2 and Sb2O3. From chemical analyses that give the oxide composition in mass%, the Seger formula can be calculated (Romanosoglou et al. 2010) by dividing the analytical value of an oxide by its molecular mass, summing the basic oxides, and dividing all other values by this sum. Hence the sum of all basic oxides (fluxes) adds up to 1. For example, the Seger formula of a glaze suitable for white earthenware fired at 950 °C is [0.2 K2O, 0.2 Na2O, 0.3 CaO, 0.3 PbO], 0.3 Al2O3, [2.1 SiO2, 0.5 B2O3] (Taylor & Bull 1986), that for stoneware fired at 1200 °C is [0.3 K2O, 0.5 CaO, 0.1 MgO, 0.1 ZnO], 0.4 Al2O3, 3.5 SiO2, and that for hard paste porcelain fired at 1430 °C is [0.2 K2O, 0.6 CaO, 0.2 MgO], 1.1 Al2O3, 10.0 SiO2 (Heuschkel und Muche 1974).

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Lead-glazed pottery was known to the Roman and contemporary Chinese civilisations. The lead glaze can be intentionally coloured yellow or brown by adding iron ions, or green by copper, blue by cobalt, purple and brown by manganese, and black by manganese and iron (Boger 1971). For example, a green lead glaze of the Romans has the Seger composition23 [0.90 PbO, 0.04 Na2O, 0.06 CaO], 0.02 Fe2O3, 1.54 SiO2 + 3.2% CuO whereas a yellow glaze was found to have a Seger formula of [0.71 PbO, 0.06 Na2O, 0.23 CaO], 0.17 Fe2O3, 2.03 SiO2 (Caley 1947). Depending on the composition of the ceramic body two primary methods of glazing were used by Roman potters. On non-calcareous24 pottery produced in central Gaul (1st to 3rd centuries CE) and the Balkans (1st to 4th centuries CE) lead oxide was used by itself (type 1 above). Adhesion of the glaze to the ceramic body was mediated by a layer of crystalline lead feldspar reaction products. The second method used a lead oxide-plus-quartz mixture (type 2 above) applied to calcareous clay bodies produced in central Italy (2nd to 4th centuries CE), and adhesion was provided by a thin layer of wollastonite (Walton and Tite 2010). In modern brashly coloured folk-style ceramic table ware the use of lead-based glazes must be tightly controlled owing to their health hazard25. Lead glazes originated in China at a comparatively late data during the Warring States period (475–221 BCE), some 5,000 years after the invention of earthenware glaze equivalents in western Asia and 1,000 later than China’s first ash-glazes developed during the Shang dynasty (Kerr & Wood 2004; p. 606). This finding accounts for the curious fact that in China potters began glazing with high-temperature ash glazes and only much later moved to lowtemperature glazes fluxed with lead and barium oxides. For a comprehensive review of Chinese stoneware and porcelain glazes see Kerr & Wood (2004; pp. 455–608). White and opaque tin-glazes were produced by adding tin oxide (‘tin ash’) to lead glaze. Such glazes were used predominately on maiolica and faience (see Chapter 13) but also on high-fired Islamic pottery from the 9th century CE onward applied to replicate the whiteness of imported Chinese porcelain (Kingery & Vandiver 1986). However, in the latter case the ceramic body was made by firing a paste of ground quartz pebbles, mixed with a premelted glass frit composed of quartz sand, and sodium and calcium carbonates, as well as highly plastic white clay. Typically it comprises 10 parts quartz, 1 part glass frit and 1 part white clay (Mason & Tite 1994). This Islamic quartz-frit-clay ware emulated an even older ceramic technology called Egyptian faience, invented between 5,000 and 3,000 BCE in Mesopotamia and Egypt. Together with overglaze lustre painting application of a lead-tinglaze resulted in spectacularly beautiful Islamic ceramic objects analysed in much detail by Kingery & Vandiver (1986) (see also Mason & Tite 1997, Vendrell et al. 2000, Pradell et al. 2008a, 2008b). Also, lead and tin-glazes are the only glazes that can be used on soft-paste porcelain (see Chapter 14) where they form a wollastonite reaction layer with high thermal

24 The definitions of calcareous and non-calcareous clays vary among different authors. Here we define calcareous clays as clays containing > 6 mass% CaO, in accord with Mason & Tite (1994). 25 The health danger of insufficiently immobilised lead in glazes applied to low-fired earthenware ceramics has been known for a long time (Westrumb 1795, Formey 1796). Based on this knowledge the Königliche Porzellanmanufaktur (KPM, Royal Porcelain Manufacture) in Berlin, founded in 1763, produced ‘sanitary’ porcelain with a lead-free feldspathic glaze during the late 18th century CE, and advertised its health virtue accordingly as an early environmentally friendly product.

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expansion matching the expansion of the highly calcareous ceramic body thus providing a well-adhering crack-free white opaque surface (Walton & Tite 2010). Historical recipes of tin oxide-opacified lead glazes used on maiolica and faience wares can be found by Piccolpasso (1557/59) (see Chapter 13.4). Ash glazing was presumably already invented during the Shang period (1550–1028? BCE) when Chinese potters discovered that wood ash heated to high enough temperatures in downdraft kilns melts into a glaze all by itself. These high-temperature ash glazes were developed in southern China to coat some wood-fired stonewares. There they formed the foundation for the ash-glazing tradition that spread to northern China in the early 6th century CE. Ash-glazing reached its zenith in the 4th to 11th centuries CE with the Yue wares of northern Zhejiang province. Subsequently limestone gradually replaced wood ash as a flux (Kerr & Wood 2004; p. 606), forming greenish-yellow glazes with spots of deeper green. The greenish colour of such famous celadon glaze is related to the iron content of the wood ash. The composition of wood ash varies greatly depending on the type of the wood and the temperature of incineration. Chemical analyses of some wood ashes are shown in Tables 4.1 and 4.2. Major oxides in the wood ash are CaO, K2O and MgO. SO3, P2O5 and MnO are present at levels below 3 to 4 mass%. Carbon is also present but was not determined analytically and thus is expressed in Table 4.1 as the difference to 100%. The nitrogen content in wood ash is normally insignificant due to the conversion of most of the nitrogen contained in the wood to NH3, NOx and N2 during combustion. Pine and aspen ashes have higher amounts of potassium compared to poplar or oak ash. The sodium content is generally low in all ash types with the exception of poplar ash with about 3 mass% Na2O. Volatile elements such as Zn, K, S, and B are consecutively lost when wood ash is fired at temperature above 1000 °C. Satisfying glazes can be made by mixing roughly equal parts of wood ash, feldspar, and clay. In some technological developments the composition of wood ash was modified by washing prior to application. During washing most of the easy soluble potassium compounds

Table 4.1. Chemical compositions in mass% oxide of the ashes of several wood types incinerated at 600 °C (recalculated from the original table given by Misra et al. 1993). * Carbon is expressed as the difference of the sum of all other oxides to 100%. Oxide

Pine

Aspen

Poplar

Red oak

White oak

CaO

40.65

29.62

35.92

51.18

43.86

K2O

19.56

13.55

9.55

7.32

12.35

MgO

11.65

5.88

15.06

8.62

12.55

SO3

2.67

1.75

2.55

4.50

3.02

P2O5

1.92

2.70

2.17

3.57

1.28

Al2O3

0.89

0.26

0.66

1.28

1 Egyptian blue is formed whereas at somewhat higher temperatures between 950 and 1150 °C and CuO/CaO ratios < 1 the ‘green frit’ appears (see also Ullrich 1979). 28 Minium was also synthetically manufactured by oxidising roasting of lead carbonate (synthetically produced ‘ceruse’, cerussite PbCO3) as described in Theophilus Presbyter’s De diversis artibus (c. 1122), Book I (The art of the painter), Chapter 37. 29 Recently there has been a priority struggle related to who first suggested that tin found in the blue and green Cu pigments may have originated from using bronze scrap as a precursor material as well as who first established the precise nature of the copper ingredient in both Egyptian blue and ‘green frit’ (Schiegl & El Goresy 2006, and references therein; Pagès-Camagna & Colinart 2006, and references therein).

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Unfortunately Egyptian blue is a pigment with rather low tinting strength. In particular, its colour depth and brilliance disappear with increasing comminution, that is, decreasing grain size. Consequently, grain sizes found on painted faience ceramics and also wall paintings in Egyptian tombs are between 5 and 50 µm (Noll 1991), rather coarse by modern pigment standards. However, a much better blue pigment was discovered and synthesised in ancient Egypt that excelled by its colour strength, fineness (0.2–0.5 µm) and environmental stability: cobalt aluminate spinel, CoAl2O4 (Riederer 1974, Noll & Hangst 1975, Noll 1981). The technology of using cobalt as a colourant for glasses30, glazes and ceramics was presumably discovered in 18th dynasty Egypt during the reign of Thutmose III (1479–1424 BCE) and became rather common during the reigns of his son, Amenhotep II (1424–1398 BCE), and grandson, Thutmose IV (1398–1388 BCE). However, it was during the reigns of Amenhotep III (1388–1350 BCE) and Akhenaten (Amenhotep IV) (1351–1334 BCE) that utilisation of cobalt blue became most abundant (Shortland et al. 2006a). It flourished during the 19th dynasty and part of the 20th dynasty after which it ceased to be used (Arnold & Bourriau 1993), presumably related to problems in the supply of raw materials31 (see Chapter 8.4). Other synthetic pigments employed in antiquity include carbon black (Vitruvius 1960) used ubiquitously since Neolithic times to decorate ceramic wares as well as effenbergerite (Chinese purple, BaCu[Si4O10]), a Ba-analogue of cuprorivaite (Egyptian blue) that was utilised in painting the Chinese Han Dynasty Terracotta Warriors (Liu et al. 2007). Faiences of the Amarna period (18th dynasty) collected by Petrie (1894) were found to be painted with Naples yellow, a lead antimonate, Pb2Sb2 O7 (Clark and Gibbs 1997), presumably synthesised by calcining an excess of a lead compound, most likely galena (PbS) with an antimony compound, most likely stibnite (Sb2S3) (Shortland 2002). It could also have been obtained by heating the mineral bindheimite, Pb2Sb2O6(O,OH) (Galli et al. 2004). Beginning from the 20th dynasty Naples yellow was increasingly replaced by lead-tin yellow (lead stannate, Pb2SnO4) that was possibly produced by melting together lead and tin metals in the presence of air (Heck et al. 2003). Technological information on synthesis and uses of post-firing paint pigments in ancient Mesopotamia, Egypt, Anatolia and Iran has been thoroughly researched by the late Walter Noll, the eminent pioneer of ancient ceramic science and technology, and collected in his posthumous treatise “Alte Keramiken und ihre Pigmente (Ancient Ceramics and their pigments)” (Noll 1991, pp. 186–220; see also Lee & Quirke 2000).

30 Recently a new cobalt colourant was discovered in 14th – 13th centuries BCE glasses from Nippur with the trace element combination Co + Fe + Al ± As, very distinct from Egyptian cobalt colourants (Co + Al + Mg + Ni + Zn). This suggests utilisation of a cobalt-bearing ore distinct from the well-characterised Egyptian source (Walton et al. 2012). 31 Noll (1991) tentatively related the sudden and massive appearance of cobalt aluminate pigment as ceramic paint during the 18th dynasty to the sun cult of Aten, championed by Pharaoh Akhenaten, the Heretic King. Use of this pigment fell into oblivion after reestablishment of the traditional Amun cult by the priests eager to erase any memory of Akhenaten.

Chapter 5

Ceramic phase diagrams Synopsis The purpose of this chapter is to introduce readers unfamiliar with phase diagrams to the qualitative and quantitative aspects, and the interpretation of the most important ceramic ternary diagrams such as the systems CaO-Al2O3-SiO2 and K2O-Al2O3-SiO2. The former system concerns most ancient ceramics, the latter is applicable to Chinese and European hard-paste porcelain. The exact meaning of relevant terms such as composition of ternary compounds, phase boundary lines, invariant points, degrees of freedom, conodes, compatibility triangles, and phase equilibria are discussed.

5.1 Introduction In mineralogy and materials science it is an established procedure to express the composition of an assembly of different phases through a ‘phase diagram’. Depending on the number of component32 there exist unary, binary, ternary, and multinary phase diagrams. Even though a natural clay contains many oxide components owing to its complex mineralogical composition the brief discussion of phase diagrams will be limited to the ternary case, in particular the systems CaO-Al2O3-SiO2 in which most ancient ceramics can be accommodated, K2O-Al2O3-SiO2 applicable to Chinese and European hard-paste porcelain, and Na2O-CaO-(Al2O3)-SiO2 typical for some alkaline glazes and French soft-paste porcelain. To some readers unfamiliar with material science the following discussion may appear highly arcane. However, knowledge of the ceramic phase diagrams provides the key to understand the complex relations between mineralogical and chemical composition of the precursor clays, the maximum firing temperature reached, the firing atmosphere utilised, and the firing time at the one side, and the microstructure, fabric, colour, and eventually the mechanical and thermal properties of the ceramic product on the other side (Heimann 1989). The pedagogical approach taken here is to discuss first the geometry and construction of ternary phase diagrams, the meaning of the terms components32, phases33, primary

32 A component C is defined as a simple oxide such as CaO, Al2O3 and SiO2 that combine to form a phase P present at equilibrium. It is the smallest number of independent variable chemical constituents necessary and sufficient to express the composition of each phase present in any state of equilibrium (Levin et al. 1964–1990). 33 A phase P is defined as that mechanically separable portion of a system in which the parameters F such as temperature and pressure and the chemical composition of matter are physically homogeneous, that is, constant and uniform.

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phases34, binary and ternary phases, phase boundaries, conodes, cotectic triangles, Alkemade lines and other rather erudite expressions and then to proceed to real ceramic phase diagrams that will also appear throughout the text of this book. For more detail the reader is referred to textbooks of physical chemistry (see for example, Aaronson 1975, Rosenberger 1979). In particular, the derivation of binary phase diagrams using the Gibbs free energy function G and its dependence on the intensive variables temperature T, pressure P, and mole numbers ni can be found in Heimann (2010, Appendix A).

5.2 Anatomy of three-component (ternary) phase diagrams 5.2.1 Composition of ternary compounds The three oxide components A, B, C are arranged at the corners of an equilateral triangle as shown in Fig. 5.1. In this case the temperature axis cannot be plotted directly but can be described only by isotherms that are projections down the temperature axis perpendicular to the triangle. Figure 5.1a shows two methods to express the compositions of a phase assembly [X] 35 in terms of the proportion of the components A, B, and C, and A, C, and D, respectively. These components can be simple oxides such as CaO, Al2O3 and SiO2. To represent larger families of ceramics manufactured from raw materials of widely variable compositions, the corners of the triangle are sometimes labelled ‘sand’, ‘feldspar’ and ‘clay’. One simple method to obtain the composition of point [X] shown in Fig. 5.1a consists of drawing lines (long dashes) parallel to the sides of the triangle through point [X]. These lines intersect the sides of the triangle at the points a’-a”, b’-b”, and c’-c”. The relative distance of [X] from the corners A, B and C can then be expressed in percentages as read from the scales along the sides of the triangle, yielding 20 mol% A, 20 mol% B, and 60 mol% C. A second method consists of letting normals [X]a, [X]b, and [X]c fall onto the sides of the triangle (short dashes). Their lengths are proportional to the percentages of the component designated at the opposite end. Sometimes it is required to express the composition of a ternary phase [X] in terms of the components, located at the apices, and a binary phase, located on a side of the triangle. Then point [X] might be expressed in term of A, C, and D (= binary phase [B3C2]35) on the line (compatibility join, conode) AD. This construction yield A = DF·100/AD = 20%, D = AE·100/AD = 34.5%, and C = EF·100/AD = 45.5%. Since the binary phase D consists of

34 A primary phase is the only crystalline phase that can exist in equilibrium with a melt of a given composition. It is the first crystalline phase to appear on cooling of a composition from the molten state or the last crystalline phase to disappear on heating a composition to melting. 35 To distinguish primary component from phases the latter are denoted within square brackets. The composition of the binary phase [B3C2] in this example refers to (3/5)·100 mol% = 60 mol% B + (2/5)·100 mol% = 40 mol% C.

5 Ceramic phase diagrams

B

B 10

10

90

a‘ 30

c‘‘ a30

70 mole% C →

X

E

90

A

A2B

30

50 c‘

mole % C

b

b‘‘

a‘‘ 90

e3

70 e2 e1 90

10 10

9

50

50

F

90 E4

5

e4

D

b‘

2

30

70

c 50

61

C

A

B3C2 = D e5 50 e6 e7 BC 2 30

Y

E3

4

mle% E C 1→

E2 Z

1

70

10

8

x

7

10 e12 e11 30

E5

6 10 e8

3 e10 50 A2C

e9 70

90

C

Figure 5.1. Left: Two ways to express the composition of a ternary phase [X] in the ternary system A-B-C given in mol%. Right: The stability fields of the primary components A, B, C (phase areas 1 to 3), binary phases [A2B], [B3C2], [BC2], and [A2C] (phase areas 4 to 7), and ternary phases [X], [Y], [Z] (phase areas 8 to 10) in a hypothetical ternary system A-B-C (Heimann 1989). The points e1, e2, ... are binary eutectics, the points E1, E2 ... refer to the ternary eutectics.

60 mol% B and 40 mol% C, one obtains approximately A = 20 mol%, B = 20 mol%, and C = 60 mol% as above.

5.2.2 Phase boundary lines, eutectic points, and degrees of freedom The point [X] in Fig. 5.1a represents the composition of a hypothetical ternary phase, [ABC3]. Exploration of the ternary system A-B-C by x-ray diffraction (XRD), optical methods, and differential thermal analysis (DTA) may have revealed two other ternary phases, [Y] with the composition AB2C, and [Z] with a composition [A4BC], as well as four binary phases [A2B], [BC2], [A2C], and [B3C2] (= D). These seven phases and their stability fields 4 to 10 are shown in Fig. 5.1b. While points [X], [Y] and [Z] represent the exact stoichiometric compositions, the areas 8, 9 and 10 are those portions of the phase diagram in which the ternary phases are stable according to Gibbs’ phase rule36. In addition, the stability fields 1 to 3 of the pure components A, B, and C have been outlined. All these phases are separated by phase boundary lines that indicate the fields of thermodynamic stability of the respective phases. It happens that three phase boundary lines intersect at five locations E1 to E5, the 36 The famous Gibbs’ phase rule P + F = C + 2 relates the number of phases P, the number of component C, and the number of degrees of freedom F, that is, the number of physical parameters that can be varied without changing the number of phases present (Gibbs 1876, 1878). Since in ceramic systems the variation of pressure does not play an important role the pressure is taken as a constant (1 atm). For this reason, frequently the condensed Gibb’s (‘mineralogical’) phase rule P + F = C + 1 is used.

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ternary eutectic37 (invariant) points. The intersections of the phase boundary lines with the sides of the triangle (binary joins AB, AC, BC) yield the twelve binary eutectic points, e1 to e12. In this simple theoretical phase diagram there are no peritectic points38. According to Gibbs’ condensed phase rule36 at the ternary eutectic points four phase coexist (number of components C = 3, number of phases P = 4, degrees of freedom F = 0): three solid phases and one liquid melt phase. For example, at the ternary eutectic point E1 the two ternary phases [X] and [Z], the binary phase [A2C], and the melt (not shown in projection) coexist. By definition this point is invariant (F = 0) to changes in temperature and/or composition as each even minor departure from its values will displace the system from point E1. A composition anywhere along a phase boundary lines is subject to a univariant equilibrium (F = 1) since it is possible to alter either the temperature or the concentration of one of the two components without leaving the boundary line between two solubility surfaces. Finally, any composition between phase boundary lines, that is, within a phase stability field is subject to a divariant equilibrium (F = 2). Temperature and compositions of two components can be manipulated without forcing the system into a different phase stability field, that is, without a new phase appearing.

5.2.3 Compatibility joins (conodes) and compatibility triangles Straight lines connecting the composition points of two primary phases34 are called compatibility joins, tie lines, or conodes. A ternary conode, sometimes referred to as an Alkemade line, must intersect the phase boundary lines between adjacent (conjugate) primary phases (see Fig. 5.2), and it determines the direction of falling temperature on the phase boundary curve. Since the direction of falling temperature is always away from the Alkemade line, the intersection of boundary curve and Alkemade line constitutes a temperature maximum or a saddle (col) point. Figure 5.2 shows the compatibility joins (dashed lines) connecting the primary phases [X], [Y] and [Z] with each other, as well as the four binary phases [A2C], [B3C2], [A2B] and [BC2], and the pure components A, B, and C. The arrows indicate the directions of falling temperatures as determined by the Alkemade theorem39. It is easy to see that the ternary eutectic point E2 constitutes the composition with the lowest possible melting temperature since the direction of the three arrows on the phase 37 A eutectic point is an invariant point at which the combination of components in a simple system has the lowest melting temperature at any ratio of the components. It is located at the intersection of two solubility curves (binary eutectic) or three solubility surfaces (ternary eutectic). 38 A peritectic point is an invariant point at which the composition of the melt phase in equilibrium with the solid phases cannot be expressed it terms of (positive) quantities of the solid phases (Levin et al. 1964–1990). This situation may arise when a solid phase reacts with the melt to form a new phase with different composition. 39 Alkemade theorem: ‘The direction of falling temperature on the boundary curve of two intersecting primary phase areas is always away from the Alkemade line. If the Alkemade line intersects the boundary curve, the point of intersection represents a temperature maximum on the boundary curve. If the Alkemade line does not intersect the boundary curve, then the maximum on the boundary curve is represented by that end which, if prolonged, would intersect the Alkemade line’ (Levin et al. 1964–1990).

5 Ceramic phase diagrams

63

B 10

30 e4 50 A2B

e3

A

9

5

70 B3C2 = D e5 50 e6 e7 BC 2 30

Y

E3

10

10 e12

8 x

Z

1

E4

2

4 E E1 2 C mle%

70 e2 e1 90

90

7

e10 50 e11 30 A2C

E5

6 10 e8

3 e9 70

90

C

Figure 5.2. Compatibility triangles (dashed lines) superimposed on the stability fields 1 to 10 shown in Fig. 5.1b, and directions of falling temperatures on the phase boundary curves (arrows) determined according to the Alkemade theorem39. For clarity not all possible arrows are shown (Heimann 1989).

boundary curves e3-E2, e10-E2, and e6-E2 point to E2 as the absolute temperature minimum within the ternary system A-B-C. The straight compatibility joins (conodes) dissect the triangle ABC into a collection of smaller triangles referred to as compatibility or composition triangles. These are extremely important in the determination of stable coexisting mineral (phase) assemblages both in petrology and ceramic science. For example, a hypothetical clay with a composition of 30 mol% A, 60 mol% B, and 10 mol% C (hexagon in Fig. 5.2) will develop on prolonged heating toward an equilibrium composition composed of the three stable phases [B], [Y] and [A2B] that constitute the apices of the compatibility triangle in which the starting composition is located.

5.2.4 Micro- or local equilibria The firing process to which clays are subjected results in a distinct state of sintering40, dependent on firing temperature, time, composition of the raw material, and kiln atmosphere (see Chapter 6). Hence it can be expected that the relationship between chemical and mechanical properties of the product and the multitude of predictor variables is exceptionally complex, and in many cases impossible to unravel. Fortunately, there are several ways to overcome this problem. One involves retrospective estimation of firing conditions by assessing temperature-dependent properties of the particular clay. Thus, the properties of the ceramic body, such as mineral content, colour, density, hardness, and strength, are compared to those of a series of similar or, ideally, identical clays fired at various temperatures. 40 Sintering is defined as densification of fired clay by formation of new mineral phases and coalescence of pores through solid state reactions, partial melting, dissolution, and crystallisation. The driving force of sintering is the tendency of the ceramic system to minimise its surface free energy.

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These series form a system of reference samples possessing a characteristic, temperaturedependent mineralogical phase composition against which the composition of the actual ceramics can be matched (Heimann & Franklin 1979). Analytical tools to determine the mineralogical phase composition are x-ray diffraction, optical and electron microscopy, vibrational spectroscopy such as infrared or Raman spectroscopy, and a host of other modern surface analytical techniques (see for example Kingery & Vandiver 1986, Kockelmann et al. 2001). Since the firing of a ceramic body is a short-term event, the time the object was subjected to the maximum firing temperature is rarely sufficient to attain an equilibrium situation with respect to the phase transitions initiated by the thermal treatment. In particular, calcareous ceramics fired for only a few minutes to a maximum temperature below 800–850 °C in a typical bonfire retains its calcite content completely even though the thermodynamic decomposition temperature has been exceeded (Maggetti et al. 2011). Consequently, kinetic retardation of the thermal decomposition reactions of primary clay minerals such as illite or kaolinite, and temper minerals such as quartz as well as the formation of high-temperature minerals such as gehlenite, diopside, or mullite, must be taken into account. The ceramic phase diagrams shown below and in Chapter 6 have been constructed from information gathered during slow cooling of silicatic melts, in which the formation of new phases from the melt is accelerated because the mass transfer coefficients are much larger in a liquid than in a solid. In fired clay, however, the temperature is usually not sufficiently high to melt the raw material completely. Even if partially melted phases existed in the ceramic body as in high-fired stoneware or porcelain, or in the slips of Cretan Kamares ware (Noll 1982) or Roman Terra Sigillata, the high viscosity of these silica-rich melts largely prevented the crystallisation of equilibrium phases. All this means that the ceramic body is left in a state of non-equilibrium. At lower firing temperatures it is composed of a succession of micro- or local equilibria that are confined to the reactive interfaces between mineral grains in an inhomogeneous clay matrix (Maggetti 1986). Figure 5.3 shows on the left an aggregate of two calcite crystals, one quartz grain, and one illite flake. The five existing microequilibria are denoted 1 to 5, and their positions in the ternary phase diagram CaO-Al2O3-SiO2 are shown on the right. This diagram is particularly important for the calcareous illitic clays widely used in antiquity, and is discussed in more detail in 5.3.1. The calcite converts on heating to CaO, and thus obtains its position in the CaO apex of the phase diagram. At the boundary between calcite and quartz, two phases of the binary subsystem CaO-SiO2 form, for example, wollastonite, Ca3(Si3O9) and rankinite, Ca3Si2O7 whose approximate positions are indicated at 2. The microequilibrium 3 is stable at the three-grain boundary quartz-calcite-illite, and the position of the reaction product is somewhere in the subsystem wollastonite-anorthite-quartz, depending on the relative proportion of the components. Here the exact position depends on the relative proportion of the original minerals. The microequilibrium 4 between calcite and illite contains less SiO2 component than microequilibrium 3, and is therefore located somewhere in the subsystem wollastonite-anorthite-gehlenite. The quartz, finally, will be located at the SiO2 apex.

5 Ceramic phase diagrams

Quartz 5 2

65

SiO2

3

5 1 Calcite

4 Illite

3 Wollastonite

2

4

Anorthite

Gehlenite

CaO

1

Al2O3

Figure 5.3. Schematic representation of microequilibria in a typical calcareous clay fired under oxidising conditions (Maggetti 1986). Left: white = calcite, stippled = quartz, laminated = illite. Right: Location of typical microequilibria 1 to 5 in the ternary phase diagram CaO-Al2O3-SiO2. For explanation see text.

The existence of microequilibria furnishes the ceramic analyst with the ability to perform mineralogical analyses of grain boundary melts, reaction rims, and pores using optical and electron-optical techniques (Heimann et al. 1980). The results often lead to conclusions about the crystallisation paths toward equilibrium. In conclusion, the useful concept of microequilibria explains why thermodynamically incompatible mineral phases can occur side by side in a ceramic body. The inherent inhomogeneity (graininess) of the clay, together with low temperatures and short firing times, are not sufficient to attain a thermodynamic equilibrium with respect to the global system. This situation reflects the universal struggle between thermodynamic and kinetic principles in that there exists the thermodynamic ‘license’ to proceed with a certain phase transformation reaction, but the low maximum firing temperature, steep heating rate, and short reaction time impose a kinetic constraint resulting in retardation of the expected reactions. Thermodynamics will control the reactions at high temperatures and for long reaction times; kinetics will exercise control at lower temperatures and shorter times. This occurs because a high amount of energy is required to nucleate a new phase. This activation energy is not available at the relatively low temperatures employed by an ancient potter, so metastable ‘frozen-in’ equilibria dominate the ceramic microstructure as evidenced, for example in Neolithic ware.

5.3 Selected model ceramic phase diagrams The phase diagrams discussed in this section relate to ancient pottery made from (a) calcareous illitic clays fired at temperatures below the onset of substantial sintering (5.3.1), (b) stoneware and porcelain made from non-calcareous kaolinitic clays fired at high tempera-

66

Part I

ture and characterised by a high degree of vitrification (5.3.2) as well as (c) soft-paste porcelain made from alkali- and alkaline earth-rich glass frit mixed with rather small amounts of clay (5.3.3).

5.3.1 CaO-Al2O3-SiO2 In the system CaO-Al2O3-SiO2 there exist only two stable ternary compounds: anorthite (An, CaAl2Si2O8) and gehlenite (Ge, Ca2Al2SiO7), and, in systems containing MgO, diopside (Di, CaMgSi2O6) that ubiquitously appears in fired calcareous clays. Much work has been expended on clarifying the complex melt and subsolidus reactions of calcareous clays subjected to ceramic firing (for example, Molera 1998, Duminuco et al. 1998, Dondi et al. 1998, Dondi et al. 1999, Jordán et al. 1999, Riccardi 1999, Cultrone et al. 2001, Bauluz et al. 2004, Hajjaji & Kacim 2004, Ionescu & Hoeck 2011). Gehlenite is the Al-rich end member of the melilite (Ca2(Mg,Al)[Si2O7]) solid solution series. Gehlenite occurs in pottery produced from low fired calcareous clays but in higher fired ware transforms readily to anorthite. The rate of formation of gehlenite depends strongly on the grain size: sizable amounts of gehlenite can be found only in coarse ware, in particular in clays containing large grains of calcite (Maggetti & Küpfer 1978). The grain size, in turn,

Figure 5.4. Cotectic (compatibility) triangles (solid lines) in the ternary diagram CaO-Al2O3-SiO2 (Heimann 2010). In the SiO2-rich portion three important triple points are indicated at the intersection of the phase boundaries (dashed lines). The arrows show the direction of falling temperatures according to the Alkemade theorem39. An anorthite, CAS2; Cr cristobalite, SiO2; Ge gehlenite, C2AS; Mu mullite, A3S2 to A2S; Tr tridymite, SiO2; Wo wollastonite, CS. AC, PC, BFS: Compositional areas of aluminate cement, Portland cement, and blast furnace slags, respectively. The binary and ternary phases are denoted according to the cement chemical notation, C = CaO, A = Al2O3, S = SiO2.

5 Ceramic phase diagrams

67

is being influenced by the degree of processing of the clay, a measure of the technological skill of the potter. During burial in humid climates gehlenite tends to decompose, forming a series of zeolites (Maggetti & Heimann 1979, Heimann & Maggetti 1978) (see Chapter 6.4.1). The binary compounds such as calcium silicates, calcium aluminates, and mullite are located along the sides of the triangle shown in Fig. 5.4. The calcium aluminate phases C3A (and C12A7), and the calcium silicate phases C3S and C2S are important clinker phases of cement. The triple point between the primary fields of An, Wo and Tr is the ternary eutectic with a melting point at 1170 °C. Other eutectics with comparatively low melting points are located at the triple points An, Ge and Wo (1265 °C) and An, Tr and Mu (1345 °C).

5.3.2 K2O-Al2O3-SiO2 This system is important to describe the development of fine stoneware and porcelain. Figure 5.5 shows a simplified version of the SiO2-rich part of the ternary phase diagram. Potassium feldspar (kfs, black point) melts incongruently at 1170 °C, slightly above the ternary eutectic Kfs-Leu-Mu at 1140 °C. The typical classic raw materials mixture for manufacturing porcelain consists of 50 mass% kaolin, 25 mass% K-feldspar and 25 mass% quartz (‘triaxial’ porcelain; see Chapter 15), corresponding to point ‘A’ in Fig. 5.5 (69.3 mass% SiO2, 26.2 mass% Al2O3, 4.5 mass%

Figure 5.5. Phase boundaries (solid lines) and coexisting phases in the SiO2-rich section of the ternary system K2O-Al2O3-SiO2 (after Schairer & Bowen 1947). The path C-B-A characterizes the reaction history of a typical porcelain body (see text). Co corundum, Cr cristobalite, Kfs potassium feldspar, KS2 potassium disilicate, KS4 potassium tetrasilicate, Leu leucite, Mu mullite, Qz quartz, Tr tridymite. The ternary eutectic is at the triple point Kfs-Mu-Tr with the melting temperature of 985°. This is also the equilibrium phase composition of porcelain. Note that the diagram has been truncated at 60 mass% SiO2. There is a peritectic point38 at 710 °C (intersection of phase boundaries Tr-Kfs-KS4).

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Part I

K2O). This composition is located within the stability field of mullite, with a liquidus temperature > 1700 °C. On heating this mixture with the composition A the first traces of melt occur at the triple point C at 985 °C. According to the condensed Gibbs’ phase rule (P + F = C + 1) at this invariant point (F = 0) one of the four phases P present (Mu, Kfs, Tr, melt) in the ternary system (C = 3) must disappear, that is, Kfs will be consumed. As more melt is formed on further heating the overall composition moves along the phase boundary between Mu and Tr towards point B. Finally, in point B at 1280 °C all Tr and/or Qz has disappeared in the melt, and the reaction path changes direction, moving along the connecting line B-Mu until it intersects the isotherm at 1400 °C, the firing temperature of hard-paste porcelain. From this temperature the porcelain body is cooled and the microstructure should theoretically consist of the phase assembly Mu + Kfs + Qz, consistent with the composition at point C, the equilibrium composition of triaxial porcelain. However, as further explained in Chapter 15 and shown in Fig. 15.12, Kfs remains dissolved in the melt that solidifies as glass in which mullite needles and residual unreacted quartz particles are suspended. More details of the crystallisation path and relevant equilibrium relations of triaxial porcelain will be shown in Chapters 6 and 15.

5.3.3 CaO-Na2O-SiO2-(Al2O3) This phase diagram is of importance to describe the phase formation in proto-porcelain compositions, in particular the French soft-paste porcelain invented and produced in the factories of Vincennes and Sèvres (see Chapter 14). The composition of the ware varies widely. An approximate value in mass% is 68–75% SiO2, 13–18% CaO, < 2% MgO, 2–7% Na2O + K2O, and < 3% Al2O3 (see Table 14.4). As shown in Fig. 5.6 these compositions are

SiO2 Soft-paste porcelain 10 20 CS + silica + liquid

30

silica + liquid

L melt

40

CS 50

CaO

Na2O

Figure 5.6. Isothermal section at 1100 °C through the simplified partial phase diagram CaONa2O-SiO2 showing the phases wollastonite (CS), silica (tridymite, SiO2) and liquid (melt) L as quasi-equilibrium phases in French soft-paste porcelain (modified after Kingery & Vandiver 1986). The composition is given in mass%. The isotherm at 1100 °C corresponds to the approximate firing temperature. The diagram is a section through the quasi-quaternary system CaONa2O-SiO2-Al2O3 at approximately 3 mass% Al2O3. See also Figs. 14.8 and 14.9.

5 Ceramic phase diagrams

69

situated in the conode triangle wollastonite (CS) – silica (SiO2) – melt L. (see also Figs. 14.8 and 14.9). At firing temperatures around 1100 °C there exists a viscous siliceous melt of composition L and the high-temperature silica phase formed is preferentially tridymite (see Fig. 14.10) that on cooling undergoes a displacive phase transformation accompanied by a moderate volume effect (Heimann 2010b) that adds to the stresses imposed by the normal high-low transition of (unconverted) quartz. These volume effects put the ceramic under considerable tensile stresses that induce microcracks through and around the transforming silica grains rather than through the confining glassy matrix (Kingery & Vandiver 1986). The typical microstructure consists of approximately 15% quartz + tridymite, 30% wollastonite, and 55% glass. The subcritical microcracks tend to grow in size under applied stress thus rendering soft-paste porcelain more prone to mechanical destruction, showing lower hardness, bending strength, and, in particular lower thermal shock resistance compared to hardpaste porcelain with about 70% glassy matrix (see Chapter 2). Owing to the considerably lower firing temperature of soft-paste porcelain vitrification is impeded and an internal graininess results that can be spotted in fracture surfaces. This inherent graininess is a distinguishing criterion between soft- and hard-paste porcelain.

Chapter 6

Materials science of ceramics Synopsis Clay-based pottery is fired clay or loam. During firing, clay minerals release their chemically bound water at temperature beyond approximately 500 °C. The submicroscopic, finegrained decomposition products have a disordered crystalline structure. Hence they react readily with partial melts formed in the temperature range of 700–1000 °C. At this stage, the thermal decomposition of hydroxides (e.g. goethite), sulphates (e.g. gypsum), carbonates (e.g. calcite and dolomite), silicates (e.g. feldspar) etc. generates oxides that either dissolve in the melts or form new silicate phases. The final state of sintering depends on many factors including the maximum firing temperature applied, the duration of firing, the firing atmosphere, and the composition of clay. Experimental firings of kaolinitic and illitic non-calcareous and calcareous clays in oxidising or reducing atmospheres show which initial phases will be stable throughout firing, which will disappear, and which new phases will appear in response to the temperature and firing atmosphere applied. Such experiments were also carried out on ceramic colouring pigments present in the ceramic body and as surface decoration. In general, increasing firing temperature leads to increasing densification of the ceramic body. These thermally induced transformations can be used to estimate the firing temperature of ancient pottery.

6.1 Ceramics as man-made ‘rocks’ Making ceramics by firing of clay is essentially a reversal of the natural weathering process. Nature over the course of millions of years breaks down rocks mechanically and chemically, forming new minerals. Potters ingeniously invented methods to recombine the chemical compounds once separated by the weathering process. As a result the potter obtains a more or less rock-like material (Heimann & Franklin 1979, Maggetti 1986, Maggetti 2001) shaped and decorated according to man’s need and preference. Nature also consolidates clays during an extremely long-lasting process known as ‘cycle of rock formation’ involving diagenetic and metamorphic transformation of clays towards minerals more stable at the pressure and temperature conditions prevailing. Man invented a ‘short-cut’ of these processes which affects the state of equilibrium of the system to be thermally transformed. As a result of this short-cut, the solidified rock-like ceramic material inherits non-equilibrium and statistical states which are best described as ‘frozen-in’. This has been described in detail in the preceding chapter.

6 Materials science of ceramics

71

6.2 Firing temperature vs. state of sintering Essentially, the high temperature applied to clays during firing results in a distinct state of sintering that is dependent on the firing temperature, the duration of firing, the firing atmosphere, and the composition of the clay. Hence the properties of the ceramic have to be assessed in a temperature-time-composition space. Composition encompasses additional variables which complicate the temperature-property relationship. Therefore no 1:1 correspondence is to be expected between the firing temperature and a particular property like mechanical strength, thermal expansion, or thermal conductivity. This is caused by the fact that a similar state of sintering can be achieved either by firing at a higher temperature for a short time or at lower temperature held for a considerably longer period. Thus comparable states of sintering do not necessarily mean identical firing temperatures. Nevertheless, the term ‘state of sintering’ is a useful reference point to assess and describe ancient ceramics. It is also useful to further distinguish clearly between firing temperatures and state of sintering. The possibility of achieving a given state of sintering in a variety of ways through manipulation of composition and/or time allows assessing the skill and the technological control of those who made the pottery. It follows that ceramic firing of clay establishes, and fixes permanently, a set of properties that are usually associated with ceramics such as hard and dense microstructure, mechanical resilience, durable and aesthetically pleasing surface decoration, as well as chemical and thermal stability. The densification of the ceramic body is achieved through sintering, with or without the presence of a liquid phase (see Chapter 5). Owing to the variability of clay composition the mineralogical processes during thermal transformation of clay minerals can be very complex, not in the least caused by the fact that most reactions occur far removed from thermodynamic equilibrium and hence are kinetically constrained, that is, they are time- and temperature dependent. A much simplified model of these phase transformations is shown in Fig. 6.1 that traces the composition of a ceramic body composed of a mixture of kaolin (china clay), quartz, feldspar, and (open) pores as a function of temperature. The final composition after firing beyond 1200 °C is that of hard-paste porcelain, consisting of mullite, glass phase, residual quartz, and closed pores. As shown in Fig. 6.1 the decrease of the pore volume causes (quasi-linear) shrinkage that can reach in excess of 20%. The properties of the ceramic body are closely related to the firing temperature and the type of atmosphere prevalent in the kiln. Oxidising firing is achieved when the fugacity of oxygen (fo2)41 exceeds the thermodynamically defined decomposition pressure of oxides with different ion valencies such as magnetite (Fe2+Fe23+O4) or hausmannite (Mn2+Mn23+O4) (see 6.4.5). On the other hand, reducing firing requires a fugacity of oxygen below the decomposition pressure of these oxide spinels. In case of iron

41 The value of fo2 (oxygen fugacity, roughly: partial pressure) determines the relative amount of oxygen in the firing atmosphere. A high value of fo2 indicates a high chemical potential of oxygen whereby fo2 of air is at 0.21 bars. Experimentally fo2 can be established and controlled by gas mixtures such as CO2/CO/H2O or by solid-state redox buffers such as Ni/NiO or Fe2O3/Fe3O4 (strongly reducing with fo2 < 10–4 atm between 816 and 1,429 K), or Mn2O3/Mn3O4 (weakly reducing with fo2 > 10–4 atm between 867 and 1,225 K; see Fig. 6.16) (Heimann et al. 1980) (see also footnote 44).

72

Part I 100

shrinkage open pores closed pores

60

ȕTXDUW]

ĮTXDUW] glass

40

feldspar

20

NDROLQLWH

PHWDNDROLQLWH

$O6LVSLQHO

Volume percent

80

DPRUSKRXV silica PXOOLWH

0 0

200

400

600

800

1000

1200

Temperature (°) Figure 6.1. Thermal history of transformation of a ceramic paste consisting of kaolinite, quartz and feldspar, and development of the composition of porcelain and its porosity as a function of firing temperature (adapted from Norton 1952).

oxides the colour of the fired ceramics is dependent on the valence state of the iron ions. The mastery of the redox behaviour of iron oxides is at the heart of the impressive artistic achievements of ancient potters throughout the ages in different world regions. Examples are Attic black- and red-figure vessels the contrasting colours of which originated from the interplay of oxidising and reducing firing cycles. Since the chemical and mineralogical composition of the clay mixture play a decisive role the development of phase composition and microstructure of the fired ceramics is notoriously complex. In this chapter the thermal transformations of kaolinitic clays during ceramic firing will be discussed in some detail. A brief survey on the phase development during experimental oxidising and reducing firing of illitic clays with low and high calcite contents is also presented as these types of clay were almost exclusively utilised to produce ancient low-fired ceramics. In addition, some information will be given on reactions occurring during firing of phosphatic ceramics as well as on redox conditions prevailing in the iron and manganese oxide systems utilised to provide colour to the surface of pottery. As valuable as the information is that can be gained from experiments, it must be emphasised that firing of pottery in electrical furnaces, well controlled in terms of temperature, atmosphere and firing times, yields results that are comparable only within limits to those attained in antiquity. Neolithic pottery produced in open bonfires or primitive pit kilns (see Chapter 7) was exposed to heat for only a very short time (Maggetti et al. 2011a), and the temperature and the firing atmosphere varied widely. Even in more sophisticated ancient kilns in which fuel and work were separated strong spatial and temporal gradients in firing temperature and oxygen partial pressure existed. As worked out by Picon (2002), in antiquity there were three basic modes of ceramic firing: (i) firing under predominately reduc-

6 Materials science of ceramics

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ing and cooling under oxidising conditions, (ii) firing and cooling under reducing conditions, and (iii) firing and cooling under oxidising conditions. Accordingly, colour and surface conditions of the pottery varied in response to varying oxygen levels, and iron and lime contents of the clays.

6.3 Thermal transformations in kaolinitic clays Literature pertaining to the behaviour during firing of kaolinite and predominately kaolinitic clays includes studies by Maggetti & Rossmanith (1981) and Maggetti (1982) in the context of ancient ceramics as well as by Iqbal & Lee (1999, 2000) and Lee & Iqbal (2001) in the context of modern porcelain. Heating of pure kaolinite leads only to a slight densification to yield a microstructure with low strength. Sintering is generally absent below 1600 °C since, owing to the lack of mineralisers such as alkali and alkaline earth ions, only solid state reactions without the presence of a liquid phase occur that proceed sluggishly. However, the situation changes dramatically for natural kaolinitic clays as the content of mineralising alkali and alkaline earth ions in always present feldspars leads to fast sintering at moderate temperature. Pure Kfeldspar starts to melt incongruently at 1150 °C but the presence of Na-feldspar (albite) reduces this temperature to 1090 °C. A mixture of kaolinite, K-feldspar and quartz, present in porcelain paste (Chapter 5), starts to melt at the ternary eutectic at 985 °C (see Fig. 5.5). In addition, even small amounts of albite lower this melting temperature by another 60 °C to 925 °C. However, at these low temperatures, the amount of liquid phase formed is still small and its viscosity is very high, thus impeding diffusion and hence the progress of the ceramic reactions. To achieve reasonable reaction rates the firing temperature must be raised to a value that allows the feldspar to enter the glass phase formed. As shown in Fig. 6.1, large scale dissolution of feldspar only happens beyond a temperature of 1200 °C. During firing a mixture of kaolinite and feldspar the first traces of mullite are obtained around 1000 °C. The exact mechanism of transformation from kaolinite to mullite has been subject of a longstanding controversy. It was known for a long time that the first step in the reaction sequence is the formation of a more or less amorphous, compositionally ill-defined dehydration product, called ‘metakaolinite’. Its presence can be inferred from an endothermic reaction peak near 550 °C in differential thermal analysis (DTA)42 plots. These studies also generally show a second, strong exothermic peak around 980 °C. Its origin has been the key issue in a flurry of research papers between 1930 and 1990. Figure 6.2 shows

42 In DTA, the material under study and an inert reference substance are made to undergo identical thermal heating and cooling cycles, while recording any reaction imposed temperature difference between sample and reference. This differential temperature is then plotted against temperature (DTA curve or thermogram). Phase changes in the sample, either exothermic (positive temperature deviation) or endothermic (negative temperature deviation), can be detected relative to the inert reference, generally alumina. Thus, a DTA curve provides data on the transformations that have occurred, such as dehydration, glass transitions, decomposition, crystallisation, melting, and sublimation (Smykatz-Kloss 1974). For application of DTA in the study of archaeological ceramics see Kingery (1974) and, more recently Odlyha (2003).

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Figure 6.2. Differential thermal analysis (DTA) traces of kaolinite-illite mixtures. 1: 90% kaolinite/10% illite, 2: 75% kaolinite/25% illite, 3: 50% kaolinite/50% illite, 4: 25% kaolinite/75% illite, 5: 10% kaolinite/90% illite, 6: 5% kaolinite/95% illite (modified after Grim & Rowland 1944).

typical DTA traces of kaolinite/illite mixtures. Illite shows endothermic reactions between 100 and 250 °C, 550 °C, and around 900 °C. Kaolinite is characterised by a very strong endothermic peak between 500 and 600 °C, related to dehydroxylation and formation of metakaolinite, and a sharp exothermic peak close to 1000 °C, representing the formation of mullite. The substitution of illite for kaolinite is indicated quantitatively by peaks representing these reactions as shown in Fig. 6.2. The metakaolinite story has centred around two main explanations. The first explanation assumes that the exothermic reaction has its origin in the formation of a ˠ-Al2O3 type defect spinel phase (Insley & Ewell 1935, Brindley & Nakahira 1959, Percival et al 1974, Leonard 1977, Brown et al. 1985, Sonuparlak et al. 1987). While agreeing on the central issue the individual approaches differ in their interpretation of the composition of the spinel phase and the question whether also mullite, formed in a parallel reaction, will contribute to the exothermic event. The second explanation concerns the assumption that mullite is formed without involvement of a spinel-type phase (Comefore et al. 1948, Bradley & Grim 1951, Roy et al. 1955). However, this hypothesis appears to have been disproved in the light of more recent experimentation (see Percival et al. 1974, Leonard 1977, Chakraborty & Ghosh 1978, Sonuparlak et al. 1987, Lecomte-Nana et al. 2013). Investigation of the transformation mechanism of kaolinite showed that in the metakaolinite phase, thought to be stable between about 480 and 950 °C, the tetrahedral Si-O layer of kaolinite expands but maintains its structural integrity whereas in the octahedral Al-O layer a loss of OH ions occurs (Fig. 6.3). The dehydroxylated kaolinite (= metakaolinite) loses its c periodicity, but retains a and b periodicities to a large extent over a wider temperature range. The distorted and thus reactive Al-O layers transform at about 950 °C (Fig. 6.1) to a

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Figure 6.3. Change of the electron density distribution of kaolinite (left) by dehydroxylation to form the highly defective metakaolinite structure (right). Note that the octahedral coordination of Al in kaolinite transforms to a (truncated) tetrahedral coordination in metakaolinite. (Iwai et al. 1971).

spinel-type phase (Brindley & Nakahira 1959). The formation of the spinel phase occurs topotaxially, that is, lattice-oriented with respect to the lattice metric of the parent kaolinite whereby [001]Kao ၌ ၌[111]Sp and [010]Kao ၌ [110]Sp. In metakaolinite the Si-O distance in the tetrahedral layer increases to 165.0 pm whereas the Al-O bond length remains essentially constant. At first glance this is rather surprising since removal of the OH groups should be accompanied by a reorganisation of the octahedral gibbsite-type layer into a tetrahedral configuration. However, even though the octahedral layer attains an extremely high degree of lattice defects it does not collapse immediately but retains a truncated octahedral structure with Al ions in a four-fold coordination (Fig. 6.3, right; Iwai et al. 1971). Hence the Al configuration is loosely tetrahedral but the perturbation of the electronic shells of the Al atom freed from the two hydroxyl ions is reduced in such a way that unusually large Al-O distances are maintained. The metakaolinite phase is thought to collapse beyond 900 °C into a spinel-type lattice. The nature of this phase has been vigorously disputed for 125 years. Already Le Chatelier (1887) noted that metakaolinite appears to behave like a mechanical mixture of silica and alumina (‘mixed oxide theory’), and that from metakaolinite alumina can be dissolved completely with dilute hydrochloric acid. Subsequently the alumina phase was identified as having an (unstable) ˠ-alumina spinel structure that collapses at higher temperature to 3:2-mullite (Brindley & Nakahira 1959) according to 3(3SiO2 · 2 Al2O3) → 2(3Al2O3 · 2SiO2) + 5SiO2 Si-Al spinel 3:2-mullite cristobalite

(6.1)

In more detail, the formation of mullite at temperatures beyond 1100 °C has also been described as the result of reaction of ˠ-Al2O3 with amorphous silica that is present in pores of the spinel structure. The presence of iron ions shifts the onset of primary mullite formation to lower temperature around 900 °C owing to the promotion of the conversion of Si-Al spinel into mullite (Lecomte-Nana et al. 2013). Also, iron ions will act as mineralisers causing a dramatic increase of grain growth of secondary mullite at 1400 °C under slightly reducing conditions, related

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to the formation of a ternary eutectic melt commencing at 1380 °C as experimentally determined by Muan (1957). It is evident that such iron-bearing kaolinitic clays require firing temperatures much below this range.

6.4 Thermal transformations in illitic clays Most structural clay products such as bricks, tile, pipe, earthenware pottery, and architectural terracotta are produced from red-firing clays and shales the main clay mineral content of which is illite (see Chapter 3). Minor amounts of kaolinite, montmorillonite and chlorite could be present but are considered non-essential for the intended properties of the fired product. The presence of feldspars and, in particular calcite provides fluxing agents that considerably lower the sintering temperature. It is thus not surprising that ancient pottery was predominately produced from calcareous illitic clays that on firing to temperatures well below 1100 °C yielded a reasonably dense and hence rather impervious body. Corinthian and Attic ware, most Roman Terra Sigillata, in particular those produced in southern Gaul (La Graufesenque, Banassac), and most Medieval earthenware products were produced from calcareous illitic clays. However, such clays possess a rather narrow softening interval so that great care had to be taken not to ‘overfire’ the ware and thus induce undesirable large-scale melting of the clay. Evidence of such failure exists since at many ancient pottery production sites copious amounts of bloated, deformed and otherwise misfired wasters have been excavated. With increasing control over the physico-chemical parameters existing in a pottery kiln and a judicious choice of appropriate clays, a shift to less calcareous clay, and hence a gain in quality control occurred. This is very evident, for example in the change from highly calcareous Neolithic pottery of Crete to much less calcareous clays in Middle and Late Minoan Kamares wares, achieved presumably by blending clays from different sources with high (northern coast of Crete, Knossos) and low (southern coast, Phaistos) lime contents to yield a consistent product (Noll 1982, Heimann 1989; see Chapter 9). A similar blending of calcareous and non-calcareous clays has been deduced from compositions of Roman Terra Sigillata (see Chapter 10) as well as Italian maiolica and French faience (see Chapter 13). Figure 6.4 shows X-ray charts of several size fractions of typical calcareous illitic clay from Otterbach, near Jockgrim, Palatinate, Germany. This clay had been used to manufacture provincial Roman Terra Sigillata ware between the 2nd and 3rd centuries CE in the eastern Gaulish settlement of Tabernae rhenanae (today’s Rheinzabern) (see, for example Schneider & Hoffmann 1976, Schneider 1978, Hoffmann et al. 1989, Schneider 1990, Schneider 1993, Daszkiewicz et al. 2001). This is indeed one of few fortuitous cases in which the original clay sources still exist so that through experimental firings the ancient technology can be ascertained with confidence (see below; Fig. 6.24, right). The coarse granular nonclay constituents quartz, K-feldspar, plagioclase, and calcite appear in the as-mined clay (Fig. 6.4a) whereas in the coarse silt (63 µm to 2 µm) (Fig. 6.4b) and clay fractions (< 2 µm) (Fig. 6.4c) the clay minerals illite, one or more members of mixed layer (ML) minerals, and iron-rich chlorite dominate. The finest fraction also contains lepidocrocite (ˠˠ-FeOOH) as the main carrier of iron. This iron-rich fraction was used as a slip, applied to the leather-hard green body prior to firing. During oxidising firing it produced, together with the iron contents of illite and chlorite, the telltale red colour of the Terra Sigillata pottery.

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I ML

I Ch

Ch

I

c

I

Ch Ch

γ-Fe

ChML Ch

I

Ch

P Qz + I

Ch Qz

b

Qz Qz Ch

I

I

I

Ch

Ch

Kf P Cc

a

5

10

20

30

o2Θ

Figure 6.4. X-ray pattern of archaeological calcareous illitic clay (O61) from Otterbach, Jockrim, Rheinland-Pfalz, Germany. (a): as-mined raw clay, (b): coarse silt fraction (63–2 µm), (c): clay fraction (< 2 µm). Cc: calcite, Ch: chlorite, I: illite, Kf: K-feldspar, ML: mixed layer mineral, P: plagioclase, Qz: quartz, ˠ-Fe: lepidocrocite (Heimann et al. 1980).

In the following paragraphs some pertinent results are described of firing of typical archaeological calcareous and non-calcareous illitic clays from Otterbach, Jockgrim, Palatinate, Germany to model the mineralogical phase development of Roman Terra Sigillata under oxidising (air, fO2 = 0.21 atm) and reducing conditions41. A second example concerns firing under reducing conditions of ± calcareous illitic/chloritic clay sampled in the vicinity of Troy, Turkey and thought to represent the raw material of Grey Mynian ware of Troy VI, likely the ‘Homeric’ level of Troy (Görres 1995).

6.4.1 Experimental oxidising firing The chemical composition of the clay used for experimental firing is shown in Table 6.1. Its phase composition has been determined by XRD as shown in Fig. 6.4. This composition, normalised for silica, alumina and the sum of calcia and magnesia is depicted in Fig. 6.5 as a circle, located approximately at the triple point anorthite-tridymitemullite with a melting point of 1345 °C (see Fig. 5.4). However, the considerable iron content (5.5 mass% Fe2O3) shifts the phase composition of the clay away from the triple point

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Table 6.1. Chemical composition of calcareous iron-rich clay (O61) from Otterbach near Jockrim, Rheinland-Pfalz, Germany. Oxides are given in mass%, elements in ppm. Heimann et al. (1980). Oxide

Mass%

Oxide

Mass%

Oxide

Mass%

Element

SiO2

61.7

Fe2O3

5.5

Na2O

0.8

Ba

ppm 514

Al2O3

19.3

CaO

7.0

K 2O

3.5

Cr

134

TiO2

0.8

MgO

2.7

P2O5

0.2

Rb

197

towards the phase field of cordierite and iron cordierite (sekaninaite), respectively. Consequently, the crystalline compounds of the ceramic formed by oxidising firing to 1010 °C shows diopside, calcium-rich plagioclase, traces of sanidine, quartz and haematite (see Fig. 6.11e) but no mullite. As shown by the DTA results (Fig. 6.2) illite-rich clays display three endothermic peaks at 100–250 °C assigned to loss of absorbed water, around 550 °C assigned to loss of structural water, that is, dehydroxylation of the octahedral sheets, and between 850 and 950 °C due to formation of an Al-Si spinel (de Araújo et al. 2004). The spinel transforms at still higher temperatures to form mullite and cristobalite (eq. 6.1). Consecutively, the iron present as primary iron oxyhydroxide (goethite, lepidocrocite) or released from the illite or chlorite lattices crystallises under oxidising conditions as haematite (˞ ˞-Fe2O3), the structural order of which increases with increasing firing temperature. This transition can be investigated and recorded by Mössbauer spectroscopy43. Fig. 6.6 shows Mössbauer spectra of an Attic clay fired between 400 and 900 °C as reported by Kostikas et al. (1976).The spectrum at 400 °C indicates features of the initially present lepidocrocite (ˠ ˠ-FeOOH) that at 600 °C shows decrease in intensity and broadening of the central doublet due to loss of hydroxyl ions bound to the iron at octahedral lattice sites. Finally, at 900 °C the central doublet has completely disappeared, leaving only the characteristic highly symmetric sextet of pure haematite. In a recent experimental study the evolution of haematite during oxidising firing of carbonate-rich illitic and chloritic clays was evaluated in much detail (Nodari et al. 2007). The thermal breakdown of chlorite initially results in an amorphous phase that retains iron at a distorted paramagnetic octahedral site. Beyond 750 °C carbonates start to decompose and react to form calcium silicates. Only at this temperature can haematite nucleate and grow in micro-domains located within former chlorite flakes. At 950 °C, decomposition of illite in the presence of an amorphous phase and CaO produces a melt in which nano-sized haematite crystallites may nucleate, although their further growth is inhibited by low diffusion rates. If higher amounts of CaO are present in the clay the composition of the resulting ceramic ware shifts into the compatibility triangle wollastonite (diopside)-anorthite-gehlenite. Indeed, such clays, when fired to 850–1050 °C contain considerable amounts of gehlenite, 43 In Mössbauer absorption spectroscopy, a solid sample is exposed to a beam of ˠ-radiation from a 57 Co source, and its intensity transmitted through the sample measured. The technique of Mössbauer spectroscopy is widely used in mineralogy to examine the valence state of iron or manganese as well as the type of coordination polyhedra occupied by the atoms. It is sometimes used to determine redox ratios in ceramics and glasses and (less successfully) in rocks (Dyar et al. 2006).

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Figure 6.5. Simplified phase diagram CaO+MgO-Al2O3-SiO2 with the composition of the Otterbach clay (O61) indicated (circle). Solid lines connecting primary phases are conodes delineating compatibility triangles, dashed lines delineate phase stability regions of an anorthite, co corundum, cr cristobalite, di diopside, ge gehlenite, mu mullite, tr tridymite, and wo pseudowollastonite. Primary phases shown in bold letter are given in cement chemical notation: CS wollastonite, CMS diopside, C2AS gehlenite, CAS2 anorthite, A3S2 3:2-mullite. Strictly speaking the quaternary system has been reduced to a ternary one by combining CaO and MgO. Addition of MgO would shift the phase boundaries although the rather small concentration of 2.7 mass% MgO (Table 6.1) may not alter them too drastically. In particular, plotting wollastonite and diopside is not permissible from a thermodynamic point of view but serves here to convey the overall picture.

Relative transmission

A

B

C

-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12

Velocity (mm/s) Figure 6.6. Mössbauer spectra of Attic clay fired at (a) 400 °C, (b) 600 °C and (c) 900 °C (Kostikas et al. 1976).

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Ca2AlIV[AlSiO7] (Peters & Jenni 1973, Maggetti & Küpfer 1978, Jordán et al. 1999). Gehlenite is metastable and reacts at higher temperature (> 1100 °C) with silica released during decomposition of metakaolinite or illite to anorthite + wollastonite (or diopside, in the presence of MgO) according to Ca2Al[AlSi]O7 + 2SiO2 → Ca2Al2Si2O8 + CaSiO3

(6.2)

The mechanism of formation of gehlenite outside its field of thermodynamic stability, and its occurrence in and disappearance from calcareous illitic clays, have been considered an analytical challenge as a thermodynamic problem. However, the solution to this problem may lie in kinetics since well-processed clays with a narrow grain size distribution and, in particular, absence of larger calcite grains such as those utilised in fine Roman Terra Sigillata do not show gehlenite whereas coarse utilitarian ware made from identical clays contain sizeable amounts of it (Maggetti & Küpfer 1978). Evidently, the reaction rate is a function of grain size which, in turn, is being influenced by the degree of processing of the clay, a clear measure of the technological skill of the potter. Since ancient earthenware ceramics were generally fired much below 1100 °C gehlenite should indeed have been formed during oxidising firing. However, this phase is rarely detected in such ceramics with some noticeable exceptions, for example when buried under strongly arid conditions (Fig. 6.7, left). Apparently, gehlenite will be dissolved under humid conditions in contact with soil solutions and/or transformed to zeolites such as wairakite

Figure 6.7. Left: XRD charts of ancient pottery buried under arid conditions, showing high concentrations of gehlenite (G, arrows). (a) Glazed pottery, Balkh, Afghanistan, 14th century CE. (b) Nabataean sigillata, Avdat, Negev, Israel. (c) Nabatean sigillata, Petra, Jordan. (d) Tomb of Intef VII, Thebes, Egypt, 17th Dynasty. Right: XRD charts of 1st century CE Terra Sigillata from La Péniche (Vidy, Lausanne), Switzerland (Küpfer & Maggetti 1978), exhibiting decomposition products of gehlenite due to burial under humid conditions. (a) Garronite/Ca-harmotome, (b) Wairakite/Ca-analcime. The labelling of the peaks are as follows: C calcite, D diopside, G gehlenite, Ga garronite, He haematite, P plagioclase, q quartz, s sanidine, sp spinel, W wairakite.

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Al2O3 + SiO2 + MgO

CO2

Gehlen ite

CaO CaO

Anorthite > 1050oC

HydroGrossularite

Gehlenite

Calcite + Smectite

Path E

Analcite, Vaterite, Aragonite

Zeolites Wairakite Garronite

Calcite + Smectite

Humid burial

Arid burial

SiO2

> 900oC

Path B

o Calcite > 700 C

81

Figure 6.8. Reactions involved in gehlenite formation during firing of calcareous ceramics (top) and cycle of gehlenite decomposition (bottom) under arid (left) and humid (right) environmental conditions prevailing during prolonged burial. Path A: Preservation of gehlenite under very dry conditions. Path B: Very humid, presence of humic acids. Path C: Moderately humid, presence of CO2. Path D: Humid conditions without CO2. Path E: Decomposition of anorthite under humid conditions (after Heimann & Maggetti 1981).

(Ca-analcime, Ca[AlSi2O6]2·2H2O), garronite/Ca-harmotome (NaCa2.5[Al3Si5O16]2·14H2O) or, in the presence of, respectively phosphate and carbonate ions, to brushite (CaHPO4·2H2O) and possibly scawtite (Ca7[CO3/Si6O18]·2H2O (Fig. 6.7, right; Maggetti & Heimann 1979, Heimann & Maggetti 1981). Depending on the composition of soil solutions interacting with gehlenite several reaction paths can be envisaged as shown schematically in Fig. 6.8. As confirmed experimentally by Heimann & Maggetti (1981) gehlenite reacts with diluted inorganic (HCl) and organic (acetic, oxalic, aspartic, tartaric) acids to hydrogrossularite (hibschite, Ca3Al2[(Si,H)O4]3), under moderate humid conditions in the presence of CO2 to zeolites such as wairakite, garronite and/or scawtite, and under very humid conditions in the presence of humic acids and CO2 to a series of calcium carbonate modifications (aragonite, vaterite) with different stabilities with respect to calcite that finally remains as the thermodynamically stable phase, together with smectites. As also shown in Fig. 6.8, in high-fired (> 1050 °C) calcareous ceramics gehlenite reacts with silica to anorthite that during burial under humid conditions and pHvalues > 8 very slowly decomposes to calcite and a smectitic phase. Chemical and mineralogical transformations during burial of ceramics were studied by many authors (e.g. Maggetti 1982, Freestone et al. 1985, Rottländer 1989, Picon, 1991, Shoval et al. 1991, Béarat & Dufournier 1994, Fabbri et al. 1994, Nuñez et al. 1994, Collomb & Maggetti 1996, Pradell et al. 1996, Buxeda i Garrigós 1999, Buxeda i Garrigós et

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al. 2001, Freestone 2001, Maritan & Mazzoli 2004, Schwedt et al. 2006, Tschegg 2009, Secco et al. 2011, and references therein). High CaO contents in clay lead during oxidising firing to a pronounced colour change towards yellow and buff, or even white hues (e.g. Peters & Jenni 1973, Peters & Iberg 1978, Meyer at al. 1984, Kreimeyer 1987, Jacobs 1992, Molera et al. 1998 and references therein). For example, the highly calcareous (22 mass% CaO) and iron-rich (8 mass% Fe2O3) brown clay of the classic Corinthian pottery changes its colour from pink (fired at 700 °C) to red (at 900 °C) to pale yellow (at 1080 °C). An originally gray clay (15 mass% CaO, 7 mass% Fe2O3) from Corfu changes from buff-cream (700 °C) to buff (900 °C) to white (1080 °C) (Maniatis et al. 1982). The reason(s) for these colour changes are still under discussion. As in the gehlenite problem they may be related to the grain size of calcite, the mineral that controls the ratio of diopside to anorthite in the fired product (Nöller & Knoll 1985). Diopside, wollastonite and mullite are all able to incorporate Fe3+ ions into their crystal lattices. Hence no free iron oxide phase such as haematite can be formed, and the yellow colouration results from a solid state charge transfer reaction caused by Fe3+ substituting Ca2+ or Mg2+ (Marfunin 1979). While, on the other hand, anorthite is unable to incorporate sizeable amounts of Fe3+ ions ceramic bodies with a low diopside/anorthite ratio will remain red. At still higher firing temperatures beyond 1200 °C calcareous and iron-bearing clays change to brown colours, thought to be generated by a superposition of the greenishblack hue of fayalite (Fe2SiO4) formed from Fe2+ exsolved during decomposition of trioctahedral illite, and the red colour of haematite (Maggetti 1975). Another example of a highly calcareous ceramic was recently investigated by Íssi (2012) who studied the ceramic remains of a 2nd century BCE Hellenistic workshop at Harabebezikan, Turkey, at the Euphrates River near the border to Syria. This site was the target of a salvage excavation in 1999, and was subsequently submerged by the reservoir lake of Karkamış. The small ceramic objects analysed are thought to be remnants of the main products of the workshop, narrow-necked amphorae. Its chemical composition ranges between 44 and 48 mass% SiO2, 10 and 12 mass% Al2O3, 20 and 25 mass% CaO, 6 and 7 mass% MgO, 6 and 8 mass% Fe2O3, as well as 2 and 5 mass% K2O and < 2 mass% Na2O. The pottery was fired under oxidising conditions in a kiln. Some pieces show evidence of overfiring since substantial amounts of a highly siliceous glassy phase were detected into which quartz grains were found to dissolve (Fig. 6.9, arrow). From this and the occurrence of very unusual phases such as leucite (Iqbal & Lee 2000) the author suggested a firing temperature in excess of 1200 °C. This would point to the ceramic samples being the result of either an accidental temperature excursion of a run-away kiln or a full scale conflagration. Apart from this, the global composition of the ceramics plots in the phase diagram CaO-Al2O3-SiO2 almost exactly on the cotectic line diopside (CMS)-anorthite (CAS2). The phases detected by XRD indicate the presence of diopside, plagioclase (anorthite), melilite (åkermanite), quartz and haematite. Traces of leucite may be also present as well as calcite, the latter being of secondary nature, likely precipitated during burial in soil. The thermodynamically incompatible phases quartz and melilite should have reacted beyond 950 °C according to mel + 2qz ൺ an + di, or mel + 2qz + co ൺ 2 an (Letsch 1982). However, different local microequilibria were established due to compositional inhomogeneity that positioned the composition either above or below the cotectic line di/an. Also, kinetic constraints tend to blur the picture. Figure 6.9 show an SEM image of sample HCR2 with quartz grains dissolving in a

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Figure 6.9. Calcareous Hellenistic ceramics HCR2 from Harabebezikan, Turkey, 2nd century BCE, showing residual quartz grains dissolving in a pool of glass (arrow), as well as anorthite and diopside neoformations (Courtesy: Dr. Ali Ìssi, Dumlupınar University, Kütahya, Turkey).

pool of glass, and anorthite lath as well as diopside crystallites with (almost) square crosssections. Both the high porosity of this ceramic as well as its phase composition suggests a firing temperature below 950 °C.

6.4.2 Experimental reducing firing Such experiments are scarce (Heimann et al. 1980, Letsch & Noll 1983, Maniatis et al. 1983, Pradell et al. 1995, Fabbri et al. 2002, Maritan & Mazzoli 2004, Maritan et al. 2006). The archaeological clay from Otterbach, Jockgrim, Palatinate, Germany fired under strongly reducing conditions (fO2 < 10–4 atm; Ni/NiO or Fe2O3/Fe3O4 solid redox buffer44) at 1035 °C exhibits a phase assembly consisting of anorthite (Fig. 6.10, left), tridymite, iron cordierite (sekaninaite) (Fig. 6.10, right), magnetite and glass (Heimann et al. 1980). Figure 6.11 shows the X-ray diffractograms of this clay fired under oxidising conditions (air, fO2 = 0.21 atm) at 600 °C (a), under weakly reducing conditions (fO2 > 10–4 atm) at 820 °C (b), under strongly reducing conditions (fO2 < 10–4 atm) at 1035 °C (c), weakly reducing conditions (fO2 > 10–4 atm) at 1010 °C (d), and under oxidising conditions (air, fO2 = 0.21 atm) at 1010 °C (e). 44 Solid redox buffers are mixtures of oxides or minerals that constrain the oxygen fugacity as a function of temperature. Solid state buffers are, for example mixtures of quartz, metallic iron and fayalite (QFFe; 2Fe + SiO2 + O2 ർ Fe2SiO4) or quartz, fayalite and magnetite (QFM; 3Fe2SiO4 + O2 ↔ 2Fe3O4 + 3SiO2) the univariant equilibrium curves of which are shown in Fig. 6.16. For ceramic applications the buffer systems Mn3O4-Mn2O3 with log fO2 = 7.34 – (9,265/K) between 867 and 1,225 K (Huebner 1971) and Fe2O3-Fe3O4 with log fO2 = 13.97 – (24,634/K) between 816 and 1,429 K are important (see also Schwab & Küstner 1981, Heimann et al. 1980).

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250 μm

250 μm

Figure 6.10. Calcareous illitic clay fired under strongly reducing conditions (fO2 < 10–4 atm; Ni/ NiO buffer) at 1035 °C. Left: anorthite crystallised from glassy phase. Right: orthorhombic pseudohexagonal crystals of iron cordierite (sekaninaite) (Heimann et al. 1980).

Although these very low oxygen fugacities were never attained in ancient kilns45 they were applied to speed up the driving force of the mineral reactions encountered. Instead, the reducing atmosphere in classic pottery kilns was governed by the so-called Boudouard equilibrium (see 6.4.4) that characterises the redox reaction of a mixture of carbon monoxide and carbon dioxide at a given temperature, that is, the disproportionation of carbon monoxide into carbon dioxide and graphite or its reverse according to 2CO ප CO2 + C. With appropriate temperature control carbon (graphite) could be precipitated at the hot surface a ceramic vessel, a technique much used to decorate reducing fired ceramic object. Deposition of graphite at a rough surface produced a dull grey sheen whereas a burnished, that is, highly polished surface would result in a shiny black coating owing to preferential orientation of the graphite platelets parallel to the vessel surface. In a second example, clays were experimentally fired under reducing conditions, established through the Boudouard equilibrium, between 745 and 940 °C, using a calcareous illitic/chloritic clay (c. 12 mass% CaO, c. 5 mass% Fe2O3; see Tables 6.2 and 9.5) obtained from the sedimentation basin of the Karamenderes and Dümrek rivers located in close vicinity of the ancient Troy, Turkey (Fig. 6.12; Görres 1995). This study is remarkable in that it is among the few archaeometric studies that employed transmission electron microscopy to identify extremely small crystals of minerals formed during firing as detailed below. Moreover, the larger crystalline neoformations were identified by X-ray diffraction and quantified by Rietveld refinement (Kroll et al. 1994). The clay, comparable to that employed in antiquity to produce so-called Grey Minyan ware, was sampled at the Troy VIa-h (1700–1250 BCE) horizon during the 1988 campaign led by the late Manfred Korfmann (University Tübingen, Germany) (see Chapter 9.3.2). At

45 However, Maggetti et al. (1981) inferred from the coexistence of two iron spinels, magnetite and hercynite, in black coatings of Campanian ware that the oxygen fugacity must have been between 10–12 and 10–21 atm.

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e

d

c b a 30

20

10

o2Θ(CuKα)

Figure 6.11. X-ray diffractograms of a calcareous illitic clay fired under reducing (b-d) and oxidising (a,e) conditions. For explanation see text. I illite, P plagioclase, Q quartz, Cc calcite, F iron cordierite (sekaninaite), S sanidine, T tridymite, Sp spinel, M magnetite, He haematite, D diopside, HA hexagonal anorthite, Kf potassium feldspar (Heimann et al. 1980). Table 6.2. Chemical composition in mass% of reference clay D of Troy obtained by X-ray fluorescence analysis (Görres 1995). LOI = loss on ignition. SiO2

59.27

Na2O

0.89

Al2O3

13.66

TiO2

0.47

CaO

12.11

MnO

0.11

Fe2O3

4.95

P2O5

0.13

MgO

4.22

LOI

0.74

K2O

2.71

745 °C thermal decomposition of calcite started, and larger grains reacted with silica and alumina released by prior decomposition of chlorite, illite, kaolinite and smectite to form thin rims of anorthite and, in particular melilite (åkermanite)46. Åkermanite apparently formed instead of gehlenite owing to the presence of Mg in the clay (Table 6.2). The K2O

46 Melilites are solid solutions of calcium aluminium (gehlenite, Ca2Al[4][AlSiO7]) and calcium magnesium (åkermanite, Ca2Mg[4][Si2O7] sorosilicates. They may serve as so-called index minerals as

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Figure 6.12. Reducing fired calcareous illitic/chloritic reference clay D from the vicinity of Troy (Turkey). (a) 745 °C, (b) 840 °C, (c) 940 °C (Courtesy: Drs. M. Görres and Ch. Evangelakakis, Münster, Germany), (d) Quantification of mineral phases as obtained by X-ray powder diffraction with Rietveld refinement. An anorthite, Di diopside, En enstatite, Ol olivine, Sa sanidine, Åk åkermanite (modified after Kroll et al. 1994, Görres et al. 2000)

liberated from decomposing illite formed sanidine throughout the entire temperature range (see Fig. 6.12d; Table 9.5). At 840 °C formation of pyroxene (enstatite, diopside) and olivine from decomposing chlorite commenced (Brindley & Brown 1984). At still higher firing temperature, starting at 940 °C, melilite reacted with silica to anorthite and more pyroxene, and thus disappeared from the ceramic phase assembly. In this study the onset of melilite formation was found to occur at a temperature approximately 100 °C lower than that shown in Fig. 6.13, right. This discrepancy can readily be explained by the fact that the stability data of Fig. 6.13 were obtained by conventional X-ray diffraction whereas those of the former were inferred from high-resolution Rietveld refinement corroborated by TEM observations, techniques able to detect those minute mineral neoformations that eludes detection by XRD. It should be emphasised that adoption of these advanced analytical techniques is essential in the quest to clarify the firing history of ancient pottery, in particular pottery fired at rather low temperature, and in turn tends to yield conclusive results beyond those obtained by the ‘classical’ X-ray powder diffraction and optical their temperature-dependent appearance and disappearance in fired ceramic shards define minimum or maximum firing temperatures.

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microscopy methods hitherto applied. Unfortunately, studies employing such advanced analytical techniques to solve technological riddles of the ceramic past are few and far between. Figures 6.12a to c show the development of the microstructure of the fired clay D. At 740 °C clays minerals begin to develop dissolution features accompanied by formation of isolated pockets of melt characterised by rounded bubbles (Fig. 6.12a). However, the morphology of the thermally dehydrated clays minerals smectite, kaolinite, and illite is still largely pseudomorphically preserved. In the vicinity of 840 °C (Fig. 6.12b) the amount of molten phase increases with pronounced formation of closed pores within melt patches, and commences at 940 °C (Fig. 6.12c) to form a frothy texture with coalescing melt regions. Within these amorphous regions aggregates of very small crystallites (100–150 nm) of so-called index minerals (anorthite, diopside, gehlenite/åkermanite) form that cannot be resolved by SEM but need to be imaged and analysed by TEM.

6.4.3 Phase formation under oxidising and reducing conditions Figure 6.13 summarizes the phase changes of several typical minerals comprising calcareous illitic clay that occur under oxidising (left) and reducing (right) conditions (see Fig. 6.5). The onset of formation of diopside and/or gehlenite in oxidising fired calcareous clays at 800 °C (Fig. 6.13) leads to a profound change of the fabric of the ceramic body as partial melts are formed the amount of which increases with increasing firing temperature. In contrast to this, calcareous ceramics that do not contain diopside were fired much below 800 °C, typical for the overwhelming majority of Neolithic pottery. However, frequently these wares were fired for only very short times, insufficient to decompose larger calcite grains into reactive CaO that would give rise to diopside formation (Maggetti et al. 2011a). Nevertheless the occurrence of diopside can, with caution, be considered an important indicator of the (maximum) firing temperature attained and thus at least a rudimentary

Figure 6.13. Experimentally determined phase stability in calcareous illitic clay fired in oxidising (left) (Maggetti 1982) and reducing (right) (Letsch & Noll 1983) atmospheres. Under oxidising conditions illite is found to be stable to higher temperatures while decarbonisation of calcite is impeded under reducing conditions. Onset of formation of diopside occurs at lower temperature in the presence of high oxygen partial pressure.

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Figure 6.14. Influence of firing temperature on the fabric of calcareous clays. Low fired Middle Neolithic ceramic from Sesklo III, Thessaly (a) and higher fired Late Minoan ceramic (LM I) from Haghia Triada, Crete (b) (Noll 1984).

marker of the technological skill of the potter. Figure 6.14 shows the microstructures of calcareous ceramics fired at an estimated temperature of 700 °C (a) and at a temperature of at least 850 °C (b). They match the microstructures of experimentally fired calcareous bodies (Peters & Jenni 1973, Maniatis & Tite 1975, Tite & Maniatis 1975, Meyer et al. 1984, Wolf 2002). The non-sintered body of a Middle Neolithic ceramic object from Sesklo III, Thessaly (see Chapter 9.1) points to a low firing temperature. It owes its strength only to a mechanical densification attained during the production process that acted to orient the more or less pristine clay mineral platelets in a parallel fashion (Fig. 6.14a). In contrast to this, the considerably sintered body of a Late Minoan I (see Chapter 9.2) ceramic from Haghia Triada, Crete (Fig. 6.14b) suggests densification by a liquid (eutectic) phase. This liquid phase solidified after cooling to a vitreous bonding agent that locked air-filled pores in their places. This liquid phase also cemented together the coarser particles such as quartz that at this rather low firing temperature likely did not participate in the reaction. As evident from Fig. 6.14b the sintered parts of the ceramic fabric mirror the orientation of the clay platelets imposed by the mechanical process of forming, for example during throwing on a potter’s wheel. Another example of such clay platelet orientation in Roman Terra Sigillata is shown in Fig. 10.9. In contrast to this the mineralogical changes of a non-calcareous iron-rich clay are shown in Fig. 6.15 under oxidising (top, dark bars) and reducing (bottom, light bars) conditions. The main differences lie in the extended stability range of illite to higher temperatures (see also Fig. 6.13, left) and the earlier onset of mullite formation under oxidising conditions. Since in contrast to calcareous clays reaction products, that is, mullite will be formed only at much higher temperatures beyond 1000 °C, the fabric of non-calcareous clays fired below that temperature threshold corresponds in principle to that of low fired calcareous clays. Hence for firing temperatures below 1000 °C attempts to relate the fabric of a noncalcareous ceramic body to its firing temperature are practically meaningless (Heimann & Franklin 1979). In summary, the onset of the formation of a partial melt phase at much lower temperature in calcareous clays compared to non-calcareous clays was the technological reason why prehistoric potters searched for clay deposits rich in calcium (and alkali ions). Such clays

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Figure 6.15. Experimentally determined phase stability in a non-calcareous iron-rich illitic clay under oxidising (top) and reducing (bottom) firing atmospheres. Under oxidising conditions illite is found to be stable to somewhat higher temperatures. Onset of formation of mullite occurs at lower temperature in the presence of high oxygen partial pressure (after Heimann et al. 1980, Letsch & Noll 1983).

could be easily processed in wood-fired kilns with a bare minimum of sophistication to yield dense, mechanically durable cooking pots, bowls, strainers and cups of all sorts that could well fulfil their function in the ancient kitchens. On the downside highly calcareous clays possess a narrow sintering interval that requires considerable temperature control. Consequently a pronounced risk of overfiring exists as evidenced by the high proportion of misfired wasters littering ancient pottery production sites. In addition, there was always the risk of ‘lime blowing’, that is, spalling of the pot by rehydration of CaO to portlandite, Ca(OH)2 and subsequent carbonation of this phase to re-form calcite (see also Chapter 17). Hence such calcareous clays, when used for cooking pots must be fired well below the decomposition temperature of calcite, that is, around 700 °C, a technology that leads to mechanically inferior wares. On the other hand, utilisation of non-calcareous clays for cooking pots, together with sufficient amounts of tempering additions, has been preferred by Neolithic potters in several regions of the world (see Chapter 2.3.1; Maggetti et al. 2011a).

6.4.4 Colour pigments of fired ceramics bodies and surfaces The behaviour of iron and manganese oxides, important carriers of colour of fired ceramics, may serve as an indicator of the gas atmosphere prevailing during firing. The relationship between the oxygen fugacity41 and temperature defines the valence state of iron and manganese oxides, that is, the temperature-dependent dissociation pressure of the oxides. Appearance and disappearance of individual oxide phases are only thermodynamically well defined in terms of oxygen fugacity and temperature for pure oxide systems M-O. However, in ceramic systems SiO2, Al2O3, CaO and alkalis liberated by thermal decomposi-

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0

250

300

400

500

700 800 1000 1400 oC

600

log fO2 (bar)

-10

-20

-30

-40

-60 2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

1000/T (K-1) Figure 6.16. Logarithm of oxygen fugacity versus reciprocal temperature for the univariant equilibria Mn-MnO, MnO-Mn3O4, Mn3O4-Mn2O3, Mn2O3-MnO2 as well as Fe-FeO, FeO-Fe3O4 and Fe3O4-Fe2O3. The line C/CO/CO2 refers to the Boudouard equilibrium, the lines QFFe and QFM refer to the oxygen buffer combinations quartz-fayalite-iron and quartz-fayalite-magnetite, respectively44. Letsch & Noll (1983).

tion of clay minerals and carbonates will react with iron and manganese oxides well before the thermodynamically defined stability range of the latter has been reached. This will be shown in more detail below.

‘Smoking’ of ceramics During cooling of the ceramics in a kiln under complete exclusion of oxygen, volatile hydrocarbons expelled by the fuel via dry distillation will be cracked thus depositing elemental carbon (soot, graphite) onto the still hot ceramic body, a process known as ‘smoking’. Frequently the graphitic flakes will be deposited in an oriented way with their basal (00.1) planes parallel to the surface, thereby creating a shiny effect that can be enhanced by burnishing the pottery in the leather-hard state prior to firing. On the other hand, at a rough surface the graphite flakes are randomly oriented thus producing a non-reflective dull appearance. A superb example of such a black-topped redware bowl from El-Badari, Upper Egypt (c.4400–4000 BCE) is shown in Fig. 8.9. In contrast to the above, the non-smoking reducing reaction occurring under strict adherence to the Boudouard equilibrium CO2 + C ർ 2 CO fixes the oxygen fugacities in such a way that ferric oxide in haematite is reduced to a spinel-type phase (magnetite) containing ferrous iron oxide. However, the system follows the univariant C/CO/CO2 curve shown in Fig. 6.16 only if airtight conditions prevail and gas-free fuels such as charcoal or coke are

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used, conditions that were never fully realised in ancient pottery kilns (Letsch & Noll 1983). In case the clay body releases oxygen and/or carbon dioxide during firing, the Boudouard equilibrium will be momentarily disturbed but quickly restored by consuming carbon. Any water released reacts with carbon monoxide until the water-gas equilibrium CO + H2O ർ CO2 + H2 is achieved, catalysed in the presence of magnetite. In ancient pottery kilns frequent refuelling, cracks in the kiln walls allowing uncontrolled ingress of air, and other changes in the air supply further disturbed the Boudouard equilibrium and created transitional states between smoking and non-smoking firing. A consequence of this is that many ‘reduction-fired’ ceramics not only contain spinel phases such as magnetite and hausmannite but also elemental carbon as black pigment phases. A telltale sign of such uncontrolled air ingress is the notorious misfired ‘red-stained’ ware.

Iron reduction pigments As shown in Fig. 6.16 the univariant equilibrium line Fe2O3/Fe3O4 lies well above the C/CO/ CO2 line of the Boudouard equilibrium. Hence during oxidising firing haematite is stable throughout the entire temperature range up to 1490 °C at which temperature it reaches a dissociation pressure of 1 bar. In a reducing atmosphere haematite is quantitatively converted to magnetite already below about 600 °C. In fired (illitic) clays magnetite is stable to between 850 and 900 °C, but converts through reaction with alumina released by decomposing illite to hercynite (FeAl2O4) (see Fig. 6.15). In contrast, in the pure oxide systems depicted in Fig. 6.16, under reducing conditions magnetite is stable to 650 °C, the point of intersection of the equilibrium lines Fe3O4/FeO and C/CO/CO2 (Boudouard equilibrium). Further temperature increase shifts the equilibrium towards the formation of FeO (wustite) that converts to metallic iron at 730 °C. Furthermore, formation of fayalite (Fe2SiO4) under ideal thermodynamically controlled conditions ought to commence already at 350 °C (point of intersection of the QFM and C-CO-CO2 univariant lines). However, at this low temperature fayalite formation is kinetically constrained by the lack of reactive silica that will be available only at much higher temperatures through decomposition of the clay minerals. Consequently, fayalite may occur in quantitative amounts in ceramic fired under reducing conditions to at least 950 °C as shown in Fig. 6.15. As stressed by Noll (1976) neither fayalite nor wustite or metallic iron have ever been found in antique paint layers, not even in painted decorations of high-fired iron-rich Samarra and Halaf wares from Neolithic Mesopotamia (see Chapter 8.2). The reason for this is simple: the primitive pottery kilns were insufficient to maintain the required Boudouard equilibrium and hence a constant level of low oxygen fugacity throughout the supposedly short firing times.

Manganese black pigments As shown in the preceding paragraph, successful application of the iron reduction technique requires rather stringent control of the oxygen fugacity prevailing in the potter’s kiln. In particular, reoxidation of the black (magnetite, hercynite) to red (haematite) paint during

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unwanted ingress of air rendered the result of reducing firing riddled with failure. Such problems could be avoided by using manganese oxides as black pigment that maintained its colour over a wide range of oxygen fugacity, from fully oxidising to moderately reducing firing conditions. A further advantage was that bichromic red-black colour combinations could be readily attained even in primitive kilns under oxidising conditions, consisting of red haematite and black bixbyite, ˞-(Mn,Fe)2O3 or solid solutions of Mn-Fe spinels (see below). While the contrast between red surface and black decoration with sharp contours could be easily maintained throughout the firing process, an additional technological ‘trick’ of ancient potters was to paint the manganese black-clay slip onto the already dried ceramic body instead of a still moist surface. The dry surface absorbed the water of the slip quickly together with the finest pigment and clay particles, so that only the coarser pigment particles were left at the surface thus providing a dull appearance that contrasted well with the shiny red underground of the partially vitrified illitic engobe (Noll 1984). However, while iron-bearing clays suitable for producing pottery were ubiquitously available throughout the ancient realm, manganese ores such as manganese ochre (pyrolusite, ˟-MnO2) could only be found in manganese-rich gossans, for example in eastern Anatolia, Cyprus, Thessalia or the southern Balkans. Indeed, there is a notion that the oldest manganese black-decorated pottery originated in the 6th millennium BCE in the East Anatolian cultural centres. However, occasionally Mn-black decorations of ancient pottery were found in other context, for example on Etruscan (Schweizer & Rinuy 1982) or Anasazi (Striova et al. 2006) vessels.

Oxidising firing As shown in Fig. 6.16 manganese ochre (pyrolusite, ˟-MnO2) transforms to bixbyite ˞ ˞-(Mn,Fe)2O3) already below 600 °C (univariant equilibrium line MnO2/Mn2O3) and is stable to between 800 and 900 °C (Fig. 6.17) whereupon it tends to react with silica released from decomposing illite to braunite, Mn2+Mn63+[O8/SiO4] and, at higher temperature, to spinel of various compositions within the solid solution series magnetite (Fe2+Fe23+O4)jacobsite (Mn2+Fe23+O4), depending on the Mn/Fe ratio. In contrast to this, in the pure Mn-O system shown in Fig. 6.16 bixbyite is stable to still higher temperatures and reaches dissociation pressures of 0.2 bars at 873 °C and 1 bar at 974 °C, forming hausmannite, Mn2+Mn23+O4 with tetragonally distorted spinel structure according to the reaction equation 3 Mn2O3 ർ 2 Mn3O4 + 1/2O2. However, in the presence of alumina, silica, and iron oxide released from decomposing illite instead of hausmannite spinel mixed crystals will be stabilised as confirmed by Noll et al. (1973) and Wang & Andrews (2002) during studies of manganese black-decorated Neolithic pottery from Thessalia and Yangshao pottery of Henan, China, respectively.

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Figure 6.17. Phase stability in Mn-enriched illitic clays (Mn/Mn+Fe = 73–91/100) experimentally fired under oxidising (fO2 = 0.21 bars; left) and reducing (Boudouard equilibrium; right) conditions (modified after Letsch & Noll 1983).

Reducing firing Under reducing conditions (Fig. 6.17, right) established by the Boudouard equilibrium, the pyrolusite (˟ ˟-MnO2) contained in the manganese-black paint slip transforms quantitatively already below 600 °C to manganosite (MnO). This highly basic oxide starts to react with silica above 750 °C to tephroite, Mn2 2+[SiO4], a member of the olivine family the concentration of which stabilises at about 900 °C. Above this temperature the garnet-type spessartine, Mn32+Al2[SiO4]3 appears in the phase assembly, formed by reaction of manganese oxide with silica and alumina released from decomposing illite and/or chlorite according to 3 MnO + Al2O3 + 3 SiO2 ൺ Mn3Al2[SiO4]3. Comparison with the pure Mn-O system under reducing conditions reveals that manganosite (MnO) commands a wide field of stability between the univariant equilibrium lines Mn3O4/MnO and MnO/Mn (Fig. 6.16), and is crossed by the Boudouard equilibrium line C/ CO/CO2. This renders MnO stable up to very high temperatures. However, as mentioned above its high reactivity towards silica and alumina leads to formation of tephroite and spessartine beyond 750 °C, the temperature at which silica and alumina become available in larger amounts. The fact that manganese silicates and aluminium silicates are conspicuously absent in the black decorations of ancient pottery suggests that the oxygen fugacity during reducing firing in rather primitive pottery kilns did not fall below the Mn3O4/MnO equilibrium line for an appreciable period of time (Letsch & Noll 1983). The implication of this is that the ceramics were fired under oxidising or weakly reducing atmospheres, confirming once more that ancient potters had considerably difficulty to attain truly reducing conditions in their kilns that were prone to air ingress by several mechanisms.

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6.4.5 Pore size distribution function As shown in Fig. 6.1 noticeable shrinkage of a fired ceramic body occurs in the vicinity of the dehydroxylation temperature of kaolinite. This shrinkage can almost completely be accounted for by reduction of open porosity, that is, pores with access to the outside of the ceramic body. At higher temperature, volume and distribution of pores change profoundly once densification through sintering has been attained. Beyond 1000 °C the increasing degree of vitrification and thus the amount of glassy phase leads to sealing of open pores and thus an increase in closed porosity. The net result is a coalescence of smaller into larger pores, that is, the large pores grow at the expense of smaller ones. The driving forces of coalescence are related to the higher gas pressure inside small pores, higher surface tension, and also depend on the viscosity of the surrounding glass phase. Thus the diffusive coalescence of small pores into larger ones leads to a reduction of the total surface area of pores and hence to an energetically favoured state (see, for example Kingery et al. 1976). Figure 6.18 shows the distribution of pore radii of calcareous illitic clay fired under oxidising (left) and reducing conditions (right). In the clay fired in air the pore radii are shifted towards larger values with increasing firing temperature as discussed in more detail by Maggetti & Kahr (1981). Here two mechanisms overlap: diffusive coalescence of smaller towards larger pores (shifting of curves to larger radii) and homogenisation of pore radii (increasing of steepness of slope). The curves shown for the reducing fired clay are more complex (right). Clay fired under strongly reducing conditions show up to a temperature of about 950 °C the same general trend as the clay fired under oxidising conditions, that is, shift of pore radii towards larger values. However, between 950 and 1010 °C many very small pores are being generated, presumably by release of oxygen following reduction of magnetite to wustite. The highly reactive wustite phase does not show up in the ceramic body as it will subsequently be incorporated into iron silicate phases such as ferrocordierite (see Fig. 6.10, right) or fayalite.

100%

100%

102

103

104

Pore radius (Å)

105

102

103

104

105

Pore radius (Å)

Figure 6.18. Pore radius distribution (open microporosity) of a calcareous illitic clay fired under oxidising conditions (air) between 550 and 1010 °C (left) and reducing conditions (weakly reducing, fO2 > 10–4 atm, 820 to 1010 °C, dashed lines; strongly reducing, fO2 < 10–4 atm, 850 to 1035 °C, full lines) (Heimann et al. 1980).

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Samples fired under weakly reducing conditions display very inhomogeneously distributed pores beyond 820 °C but otherwise conform to the same pattern as the strongly reduced sample.

6.5 Thermal transformations in phosphatic ceramics From the middle of the 18th century onward bone ash was used to produce porcelain-like bone china in England for durable, mass-produced tableware (see Chapter 16). Its main ingredients are calcined animal bone (20–45%) together with china clay (kaolinite, 20– 35%), Cornish (china) stone, that is, weathered feldspar-rich granite (20–45%), and small amounts of fluxing alkali carbonates. This special development of ceramic technology deserves a closer look. The calcining temperature of about 1000 °C applied to decompose the bone was not high enough to alter the chemical nature of the mineral constituent of bone, hydroxyapatite (Ca10[(OH)2/(PO4)6] other than abstracting the hydroxyl ions, forming oxyhydroxyapatite (eq. 6.3) and subsequently oxyapatite (eq. 6.4) with the same Ca/P ratio of 1.67 as that of hydroxyapatite. Only during ceramic firing well beyond 1200 °C apatite thermally decomposes, forming tri- and tetracalcium phosphates (eq. 6.5), and eventually highly fluxing CaO and P2O5 (eqs 6.6 and 6.7), the latter dissolving to a small extent in the glass phase (Owen et al. 2011) while the majority will evaporate. The following equations show the sequence of decomposition steps (Heimann 2007): Step 1: Step 2: Step 3: Step 4a: Step 4b:

Ca10[(OH)2(PO4)6] → Ca10[(OH)2–2xOx ႒x(PO4)6] + xH2O Ca10[(OH)2–2xOx ႒x(PO4)6] → Ca10[Ox ႒x (PO4)6] + (1 – x) H2O Ca10[(Ox ႒x(PO4)6] → 2Ca3(PO4)2 + Ca4 O(PO4)2 Ca3(PO4)2 → 3CaO + P2O5 ↑ Ca4O(PO4)2 → 4CaO + P2O5 ↑

(6.3) (6.4) (6.5) (6.6) (6.7)

The squares ႒ in the formulae refer to lattice vacancies in the OH positions along the crystallographic c-axis in the structure of hydroxyapatite. These vacancies support diffusion of ions and thus increase the reactivity of calcium phosphate during the ceramic firing process. According to the binary CaO-P2O5-(H2O) phase diagram (Fig. 6.19) hydroxyapatite melts incongruently above 1570 °C and decomposes into the high temperature phase ˞’-TCP and tetracalcium phosphate (see eqs 6.6 and 6.7). However, such high temperatures will not be reached during firing of bone china ceramics. Instead, the development of the microstructure of bone china starts already beyond 1000 °C by progressive thermal decomposition of the Ca-deficient hydroxyapatite of the bone to form ˟-TCP (whitlockite) and free CaO (eq. 6.8). The latter reacts quickly with active alumina and silica released from decomposing kaolinite and metakaolinite, respectively (eq. 6.9) to crystalline anorthite (eq. 6.10) according to Ca10[(OH)2(PO4)6] → 3Ca3(PO4)2 + CaO + H2O Al2[(OH)4Si2O5] → Al2O3 · 2SiO2 + 2H2O Al2O3 · 2SiO2 + CaO → CaAl2Si2O8

(6.8) (6.9) (6.10)

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Fig. 6.19. High-temperature portion of the phase diagram of the quasi-binary system CaO-P2O5H2O at a water partial pressure of 65.5 kPa (after Riboud 1973). Note that at 1570 °C, incongruent melting of hydroxyapatite (HAp) occurs under decomposition into ˞’-C3P (˞’-TCP) and C4P (TTCP). The shaded areas indicate the regions of relative stability of hydroxyapatite.

TCP

V

V

V

V

V

V

CSP2

V

V

V V

CSP1 V

V

V

V

V

V

V

V V

An

V

V

V

V

V

V

V

V

SiO2

Fig. 6.20. Subsolidus portion of the phase diagram anorthite (An)-tricalcium phosphate (TCP)silica (SiO2) (St. Pierre 1954; adopted from Owen et al. 2011) showing the composition of sulphate-containing (full triangles) and sulphate-free (empty triangles) phosphatic porcelain imported into North America during the late 18th century. The squares denote the melt compositions close to the ternary eutectic (full squares: melt has corroded quartz grains; open squares: melt has corroded TCP).

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The composition of the average modern bone china body thus consists of about 40% tricalcium phosphate (˟ ˟-TCP), 20% anorthite and 40% calcium aluminium silicate glass phase. Figure 6.20 shows the composition of late 18th century CE phosphatic porcelain (Owen et al. 2011). The anorthite phase formed during firing is found as tiny microlites crystallised from the phosphatic melt phase of close to ternary eutectic composition.

6.6 Densification during firing The increasing densification of non-calcareous illitic clay with stepwise increasing firing temperature is shown in Fig. 6.21 (see also Fig. 6.12). The archaeological clay used in these experimental firings comes from the Thu horizon of the Otterbach (Rheinland-Pfalz, Germany) clay deposit (see also Fig. 6.4) with the nominal composition (in mass%) of 66.7% SiO2, 17.4% Al2O3, 4.6% Fe2O3, 3.1% K2O, 1.1% CaO, 0.7% TiO2 and 6.4% LOI. At 850 °C the clay mineral particles start to react in the solid state without noticeable volume change. In such reactions preferentially small particles are involved whereas the larger clay mineral platelet stacks retain their form pseudomorphically up to high temperatures (Kromer & Schüller 1974, Noll 1984). At about 950 °C first traces of a eutectic melt appear, leading to grain boundary melts, rounded platelet edges and to onset of diffusion of alkali and alkaline earth ions. At 1000 °C the fabric of the clay body is largely densified, with closed pores aligned parallel to the extrusion direction of the clay body. Beyond this tem-

Figure 6.21. Non-calcareous illitic clay (Otterbach O49) fired in air between 850 and 1100 °C. First traces of a grain boundary melt occur around 950 °C. For calcareous illitic clays the onset of melt formation has been found to occur at considerably lower temperatures, possibly as low as 750 °C (see Fig. 6.12; Görres et al. 2000).

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perature more melting occurs, accompanied by the diffusive coalescence of smaller pores towards increasingly larger ones (see 6.4.5). At 1100 °C considerable bloating can be observed with a large proportion of melted phase that eventually renders the ceramic fabric unstable. These sintering and, eventually, vitrification processes can be quantitatively described by defining two stages during which the microstructural changes are brought about by different mechanisms. Stage 1 between ambient temperature and 900 °C is governed by dehydroxylation of clay minerals, leading to a mass loss that causes shrinkage-related changes of the fabric imposed by the forming process of the ceramic body. In Stage 2 beyond 900 °C the reactions associated with mass loss have ceased to occur and microstructural changes are essentially brought about by changes of the pore volume, that is, sintering sensu strictu caused by sealing of open pores. The densification within these two stages can be expressed by the volume quotients V900/V0 and VT /V900 as V900 ⎛ ˂m ⎞ ˮ900 – R900 ˮ0R0 = 1– ⎝ V0 m ⎠ ˮ0 – R0 ˮ900 R900 VT ˮT – RT ˮ900R900 = V900 ˮ900 – R900 ˮT RT

(6.11a)

(6.11b)

with m mass of clay, ˂m mass loss between ambient temperature and 900 °C, V0 pore volume of dried ceramic body, V900 and VT pore volumina at 900 °C and T > 900 °C, respectively; ˮ0 specific gravity of the dried ceramic body, ˮ900 and ˮT specific gravities at 900 °C and T > 900 °C, respectively; Ro volumetric density of the dried ceramic body, R900 and RT volumetric densities at 900 °C and T > 900 °C. Plotting VT /V900 against firing temperature yields an S-shaped curve whereby VT /V900 = 0.8 is defined as the temperature of the onset of densification and VT /V900 = 0.5 to the mid-point sintering temperature (Hinz 1974, Heimann 1978/79) (Fig. 6.22). The steep negative slope of the sintering curve of a calcareous clay suggests an extremely narrow firing interval of only 15–25 °C as suggested by the small difference between the

Figure 6.22. Typical sintering curve of calcareous illitic clay fired in air. (Hinz 1974).

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onset of densification (VT /V900 = 0.8 at 1015 °C) and the mid-point of sintering (VT /V900 = 0.5 at 1035 °C). Hence such clays need very strict control of the firing temperature to avoid undesirable slumping of the ware owing to rapid formation of grain boundary melts with subsequent destabilisation of the vessel. The rise of the sintering curve beyond 1150 °C can be explained by bloating (see Fig. 6.21), that is, swelling owing to the expansion and expulsion of air trapped in closed pores (Maniatis & Tite 1975).

6.7 Determination of firing temperatures The determination of the firing temperature of ancient ceramics is an important (and often neglected) aspect of the investigation. Archaeothermometry involves the study of the changes of properties of clay with temperature (Roberts 1963, Tite 1969, Maniatis & Tite 1975, and many others). These changes are investigated hoping to find an indication of the firing temperature of the ceramic object. One searches for points of discontinuity of a physical property, discontinuities that can be related to specific temperatures (or more likely to a temperature range) up to which the materials have been heated in antiquity. While the actual measurements of such diagnostic properties are exact in the physical sense of the word, the results are not always as precise as the technique employed. This is due to the fact that the properties in question may be influenced by a variety of external factors that can be related to the events between the making and the investigation of the object. Use, burial, excavation, cleaning, conservation and storage are in this context critical interventions (Heimann & Franklin 1979). There are many analytical techniques available to monitor the change of physical properties of clay during firing. However, their easy availability and the seemingly exact nature of the data produced at various levels of sophistication and complexity may lead to uncritical and overenthusiastic interpretation. Moreover, the analytical techniques applied can introduce spurious variations on a scale that is too fine for the essentially heterogeneous or non-representative ancient ceramic samples. Here is a strong need for cooperation and mutual education between scientists and archaeologists as stressed many years ago by Anna Shepard (1971) and also Ursula M. Franklin (1977). More details on pros and cons of archaeothermometry can be found in Heimann & Franklin (1979) and Heimann (1982b). In many cases it suffices to estimate a tentative firing temperature that may carry an error margin of ± 50 °C or less. Sometimes the rare but fortuitous situation arises that the clay from which the ancient pottery has been produced is still available. Then clay tablets can be fired at ascending temperatures and the microstructure of the experimentally fired samples compared to that of the original pottery (for example, Tite and Maniatis 1975). Such an example is shown in Fig. 6.23. Panel (A) shows calcareous TS clay (5.2 mass% CaO, 4.6 mass% Fe2O3) from Rheinzabern (the ancient East Gaulish Tabernae) fired at 850 °C. Panel (B) shows the microstructure of a TS dish attributed to the Roman potter Secundinus Aviti who worked in Tabernae during the 2nd century CE. Its firing temperature was determined by a modified thermal expansion method (Heimann 1976, 1978/79) to be 866 ± 25 °C. The textural similarity between the two samples is striking and is particularly well illustrated in the pseudomorphologically maintained illite platelets. Panel (C) shows non-calcareous clay (1.1 mass% CaO, 4.4 mass% Fe2O3) fired to 1,000 °C, whereas (D) relates to a Roman

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Figure 6.23. Experimentally fired clay tablets (left) and Terra Sigillata shards (right). (A) Calcareous TS clay from Rheinzabern fired at 850 °C, (B) TS dish, Secundinus Aviti, Tabernae, 2nd century CE, (C) Non-calcareous clay fired at 1,000 °C, (D) Roman coarse pottery, Tabernae (Heimann & Franklin 1979).

coarse pottery from Tabernae, the firing temperature of which was determined to be 1,020 ± 20 °C. The textures of these two high fired samples are again very similar. The direct observation and comparison of ceramic shards allows to state that a comparable state of sintering has been obtained. However, this does not imply that the firing temperatures used were the same, because of the strong influence on sintering of mineralisers (fluxes), compositional variability, and variations in the kiln atmosphere. The latter, in particular, is crucial: in primitive kilns the oxygen partial pressure at different levels of the kiln shows strong gradients. X-ray investigations of fired non-calcareous illite-containing kaolinitic clays from the Westerwald region revealed that the thermal breakdown of kaolinite starts between 500 and 600 °C (see Fig. 6.2, 6.3) but that the basal (00l) reflections of illite will be retained to 1000 °C and the (110) reflections even to 1050 °C (Maggetti & Rossmanith 1981). This finding may suggest that kaolinitic clays are less useful for thermometric investigations since there are no other profound mineralogical changes observable between 600 and 1000 °C. However, the presence of 2M illite47 renders X-ray diffraction a useful tool to estimate the original firing temperature of a ceramic object. As shown in Fig. 6.24, left the ratio of the 47 2M illite is an ordered two-layer monoclinic polytype of commonly detrital origin (Austin et al. 1989).

6 Materials science of ceramics

400

E

40

3 300

30

MPa

Height (110)/FWHM (110) of illite

50

101

2 1

G

200

20

100

10

0

0 500 600 700 800 900 1000

°C

300

500

700

900

1100

°C

Figure 6.24. Left: Variation of the ratio of peak height of the (110) interplanar spacing of 2M illite and its FWHM (full width at half maximum) with firing temperature. The lines connecting the data points are a guide to the eye only (Maggetti 1982). Right: Variation of the modulus of elasticity (E) and the shear modulus (G) of an experimentally fired calcareous illitic clay (Otterbach clay O61) and Terra sigillata shards from Rheinzabern, 2nd century CE (samples 1–3) (Hennicke 1977, Heimann 1978/79, Heimann 1982b).

height of the (110) interplanar spacing of 2M illite to its FWHM (full width at half maximum) value decreases with increasing firing temperature and thus provides a rough estimate of the ancient firing temperature, in this case between 800 and 900 °C (Maggetti 1982). During firing of a green ceramic body many properties change: colour, density/porosity, hardness, phase composition, that is, mineral content, magnetic properties of ferromagnetic compounds, coefficient of thermal expansion, texture and microstructure, coefficient of elasticity (Young’s modulus) and others. Accordingly, among the various methods to estimate firing temperatures are (i) psycho-physical determination of colour changes in response to firing temperature, (ii) objective measurement of colour changes through remission spectroscopy, (iii) measurement of density changes, (iv) measurement of hardness changes, (v) determination of phase composition with firing temperature using optical microscopy, thermal analysis (DTA, DTG, DSC), and X-ray (Fig. 6.24, left) and neutron diffraction, (vi) measurement of the changes of the magnetic properties (saturation magnetisation, remanence, coercive force) of para- and ferromagnetic constituents, (vii) observation of changes of the coefficient of thermal expansion during refiring of ceramics, (viii) determination of textural changes depending on firing temperature and atmosphere, and (ix) nondestructive measurement of the moduli of elasticity and shear by observation of the travelling time of an ultrasonic impulse (Hennicke 1977). Details on these methods and references can be obtained from Heimann (1978/79), Heimann & Franklin (1979), and Heimann (1982b).

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Fig. 6.24, right shows a calibration curve obtained by firing of calcareous illitic clay (Otterbach clay O61; composition see Table 6.1) that was reportedly used to manufacture TS in eastern Gaul Tabernae (today’s Rheinzabern) in the 2nd and 3rd centuries CE (see Fig. 10.9). The fired test clay samples as well as the TS shards were subjected to measuring the propagation velocity of an ultrasonic pulse that is a function of the density, and the elastic constant48 (stiffness) E. The steep slope of the lines above 900 °C is an indication of the increasing densification by consecutive sintering that in turn causes an increase in elastic constants. In the diagram three TS shards are included the original firing temperatures of which are located in the temperature range between 980 and 1090 °C, typical firing temperatures of Roman Terra sigillata. This elegant, non-destructive technique to estimate firing temperatures is applicable only if the original clays are available, a fact that is rarely realized.

48 For simplification ceramics are considered linear-elastic isotropic materials. Measuring the longitudinal ultrasonic wave propagation velocity V yields the elastic modulus E as a function of the density of the ceramics ˮ and a complex expression containing the Poisson ratio ˪, usually taken ˪)/(1–2˪ ˪)(1+˪ ˪)]. The shear as a constant K. Then the equation reads as E = ˮ·V2/K, where K = [(1–˪ ˪ < 0.5. Comparmodulus G is defined as G = E/2(1+˪˪) and is approximately ൈE < G < ½ E for 0 90 mass% silica content. Typical analyses of the ceramic body show 94.0–94.2 mass% SiO2, 0.6–1.8 mass% Al2O3, 0.9–1.6 mass% Fe2O3, 1.7–2.0 mass% CaO, 1.1–1.8 mass% MgO, and traces of alkalies (Bakr 1956). Owing to its lack of clay-based constituents the name ‘faience’ is com-

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Figure 8.11. Red-slipped pottery bowl of ‘Meidum’ type, showing a flaring body, drawn in beneath the everted rim. El-Badari, Upper Egypt. Old Kingdom (2650–2150 BCE). Diameter 21.6 cm. Reg. no. 1925,1012.11. © The Trustees of the British Museum.

pletely misleading. This type of ‘pottery’ was produced from sand grains or crushed quartz pebbles that were mixed with a pre-melted alkali silicate glass frit and a few percent of refractory kaolinitic clay. The ceramic body was coated by a alkali-lime silicate glaze coloured blue-green by copper that was arguably applied to simulate scarce and highly prized lapis lazuli and turquoise (Tite 1992). The first glazes of this kind have to be seen in context with the copper ore deposits in the Sinai peninsula, where in the Timna Valley ore was mined and processed from the Chalcolithic period (5th–4th millennium BCE) to the Egyptian New Kingdom (late 14th to mid-12th centuries BCE) (for example Conrad & Rothenberg 1980, Hauptmann 2000, Drenka 2003). Details on production technology and microstructure of Egyptian ‘faience’ were provided by Kingery & Vandiver (1986), Vandiver & Kingery (1987), and more recently by Nicholson (2009). In this context it should be mentioned that the ancient Egyptian artisans appear to have invented two synthetic ceramic colour pigments, Egyptian blue (cuprorivaite, CaCuSi4O10, Tite 1985) and cobalt blue (cobalt aluminate spinel, CoAl2O4) (see also Chapter 4.6). The former was almost exclusively used in wall painting as its low colour intensity, in particular in fine grained products, precluded its use as a pigment for cold-painted ceramic decoration (Noll 1991). It was arguably invented already during the 4th dynasty (c.2575–2467 BCE). In Roman time it was also widely used under the name of caeruleum (Gettens & Stout 1966) and its manufacture was (incorrectly) described by Vitruvius (1960). Knowledge of the existence of Egyptian blue disappeared in the 4th century CE (Chase 1971) and the material was only reinvestigated in the early 19th century CE by Sir Humphrey Davy (1815). For

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additional information see Noll 1982a, Ullrich 1987, Tite & Hatton 2007, Rehren 2008, and Hatton et al. 2008. The second pigment, cobalt blue with superior quality and higher colour intensity compared to the classic Egyptian blue was copiously used from the 18th to the 20th dynasties (1550–1070 BCE) to decorate faiences (Tite & Shortland 2003) and was almost certainly derived from rare cobaltiferous alums found in the Western Oases of El–Kharga (Fig. 8.8, site 7) and Dakhla (Kaczmarczyk 1986, Shortland et al. 2006b). Prior to the 18th dynasty, the synthetic blue cobalt spinel pigment was rarely used to paint pottery surfaces, presumably owing to its scarcity and hence high cost. Interestingly, Noll (1991) tentatively related the sudden and massive appearance of this blue pigment as ceramic paint during the 18th dynasty to the sun cult of Pharaoh Akhenaten, the Heretic King. The vivid colour of cobalt aluminate was supposed to resemble the heavenly blue, a symbol of the god of the sun disc, Aten, but its use fell, like Akhenaten himself, into oblivion after reestablishment of the traditional Amun cult by Tutankhamun in 1334 BCE. However, the continuing use of this pigment up to the 20th dynasty casts some doubt on this hypothesis. Still, it is rather mysterious why this better product did not conquer the antique market since it produced painted surfaces with high abrasion strength and as the only thermally stable blue pigment may also have been used to colour the ceramic body during firing (Noll 1984, 1991). Alas, this material was forgotten for almost two millennia until it was rediscovered as a ceramic pigment during China’s Tang dynasty (Kerr & Wood 2004) and in overglaze-painted Iranian lustre ware of Mina’i and Lajvardina styles (Kleinmann 1991, Mason 2004; see 13.2.3). It was discovered a third time in 1799 by the French chemist Louis Jacques Thénard (Thénard’s blue) and used as brilliantly blue pigment by the Sèvres and Vienna’s Augarten porcelain manufactures. During the 18th dynasty Tell-el-Amarna (Fig. 8.8, site 2) period around 1300 BCE plaster-ofParis moulds for slip casting of pottery were invented, the knowledge of which was, maybe for the same reason, rather quickly forgotten and reinvented only much later. In contrast to this, the Egyptian potters stuck to the ancient manganese-black pigment even though their trade relation with Minoan Crete certainly had made them aware of the technically superior and more variable iron oxidation/reduction colour palette used there since at least the Early Minoan (EM II, 2500–2300 BCE) Vasiliki ware (Noll 1982). The polychrome pottery of the 18th/19th dynasties was decorated lavishly with black, red, white and blue colours. As discussed above the blue pigment is cobalt aluminate spinel63; black is related to manganiferous iron oxide with haematite structure and varying Mn/ (Mn+Fe) ratios; red colours are generated by haematite crystallites obtained by heating of Mn-free iron ochre; white impure decoration may be related to a mixture of gypsum, calcite and diopside (so-called ‘lime silicate white’, Noll 1982b). Apart from the various ways the Egyptians decorated their pottery the ceramic bodies were chemically remarkably homogeneous, with clay raw materials closely associated with lime-

63 As discussed by Kerr & Wood (2004; p. 663) cobalt spinel could have been formed in situ during glazing rather than being introduced as a prefabricated pigment the production of which was complicated owing to a complex roasting and fritting process of sulphide and/or arsenide cobalt ores (Kleinmann 1991).

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SiO2/Quarz

Nile mud Pre-dynastic Old kingdom Middle kingdom New kingdom Marly clay (Qena) Late Egyptian

10

90 80

20

70

30

Nile mud

60

40 Diopside Wollastonite

50 Anorthite

60

40 30

70

Mullite

Gehlenite

80

20

10

90 CaO+MgO 10

20

30

40

50

60

70

80

90

AI O 2 3

Figure 8.12. Position of Egyptian pottery in the ternary phase diagram (CaO+MgO)-Al2O3-SiO2 (Data from Noll 1984). The composition of Nile mud was obtained from Hangst (1979).

poor Nile silt, the composition of which is nearly constant over long distances (Table 8.3). The clay has been deposited between the Upper Pleistocene and the present. As a consequence the deposits can be found well away from the present course of the Nile as well as within the modern flood plain (Bourriau et al. 2000, Michelaki & Hancock 2013). As shown in Fig. 8.12 the composition of Nile silt (normalised for loss on ignition, LOI) is approximately 68 mass% SiO2, 24 mass% Al2O3 + Fe2O3, and 8 mass% CaO + MgO (see also Kemp 2000). Hence the ceramic compositions are straddling the cotectic line quartz-anorthite. A substantial number of pottery analyses are found in the cotectiv triangle quartz-anorthitemullite. The fact that the Nile silt composition is lowest in SiO2 compared to the analyses of the Egyptian wares is certainly related to the fact that fine quartz sand was used as an intentionally added temper during production of the pottery64. It should be noted that in Fig. 8.12

Table 8.3. Chemical analyses in mass% oxide of Nile mud and marly clays from Qena and ElBallas (Bakr 1956, Lucas & Harris 1962, Shortland 2000; see also Bourriau et al. 2000). Origin

Analyst

SiO2

Bakr

43.1

Nile mud

Hangst

57.2

13.4

2.1

10.4

5.2

3.2

Nile mud

Shortland

59.7

14.2

2.8

12.0*

5.2

3.4

Nile mud

Shortland

62.8

15.8

1.7

11.2*

3.3

3.1

1.1

1.0

Nile mud

Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O 14.8

15.7

3.3

3.2

2.3

LOI

1.1

15.5

1.5

1.5

5.0

1.6

1.2

Qena marly clay

Bakr

33.0

15.0

8.1

17.5

2.0

1.0

1.0

20.0

Ballas marly clay

Lucas

34.8

20.6

6.1

17.7

0.4

1.3

1.0

21.4

*Expressed as FeO

64 A recent study (Michelaki & Hancock 2013) showed that sediment samples collected from the Nile River delta, potential raw materials of Egyptian ceramics, could be assigned by principal component analysis (PCA) to three groups: unaltered Nile alluvium, lime-diluted Nile alluvium, and silica (quartz sand)-diluted Nile alluvium. This finding underscores the complexity of the interpretation of archaeological data of ancient Egyptian ceramics (see also Hancock et al. 1987).

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the compositional point of Nile silt is shifted towards the SiO2 apex since the high content of Fe2O3 (Table 8.3) has not been considered in the ternary diagram, that is, the composition of Nile silt and the ceramic bodies produced from it must be displayed correctly in the quinary phase diagram CaO-MgO-Al2O3-Fe2O3-SiO2. The New Kingdom wares produced from very lime-rich marly clays of Qena (Dendara, Fig. 8.8, site 11)) and El-Ballas (Fig. 8.8, site 5) form a clearly separated group (Fig. 8.12) that extends from the cotectic triangle di-qz-an to the calcareous triangle di-an-ge. Table 8.3 shows analyses of silty Nile mud and marly clays from Qena and El-Ballas (Bakr 1956, Lucas & Harris 1962, Bourriau et al. 2000, Shortland 2000).

8.5 Iran Iran is home of one of the oldest civilisations on Earth, located at the eastern branch of the so-called ‘Fertile Crescent’, a region stretching from Egypt in the West to Anatolia and eastwards to the deserts of eastern Iran. To date in the mountainous regions of central Iran only few remains from the earliest periods are known. Notable exceptions are Tepe Ghabristan and Tepe Hissar in the North, Tepe Yahya and Tal-i-Iblis in the South, and, in particular Tepe Sialk and Arismān in the west-central part (Fig. 8.1; Schreiner et al. 2003). Since the Central Iranian Plateau is surrounded by natural barriers, its mountains have helped to shape and sustain the political, cultural and economic unity of the region for most of its history. The oldest pottery-producing cultures go back to the Neolithic, in the Zagros Mountains as far back as the 7th millennium BCE (Wulff 1966). In this area arguably the oldest pottery painted with iron oxihydroxide (ochre) originated. It is interesting that these ancient potters did not use the readily available iron-rich clays to obtain a red surface decoration but instead resorted to painting the surfaces with (yellow) iron ochre that turned red during oxidising firing (see Chapter 4.6). This is known among potters as ‘intentional red’ (Hofmann 1966). Noll (1982a) suggested that this practice could have been a carry-over from the ancient way of decorating cave walls and dead bodies with red iron ochre. The Chalcolithic (c. 5500–3500 BCE) produced important production centres of painted pottery. In the North and central regions wheel-turned vessels were manufactured and decorated with an engobe technique at Tepe Sialk, Tepe Hissar and Tepe Guran since 3200 BCE (Figs. 8.15, 8.16). In the South at Tell-i-Bakun (Fig. 8.14, right), Bampur, Persepolis, and, most importantly, Susa potters produced their finest painted wares, developing a variety of jars, bowls (Fig. 8.13), chalice and goblet forms, some of which rivalled those of the earlier Mesopotamian Halaf culture (see above). These technological achievements must be seen in context with the widely varying clay compositions the early Iranian potters had to cope with, in contrast to the uniform compositions the Mesopotamian potters enjoyed. This difference is also manifest in the firing temperatures applied: whereas the Mesopotamian Ubaid ware was fired around 1100 °C to attain a dense, partly vitrified body, wares of the Iranian Tepe Hissar and Tureng Tepe kilns were fired between 750 and 1000 °C as determined by saturation magnetisation vs. magnetic coercive force plots by Coey et al. (1980) (see also Heimann 1978/79).

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Figure 8.13. Painted pottery bowl made from very calcareous clay. The interior is decorated with cross hatching and three roundels, each enclosing a large elliptic motif with multiple wavy lines parallel to its long axis. The exterior is plain apart from a band painted around the foot. Made in Susa, 4200–3800 BCE. Height: 8.9 cm, diameter: 20 cm (rim), diameter: 7.3 cm (at the base). Reg. no. 1924,0902.4. © The Trustees of the British Museum.

Figure 8.14. Left: Pottery fragment with human and equine figures. Sialk III (early 4th millennium BCE). Near Eastern Antiquities, Louvre, Paris. Object no. AO17865. Use of this image is licenced under the Creative Commons Attribution 2.5. Photo: Jastrow. Right: Thin-walled pottery bowl with dark brown matt painted decoration showing three dancing figures with stylised heads and raised hands as in adoration. Tell-i-Bakun, southern Iran. 5000–4000 BCE. Design typical of Tepe Sialk III. Height 16 cm, diameter 27 cm (rim), diameter 5.5 (base). Reg. No. 1936,0613.2. © The Trustees of the British Museum.

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During the reign of the Sassanide and Parther kings (500 BCE) the ceramic tradition artistically declined somewhat but reached a new high level with the revival of the glazing techniques during Islamic time (see Chapter 13.2.1; Mason 2004). One of the most important Iranian archaeological sites was excavated at Tepe Sialk starting in 1933 (Ghirshman 1938/39) and continuing at present time after the excavations resumed in 1999 (Shahmirzadi 2002). Tepe Sialk is situated at two neighbouring hills southwest of Kashan. While earlier research has identified four main phases of occupation (Majidzadeh 1981), today six phases are recognised between the first half of the 5th millennium BCE and the 8th century BCE (Sialk I to VI), spanning the four main epochs of the Chalcolithic (c. 5500–3500 BCE), Proto-Elamite (3500–2800 BCE), Bronze Age (3000–1350 BCE), and Iron Age I–II (1350–800 BCE). The Sialk III period saw the introduction of the potter’s wheel and the production of beautiful terracotta pottery adorned with animal and human figures (Fig. 8.14). Important centres of pottery production were scattered throughout ancient Iran including Amlash, Rajj, Tepe Ghabristan and Tepe Hissar south of the Caspian Sea, Hasanlu in the Northwest, Tepe Giyan in the West, Susa in the Southwest as well as Nishapur in the far Northeast. However, among the large number of pottery traditions in ancient Iran (Reindell & Riederer 1983) one appears to be of particular importance and will therefore be discussed in more detail: Arismān near Kashan in west-central Iran (Fig. 8.1). Here fine ceramics of the Sialk III/IV periods were discovered by a local hobby geologist in 1996. Subsequently an archaeological excavation campaign brought to light a large industrial copper smelting complex, dubbed the ‘Ruhr of the Bronze Age’ (Zick & Bick 2001) and considered to be the oldest metallurgical centre in the world, and hence presumably the cradle of the raw materials basis of the contemporary high cultures in the Near East. This particular important technological centre existed between the mid-4th and the early 3rd millennia BCE. The excavation of this vast prehistoric copper smelting and working centre of the Sialk III/ IV periods led by Deutsches Archäologisches Institut (DAI) in cooperation with the Geological Survey of Iran and the Iranian Cultural Heritage Organisation (ICHO) so far revealed the existence of no less than 34 smelting furnaces in which copper ores from distant deposits such as Vesnoveh and Nakhlak were processed. Arguably Arismān was the main copper supplier of the Mesopotamian and Egyptian high cultures and may have traded copper ingots and weapons, tools and jewellery made from copper with the Mohenjo-Daro culture of the Indus valley. Specific details of the origin of the Arismān culture and the work organisation of their copper smelting, casting and trading systems are still obscure and need much additional research (Chegini et al. 2004). Next to numerous artefacts related to metal working activities five circular pottery kilns were excavated with stoking channels attached and a central pillar supporting a holey firing platform, similar to the kiln-type shown in Fig. 7.9. The yellow-brown Sialk III Arismān pottery shown in Figs. 8.15 and 8.16, right was apparently made from very calcareous clay covered by a light-coloured engobe (slip), and burnished prior to firing under oxidising atmosphere. This technique was widely applied throughout Neolithic cultures of the Fertile Crescent, Anatolia, Egypt and Iran as well as Minoan Crete and Cyprus. Even today potters in Crete polish their vessels in the leatherhard state with a wet smooth pebble (Hampe & Winter 1962, Noll 1982). This burnished

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Figure 8.15. Painted ceramic vessels of the Sialk III period (late 5th to early 4th millennium BCE) from Arismān, Kashan, Iran. Left: Height 16 cm, max. diameter 13.5 cm. Centre: Height 17 cm, max. diameter 17 cm. Right: Height 11.5 cm, max. diameter 8 cm (Photos: Helwing/Boroffka, DAI Fotoarchiv Arismanprojekt. All rights reserved).

surface provides a solid ground for an engobe made from fine, very calcareous clay. The similarity of the coefficients of thermal expansion of the ceramic body and the slip assured excellent adhesion of the latter. After drying the slip served as an ideal canvass for painting, in this case with black manganiferous pigments. As discussed in Chapter 6.4.4 the decoration with sharp contours could be easily maintained throughout the oxidising firing process by painting a manganese oxide (pyrolusite)-bearing thick clay slip onto the already dried ceramic green body instead of a still moist surface. The dry surface absorbed the water of the slip quickly together with the finest pigment and clay particles, so that only the coarser pigment particles were left at the surface thus providing a dull appearance that contrasted well with the shiny yellowish underground. The use of manganese-black decoration technique originated presumably in the mountainous regions of East and Central Iran, and spread via eastern Anatolia to Greece and the southern Balkans (Noll 1984). There is some indication that the Sialk III culture found its abrupt end after the site had been raided by advancing Elamite (?) people and burned to the ground. The following Sialk IV period was approximately contemporary with the Early Dynastic period in southern Iraq, dating from about 3000 to 2500 BCE. During this time the great ziggurat was constructed as the largest and tallest structure on the southern hill, and the oldest monument of this type in Iran with three platforms and ascendance from the south. The pottery of the Sialk IV period (late 4th millennium BCE) is much less sophisticated compared to Sialk III ware. Apparently emphasis was put more on functionality than an artistic beauty, an indication that pottery production moved from a holistic to a prescriptive technology, from a craft to an industry (see Chapter 1). This change also marks the transition from an agricultural to an early industrial society when Bronze metallurgy was developed and brought to perfection. After decline of the Sialk IV culture the site was abandoned for the remainder of the Bronze Age and resettled only during Iron Age II at the end of the 2nd millennium BCE (Sialk V). From this period some remarkably beautiful ceramic object are known such as the vessel shown in Fig. 8.16, left. While again a light slip was applied this time an iron-bearing paint was used that during oxidising firing produced a red decoration.

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Figure 8.16. Left: Painted beaked vessel from Tepe Sialk, early 1st millennium BCE. © Rijksmuseum van Oudheden, Leiden. Right: Painted stemmed drinking cup, Sialk III period, Arismān, Kashan, Iran. Height 15.5 cm, rim diameter 17.5 cm (Photo: Helwing/Boroffka, DAI Fotoarchiv Arismanprojekt. All rights reserved).

In conclusion, despite the large diversity of Iranian pottery, in a geographical and stylistic context there are some common traits with neighbouring cultures. The ceramic bodies of Iranian pottery are very similar to those of Anatolian ware in terms of their positions in the ternary diagram SiO2-Al2O3-(CaO+MgO), mineralogical phase content, and firing temperatures. The oldest Iranian ceramics from Zagros Mountains are highly calcareous and extremely low fired, maybe as low as 600 °C. Later wares from Tepe Sialk, Tell i Bakun and Tepe Giyan suggest somewhat higher firing temperatures between 800 and 850 °C as evidenced by partial sintering and formation of traces of gehlenite and diopside. Hence these ceramics as well as the Anatolian wares are clearly inferior in their materials properties compared to Mesopotamian pottery from Hassuna, Samarra, Tell Halaf and Ubaid that were fired at temperatures as high as 1050–1100 °C to attain dense, fully sintered ceramic bodies. Also, the technology of ceramic decoration in Iran is comparable to that of Anatolian pottery (see 8.3) as evidenced by the almost exclusive use of manganese-black pigments based on widespread but small deposits of manganese ores in the Iranian highlands. This is in stark contrast to Mesopotamian pottery that obtained their black decoration singularly through iron reduction technique. However, some evidence exist that pottery found at Tepe Giyan in Luristan, and Susa were decorated with a mixed manganese-black/iron reduction technique. Here technological diffusion from nearby Mesopotamia may have played an important role (Noll 1991). The reddish-brown decoration of the bowl from Susa shown in Fig. 8.13 suggests the exclusive use of iron hydroxide pigment, a possible regress to the much older Neolithic ceramics from the Zagros Mountains region discussed above.

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8.6 Hidden messages from Neolithic cooking pots 8.6.1 Food in ancient Mesopotamia Three clay tablets (YBC 4644, YBC 8958 and YBC 4648), written in cuneiform letters from ancient Mesopotamia preserve 35 recipes dating from the 18th to the 17th centuries BCE (Bottéro 1995; Fig. 8.17). Apart from these ancient texts that record the oldest culinary recipes known to date, our information about supplies and processing of food is only indirect, the main information originating from both dictionary-like lists of food stuff and administrative texts recording the acquisition and expenditures of raw staples and kitchen supplies. Although a large number of names of edible plants, animals, and condiments could be identified, in many cases their exact identification is still not possible. As maintained by leading archaeologists, cooking in Mesopotamia is considered to mark, alongside with clothing, the beginning of civilisation (Encyclopaedia of Food & Culture; Contenau 1954). In particular, whereas in pre-Babylonian times food preparation was done by baking, roasting, broiling and grilling, all recipes preserved in the Yale culinary tablets (Bottéro 1995, see below) have one thing in common: cooking in water or some other kind of liquid. This was indeed a true revolutionary paradigm shift in food processing, opening up new opportunities to make meat and vegetables more palatable by adding richer flavour and succulent taste as well as a more gentle, easier to control treatment of heat sensitive food ingredients thereby retaining their nutritional value. As Alice Slotsky put it: ‘The sophisticated refinement it [cooking in water] introduced added a whole new dimension to the practice of cooking and brought Mesopotamian cuisine across the fine food frontier’ (Slotsky 2007). An important prerequisite of cooking meat in liquid was the development of suitable cooking pots (Fig. 8.18). As mentioned above in section 8.2 the low firing temperature applied to the Mesopotamian pottery rendered the ceramic body porous. This initial drawback was remedied by sealing the outside of the pot by painting with a clay slip or, much later, by

Figure 8.17. Left: Yale culinary tablet YBC 8958. The wild pigeon recipe, written in Akkadian, and translated below is described in lines 50–55 (Bottéro 1995, p. 230). Right: Yale culinary tablet YBC 4644. The recipe of ashshuriâtum shirum (Assyrian meat), translated below is described in lines 3 and 4 (Bottéro 1995, p. 226).

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Figure 8.18. Cooking vessels from Arismān, Kashan, Iran. Sialk III period. Left: Height 17.5 cm, max. diameter 23.5 cm. Right: Height 18 cm, max. diameter 19 cm. (Photos: Helwing/Boroffka, DAI Fotoarchiv Arismanproject. All rights reserved).

application of a vitreous glaze to make the pots water-tight. On the other hand, fat and other organic food residues that impregnated the porous inner walls of a cooking pot would carbonise with time and seal efficiently the pores. According to Mesopotamian lore food recipes were invented by Enki, god of wisdom and knowledge as mentioned in a literary text describing the preparation of a popular type of cake (Sumerian, ninda-ìdé-a; Akkadian, mirsu) : ‘King (?) Ulgi will have a banquet in his pleasant palace after large beautiful dates mixed with raisins, butter from the holy sheepfold, and the sweetest (date) honey have been worked together, and the sweets have been mixed with fine flour, according to the good instructions of god Enki’ (Encyclopaedia of Food & Culture).

Mirsu (traditional cake) Ingredients: 3 cups of fine flour, ¼ cup of butter, 1 cup of dates, small amounts of cheese and resins.

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Wild pigeon (amursânu) in broth This ancient Mesopotamian recipe, recorded in Akkadian cuneiform (Fig. 8.17, left), has been translated by Bottéro (1995) as follows:65 66 ‘Slaughter the wild pigeon and, after soaking it in hot water, pluck it. Once plucked, wash it with cold water and skin the neck, leaving attached to the body the skin with its meat. Cut out the ribs. Open the underbelly and remove the gizzard. Wash and soak the bird in cold water, and wipe carefully. Open and peel the membrane from the gizzard. Cut open and chop the intestines. To cook in broth, first put in a kettle the gizzard, intestines, and head, as well as a piece of mutton fat, and cook it. Remove from the fire. Sprinkle [the bird] with salt and assemble all the ingredients. Prepare the broth, adding a piece of fat from which the gristle has been removed, some vinegar, samîdu65 flour, leek and garlic, mashed with onion, and if needed add some water. Let simmer. When it is cooked, pound and mash together leek, garlic, andahöu66 and kisimmu to add to the dish [follows a damaged passage in which apparently the pigeon is disjointed and the legs are covered with dough]. When everything is cooked, remove the meat from the fire, and before the broth cools, serve it accompanied with garlic, greens, and vinegar. Carve and serve. The broth can be consumed later by itself’. (Encyclopaedia of Food & Culture, Bottéro 1995).

Meat Assyrian style (ashshuriâtum shirum)67 The two original recipes have been written in Akkadian on tablet YOS 4644 (Fig. 8.17, right). Bottéro’s translation reads as follows: Meat (cooked in) water (me-e shirim shi-rum). Meat is used. Prepare water; add fat, [break in tablet], mashed leek and garlic, and a corresponding amount of raw shuhutinnû67.

65 It is not clear what samîdu means; it could be a type of flour preparation or a spice (Encyclopaedia of Food & Culture). 66 Andahöu is presumably a kind of wild onion (allium desertorum); kisimmu refers to a type of cheese or coagulated (non-liquid) milk. 67 Shuhutinnû (šuhutinnû) is presumably of the allium (onion) family; the meaning of zurumu is unknown, likely a spice.

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Meat Assyrian style. Meat is used. Prepare water; add fat [break in tablet], garlic and zurumu67 with [break in tablet], blood, and mashed leek and garlic. Carve and serve’.

A modern, palatable version of ‘Meat Assyrian style’ has been provided by Alice Slotsky (2007): 1. Chop many onions, shallots, garlic, chives, leaks and scallions. Fry them in vegetable oil until soft. 2. Brown all sides of an eye round pot roast in this mixture. Add salt to taste. 3. Turn down the heat and simmer until done in a small amount of water to which a quarter to a half bottle of Guinness stout [to replace the blood asked for in the original recipe] has been added, turning once or twice during cooking. Remove meat. 4. Reduce onion-beer mixture to yield a thick gravy. 5. Carve and serve.

Boiled lamb ‘Shamshi-Adad’68 (Fig. 8.19). Our own version will dispense with the Guinness stout to be closer to the original. This alteration we will do in the spirit of Slotsky who wrote: ‘There is the question of how the raw ingredients of today match their ancient counterparts as well as what is lost or incomprehensible. It helps to look at it this way: even faux Babylonian food is better than no Babylonian food’ (Slotsky 2007). Here follows the very detailed recipe:

Ingredients: 1 lamb forequarter (1–1.5 kg), 1 leek with abundant green, 2 medium-sized onions, 3 cloves of garlic, 4 dates dried and pitted, 1 tbls sesame oil, 2 tsp cumin, 1½ tsp coriander seeds, 5 leaves of mint, 2–3 tsps of salt, 1 liter water, pepper to taste.

68 Shamshi-Adad I., King of Assur, reigned 1750–1717 BCE or 1813–1791 BCE (short chronology). He founded an empire encompassing much of Mesopotamia, Syria and Asia Minor (Kingdom of Upper Mesopotamia).

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Preparation: 1. Pad meat dry, remove abundant fat and save. 2. Roast 1 tsp each of coriander seeds and cumin lightly in a coated pan, and pound subsequently in a mortar. Squash 2 dates and chop finely 1 garlic clove. Add sesame oil and 2 tbls of hot water. Work everything to a paste. 3. Rub paste thoroughly into the meat, wrap in clinging foil and let rest for at least 1 hour at room temperature. 4. Cut leek lengthwise and then in 5 cm long sections. Divide onions into eight pieces each, cut remaining garlic cloves in half. Heat water. 5. Add reserved fat to large, coated pan (if not sufficient, use goose grease), heat to medium, add meat and brown it throughout. Move meat to a preheated lidded casserole of appropriate size. 6. Brown onions first, then leek and finally garlic, stirring constantly. Add to meat. Deglaze with hot water, cook the drippings loose and pour over meat. Season sparingly with salt and pepper, add remaining dates and bring quickly to a boil. Cover casserole but leave lid slightly ajar, and simmer over low heat for 120 minutes. 7. Remove meat and keep warm, wrapped in aluminium foil. 8. Reduce liquid in an uncovered pot. Pound coriander seeds (½ tsp) and cumin (1 tsp) in mortar and add to boiling liquid. If desired, add salt. 9. Pass liquid through a sieve, pressing the solid leek, onion and date ingredients lightly with the backside of a tablespoon to recover all liquid. Thicken with roasted flour, mix-in chopped mint leaves. 10. Serve meat with the gravy and bread.

Figure 8.19. Boiled lamb ‘Shamshi-Adad’ with gravy. The dish is similar to the shallow carinated Neo-Assyrian bowl with a ring base shown in Fig. 8.5.

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Tarru (fowl)-bird stew (Rivera 2009). Ingredients (4 servings): 2 small fowl such as Cornish game hen, quail, partridge or poussin (chick); 1 small onion, peeled and finely chopped; 1 whole leek, rinsed and finely chopped (green part only); 2 cloves of garlic; 4 tbls milk; 4 cups beef broth or bouillon; 2 tbls vegetable oil; salt to taste; 2 tsps malted milk powder. Preparation: 1. Rinse the birds under cold running water and pat dry with paper towels. Split hens in half. 2. Put the onion, leek, and garlic into a mortar and pound until everything is crushed. Add the milk and mix. Avoid using a blender as this will render the mix too watery. 3. Place the birds in a large pot, casserole, Dutch oven, or clay pot. Add the beef broth, oil, salt, and malted milk powder. Bring everything to a boil, stirring constantly. Lower the heat and simmer, covered, for ten minutes. 4. Add the onion mixture. Cover and continue simmering until the birds are tender (10 to 15 minutes). 5. Place the birds whole on a serving platter or carve them into smaller pieces, with the broth either served separately or poured over the birds.

8.6.2 Food in ancient Egypt The ancient Egyptians are credited with the invention of bread as we know it today (PetersDestéract 2005). Prior to this for thousands of years people found nourishment in the form of gruel made first from millet, emmer and oats, later from barley and wheat. Subsequently round flat dough cakes were baked that were eaten warm. The transition from such unfermented, heavy and chewy pancakes to fermented, light and fluffy bread was achieved by the use of yeast. Emmer or wheat grains were crushed in stone mills, mixed with yeast and water, or sometimes with eggs and milk. For sweet taste the dough was enriched with dates or lotus seeds. Some 140 kinds of bread with different ingredients, and of different sizes and shapes are known to have been eaten in ancient Egypt (Samuel 1999). Bread was also the precursor of beer, considered to be the ‘national’ beverage of ancient Egypt (Hirschfelder 2005, Geller 1993). Milled barley or wheat was mixed with water and the resulting dough was poured into a mould and slightly baked to yield a product that was crisp at the outside but still soft and unbaked inside. This semi-baked bread was then ripped apart, mixed in a bowl with sweet date juice, and the liquid separated by straining. When fermentation had ceased the finished ‘beer’ was filled in jugs and sealed airtight (Helck 1971). The taste of this concoction was certainly very different from our present-day beer.

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Figure 8.20. Left: Wall painting of a sacrificial altar from an Egyptian tomb (Tomb of Menna, 18th dynasty, c. 1400 BCE). On top a bowl of eggs, gourds, and river fish, below two ducks and two covered bowls, and at the bottom three wine amphorae, and three baskets filled with various fruits and berries (http:// en.wikipedia.org/wiki/Ancient_Egyptian_cuisine; accessed June 21, 2013). Right: Clay tablet OMM 606 + 791(ostrakon) written in demotic script with the recipe of sweet bread called elgi (Gallo 1997).

Most of what we know about the diet in Egypt has been gleaned from wall paintings in tombs (Fig. 8.20, left). For example, the Tomb of Rekh-mi-Re’ at Thebes depicts the making of cake from crushed roots of galangale that are sweetened with honey and baked in fat or vegetable oil (Bauer 1967). In a very early tomb food for the afterlife of the deceased was found placed on plates alongside the coffin (Emery 1962). Murals give us an impression of what was available to the people of this period. A succulent funeral repast from a 2nd dynasty (2820–2670 BCE) tomb includes barley gruel, boiled fish with bread, pigeon ragout, snipe, beef kidneys, beef shanks, figs, berries, honey cakes, cheese and grapes (Spencer 1983, Hirschfelder 2005). Despite all this, to this date no real cooking recipes were discovered. All recipes-like descriptions known at present do not relate to food preparations but to concocting balms, medicinal potions or incense (von Lieven 2011). The papyrus Anastasi I (papyrus British Museum 10247) written in the Ramesside Period (19th and 20th dynasties) reports the kind and amounts of food required for a reception of a pharaoh and his entourage. It lists the astonishing number of 29,000 loaves of bread of 10 different kinds, 100 baskets of dried meat, 250 ‘fistfuls’ of entrails, 60 measures of milk, 60 measures of cream, 100 heaps of cabbage, 50 geese, 70 rams as well as grapes, pomegranates and figs (Bauer 1967). However, since the overall tone of the papyrus is satirical, deriding a fellow soldier-scribe as inept and irresponsible, the content of truth of such information is questionable. The food generally available to the (rich) Egyptians comprised fowls such as goose, duck, pigeon, snipe and small song birds, domesticated animals such as cattle, sheep, goats and

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pigs, wild animals such as antelope, gazelle and ibex, vegetables such as lettuce, leek, garlic, cucumbers, onions, cress, seeds of lotus and parts of the papyrus plant, fruit such as figs, dates, melons, grapes and pomegranate, as well as legumes such as lentils, beans, peas, chickpeas and fenugreek. Wine (irep) was made from grapes and presumably available only to a selected group of high-ranking officials. There is also evidence that a special, possibly ritual beverage was produced from red vitis vinifera grapes with the Egyptian name shedeh. According to an ancient Egyptian text (papyrus Salt 825, Derchain 1965) this beverage akin to red wine was a gift from the sun god Ra to his sons, the pharaohs to confer onto them a share of divine power. A two-handled amphora excavated in 1922 by Howard Carter at the tomb of Tutankhamun carried a hieratic inscription indicating its content as high quality, five year old shedeh and providing also the names of the vineyard it came from and its chief vintner. The amphora contained a black residue that was only recently analysed by liquid chromatography combined with mass spectrometry in tandem mode (Guasch-Jané et al. 2006). The results showed unequivocally the presence of chemical markers of wine such as tartaric acid and, in particular, syringic (trihydroxybenzoic) acid derived from the anthocyanine compound oenin (malvidin-3-glucoside) found in the skin of purple grapes.

An ancient Egyptian recipe As already mentioned above, very little is known about the preparation of food in ancient Egypt. As much as the inscriptions in temples and tombs expound the heroic deeds of pharaohs, high ranking officials and priests they are silent about culinary details. A noticeable exception is a terracotta ostrakon from Narmouthis (Medinet Medi) written in demotic script, possibly describing a recipe of a sweet called elgi (Fig. 8.20, right; Gallo 1997). A tentative translation of the recipe of this sweet based on beer and flour reads as follows: ‘This is the way [to] make the sweet elgi: mix these [...] (in) the evening, ass[uming (?)...]… Take [for you] [...]. dough [for] bread kaka (?) mix (with ?) beer; a pot (keb) [of beer] for one oipe of sieved flour and put it (?) in the middle (?) of the dough’.

Comment (Gallo 1997): This is a recipe for a food named elgi; it seems to be a sweet, made by mixing beer and sieved flour. The text specifies that the same dough is used to make the bread-kaka. This is, admittedly, not much to go on. Possibly this recipe is not really typical for the Egyptian cuisine but may be only advice to a novice priest how to sustain himself with meagre food when on mission in remote outposts (Quack 2011).

Chapter 9

Aegean Neolithic, Bronze and Iron Age pottery Synopsis In Thessaly, the archaeological sites of Sesklo and Dimini are early highlights of the European Neolithic ceramic development. Pottery production there was already standardised in terms of selection of raw materials (low and high calcareous clays), firing technology with firing temperatures up to 1000 °C, as well as abundance and diversity of decorating pigments and their application technique. Middle Bronze Age (about 2000 BCE) Cretan potters were the first to use a fast-spinning potter’s wheel. The composition of Cretan pottery changed with time in response to technological development. Densely fired Kamares ware was the wonder of the Middle Minoan (2000–1600 BCE) world. Greek Iron Age pottery from both Attica and Corinth was turned on a fast-spinning wheel, painted and fired around 950 °C, but was generally not glazed. Attic ware can be distinguished from Corinthian pottery by its much lower CaO-content. Attic black- (best qualities in the mid-6th century BCE) and red-figure (invented around 530 BCE) wares were highly successful and widely traded throughout the ancient world. The well-sintered, highly glossy black slip coating and the red, porous body of these wares attest to close control of materials as well as firing atmospheres and temperatures. The red-figure technique is essentially the reverse of the older black-figure technique with its three-phase firing. During the oxidising first heating phase, body and slip attained a reddish colour that changed to black during the following reducing second firing phase at maximum temperatures. Controlled ingress of air into the kiln during the third (cooling) phase caused re-oxidation of the porous body from black to red, but did not affect the well-sintered, non-porous surface layer that was protected from reaction with oxygen at lower temperatures.

9.1 Setting the stage The Aegean Sea provided a fertile ground for the development of thriving cultural centres including the main islands of Greece, Cyprus and Crete (Fig. 9.1). In the Bronze Age, the Aegean spawned major cultural sites, raw materials resources, and the import of luxury items from Egypt and the Near East. The Minoan and Mycenaean civilisations had easy access to lead and silver but needed to import copper, tin and gold from extraneous sources in exchange for particularly fine pottery. On the other hand, the Iron Age Greece traded their cherished Attic pottery for grain to feed an ever growing population. In this chapter we will attempt to trace the ceramic traditions of the Aegean realm from the Neolithic Thessalian pottery of the 6th to the 3rd millennia BCE through the Minoan period from the 3rd millennium BCE onward to the height of Mycenaean power in the 13th century BCE, and to

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Troy Sesklo Volos Dimini

Corinth Mycenae Argos

Athens Tyrins

Hersonissos Knossos Phaistos

Pyrgos Viannos

N 100 km

Figure 9.1. Map of Greece and Crete with major archaeological sites indicated.

the Iron Age Greece between 800 BCE and the classic period of the 5th century BCE, including ceramic developments in Corinth and Attica. Historically, Iron Age Greek pottery made between 1000 and 400 BCE comes in four main groups: Geometric, Orientalising, Blackfigure, and Red-figure. The Geometric style is characterised by designs executed with ruler and compass, placed strategically on the surface of a pot and organised in distinct zones and panels, often with stepped meander motifs. Trade contacts established with Egypt, Phoenicia, and Asia at the end of the 8th century BCE gave rise to the Orientalising style, with stylised ornaments of human, animal and plant forms. This was followed by two inventions attesting to the outstanding artistic and technological qualities of Corinthian and Attic pottery, the black-figure technique invented around 700 BCE in Corinth and perfected in mid-6th century BCE in Attica, and the red-figure technique discovered around 530 BCE by Athenian potters. Although black-figure style pottery continued into the 5th century BCE its form canon, artistic design and technical quality declined. This was also the destiny of the red-figures style pottery in the 4th century BCE that diminished in quality and quantity, and was eventually supplanted by more popular wares adorned with decoration in relief (Boger 1971).

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9.2 Neolithic to Bronze Age Thessalian pottery 9.2.1 Chronology and stylistic character The ceramic competence achieved by Bronze and Iron Age Aegean potters cannot be thought without the achievements of their Neolithic predecessors. Hence a rather extended section on Neolithic Thessalian pottery will be presented here. The archaeological sites of Sesklo and Dimini are two early highlights of the European Neolithic ceramic development. They are considered a crucial fulcrum of the expansion of the Neolithic ceramic tradition from Anatolia to the remainder of Europe (see Chapter 8). Hence the work of potters in the settlements of the pre-Sesklo culture can be seen, at least partly, as the origin of the Mediterranean Neolithic Cardium pottery. Indeed, with few exceptions, all other European Neolithic ceramics are thought to have originated in Thessaly. From its first appearance during the Early Neolithic (EN) period (6000–5500 BCE?), Thessalian pottery was of surprisingly high quality. While the early wares in the form of bowls were still monochrome (EN I), subsequently ceramic vessels with painted (EN II, ‘Protosesklo’) and incised (EN III,’Presesklo’) decorations appeared. The EN II ware includes the so-called ‘porcelain ware’, the EN III ware includes pottery imprinted with the shell of Cardium edulis, a marine mollusk (Cardium ware). The Middle Neolithic period (5500?–4500/4300 BCE) or ‘Sesklo Culture’, is characterised by painted pottery. This pottery chiefly took the form of red iron-bearing paint on its light surface produced by firing iron-poor white illitic clays (Fig. 9.2, left). Frequent decorative

Figure 9.2. Left: Monochrome red-slipped collar-necked jar, incised with linear zigzag lines. Middle Neolithic Sesklo culture (5500–4300 BCE). © Volos Archaeological Museum. Right: Ceramic ‘scoop’ (?). Final Neolithic Sesklo culture (4500–3300 BCE). Inv. No. 1880. Partly restored. Height 17 cm, length 19 cm. National Archaeological Museum, Athens. © Hellenic Ministry of Culture and Tourism /Archaeological Receipts Fund.

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motifs are variations of zigzag lines (Solid style, Sesklo I), flame pattern (Sesklo II) and linear designs (Linear style, Sesklo III). Sesklo III includes also pots with scraped decoration found at Lianokladhi in Phthiotis. The function of the ceramic object shown in Fig. 9.2, right is highly enigmatic. According to Christos Tsountas, excavator of Sesklo over a hundred years ago, such rare objects were used as a kind of ‘scoop’ to transfer grain during a possibly ritualistic procedure (Kotsakis 2012). It is fashioned from three separate parts, burnished and decorated with cut-out geometric designs hollowed out on the upper surface and the handle. It may imitate a wooden prototype even though its original purpose remains unknown (Christopoulou 2012). Archaeologists have divided the Thessalian Late Neolithic (4400–3500 BCE) or ‘Dimini’ culture into four phases according to their pottery styles: Dimini I (Tsangli-Larissa), Dimini II (Arapi), Dimini III (Otzaki), and Dimini IV (Classical Dimini) periods (see Fig. 9.6; Schneider et al. 1990). The Earlier Late Neolithic (c. 4300–3800 BCE) includes the Tsangli-Larissa (Dimini I) phase. Its pottery is either dark surfaced, plain and incised, or light surfaced with dark-on-light pattern-painted decoration executed in a matt paint. Grey-on-grey decoration is also found on presumably high fired pottery (see below). Dimini II (Arapi) shows mostly black pottery, painted in chocolate on cream or polychrome hues. A cemetery of cremation burials of the Tsangli-Larissa phase at Plateia Magoula Zarkou on the northern Thessalian plain provides evidence for some sort of social differentiation, probably gender-based, in the form of a mutually exclusive distribution of collar-necked jar and concave-sided bowl shapes among the tombs (The Prehistoric Archaeology of the Aegean 1997). The Later Late Neolithic (c. 3800–3300 BCE) is divided into the Otzaki (Dimini III) and the Classic Dimini (Dimini IV) phases. The pottery shows a marked preference for spiraliform

Figure 9.3. Two-handled spherical vase from Dimini. Late Neolithic era (5300–4800 BCE). Courtesy National Museum Athens, Greece. Inv. no. 5922. © Hellenic Ministry of Culture and Tourism/Archaeological Receipts Fund.

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and meandroid patterns and belongs to the Classic Dimini phase, typical of east Thessaly only. Pottery shapes include bowls, strainers and ladles found in kitchen areas as well as storage jars. Common design elements include complex repeated geometric patterns, with use of the curve in combination with squares, lines and polygons as evident in the decoration of the famous spherical vase69 shown in Fig. 9.3 (Papathanassopoulos 1996, Preziosi et al. 1999). The Thessalian Final Neolithic period (< 3300 BCE) or ‘Rachmani phase’ extends well beyond it so that its end is contemporary with the phase of the southern Greek Early Bronze Age known as Early Helladic II (see Chapter 9.4.1). The pottery of this period is characterised by a thick collared pasty engobe instead of paint coating the vases (‘Crusted’ ware) and by the appearance of horn-like lugs instead of handles on the monochrome ware. The ‘engobe’ can be scraped off relatively easily. This ‘Crusted’ ware has technological parallels in the Final Neolithic of Franchthi Cave in Peloponessus.

9.2.2 Clay types and correlation to pottery styles The earliest excavations of Neolithic sites in Thessaly were conducted by Christos Tsountas (Sesklo 1901/02; Dimini 1903) and published in 1908 (Tsountas 1908). Archaeometric investigations of the Sesklo and Dimini wares started in 1912 with the chemical analysis of the main elemental composition of a small suite of ceramic shards (Wace & Thompson 1912). Investigation of painting materials and techniques as well as the firing technology of Thessalian pottery were done in much detail by Letsch & Noll (1983). Information on clay sources, and chemical and mineralogical analyses of pottery was provided by Schneider et al. (1990). In particular, their work set out to relate social, regional and chronological aspects of the numerous decorative styles found among Thessalian Neolithic pottery with the technology of pottery manufacture and, last but not least, with the types of clay found within the area of interest. Close to the archaeological sites of Sesklo and Dimini three types of clay occur (Table 9.1), and all of them were apparently used at one or the other time to produce pottery in the Neolithic (Schneider et al. 1990). • White illitic clay deposits with very low iron content were found near the Sesklo site, formed as weathering product of mica schist and gneisses, and hence are very poor in calcium and chromium. The composition of this clay matches closely that of non-calcareous locally produced red-and-white painted pottery. • Red tertiary clays with very high iron content exist between the Sesklo and Dimini sites. These clays are characterised by low calcium and high chromium contents. • Alluvial clays found near Dimini are high in chromium but show rather variable calcium contents. On first glance it appears that the various pottery styles could be distinguished from each other by their different calcium and chromium contents (Fig. 9.4, left). Chromium content 69 The original meaning of the term ‚vase‘ is synonymous with that of a useful pot.

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Table 9.1. Composition of a white illitic clay (1) and a red tertiary clay (2) found near the Sesklo site (Schneider et al. 1990). Oxides are given in mass%, trace elements in ppm (see also Fig. 9.4). Type

SiO2

TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O

Cr

Ni

Rb

Y

1

72.6

0.44

17.9

2.20

0.03

1.09

0.54

1.86

2

66.0

0.80

16.6

6.54

0.12

2.18

1.95

3.37

3.31

51

11

179

13

2.32

164

76

96

51

is related to the mineral serpentine typical for ophiolitic rocks that are common throughout the mountains surrounding the Thessalian plains to the East, South and West. Only the northern parts of the plains, covered by Pleistocene terraces, yield clays that are low in chromium but with differing yttrium contents. Hence coarse ware of the Tsangli-Larissa phase (Dimini I) and related clays found in the North could be subdivided by their yttrium content (Fig. 9.4, right): Plateia Magoula Zarkou (low yttrium, < 40 ppm; low chromium, < 250 ppm) and Makrychori (high yttrium, > 40 ppm; low chromium, < 250 ppm). The differences in the yttrium content of the local clays and the pottery excavated is thought to be related to the geochemical difference of the metamorphic rocks nearby (Schneider et al. 1990). Likewise, clays and pottery from the northern site of Soufli are low in yttrium but rich in chromium (> 250 ppm), a fact that attests to the alluvial nature of the clay, being derived from the mountains to the East and West. Painted Classic Dimini IV ware forms a narrow compositional group rich in CaO (> 6%) and chromium (> 250 ppm), indicating a high degree of standardisation. Indeed, according to

Makrychori 2

60 25

5

Northern Plains 1

Sesklo

40

Platia Margoula Zarkou

2

10

Y (ppm)

CaO (mass%)

15

Plains and hills

50 20

30

20

3

Plains and hills

Dimini

Soufli

4

10

0 0

200

400

600

Cr (ppm)

0

200

400

600

800

Cr (ppm)

Figure 9.4. Bivariate CaO-Cr and Y-Cr plots of clays and associated pottery. Left: Calcium and chromium contents of clays from areas surrounding the Sesklo and Dimini sites. The circled numbers refer to alluvial clays still used today by local potters: 1 Tyrnavos, 2 Agia, 3 Volos, 4 Phanari. Right: Yttrium and chromium contents of Dimini I (Tsangli-Larissa period) coarse ware and of local clays. The chain lines mark the compositional spread of the analyses. Modified after Schneider et al. (1990).

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Figure 9.5. Calcareous pottery shard of a vessel from the Middle Neolithic Sesklo culture (5500– 4300 BCE). The low firing temperature (c.700 °C) prevented sintering of the clay matrix that retains its fabric imposed by the manufacturing process (Noll 1984).

Schneider et al. (1990) this ware is one of the earliest examples of considerable pottery specialisation in the European Neolithic. In contrast to this, the Incised Classic Dimini ware spreads out widely in a CaO-Cr plot, indicating that pottery of this particular style has been produced at many sites outside Dimini proper, and was widely exchanged and distributed. A tentative explanation was offered by the authors: the ancient potters realised that a lightcoloured shard was more important as a background for a visually contrasting red-painted decoration than for incised patterns. Thus there was no need for standardising the calcium content of the clay destined to produce incised pottery. From chemical analyses of the pottery and relating their results by multivariate clustering in a dendrogram a much more complex picture emerged than generally assumed previously. Neolithic pottery production in Thessaly was already surprisingly standardised in terms of selection of raw materials, firing technology that may have achieved temperatures as high as 1000 °C, at least for Dimini I grey-on-grey pottery, as well as abundance and diversity of decorating pigments and their application techniques. The transition from rather crude, low fired Middle Neolithic Sesklo III pottery (Fig. 9.5) to Late Neolithic Tsangli-Larissa (Dimini I) fine decorated ware was accompanied by impressive and intensive technological and stylistic innovation (Schneider et al. 1991). The different pottery types, distinguished by their Ca/Si ratios, are collected in Fig. 9.6. During the Late Neolithic to Early Helladic periods Thessalian potters learned how to use calcareous clays to brighten the appearance of their pots. Calcareous clays provide both artistic and technological benefits. On these light surfaces black and red pigments could create strikingly beautiful contrasting colourations. On the other hand, the combination of calcareous ceramic bodies and illitic slips was very fortuitous as the engobe surface layers adhered well to the body owing to the small gradient in the coefficients of thermal expansion, and lower firing temperatures were required to obtain dense ceramics owing to the fluxing properties of CaO.

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Fig 9.6. Ca/Si ratios of the ceramic body of different Thessalian pottery wares (Noll 1984). .

9.3 Cretan pottery While Neolithic Aegean pottery was produced using free-forming techniques such as coiling, slab building and paddling (see Chapter 4.3) only in about 2000 BCE, early in the Middle Bronze Age (Late Early Minoan, EM II/III), did Cretan potters regularly use a fast-spinning potter’s wheel, at this time a radical advance in ceramic technology that enabled the artistic expression of clays in a much more sophisticated manner (Williams 1997). The use of the potter’s wheel is thought to have spread from Crete to mainland Greece around 1800 BCE (see Chapter 4.3.2). Such advances also pertain to the selection of raw materials. The examples of Early to Late Minoan pottery show clearly the desire of the ancient potters to boost the quality of their products by beneficiation of clay raw materials even though the rather primitive kiln technology (see Fig. 7.11) frequently counteracted their endeavours. The main materials development paths may in detail be studied by reading the account on the mineralogy and technique of the Cretan pottery worked out in much detail by Noll (1982). This author used in his chronological allocations the somewhat outdated chronology of Evans, the excavator of the palace of Knossos. One of several comparative chronologies is shown in Table 9.2 (Marangou 1992). The chemical composition of some Minoan pottery can be plotted on a simplified ternary diagram as shown in Fig. 9.7. The historical development of this particular kind of pottery is being discussed rather exhaustively to document the power of phase diagrams to characterise pottery and to derive from this knowledge insight into technology development trends.

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Table 9.2. Comparative chronology of the Minoan civilisation. Periods (after Platon)

Pre-palatial (3650/3500–20th century BCE)

Old Palace (19thcentury–1600 BCE)

New Palace (1600–1390 BCE)

Post-palatial (1390–1070 BCE)

Periods (after Evans)

Chronology, BCE (after Warren & Hankey 1989)

Final Neolithic

pre 3650/3500

Early Minoan I

3650/3500–3000/2900

Early Minoan II

3000/2900–2300/2150

Early Minoan III

2300/2150–2160/2025

Middle Minoan IA

2160/1970–20th century

Middle Minoan IB

19th century

Middle Minoan II

19th century–1700/1650

Middle Minoan IIIA

1700/1650–1640/1630

Middle Minoan IIIB

1640/1630–1600

Late Minoan IA

1600/1580–1480 (Thera event ?)

Late Minoan IB

1480–1425

Late Minoan II

1425–1390

Late Minoan IIIA1

1390–1370/1360

Late Minoan IIIA2

1370/1360–1340/1330

Late Minoan IIIB

1340/1330–1190

Late Minoan IIIC

1190–1070

Sub-Minoan

1070–after 1015

The composition of Cretan pottery changed with time in response to technological development (Noll 1982). Figure 9.7 depicts the “ceramic triangle” (CaO+MgO)A12O3-SiO2 with the binary compounds diopside (CaMgSi2O6, labelled Di), wollastonite (CaSiO3, Wo), and mullite (3A12O3·2SiO2, Mu), as well as the ternary com-

Figure 9.7. Position of the composition of Neolithic and Minoan ceramics in the ternary phase diagram (CaO+MgO)-Al2O3-SiO2 (Heimann 1989; data after Noll 1982).

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pounds anorthite (CaA12Si2O8, An) and gehlenite (Ca2A12SiO7, Ge). The open circles refer to Neolithic pottery made from low-calcareous clays found at Phaistos in southern Crete, the closed circles to pottery from very calcareous clays found at Knossos in northern Crete. The Phaistos samples are situated in a compositional triangle formed by the conodes (see Chapter 5) quartz-anorthite, anorthite-mullite, and mullite-quartz. Thus, the equilibrium composition should consist of the mineral assembly quartz + mullite + anorthite (glass) similar to stoneware or porcelain. However, low firing temperatures, presumably not higher than 600 °C, prevented the attainment of an equilibrium composition. In addition, since small amounts of residual primary dolomite occur in the shards and dolomite decomposes at 550 °C in oxidising ceramic firing, its preservation also points to low firing temperatures. The Neolithic Knossos shards are highly calcareous, and occupy the compositional triangle anorthite-gehlenite-diopside (wollastonite). Again, the very low firing temperature used by the ancient potters prevented the formation of equilibrium high-temperature phases such as gehlenite and diopside. Large amounts of pristine calcite grains are still present in the shards (Noll 1982). Here the solidification has been attained solely by shrinkage due to dehydration of the clay matrix and the concomitant increase of packing density of the clay particles with only marginal mediating action of true sintering processes (see also Fig. 9.5). The characteristics of ceramics changed considerably during the Early Minoan period. Although Pyrgos style ware of Late EM I (c. 2600 BCE) still had many features like those of the Neolithic Phaistos pottery whose compositional field it partly shares (open ellipses), sintering of the clay matrix attests to an improvement of the technology through higher firing temperatures. The Vasiliki ware of EM II (c. 2000 BCE) became more limerich with a comparatively narrow distribution of the A12O3 content. The presence of small amounts of the equilibrium phase diopside in the shards points to a firing temperature in excess of 850 °C. Early Minoan potters may have realised that it was more advantageous to use calcareous clays rather than non-calcareous ones as the former resulted in a denser fabric at moderate firing temperatures. On the other hand, highly calcareous clays, like those from which the Neolithic Knossos pottery was made, yielded pottery prone to lime spalling because of the presence of residual free CaO from the thermal decomposition of calcite. Only if firing temperatures exceed 850 to 900 °C and was maintained for sufficient time can the free CaO combine with alumina and silica released by the thermal decomposition of clay minerals to form high temperature minerals such as diopside, gehlenite, and anorthite. In eastern Crete a lively ceramic technology evolved at Gournia. As opposed to earlier darkon-light pottery, its typical white-on-dark ware (Betancourt 1982) can be traced from the end of EM II, roughly contemporary with Vasiliki ware, to the MM IA-B period, just prior to the brilliant palacial civilisation that began in MM IB-II (Table 9.2). Thus the artistic style of the white-on-dark Gournia ware with its characteristic curvilinear patterns appears to have been the immediate predecessor of the elaborate floral and geometric decoration of polychrome Kamares ware (Fig. 9.8). However, in contrast to the Mg-rich white decoration applied to Kamares ware (see below) the white pigment of white-on-dark ware from Gournia and Palaiokastro is rich in Al, Ca, Si and Fe, suggesting the use of mixtures of calcium silicates and aluminosilicates (‘asprochoma’, kaolinite?), possibly with some quartz added (Swann et al. 2000).

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The Middle Minoan (2160/1970–1600 BCE) high quality Kamares ware (mottled grey ellipse in Fig. 9.7) is squarely located in the compositional triangle quartz-anorthite-diopside. The shards generally contain less alumina than Early Minoan ware. Diopside is abundantly present, calcite is usually absent, and gehlenite is only rarely visible in x-ray diffractograms. All these features point to a firing temperature in the range of 900 to 950°C. Later Kamares ware may have been fired at still higher temperatures around 1050°C because in the white parts of the decoration protoenstatite (MgSiO3) has been formed from talc (Mg3[(OH)2Si4O10]) painted onto the unfired leather-hard body (Noll 1982). Hence the occurrence of protoenstatite constitutes an important temperature marker for this type of pottery. In the Late Minoan period (1600/1580–1070 BCE), the processing and firing technologies of the pottery have further improved. The variation ellipse (light grey with crosses in Fig. 9.7) has considerably narrowed pointing to a very effective compositional control of the alumina/silica ratio, presumably by standardising the ratio of plastic clay to non-plastic inclusions. This suggests that the Late Minoan potters had learned to blend clays from different sources with different lime contents and workabilities to achieve a consistent product. This conclusion has been bolstered by the study of the distribution of foraminifera in archaeological Minoan clays (Quinn & Day 2007). The samples lower in lime and richer in silica shown in the variation ellipse come from Mochlos, Gournia, and Pseira on the northern coast of Crete, whereas the samples richer in lime and lower in silica were found at Knossos, Haghia Triada (Belfiore et al. 2007), and Phaistos. The firing temperatures can be estimated to have been around 1050°C (Heimann & Franklin 1979) judged by the high proportion of closed pores, and the highly sintered and slightly vitrified shard matrix. During the Neolithic the available clays, calcareous in the north of Crete (Knossos), lowcalcareous in the south (Phaistos), were indiscriminately used with little or no attention to any conceivable modification. However, the Pyrgos ware of the Early Minoan (EM I; 3650/3500–3000/2900 BCE) with a uniformly black surface and occasionally decorated with incisions demonstrated that utilisation of calcareous clays provided the technological advantage of requiring lower firing temperatures to arrive at a dense ceramic body. This was also true for the likewise monochromic Vasiliki ware of EM II (3000/2900–2160/2025 BCE). Much more detailed analyses of the geochemistry of clay deposits in Crete have been carried out by Jones (1986) and Hein et al. (2004) that put the more cursory division suggested by Noll (1982) into perspective. The earlier work by Jones (1986) proposed the existence of three broad compositional zones of West, Central and East Crete whereby the clays of the Central zone were described as calcareous with high contents of Cr, Ni and Mg clearly related to ophiolitic outcrops ubiquitously scattered throughout the marly clays of Neogene sediments. This feature provides a useful distinguishing criterion among clays and pottery produced from these clays in the Central Cretan areas, and the Western and Eastern regions. However, as pointed out by Hein et al. (2004) the situation is even more complex. Apart from the Ca concentration related to marine nannofossils (Quinn & Day 2007) it is more difficult to separate the Middle Miocene clay deposits of the north-central Heraklion Basin (Hersonissos, Agios Syllas, Vathypetro) from those of the south-central Mesara Basin (Avli, Skinias, Viannos, Pitsidia). In addition, Eastern Cretan Upper Miocene clays (Vasiliki, Skopi)

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Table 9.3. Calcium, magnesium and iron contents (mass %) of selected clays from northern (Heraklion, Hersonissos, Vathypetro) and southern (Pitsidia, Zaros, Viannos) Crete (Hein et al. 2004). Oxide

Heraklion

Hersonissos

Vathypetro

Pitsidia

Zaros

Viannos 4.9

CaO

32.0

18.5

11.9

2.2

1.0

MgO

2.7

5.4

6.3

4.5

5.0

7.6

Fe2O3

2.8

6.2

5.3

8.5

8.2

8.2

show higher concentrations of REE, low Cr/Th and Co/Th ratios, and higher Th/Sc ratios whereas the geologically younger Central Cretan deposits are characterised by low Th/U ratios. Hence the claim by Noll (1982) that in their totality northern Crete clays are calcareous and southern Crete clays are non-calcareous cannot be upheld in its apodictic exclusiveness. As shown in Table 9.3 this statement may pertain only to highly calcareous Minoan ceramics of Knossos produced from clay deposits of Heraklion, Hersonissos or Vathypetro and lowcalcareous ceramics from Phaistos likely produced from clay deposits of Pitsidia or Zaros. A somewhat intermediate lime content is found in ceramics from Pyrgos that likely reflects the geochemistry of the clay deposits from Viannos. During the Middle Minoan (MM IB, Old Palace Period; 19th century BCE) and partially spilling over to the New Palace Period the thin-walled (‘eggshell’ ware with wall thickness down to < 1mm), densely fired and otherwise well-processed Kamares ware was the wonder of the ancient world (Fig. 9.8). The characteristics of this ware were the highly vitrified black engobe (Floyd 1995, Swann et al. 1995) applied to the ceramic body coloured by hercynite (FeAl2O4) and/or magnetite (Fe3O4) (Fig. 9.9), and the figurative and/or ornamental paint in red (hematite, ˞-Fe2O3) or white (talc, Mg3[(OH)2/Si4O10] that converted on firing to protoenstatite, MgSiO3) (Fig. 9.8, right). The yellow pigment visible in Fig. 9.8, left may be traced to ‘lime silicate white’, consisting of diopside (CaMgSi2O6) coloured yellow by incorporation

Figure 9.8. Left: Kamares-style cup of MM IB (1950–1850 BCE) with white and red decoration. Crete. Height 5.5 cm, width 10.2 cm. Reg. No. 1906,1112.73. © Trustees of the British Museum. Right: Kamares-style tumbler with red and white stripes. Middle Minoan (MM IB; 19th century BCE). Knossos. Crete. Height 4.4. cm, width 7.7 cm. Reg. no. GR 1906.11-12.74. © Trustees of the British Museum.

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Figure 9.9. SEM image of black vitrified engobe (left) on a shard of Middle Minoan Kamares eggshell ware (Noll 1982).

of Fe3+ ions (Nöller & Knoll 1985). Kamares pottery is characterised by a black or dark brown body with white, red, and yellow painted-on floral and marine motifs. The shards generally contain less alumina than Early Minoan ware. Pottery of the late Middle Minoan (MM IIIB; 1640/1630–1600 BCE; post-Kamares ware) surprises with three-dimensional barbotine-like applications whereas ceramic wares of the Late Minoan show polychromic black, brown and red paint on the plain ceramic body, forming the naturalistic Floral and Marine styles of LM I (1600/1580–1480 BCE) and the Palatial style with strongly stylised geometric elements of LM II (1425–1390 BCE), presumably influenced by the short-lived Mycenaean control between about 1450 and 1400 BCE. During this time the Minoan tradition was amalgamated with the Greek way to see the world. Indeed, the Greeks appear to have adapted the peaceful Minoan culture to their more warlike spirit (Boger 1971). Whereas the Floral and Marine styles remained to be popular decorations the motifs became increasingly stylised, with the sacred axe and helmets being painted on jars and vases. Owing to their high quality, Kamares ware has traditionally been interpreted as a prestige artefact, possibly utilised as an elite table ware in the palaces of the Minoan kings. It is remarkable that the quality of craftsmanship continued to flourish even after the palaces were destroyed around 1700 BCE and again around 1450 BCE by earthquakes and most, strikingly in the wake of the Thera eruption thought to have occurred around 1630 BCE70. Hence the general picture of the ceramic production in Minoan Crete corresponds to that of Egypt and Mesopotamia with respect to its uniformity and continuity, and thus contrasts sharply with the unsteady development of ceramic technology in the Neolithic-Chalcolithic Thessaly of Greece (see Chapter 9.2; Letsch 1981, Schneider et al. 1990). However, contrary to the uniform wares of Mesopotamia and Egypt caused by the compositional uniformity of the clay sources the compositional homogeneity of Late Minoan ware was achieved by

70 Archaeologists usually had set the date of the Thera eruption to around 1500 BCE (Sivertsen 2009) whereas recent radiocarbon dating fixed the date between 1627 and 1600 BCE (Friedrich et al. 2006).

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conscious selection and mixing of different clays and careful levigation thus guaranteeing a narrow distribution of the alumina and silica contents (see Fig. 9.7).

9.4 Bronze Age (Helladic) pottery The term ‘Helladic’ used by modern archaeologists refers to the culture of mainland Greece during the Bronze Age and hence complements the parallel sequences of the ‘Cycladic’ culture of the Aegean Bronze Age and the ‘Minoan’ period (Table 9.2) with reference to the Cretan civilisation. As recently stressed by Mommsen (2012), at present the different chronologies cannot always be satisfactorily correlated since absolute dates for the Aegean Bronze Ages are not yet very reliable. In addition, different sets of dates are frequently used for identical phases or periods. Hence it is preferable to describe an archaeological assemblage in terms of a relative chronological label such as Early Helladic II, Late Minoan IA, etc., rather than in calendar years BCE that may change with progressing research. A particularly important time marker is the Thera vulcano eruption, the absolute dating of which is still being a topic of debate. Traditionally its date has been set to around 1500 BCE (Sivertsen 2009) but recent radiocarbon dating of the remains of a charred olive tree found in the Plinian destruction level on Santorini (Thera) yielded a calibrated date between 1627 and 1600 BCE with 95% probability (Friedrich et al. 2006). If this date is accepted, the end of LH IB that has been related to the eruption must be moved back by approximately 100 years

Table 9.4. Tentative chronology of the Helladic culture (Manning 2001). Archaeological period

Chronological label

Approx. Date (BCE)

Early Helladic I

EH I

2800–2500

Early Helladic II

EH IIA

2500–2400

EH IIB (Lefkandi I)

2400–2200 2200–2100

Early Helladic III

EH III (Tiryns)

Middle Helladic

MH

2100–1550

Late Helladic I

LH I

1550–1500

Late Helladic II

LH IIA

1500–1450

LH IIB

1450–1400

LH III A1

1400–1350

LH IIIA2

1350–1300

LH IIIB1

1300–1230

LH IIIB2

1230–1190

Early LH IIIC

1190–1130

Middle LH IIIC

1130–1090

Late LH IIIC

1090–1060

Late Helladic III

Sub-Mycenean

1060–1000

Protogeometric

1000

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to 1650/1600 BCE and all other dates revised accordingly for a ‘high’ chronology71. A case in point is the dating of the famous Lerna shaft graves excavated in the 1950th (Lindblom & Manning 2011).

9.4.1 Early and Middle Helladic pottery Early Helladic (2800–2100 BCE; see Table 9.4) Bronze Age pottery in Greece shows Western Anatolian influences. However, a distinct local Greek pottery emerged during the Middle Helladic (2100–1550 BCE) period known as Minyan ware72, a form of monochrome burnished pottery produced from extremely fine or moderately fine clay. Specifically, Grey Minyan ware mostly found in central Greece shows angular forms possibly reflecting the

Figure 9.10. Middle Helladic Grey Minyan goblet from Mycenae, Greece. (2100–1550 BCE). Height 18.6 cm, diameter 25.2 cm. Reg. no. 1912,0626.35. Much restored. © The Trustees of the British Museum.

71 It is conceivable that the Thera eruption itself did not trigger the devastating tsunami that is thought to have led eventually to the demise of the Minoan civilisation. Instead, it could have been the subsequent collapse of the emptied magma chamber that was responsible for the tsunami. If that had occurred a hundred year after the eruption the ‘archaeological’ date of 1500 BCE could be reconciled with the radiometric data of around 1630/1600 BCE. 72 The term Minyan ware was given by Heinrich Schliemann to a type of fine, smooth, grey or yellowcoloured pottery, made on the wheel, found widely from about 2000 BCE in Thessaly and Macedonia to the Argolid and considerably at Troy (Darvill 2010). Schliemann erroneously connected this type of pottery to the fabulously rich King Minyas whose exploits are chronicled by epic and lyric poets in classical times.

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desire to copy metallic prototypes (see Fig. 9.10). There is the notion that the angular forms of this particular pottery style may be derived from the common use of the fast potter’s wheel invented during the preceding Early Helladic period. It is thus the first wheel-thrown pottery in Middle Bronze Age Greece and a successor of the Tiryns culture ware of Early Helladic III (2200–2100 BCE). Minyan ware of the Middle Helladic I (2100–1900 BCE) period is decorated with grooves on the shoulder part of kantharoi and bowls whereas later during the Middle Helladic II (1900–1700 BCE) period stamped concentric circles and festoons (parallel semicircles) became a common characteristic of decoration especially on Black (or Argive) Minyan Ware. Reduced fired Black Minyan Ware is common in northern Peloponnese while oxidising fired Red Minyan Ware is found in Aegina, Attica, the northern Cyclades and Boeotia. High calcareous, oxidising fired Yellow or Tan Minyan (‘matt-painted’) Ware first appeared during the Middle Helladic II and continues into the Middle Helladic III (1700–1550 BCE) periods. The type of pottery found in Troy and the Troas (Schubert 2010) is commonly differentiated into early Anatolian Grey Ware (AGW I) and classic Anatolian Grey Ware (AGW II). AGW I typically found in Troy VIa levels contains large amounts of mica and is similar in its form canon to Middle Helladic III ware of mainland Greece, AGW II comprises Tan Ware found at Troy VIc and d horizons. Later Tan Ware (Troy VIIa) was coated with a brown slip.

9.4.2 Late Helladic (Mycenaean) pottery Mycenaean culture was initially under the artistic domination of the Minoan civilisation and consequently the general aspect of Mycenaean art and technology may be regarded as an extension of Minoan. From about 1400 to 1200 BCE the Mycenaean Greeks ruled the Aegean world, following in the footsteps of the Minoan empire they took over after about

Figure 9.11. Mycenaean stemmed goblet with stylised cuttlefish painted in a linear Marine and Floral style. Late Helladic III B or A2 (1230–1190 BCE). Kalymnos, Dodecanese, Greece. Height 18.2 cm, rim diameter 16.9cm, base diameter 9 cm. Reg. no. 1886,0415.15. © The Trustees of the British Museum.

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1450 BCE. By and large the Mycenaean’s adopted the Minoan system but created a complex economy based on and controlled by records written in Linear B script (Chadwick 1990). The style of painting of the early Mycenaean vases is very similar to that of Minoan Floral and Marine styles of LM I (see 9.2). This similarity of styles is obvious when looking at the kylix (Fig. 9.11) dated around 1200 BCE even though Mycenaean restraint has reduced the Minoan marine and floral motifs to a rather lifeless, nevertheless striking linear arrangement (Boger 1971). The high-stemmed form of this type of kylix has evolved from the so-called Ephyrean goblet, a name derived from the old name of Corinth, Ephyre. Ephyrean goblets appeared first in Minoan Crete in LM II (1425–1390 BCE) and are characterised by a single motif placed in the centre of the goblet shown in Fig. 9.11 (Betancourt 1985). During the Late Helladic IIIA and B periods (1400–1200 BCE), Mycenaeans were particularly active in maritime trade. Products such as olive oil, wine, and perfumes, stored in high-quality decorated containers, were exported to the major ports of the Eastern Mediterranean and southern Italy in exchange for metals, ivory, and finished luxuries. The style of decoration of ceramic vessels was further simplified, and the more naturalistic motifs of the Late Helladic II pottery were linearized and evolved into a stylistic uniformity, the so-called Mycenaean koine. The decorations do not cover the entire surface of the vessels but are organised in circumferencial bands and friezes (‘Open style’) (Fig. 9.12) The vessel shown in Fig. 9.12, right is a fine example of Mycenaean Pictorial (open) style pottery. Such vases, painted with scenes of humans and animals, were popular exports from Mycenaean Greece to the island of Cyprus. The upper zone of the vase is painted with a frieze of chariots, pulled by elongated horses, in which ride a charioteer and a passenger.

Figure 9.12. Left: Stemmed bowl (krater) decorated with running spirals (‘running dog’ motif) and bivalve shells. Late Helladic IIIA2 (Mycenaean, 1350–1300 BCE). Enkomi, Cyprus. Height 25,5 cm, diameter 31 cm. Reg. no. 1897,0401.1151.© The Trustees of the British Museum. Right: Mycenaean amphoriod krater, decorated with four chariots and riders in a procession. Late Helladic IIIA2 (1350–1300 BCE). Maroni, Larnaka, Cyprus. Height 42 cm, diameter 32 cm. Reg. no. 1911,0428.1.© The Trustees of the British Museum.

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Such chariot processions on vases may well have been inspired by contemporary frescopaintings which decorated the walls of Mycenaean palaces in Knossos or Phaistos. Later stylistic developments in Late Helladic IIIC (1200–1100 BCE) saw the disruption of artistic uniformity and evolution of a number of local workshops with their own traits that created a certain degree of polymorphism. So-called Granary style pottery shows simple linear decoration. Argivean workshops produce the so-called Closed style, characterised by numerous tiny abstract motifs such as concentric semicircles and triangles that cover the whole surface of a vessel. Whereas the Pictorial style continued to evolve in the Argolis, the Octopus style, with representations of octopoi, fish, and birds was particularly popular in the Dodecanese, the Cyclades and Crete. Towards the end of the Late Helladic period, the transitional sub-Mycenaean pottery of the 11th century BCE comprised mainly vessels with simple linear decoration, the precursors of Greek Protogeometric and Geometric styles (Table 9.4). Analyses of Late Helladic pottery revealed that ceramics from the Argolid (named Mycenae/ Berbati, MB) and Achaia (ACH-a) are very similar in chemical composition and also similar to pottery found at other Greek sites and even the Troas (Hein et al. 2002). While the archaeological classification of these wares clearly contradicts a common origin, the stratigraphic setting and geological age of clays deposits likely used to produce these pottery types suggest preferred selection of a specific type of clay deposits by the ancient potters (Jones 1984). For example, whilst the clay from Katakolo in Elis (northwest coast of the Peloponnese) appears to match well the chemical composition of both the MB and ACH-a wares (Hein et al. 2002) there exist many other clay deposits of comparable geological age and thus similar or even identical chemical and mineralogical composition that were utilised in Greek antiquity to produce Mycenaean-style pottery (see also Mommsen et al. 1988, Mommsen 2003, Buxeda i Garrigós et al. 2003, Mommsen 2004).

Figure 9.13. Concentration range of cobalt in Argolid fineware of the Later Helladic III (after Mommsen et al. 1988).

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Separation of otherwise very similar ware likely produced within the Argolid realm appears to be possible when looking at small but statistically significant differences in the concentrations of trace elements such as Cs, Rb and Co. For example, Fig. 9.13 shows the concentration distribution of cobalt in Late Helladic III A to C Argolid fineware that allowed distinguishing Tiryns-Asine from neighbouring Mycenae-Berbati wares (Mommsen et al. 1988, see also Mommsen et al. 2011). A clay collected from locations close to the excavation site of Troy VI a-h/VII (1700–1250 BCE), almost certainly the ‘Homeric’ Troy, was subjected to experimental firing under reducing conditions (Boudouard equilibrium) between 745 and 940 °C (Görres et al. 2000) and investigated by XRD using Rietveld refinement, and TEM. Table 9.5 shows the temperature-dependent development of the phase composition of the ceramics between firing temperatures of 745 and 940 °C. More details on this study can be obtained from Chapter 6.4.3. The raw calcareous clay (reference clay D, Table 9.5) contains the sheet silicates illite, chlorite, talc and minor amounts of smectite and kaolinite as well as quartz and calcite. At 745 °C chlorite has already disappeared, consistent with experimental results by Jornet et al. (1985) and Freudiger-Bonzon (2005). At temperatures beyond 745 °C sintering of clay minerals set in concurrent with the thermal decomposition of calcite and formation of an x-ray amorphous phase. Between 745 and 940 °C very small (~ 150 nm) crystals of anorthite (An) and pyroxene (Px, diopside) form, accompanied by minor amounts of olivine (Ol), sanidine (Sa) and åkermanite (Åk) (Fig. 6.12d). These tiny crystalline phases could be detected only by transmission electron microscopy. Comparison of the phase composition with that of pottery from Troy VI suggested a firing temperature of 840 °C for the ancient ware (Görres et al. 2000).

Table 9.5. Phase composition in mass% of reference clay D (Troy, Turkey) and development of the phase composition of experimentally reducing fired samples (Kroll et al. 1997). SEM micrographs are shown in Fig. 6.12. Mineral

Clay D

745 °C

840 °C

875 °C

940 °C

Illite

55.0

17.8

–

–

–

Talc

12.9

–

–

–

–

Calcite

9.6

4.6

–

–

–

Chlorite+

8.3

–

–

–

–

Quartz

7.1

9.0

6.3

6.6

6.4

Magnetite

–

Åkermanite

–

–

–

6.0

5.7

–

Anorthite

21.1

34.8

36.8

43.4

Diopside

–

16.0

12.8

11.4

Enstatite

–

12.6

19.4

19.8

Olivine

–

5.8

7.7

7.9

Sanidine

12.1

9.6

11.0

9.1

31.4

8.8

–

2.0

Amorphous +

2.1 1.8

7.2*

Contains kaolinite *Contains smectite and unidentified accessory minerals

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Figure 9.14. SEM micrographs of Grey Minyan pottery from Troy VI level (Görres 1995) (Courtesy: Dr. M. Görres).

Figure 9.14 shows SEM micrographs of a porous (left) and rather dense (right) pottery from Troy VIa-h horizon (Görres 1995). More information on physical and technological features of Mycenaean pottery and their analyses can be found in the extended literature, for example Catling et al. (1963), Catling & Millett (1965), Millett & Catling (1967), Harbottle (1970), Boardman & Schweizer (1973), Bieber et al. (1976), Jones & Mee (1978) and Maniatis & Tite (1978).

9.5 Iron Age Greek wares The period between 1100 and 800 BCE is considered the Greek ‘dark age’ because the scant archaeological record points to a cultural recession between the Mycenaean era and the Greek renaissance of the 8th century BCE. The material legacies of Mycenaean civilisation included painted pottery. By 900 BCE the Protogeometric style has spread to most parts of the Greek world, with Athens’ potters being the leading producers and possibly inventors of this style. Subsequently the Geometric style pottery provided a chronological framework for cultural changes (Cartledge 1980). Historically, fine pottery produced by Greek potters between 1000 and 400 BCE can be subdivided into four main groups: Geometric, Orientalizing, black-figure, and red-figure wares (Boger 1971). The Geometric style ware was painted in black or brown monochrome from around 1000 to 700 BCE. Towards the end of the 8th century BCE the pottery underwent a profound modification owing to increasing trade contacts with the Near East cultural circle of interest, expressed by the stylised ornamentation of human figures, animals and plants of the Orientalizing ware. Indeed, by 700 BCE the ‘orientalising’ phase of Greek ceramic art had begun that was Greek in spirit but oriental in flavour. In its wake the Geometric tradition of Athens and Argos lost its vitality and gave way to an orientalising blackfigure style executed in rivalling Corinth (see 9.5.1). An introduction to Greek pottery including detailed discussion of its history, styles, shapes, and functions was given by Sparkes (1991).

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Greek Iron Age pottery from both Attica and Corinth is turned on a fast-spinning wheel, painted, unglazed and fired at relatively low temperatures. Attic ware was made from predominately lime-poor clays (Table 9.7) and hence can be accommodated in the cotectic triangle di-an-qz of the ternary diagram CaO-Al2O3-SiO2 (Fig. 5.4) whereas Corinthian pottery was manufactured from very calcareous clays (Table 9.6). As the latter type of pottery is characterised by a very light coloured yellow-greenish body, colour serves to easily distinguish Attic and Corinthian wares (Mirti et al. 2006).

9.5.1 Corinthian pottery Around 700 BCE potters started to play with the colour palette that iron-rich clays provided through judicious choice of the firing atmosphere. This resulted in the advent of black-figure vases69 in Corinth in about 720 BCE that dominated Greek pottery technology for almost two hundred years. The Corinthian potters developed an orientalising style with typical animal friezes and floral ornaments drawn in silhouette with incised details (Fig. 9.15; Cook 1997). The highly calcareous and iron-rich Corinthian clays provided a light-coloured, slightly yellowish-greenish canvas on which the artistically inclined potters could draw their figures with high contrast. This was based on the fact that during oxidising firing the ceramic body did not turn red as in a CaO-poor clay but a pale yellow with the telltale olive-green tinge of Corinthian vases, depending on the firing temperature. For example, the highly calcareous (22 mass% CaO) brown clay (8 mass% Fe2O3) of the classic Corinthian pottery changes its colour from pink (fired at 700 °C) to red (at 900 °C) to pale yellow (at 1080 °C). A gray clay (15 mass% CaO, 7 mass% Fe2O3) from Corfu changes to buff-cream (700 °C), buff (900 °C), and white (1080 °C) (Maniatis et al. 1982). The reason for this dramatic colour change has been found in the fact that at higher temperature Fe3+ ions, normally colouring the body red by finely dispersed haematite (˞ ˞-Fe2O3) crystals, dissolve in highly calcareous clays in the crystal lattice of newly formed diopside (CaMgSi2O6) (Nöller & Knoll 1985). As found by Farnsworth et al. (1977) fine painted Corinthian pottery is surprisingly constant in composition over several hundred years, showing CaO contents of about 20 mass% and Fe2O3 contents of about 7 mass% consistent with the analyses by Maniatis et al. (1982). However, some Pleistocene clay samples (Whitebread 2003) taken immediately west of the Potter’s Quarter in ancient Corinth showed much higher (Table 9.6) CaO and lower Fe2O3 contents (Heilmeyer 1981), suggesting mixing of clays with high and low calcium and iron contents by the ancient potters (Sanders 2006). The complex solid state reactions leading to differently coloured figures and decorating elements were the result of a complicated, and time and energy consuming three-phase firing process fraught with many obstacles and error sources. The body of the ceramic vessel was first painted in its leather-hard state using slurries of fine iron- and K2O-rich clay of illitic composition. After drying, structural details and internal outlines of the figurative decorations were incised into the slip so that the clay body underneath the slip became visible.

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Table 9.6. Chemical and phase compositions in mass% as well as firing properties in relative % of Corinthian clays (Heilmeyer 1981). Oxide

# 31a

# 31b

SiO2

23.0

21.3

Quartz

8.0

Expansion @ 800 °C

+ 4.8%

TiO2

0.23

0.23

Calcite

60.3/64.3

Expansion @ 900 °C

+ 4.5%

Al2O3

5.80

5.10

Dolomite

4.0

Expansion @ 1000 °C

+ 3.3%

Fe2O3

2.02

2.00

Kaolinite

20.2

Shrinkage @ 1100 °C

– 5.4%

Illite (?)

8.0 Loss on ignition (LOI)

31.7

MgO

1.91

2.16

CaO

33.8

36.0

Na2O

0.25

0.10

K2O

1.05

0.96

Mineral content

Properties

Other red and white clay-based pigments were added to accentuate parts of clothing, hair, and in particular, women’s skin. During the first oxidising firing in a kiln at about 800 °C the slip turned reddish-orange as well as the unpainted parts of the vessel. Then in a second firing step the kiln temperature was raised to about 950 °C, green wood added to provide a reducing environment, and the

Figure 9.15. Corinthian pottery. Left: Black-figure oinochoë decorated with fighting warriors between horsemen, and spotted snakes twisting up the handle and around the rim. Attributed to The Tydeus Painter, dated to 575–550 BCE. Height 24.5 cm, width 20 cm. © The Trustees of the British Museum. Right: Black-figure amphora with lid, decorated with two cocks facing each other over a double palmette decor. Height 36 cm, width 24 cm. Around 600 BCE. © Museum of Corinth, Greece.

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top vent of the kiln closed (Fig. 7.12) to prevent ingress of oxygen. This reducing firing caused the vessel to turn to an overall black. In the third firing step, the vents were reopened to allow oxygen to enter the kiln; then the kiln was allowed to cool down to ambient temperature. The unpainted, porous parts of the vessel were quickly oxidised back to either the red colour of CaO-poor clays as in Attic pottery or the yellow-olive green colour of CaOrich clays as in Corinthian pottery. In contrast to this, the vitrified decoration retained its black magnetite or hercynite (depending on the alumina content of the slip; Tang et al. 2001) colour owing to the inhibited oxygen diffusion. The chemical processes accompanying these procedural firing steps were studied in much detail by Theodor Schumann (1942) and later Ulrich Hofmann (1962, 1966), and experimentally confirmed by Winter (1956, 1980) and Hampe & Winter (1962) (see also Chapter 10.1). Figure 9.15, left shows a splendid example of a black-figure oinochoë, a jug used for ladling and pouring wine. It is characterised by a handle extending from the lip rim to the top of the shoulder. Note the trefoil-shaped pouring projection that was designed to prevent spilling of liquid during pouring. On the right of Fig. 9.15 a Corinthian amphora is shown. Such vases were important export articles to pay for grain imports needed to feed a steadily growing population. These black-figure vessels demonstrate very clearly the mastery of the ancient potters to produce glossy black and red colourations on the same vessel during an oxidising-reducing-oxidising firing cycle (see Chapter 9.5.2). The simultaneous occurrence of black and red colours on the same Corinthian and Attic vessels has prompted much research in the past, and several theories have been put forward to explain the chemistry and technology of the process. These approaches were recently reviewed by Walton et al. (2009) (see 9.5.2 for details).

9.5.2 Attic pottery The cultural position occupied by Corinth in the 6th century BCE was taken over by Athens in the 5th century BCE. As one of the results, fine Corinthian pottery had been virtually driven out of production by its Athenian rival by 550 BCE. By the mid 6th century BCE the quality of Corinthian ware had fallen away significantly to the extent that some Corinthian potters would disguise their pots with a red slip in imitation of technologically superior Attic ware. Initially the Attic potters perfected the black-figure technique in a style that inherited the old Attic interest in human figures. In the mid-6th century BCE a generation of master vase painters brought the black-figure technique to perfection, culminating in the works of the painter Exekias. This artistic revolution marked by the use of ambitious figured scenes mirrors the interest the ancient Greeks had always shown in the human body. In this use of the human form as a major decorative motif Greek pottery is different from almost every other pottery produced anywhere (Williams 1997). Later, in a search for a medium more subtly expressive, the formerly highly successful and widely traded Attic black-figure vases were succeeded by the red-figure technique invented around 530 BCE in Attica and en vogue until the late 3rd century BCE. Its invention was rather inevitable as the Attic clays were quite rich in iron with moderate CaO contents and

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thus red-firing under oxidising conditions. This was in contrast to the Corinthian clays that were CaO-rich (see above). Initially, a transitional pottery technique combined both styles (so-called bilingual vases), with black-figure scenes painted on one side of the vase, and red-figure details on the other. Compositions and techniques of the older black-figure style remained in use. Thus, incised lines are quite common, as is the additional application of red pigment such as ochre (‘intentional red’) to cover large areas (Farnsworth & Wisely, 1958, Hofmann 1966). The red-figure technique is based on the figural depictions in red colour on a black background, in contrast to the preceding black-figure style that showed black figures on a red background. While the most important sites of production, apart from Attica, were in Southern Italy, the new style was also readily adopted in other parts of Greece. Outside the Greek world Etruria became a leading centre of its production (Giorgetti et al. 2004). In this part of the ancient word the red-figure technique became an important predecessor of Terra Sigillata (see Chapter 10). The outlines of the intended red figures were drawn on the unfired vessels dried to a leather-hard texture either with a stylus or with charcoal that would disappear during oxidising firing. Then, the contours were redrawn with a brush, using a glossy clay slip. Important contours were often drawn with a thicker slip, leading to a slightly protruding outline, the so-called relief line. Less important lines and internal details were drawn with diluted clay suspension. Detail in other colours, like white or red, were applied at this point. The relief line was probably drawn with a bristle brush, a feather or a hair, dipped in viscous, but still fluid paint. The application of relief outlines was necessary since the rather liquid glossy clay would otherwise have turned out too dull. The space between figures was filled with a grey clay slip. Then, the vases underwent the three-phase firing known from the black-figure technique, during which the clay reached its characteristic black or black-brown colour through reduction, the reddish colour by a final re-oxidation during controlled ingress of air into the kiln. Since this final oxidising firing phase was applied at lower temperatures, the slipped parts of the vase did not re-oxidize from black to red since their finer surface was sintered during the reducing phase, and was now protected from reaction with oxygen. In this the red-figure technique is essentially the reverse of the older black-figure technique. Figure 9.16, left shows a superb example of an Attic red-figure oinochoe from the Etruscan city of Vulci. The chemistry and technology of highly glossy black coatings on Attic and Campanian pottery has been studied by several research groups, including Bimson (1956), Noble (1960), Oberlies & Köppen (1962), Maggetti et al. (1981), Tite et al. (1982a), Jones (1986), Kingery (1991), Vendrell-Saz et al. (1991), Maniatis et al. (1993) and Walton et al. (2009). Generally, close control of materials as well as firing atmospheres and temperatures (Jones 1986) was required to achieve the striking contrasts between surface decoration and ceramic body of ancient Greek pottery. As demonstrated in Table 9.7 the slip of Attic ware contains more alumina, iron oxide and potassia compared to the body (Tite et al. 1982, Williams 1997). This is consistent with the illitic nature of the slip. To explain the simultaneous occurrence of black and red glossy colours on a single vessel of Attic and also Corinthian pottery basically five competing mechanisms were proposed.

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Table 9.7. Average composition in mass % of body and slip of Attic red or black slipped wares (Tite et al. 1982a, Williams 1997) SiO2

Al2O3

FeO

MgO

CaO

K2O

Body

55

Slip

46

21

9

5

5

5

29

15

2

1

7

(i)

To produce the beautiful red colours, yellow (hydrated iron oxihydroxide, FeOOH·H2O) or red ochre (anhydrous iron oxihydroxide, ˞-FeOOH, goethite) were painted in the spaces not intended to be covered by glossy black, producing during re-oxidation red haematite known as ‘intentional red’ (Farnsworth & Wisely 1958, Farnsworth & Simmons 1963, Hofmann 1966). (ii) The potters used clays of different composition, that is, kaolinitic, more refractory clays for the red parts but illitic, more fusible clays for the black parts (Winter 1978). (iii) Addition of fluxing plant ash rich in potassia to the painted parts intended to be black promoted vitrification and thus hindered the ingress of oxygen during the re-oxidation step (Kingery 1991). (iv) Tite et al. (1982a) suggested that colour development has to do with grain size: the finest, more easily vitrified fraction of the levigation-derived clay slip was used on the black parts whereas the coarser illite particles behaved more refractory, did not sinter easily and thus developed their red colour during the final re-oxidation step. (v) A recent study on coral red slips on Greek Attic pottery (Walton et al. 2009) found two distinct compositions of red gloss slips, named LCM (low calcium magnesium) and HCM (high calcium magnesium) coral reds, the former being similar in composition to the black gloss. Both LCM and HCM coral red glosses are characterised by a significantly more porous microstructure compared to the black, that is, a lower degree of sintering. The similarity in composition of the black gloss and the LCM coral red does not support the assumption of a single three-stage firing but instead suggests that the black gloss and the LCM coral red gloss was generated during two separate firings, that is, a first reductive firing to produce the black gloss, and, after cooling and application of the LCM, a second oxidative firing to form the red gloss. On the other hand, the slightly more refractory HCM coral red pigment may have enabled the red gloss to be produced together with the black in a single three-step firing. The authors suggested that both technologies may have been applied simultaneously by different workshops and concluded that the art of making coral red glosses was essentially workshop-specific and not commonly held knowledge. On the right of Fig. 9.16 an Attic black-figured dinos is shown, a vessel on a ceramic stand used at a symposium to mix wine and water as the Greeks made a very strong wine that they always drank diluted. The decoration depicts the wedding procession of Peleus and Thetis who became the parents of the Greek super-hero Achilleus. Apart from the black- and red-figure techniques described above, a third technique existed that outlined the figures on a white ground prepared by applying iron-free clays over the common iron-rich and hence red-firing clay (Walton et al. 2010). Even though this so called white-ground technique has quite a long history, it was common only in Athens during the

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Figure 9.16. Left: Attic red-figured oinochoe, showing Eos (Dawn) pursuing Tithonus. 470–460 BCE. From Vulci. Louvre, Paris. Courtesy: Marie-Lan Nguyen / Wikimedia Commons (Public Domain). Right: Attic black-figured dinos and stand, showing the wedding procession of Peleus and Thetis, the parents of Achilles. c. 580 BCE. Reg. no. GR 1971.11-1.1. © Trustees of the British Museum.

5th and 4th centuries BCE. It provides a more realistic effect than black- and red-figures and also permits the addition of colours ranging from yellow to pink, red, violet and even blue. Thus it probably comes closest to painting on panels and walls. This white-ground technique gained great importance despite the fact that the white slip was not very resistant to wear. Hence this technique was specifically applied to ‘one way’ funerary vessels or ritual lekythoi that became typical grave offerings (Fig. 9.17). Beyond the artistically ambitious painted pottery described above during the 4th century BCE Athenian plain black-slipped ware became elaborately decorated by raised ornaments attached to the surface of the pots. This new Hellenistic so-called West Slope style eventually developed around 220 BCE into the mould-thrown black-slipped hemispherical Megarian bowls (Fig. 9.18) adorned with relief decoration (Williams 1997), possibly a lowbudget answer to cherished expensive silver vessels of comparable shape. This innovative technique was somewhat later perfected in Italy where potters had begun to emulate the production technique of the Megarian bowls. Moreover, they changed the black- to a redslipped surface that was easier to produce and thus laid the foundation of the hugely popular Terra sigillata ware (see Chapter 10) that conquered the Roman world soon after and thrived for almost five hundred years.

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Figure 9.17. Attic lekythoi in white ground-technique with red-and black-figure decoration, c. 470–450 BCE Attributed to the Painter of The Villa Giulia. Gela, Sicily. Height 35 cm. Reg. no. 1863,0728.312. © The Trustees of the British Museum.

Figure 9.18. Black-slipped Megarian bowl, showing in relief the abduction of Persephone by Hades, the ruler of the underworld. The black slip has been worn in places. Hellenistic period, 225–175 BCE. Made in Attica. Diameter 17.8 cm, height 10.8 cm. Reg. No. 1897,0317.3, © The Trustees of the British Museum.

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9.6 Culinary traditions: Greek delicacies revealed 9.6.1 Historical remarks Information on the food available in ancient Greece, eating habits, and, in particular, recipes is difficult to come by. Few written texts have come down to us describing the everyday life 2500 years ago. However, many ancient Greek potters have faithfully depicted typical scenes of a symposion on black- and red-figure kraters, oinochoë, and other types of vessel (see Fig. 9.20). Traditionally, any proper Greek meal consisted of three components: sitos, that is, staples such as barley gruel or wheat bread; opson, i.e. relishes such as fish, meat, vegetables, cheese or simply olive oil; and oinos, i.e. wine (Encyclopaedia of Food & Culture). Hence, there is little doubt that the Greeks enjoyed a rather simple but healthy fare: seafood, fish, lamb, wine, olive oil, cheese and bread, in much the same way as their descendants do today. In Homer’s Odyssey the hero Ulysses, king Alcinous’ proxenos (guest-friend), reflects on the pleasures of (communal) eating and drinking:        

ಯழ˨˧గ˪ˬˢ˧ˮˢ౰ˬ˪ ˬ௴ˠఐˮ஼ˠఝˠఓ˱గ˳ˤ˩˦˱ఓ˨ˬ˯˴˞ˮ˦ఓ˰˱ˢˮˬ˪ˢ௞˪˞˦ ொ௭˱ౚ஼˲˳ˮˬ˰ఛ˪ˤ˩ఒ˪ீ˴ౠ˧఑˱˞ˡౢ˩ˬ˪ற˭˞˪˱˞ ˡ˞˦˱˲˩ఙ˪ˢ˯ˡౚ஬˪ఐˡఝ˩˞˱ౚ஬˧ˬ˲఑ˣ˶˪˱˞˦஬ˬ˦ˡˬ౿ ்˩ˢ˪ˬ˦஽˫ˢగˤ˯˭˞ˮఐˡఒ˭˨క˥˶˰˦˱ˮ఑˭ˢˣ˞˦ ˰గ˱ˬ˲˧˞ఖ˧ˮˢ˦ಌ˪˩ఓ˥˲ˡౚ஼˧˧ˮˤ˱ౢˮˬ˯஬˳ఛ˰˰˶˪ ˬ௘˪ˬ˴ఙˬ˯˳ˬˮఓౠ˰˦˧˞ఖ஼ˠ˴ˢగౠˡˢ˭఑ˢ˰˰˦ʷ ˱ˬ౿˱ఙ˱గ˩ˬ˦˧఑˨˨˦˰˱ˬ˪஼˪ఖ˳ˮˢ˰ఖ˪ˢ௜ˡˢ˱˞˦ˢ௞˪˞˦ರ

‘King Alcinous, ...there is nothing better or more delightful than when a whole people make merry together, with the guests sitting orderly to listen [to a bard], while the table is loaded with bread and meats, and the cup-bearer draws wine and fills his cup for every man. This is indeed as fair a sight as a man can see’ (Homer, Odyssey, 9:5-11). Communal feasting in the Aegean world was very powerful social glue. Different types of banquets existed in discrete spheres of social exchange and served distinct strategies of power mobilisation and management, involving both the maintenance of the status quo and competition for definition of status. However, as discussed by Elisabetta Borgna (2004) feasting activities in Minoan Crete were even more suited than those of mainland Greece to facilitate community solidarity through widespread attendance. The Homeric as well as the Minoan societies were characterised by social groups founded on aggregations of male warriors, and supporting dominant individuals. These individuals, the ‘big men’ or chiefs, held communal banquets as a vehicle of attracting consent, maintaining cohesion, forming alliances, or striving for dominance among equal individuals or oikoi. Archaeologists stress the importance of using pottery style to monitor such social and ideological relationships within the framework of use and circulation of drinking vessels for ritual consumption of wine. Such stylistic differentiation serves to reconstruct the political development of Cretan and mainland Greek societies to the end of the Bronze Age and into the beginning of the Iron Age. The stylistic display and decoration of LH I and LM IIIC ves-

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sels suitable for distribution (kraters, oinochoë) and individual consumption (deep bowls, kylikes) is thought to reflect the openness of the communities and the strong interaction among providers and receivers within the social framework of feasts including mortuary meals (Lindblom 2004), in which intense social and ideological exchanges took place targeted at creating, maintaining, strengthening and modifying social relationships (Borgna 2004). In several cases the ceramic shards found in graves have been interpreted as the remains of mortuary meals held by kin groups of the deceased in connection with the burial rites (Graziadio 1988). A case in point are the Early Mycenaean (LH I) shaft graves of Lerna, near the east coast of the Peloponnesus, south of Argos. Before the shaft graves were filled in, ceramic vessels (plain cooking pots as well as bi- and polychrome kraters, cups, jars and jugs) likely used during the ceremonial mortuary meal were thrown into the shafts and/or scattered in their vicinity, together with a large number of animal bones and other organic remains (Lindblom 2004). These ritually discarded objects are different from funeral offerings carefully deposited to serve the deceased in the afterlife. Two LH I mainland polychrome cups (Fig. 9.19) were found (almost) intact on the floor of shaft grave #2 at the Lerna VI site (Lindblom 2004). Re-cut and smoothed out ceramic shards of broken vessels known as pessoi (pebbles), were reused to wipe clean ones buttocks after defecation literally at both the ‘bottom’ end of the food chain and the communal feasting activities (Charlier et al. 2012). In Classic times social exchange was facilitated by the symposion (from Greek ˰˲˩˭˜˪ˢ˦˪ sympinein, ‘drinking together’), a banquet designed to reinforce communal cohesiveness and to provide a forum for man of standing to debate philosophical issues, plot strategic political moves, celebrate victories in poetic or athletic contests, boast about their own achievements, or simply revel with cronies and have good time. Hence the symposium was a key Hellenic social institution. Figure 9.20 shows a symposion scene such as the one described in the Deipnosophistoi (Braund & Wilkins 2000), painted on an Attic red-figure crater, with banqueters playing kot-

Figure 9.19. Late Helladic (LH I) semiglobular (left; height 15 cm, maximum diameter 22 cm) and Vapheio-type cups (right; height 16.5 cm, rim diameter 22 cm) found as grave offerings in shaft grave #2 of Lerna. Mainland (Boeotian) polychrome ware. c. 1620 BCE (Lindblom 2004, 2011). Courtesy of the American School of Classic Studies at Athens.

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Figure 9.20. A ‘Banquet of the Learned’. Attic Red-Figure crater, decorated by the Nikias painter, ca. 420 BCE (Courtesy: Marie-Lan Nguyen (user: Jastrow)).

tabos73 while a musician plays the Aulos, the Greek double-reeded instrument, possibly similar to an oboe. According to Homer the Greek heroes feasted on heaps of meat. In contrast to this, the most important food of ordinary people was barley gruel, made from de-husked grains, crushed in a mortar or ground in a hand-operated mill made from stone. Also, barley was used to make pearl barley that Homer called the ‘core of men’ (Bauer 1967). Since barley flour does not make good bread it was made into various kinds of broth, porridges, gruels, and mashes. However, rich cities such as Athens imported wheat from Sicily, North Africa, and the northern Black Sea coast, and in turn the Athenian market became famous for its fine, white, industrially produced bread enjoyed by the affluent part of the citizenry (see below). Cabbage, leaks, onions, garlic, fruits such as pears, apples, figs and grapes completed the rather vegetarian everyday fare of the average Greek. Archestratos74 described a somewhat peculiar wine soup into which goat cheese was grated and barley flour mixed in. This recipe75 of a dish called kykeon was already described by Homer in the Iliad (Book XI). It was 73 Kottabos was a popular party game during which the last drop of an emptied kylix was flicked by a swift twist of the hand onto a tiny statuette on top of a stand with outstretched arms delicately holding a small disc called a plastinx. Halfway down the stand was a larger disc called the manes. To be successful the player had to knock off the plastinx so that it would fall to the manes and make a bell-like sound. 74 Archestratos was a gourmet from Sicily travelling around 350 BCE to the markets of the Mediterranean and wrote about the local delicacies he found along the way (Dalby & Grainger 1996). 75 The text translated by Samuel Butler reads: “Fair Hecamede … set for them a fair and well-made table … on it there was a vessel of bronze and an onion to give relish to the drink, with honey and cakes of barley-meal. There was also a cup of rare workmanship… In this the woman, as fair as a goddess, mixed them a mess with Pramnian wine; she grated goat’s milk cheese into it with a

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suggested that the unusual combination of wine, barley flour and goat cheese was a ritualised reverence to the three nutrition providing gods Dionysos, Demeter and Pan (Dalby & Grainger 1996). Honey, thought by the ancients to be a kind of dew dripping from trees and collected by bees, was very popular, and associated with the gods and their divine fare. The food and drink served on Mount Olympus, ambrosia and nectar, were based on honey and supposed to confer eternal youth and immortality to the ancient Greek gods and to those heroes that were found worthy to be among them, where they ‘live untouched by sorrow in the islands of the blessed along the shore of deep swirling Oceanos, happy heroes for whom the graingiving earth bears honey-sweet fruit flourishing thrice a year’ (Hesiod 8th century BCE). In Greece honey served many purposes: besides eating it as a sweet (see recipes below) and adding it to a large variety of dishes, it was used to conserve fruits, make ointments, and even embalm corpses. While Homeric heroes apparently dined on large quantities of meat as mentioned above, normal people supported themselves mostly with vegetarian food. The meat of husbanded animals such as cattle, sheep, goats and pigs was consumed only when the animals were sacrificed to the deities. It was roasted on sticks and sprinkled with salt won from the sea. The use of spices and herbs appears to have originated much later in Greek history. Then coriander, dill, pepper, caraway seeds, thyme, fennel and liquid condiments such as vinegar, honey, wine, and fig juice were copiously used, sometimes to such an extent as to overpower the natural taste of the dish. In his comedy ‘The Birds’, Aristophanes makes fun of this custom by advising the birds complaining about the relentless pursuit by humans with slings, nets and lime-twigs, to be careful ‘in as much as people not only simply roast you but add cheese and oil, condiments and vinegar, and sauces mixed from acids and sweets. When it boils and sizzles, this concoction will be poured over your skin as you were stinking carrion’ (Bauer 1967). While seafood and fish replaced meat in the diet of the average person, it apparently was less a favourite with the upper class who liked to dine on birds such a seagulls, wild doves and fieldfare, and also on hunted for food animals such as ibex, stag, hare and wild boar. Even simpler was the fare in Sparta, conforming to the ‘heroic’ and austere lifestyle cherished in this militarised society. Characteristic for these attitudes is the infamous black soup, consisting of pork cooked in pig’s blood and seasoned with vinegar and salt. This frugality was the object of ridicule in Attica and, in particular, Sybaris. There the story was told of a travelling Spartan who in the evening handed over to his hostel warden a small fish with the request to cook it for him. After the admonition of the warden that to accomplish the task he would still need oil, vinegar and cheese, the Spartan replied: ‘If I had had all of this I would not have dreamt of buying this fish’ (Bauer 1967). With time the culinary prowess of the Greeks improved, related to their colonisation effort throughout the Mediterranean. As mentioned above, trade relations from the Caucasus to Sicily and Campania brought lots of wheat to Greece where it replaced barley for all but the

bronze grater, threw in a handful of white barley-meal, and having thus prepared the mess she bade them drink it” (Homer, The Iliad, Book XI, 638-641).

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poorest people. Hence white bread became more widespread, and sweet cakes and pastries were enjoyed by many. Also, oxen, sheep, goats and pigs were now slaughtered for their meat rather than being ‘by-products’ of sacrificial ceremonies as previously. Athenaios, a celebrated orator and grammarian, described what may be considered the first ‘patent’ on food preparation. He mentions that in 500 BCE, in the Greek colonial city of Sybaris in southern Italy, there were annual culinary competitions the excessive elaboration of which led to the proverbial term ‘sybarite’, a person only devoted to pleasure and luxury. The victor was given the exclusive right to prepare and sell his dish for one year.

9.6.2 Ancient Greek recipes ‘Fig leaf’ (Bauer 1967) 1. Mix pork fat cooked in milk with thick pearl barley. 2. Knead this mixture with fresh cheese, egg yolk and sheep’s brain, 3. Wrap the dough in pleasantly smelling fig leaves so that small parcels will be formed. Tie with a thin string. 4. Cook the wrapped parcels in a broth of poultry or goat meat. 5. Take them out, remove the leaves, and coat the paste in a pot with boiling honey.

Kid goat, lamb or chicken in broth The Greek physician Nikander (early 3rd century BCE) described in a fragment published by Athenaios Naukratios (Gulick 1827–41) a recipe to prepare goat, lamb or chicken meat as follows (Hirschfelder 2005). But when you prepare freshly butchered kid goat or lamb or also chicken, add some freshly crushed wheat grains to a deep pan and stir them together with fragrant oil (oil in which herbs were steeped). When the dish boils (when the separately boiled meat is tender) pour it over the crushed wheat grains and cover with a lid because when this heavy dish is prepared in the described way, it will swell. Serve it warm with bread.

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[Stuffed] Kid or lamb another way (Fig. 9.21, top). Aliter haedus sive agnus syringiatus: lactis sextarium unum, mellis unc. IV, piperis unc. I, salis modicum, laseris [silphium, asa foetida] modicum. ius in +ipsius:* oleum acetabulum, liquaminis acetabulum, mellis acetabulum, dactilos tritos octo, vini boni heminam, amulum modice (Apicius, Book VIII, Chapter VI, No. 7).

Translation: Kid or lamb is thus prepared and seasoned. Take 1 pint milk, 4 ounces honey, 1 ounce pepper, a little salt, a little laser. Gravy [of the lamb] 8 crushed dates, a spoonful oil, a little broth [liquamen], a spoonful honey, a pint of good wine and a little roux (Dalby and Grainger 1996).

This recipe is one of the very few that reports exact quantities of ingredients. Although contained in a Roman cookbook it is likely an original Greek recipe as also suggested by the comparatively sparing use of seasonings and condiments (Dalby & Grainger 1996). We have tested this recipe and found the sauce much too sweet for modern taste. Hence we recommend a slightly altered recipe as follows: Ingredients: Take 1 kg boneless saddle of kid (or lamb). For the marinade: ½ L milk; 100g honey; 25 g pepper; ½ tsp salt; ½ tsp asa foetida powder (hing). For the sauce: 2 x 30 ml olive oil; 60 ml liquamen (fish sauce); 30 ml honey; 4 dried dates; 1–2 tbls tapioca flour; 125 ml hot water. Preparation: 1. Prepare marinade, mix well and pour into a large (6 l) freezer bag. Add meat and shake well. Keep refrigerated overnight. 2. Soak dates in red wine overnight. 3. Preheat oven to 200 °C. Dab meat dry with kitchen towel. 4. Heat slightly 30 ml olive oil in a coated pan or casserole. Add meat and brown on all sides at medium heat. 5. Transfer meat to a well-soaked clay pot. Cook loose drippings with the hot water and pour liquid over meat. 6. Close clay pot with its lid and roast without opening at 200 °C for 150 min.

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7. Strain red wine with the dates, crush softened dates through the sieve and remove hard parts. 8. Remove all liquid from clay pot. Keep meat warm in clay pot during preparation of the sauce. 9. Mix cooking liquid with the red wine, the fish sauce, honey and the remaining 30 ml olive oil. Stir in tapioca powder and heat until thickened. 10. Cut meat into slices or bite size pieces. Pour sauce in bowls and serve everything with barley or wheat bread, green and black olives, and grapes.

Gastris (honey-nut-poppy squares) (Fig. 9.21, bottom). This easy to prepare recipe originates from Chrysippos of Tyana, quoted by Athenaios Naukratios (Dalby & Grainger 1996). ‘In Crete they make little cakes called gastris. And this is how they are made: roast sweet almonds, hazelnuts, bitter almonds and poppy seed without letting them burn, and crush them well in a mortar. After the mixture is kneaded with boiled honey, add pepper lavishly. The dough will turn black because of the poppy seeds. Roll it flat. Now crush white sesame seeds, add boiled honey and draw it out to form two flat pancakes, one for the bottom and one for the top, and add the black one in the middle. Then cut it into pieces’. Below there is a modern rendering of the recipe as reported by Dalby and Grainger (1996). Ingredients (sufficient for about 15–20 pieces): 1 cup (120 g) crushed almonds; 1 cup (120 g) crushed hazelnuts; 1 tbs. (30 g) bitter almonds; 1.5 cups (170 g) sesame seeds; 1 cup (120 g) poppy seeds; 7 tbs. (210 g) clear honey, preferably Greek thymeli; up to 1 tsp ground black pepper to taste. Preparation: 1. Preheat oven to 180 °C. 2. Roast almonds, nuts and poppy seeds separately in the oven on a baking sheet until coloured. Set aside. 3. Roast sesame seeds quickly, let them cool and crush in a mortar. 4. Give 3 tbs. honey in a small pan, bring to a boil and let simmer for 7 minutes until bubble formation almost ceases. Stir constantly. 5. Stir crushed sesame seeds into the boiling honey.

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6. Pour the mixture onto an oiled tabletop or marble plate, and knead it with greased hands until firm but still warm. 7. Divide into two parts and keep warm. 8. Coat a baking sheet or large flat pan with olive oil. Roll out the cooler portion of the sesame paste so that it will fit the baking sheet or the pan. 9. Crush all roasted almonds, nuts and poppy seeds together with the ground pepper for 1–2 minutes in a blender until a fine mixture has formed. 10. Cook the remaining 4 tbs. honey for 7 minutes as above and stir into the nut mixture. 11. Coat the nut and honey mixture over the still warm sesame paste and smooth out. 12. Roll out the second portion of the sesame paste and coat over the nut layer. 13. Let the dough rest for 1 hour, and then cut it immediately into rectangular pieces to avoid that the honey-nut-poopy squares get too hard.

Fig. 9.21. Ancient Greek delights. Top: Haedus syringiatus (kid roast) with date sauce and wheat bread, accompanied by olives and grapes. Bottom: Gastris (honey-nut-poppy squares)

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Roman earthenware Synopsis Mass-produced Roman pottery was exported from Italy since the 4th century BCE, first as so-called Campanian Ware, a black-surfaced pottery type, followed in the mid-1st century BCE by Terra Sigillata (TS), a red slip ware produced initially in Arretium (modern Arezzo). TS pottery workshops flourished between the 1st century BCE and about the 4th century CE in all parts of the Roman Empire. The mass-produced pottery was highly standardised and limited to few practical and stackable forms for easy transport. It was predominately produced from calcareous illitic clays that were carefully collected, processed by elutriation and settling, and fired between 950 °C and 1050 °C in a kiln. The finest clay fraction (400 (Schifer 2003) confirming the assumption that the ‘Falke group’ ware was produced in or around Zittau.

88 For the stepwise canonical Mahalanobis D2 discriminant test the squared distance is used of a data point within a k-dimensional hyperspace from a (k-1)-dimensional plane separating the data points (Huberty & Olejnuk 2006). This test is an important tool for discriminant analyses that, through combination of variables, sets out to separate groups in an object hyperspace as much as possible. It is desired to minimise the Mahalanobis distances within groups but to maximise the distances between groups. The dimensions are displayed by discriminant functions Dim with associated eigenvalues of the in-between group variances. Whereas by this technique nice separation diagrams can be created caution has to be exercised along the lines expressed by Maggetti and Messiga (2007) in the preface to the book ‘Geomaterials in Cultural Heritage’: ‘Often archaeologically irrelevant questions are investigated, insufficient numbers of samples are analysed and it can happen that “poverty of measurements is sometimes hidden behind sophisticated data processing displays” (quoted after F. Widemann 1982)’. This means that in many cases simple bivariate plots of elemental concentrations will yield a more penetrating insight and should preferentially be used over highly sophisticated data massaging (Michelaki & Hancock 2011). It has also been established that the method of data exploration and the number of elements included in analysis can influence chemical group membership (Michalaki et al. 2013), suggesting a crucial impact on archaeological interpretations. The authors thus made a strong point ‘that we must apply as much attention to assessing and sorting analytical data as we do to creating data sets’.

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11.4 Bunzlau stoneware Ancient Bunzlau stoneware (Endres et al. 1997) is distinguished by its salt-glazed bright and rich rust- to coffee- or chocolate-brown colour. While pottery activity went on in the area since the 7th century CE the high point was reached during the 18th and 19th centuries CE when simple storage vessels for farmers were produced in large numbers and sold in the local markets. Their telltale shiny brown glaze (Fig. 11.15) was generated by dipping the leather-hard green body into a wash of ferruginous mud prior to firing. At the end of the 19th century CE increasing urbanisation, industrialisation and also strong competition from other pottery producing regions resulted in new production lines such as pear-shaped tankards, coffee pots and wine jugs (Fig. 11.16) that quickly conquered the parlours and dining rooms of the affluent city burghers. Decorative motifs included relief applications of leaves and flowers, and coat-of-arms made from yellowish clay similar to the technique used at Waldenburg (see 11.3.1). Besides the monochrome brown glaze polychrome glazes of red, blue and black, sometimes with touches of gilding were introduced. Bunzlau stoneware enjoyed a renaissance during the early 20th century CE with Art Nouveau (‘Jugendstil’) decoration sporting the famous peacock’s eye design. In the 1920th the Bunzlau potters introduced a trend towards increased colouration following Art Deco designs. After WW II when Silesia had to be ceded to Poland, the town of Bunzlau was renamed Boleslawiec, the factories reopened and the German pottery tradition revived and continued. The pottery production benefitted also from addition of Polish pottery traditions and a remarkable school of artists was established at modern Boleslawiec.

Figure 11.15. Ferruginous mud-glazed Bunzlau-style Braunzeug (Courtesy: Töpferhof Meißner, Trebus, Oberlausitz, Germany). Photo: Heimann.

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Figure 11.16. Stoneware jug, brown glaze with moulded clay applications, showing the coat of arms of the Electorate of Saxony (Kursachsen, 1356–1806). c. 1700–1800 CE. Bunzlau, Silesia. Reg. No. C.62-1925. © Victoria and Albert Museum, London.

Today several small German potteries continue the Bunzlau stoneware tradition by replacing salt-glazing by other technologies owing to environmental concerns. The famous highfired Bunzlau stone- and earthenware is now glazed by a low-melting iron-rich mud glaze that produces the telltale warm, coffee-brown, shiny surface of the ‘Bunzlau Braunzeug’ (Fig. 11.15).

11.5 Of late medieval broth and mush 11.5.1 Traits of medieval cooking Medieval life style was characterised by the fact that it was strictly subordinate to the position of the individual within the rigid framework of the ordo, the authority-imposed corporate hierarchical order that prescribed the rules and regulations according to which its members had to live and to behave, what they were allowed to wear and to eat, and what their role in society was supposed to be. Nature, acquisition and consumption of food depended on the status of the eater: members of the nobility and the clergy had other means to organise their menu than peasants or burghers. Consequently the recipes laid down in

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early cookbooks reflect the customs and culinary taste developed and refined in monasteries and at kingly and princely courts rather than the everyday food consumed by the less affluent. Peasant food was ridiculed in contemporary poetry, and middle-class fare was only reported in printed cookbooks from the 16th century CE onwards. A recent treatise on the history of European culinary culture traces the ups and downs of eating customs and taste (Hirschfelder 2005). However, there was one common overriding trend: reliance on food products obtained from fields, farms and kitchen gardens. Transport of agricultural mass products was beyond the capability of society at the time. Food stuff consumed the most derived from grain such as rye, oats, millet and barley. Wheat in the form of ‘schoenez brot’ was considered expensive food for the rich the same way as it was in Greece some two thousand years ago. Porridge made from available cereals was the daily nourishment of poor people, and even bread was considered a kind of luxury during early medieval times – monks abstained from eating bread as a sign of particular severe penitence and ate mush instead. Beans and peas were eaten copiously during Lent. From medieval cookbooks the impression could be gained that vegetables were rarely eaten. However, this is far from the truth. Marx Rumpolt, personal cook to the Prince-Bishop of Mainz lists no less than 50 different salads and 225 kinds of accompanying vegetable dishes in his instructional textbook to apprentice cooks, Ein New Kochbuch of 1587. The explanation of the lack of mentioning vegetables is simple: they were taken for granted. On the other hand, many vegetables we enjoy today were not consumed for their nutritional value but for their true or assumed medical effects. Cabbage and turnips were considered peasant food but were also eaten by monks and members of the lower gentry. Herbs such as sorrel, lamb’s lettuce, common scurvy grass, dandelions, purslane and lettuce were eaten as salad or cooked. Mediterranean cultured vegetables such as cucumbers, turnip, cabbage and pumpkin were introduced by travelling monks and grown in monastery kitchen gardens. Pork, beef, sheep and poultry were widely raised but eaten by all in very moderate amounts, very different from our present overindulging in meat of all sorts. Slaughtering of larger animals was a seasonal business and preferably done during the late autumn/early winter months to avoid feeding the animals through the winter and also to preserve the meat for extended periods without health risk. Conservation of the meat was accomplished by smoking, drying or curing, sometimes by immersing in honey. Huge amounts of milk and eggs were consumed, and numerous contemporary recipes attest to their use. Game was generally available only to the nobility that possessed the right to hunt. This right was strictly enforced and many sad examples confirm that transgressions were mercilessly punished. Game hunted included deer, hare, wild boar, and wild birds such as mallard, wild goose, pigeon, snipe, lark, bustard, pheasant, quail, peacock, swan, and crane. Similarly the right to fish was not granted to everybody. Domestic freshwater fish was consumed during Lent, generally on Fridays and during church holidays. Hence demand was strong since the church forbade eating meat on up to 150 fast days per year. It is thus not surprising that freshwater fish had to be supplemented by saltwater fish that was widely traded from the coast to the interior land, mostly in dried or cured and salted form. Several seafaring

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nations with economies based on fishing such as Norway exported huge amount of stockfish (dried cod) as its longest sustained export commodity, and the socioeconomically most profitable export product over the centuries. This product represented most of Norway’s national income from Viking age throughout medieval age. Occasionally fish was raised and bred in fishponds. After the first waves of the ‘black death’, during the second half of the 14th century CE fishponds were established in a wide swath ranging from Bohemia, Poland and Silesia via Lusatia to Württemberg and Lorraine (Hirschfelder 2005). Apart from written records preserved in cookbooks or sales bills information on the nature of food stuff consumed can be obtained from the contents of excavated middens, refuse heaps and pit latrines associated with medieval urban environments. These studies can provide very detailed knowledge on the kind of domestic and wild animals available for human consumption. For example, a latrine used between 885 and 911 excavated in Konstanz, Germany yielded bones of a variety of animals of which 94% were of domestic origin, among them 21% were sheep and goats, 16% pigs, 32% chicken, 7% cattle but also a surprisingly high percentage (12%) of cats and some dogs (3%). Since almost exclusively paired extremities of cats and crushed dog bones were found but only very rarely skulls and torsos the author assumed that in these times cats and dogs also belonged to the animals eaten (Kokabi 1992). Medieval dishes were most often liberally seasoned to an extent as to mask their natural flavours. Domestic seasonings included chervil, garlic, caraway seed, parsley, mint, anis, sage, onion, horseradish, fennel, mustard seed, leek, galangal and pennyroyal. During the crusades foreign spices were discovered and brought home such as pepper, ginger, cloves, cinnamon, nutmeg, mace and saffron. These much more expensive foreign products were held in high esteem and at princely courts replaced the traditional domestic seasonings as the former were seen as a sign of savoir-vivre. While the overuse of spices has traditionally been linked to attempts to mask the smell and taste of meat rotting due to insufficient conservation methods it is now acknowledged that there were medical reasons too: people were convinced that spices furthered health and in particular immunised against recurrent bouts of epidemic plagues. In medieval Germany culinary influences sometimes came from abroad, mostly from France. Hence in medieval German cookbooks many recipes bear French names such as gramangir, pittit manger, agraz, condiment and, in particular, blancmange. However, these influences did not really alter the rather crude, and hearty and boorish ways of food preparation typical for old German cooking. A more pronounced effect was noticeable only during the 16th/17th centuries when new recipes, new ways of cooking and novel food stuff, condiments and spices were introduced from Italy and France. In particular, the refined ways of French cuisine were adopted by the princely courts and, later, by the well-to-do burghers and thus totally transformed German cooking the same way it did in contemporary Britain (see Chapter 16). It might be argued that German stoneware was rarely used for cooking since it could not tolerate the direct heat of a cooking fire. As discussed in more detail in Chapter 2.3.1 an ideal cooking pot should possess high thermal shock resistance as well as high mechanical

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Figure 11.17. Tripod earthenware cooking pots, so-called grapen, fired under oxidising (left) and reducing (right) atmospheres. 13th/14th century CE. Konstanz, Germany (Junkes 1992). © Landesdenkmalamt Baden-Württemberg, Stuttgart.

fracture toughness. However, the dense ceramic body of stoneware was not well suited to withstand steep thermal gradients even though its mechanical strength was rather substantial. Hence on the one hand, stoneware was predominately used as tableware, storage containers for liquids, drinking vessels, and multipurpose jugs and jars plentifully around in medieval kitchens. On the other hand, typical cooking utensils produced from red-firing clays were the grapen, round, frequently bulbous earthenware tripod vessels (Fig. 11.17) that were fired at rather low temperatures and could be directly positioned in a hearth over glowing embers and so avoiding direct contact with a flame that would have created deleterious thermal stresses in the ceramic body. The tripod feet allowed a secure stand and the rounded form of the bottom assured that the liquid content of the grapen was always at the lowest position thus preventing scorching of the solid content. Since in some cases the inner surface of a grapen was covered with an impervious engobe or glaze it was suggested that these grapen were used to prepare special dishes (Junkes 1992).

11.5.2 Blancmange – a hugely popular medieval dish Blancmange, blamensir or ‘whitedish’ was a hugely popular upper-class dish in the Middle Ages and well into the present. Since it was frequently sweet, smooth and white it was considered a ‘frouwen spîse’ (ladies’ dish). The true origin of the blancmange is obscure, but it is believed that it was a result of the introduction of rice and almonds by the Arabs into early medieval Europe. It occurs in countless variations in recipe collections from all over Europe (see recipes below) and is also mentioned in the prologue to Geoffrey Chaucer’s Canterbury Tales (see Chapter 16.5) and in an early 15th century cookbook The Forme of Cury written by the chefs to the court of king Richard II. One of the oldest recipes translated

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from German by the Danish author and scientist Henrik Harpestraeng († 1244) goes back to the early 13th century CE. A more recent recipe from Das bůch von gůter spîse 89, a Würzburg manuscript written before 1350 reads in the Middle High German of the time as follows.90 91 Wilt du machen einen blamensir90 Wie man sol machen einen blamenser. Man sol nehmen zigenin milich vnd mache mandels ein halp phunt. einen virdunc ryses sol man stozzen zů mele, vnd tů daz in die milich kalt, vnd nim eines hůnes brust, die sol man zeisen vnd sol die hacken dor in, vnd ein rein smaltz sol man dor in tůn, vnd sol ez dor inne sieden. vnd gibs im genůc vnd nime es denne wider. vnd nim gestozzen violn vnd wirfe den dor in. vnd einen vierdunc zuckers tů man dor in vnd gebs hin. Also mac man auch in der vasten machen einen blamenser von eime hechede. Translation: To prepare a blancmanger, add to [cold] goat milk ½ pound of [peeled] almonds and 1 virdung91 rice, both pounded in a mortar to a fine flour. Pound chicken breast, slice it and add to the almond milk. Add pure lard and cook long enough [until tender]. Remove [from fire]. Add crushed petals of violets [for colouration] and ¼ pound of sugar. Serve. Instead of chicken, pike could be used during Lent (see also Adamson 2000).

Here follows a modern variant of this recipe (Fig. 11.18). Ingredients: ½ chicken breast, ¾ l goat milk, 20 g goose lard, 150 g almonds (peeled and crushed), 75 g rice flour, a handful of violet petals, 75 g sugar, ¼ l hot water. Preparation: Steep the violet petals for 30 minutes in hot water. Strain and pour the bluish liquid into the goat milk. Add lard and sugar to liquid and heat the mixture in a casserole, add chicken breast cut into thin strips, reheat, cover

89 The book has been written in Middle High German. Many recipes taken from the book were translated and adapted to modern usage by Fahrenkamp (2009). 90 Translation into modern German was provided by Lemmer & Schultz (1969). 91 Vierdung (or verding) is the fourth part of some unit measure, in particular ¼ pound. This division was also used in contemporary coinage: in the 14th century, 4 verding/vierdung = 1 mark = 16 Bohemian groats (groschen).

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Figure 11.18. Blancmange (blamensir) in shape of a fish and coloured by violets, served on a salt-glazed Siegburg stoneware plate with grapes and carrots, and accompanied by ‘schoenez brot’ (wheat bread).

and let cool for about 10 min. Then stir in the rice flour and heat over low heat and under constant stirring until thickened, for about 5 min. Fold in the crushed almonds, pour into suitable mould and let it cool (If desired the blancmange can be enjoyed warm, too). The solidified mush can easily be removed from the mould by turning it over.

Another, more frugal, variant of this dish appears under the title ‘Blamůß von mandel gemacht’ (Blue mush made from almonds) in a compilation of cooking recipes called ‘Kuchenmaistrey’ (kitchen mastery), published in Nürnberg around 1490 (Fig. 11.19). In this recipe the dish is coloured blue by cornflower petals instead of violets. Neither milk nor sugar or chicken are being used in this recipe, and salt is added instead of sugar.

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Figure 11.19. Recipe of different version of blancmanger, actually ‘bleumanger’ (plabes (= blue) můß) from ‘Kuchenmaistrey’, c. 1497, Augsburg. © Bayerische Staatsbibliothek, München.

The translation reads as follows: A blue mush is made from almonds and rice. Pound cornflowers well with [hot] water, press the liquid through a cloth and save it. Pound almonds with the same [reheated] water and press it through a cloth to obtain a blue milky liquid. Then make a mush with rice or wheat [flour]. If desired garnish with grapes. Do not oversalt and let it not scorch. Serve the mush in a new [clean] tin bowl or in a white [cleanly scrubbed?] wooden bowl.

Blamensirs were prepared in many different ways throughout medieval Europe, in France, Germany, England and Italy. A North German/Dutch cookbook of the 15th century CE contains a recipe of a blamensir made from chicken and hare to which wine was added. Instead of sugar spices such as white ginger, mace, cardamom and cloves were used: ‘Item wiltu maken blaemantir, so nym rises eynen halven verding91 unde also vele mandelkerne. So nym dat rysz unde wassche id reyne unde wirff de hulsen alle wech. So nym unde lat dat risz droghen. So nym de kerne unde make reyne. Unde make de dicke myt wyne. So nym de dunne melk. Do in dat risz. Rore dat sere. So nym dre bruste van dren braden honeren unde plucke clene also eyn har. Rore de in dat risz. So nym witten ingever, muschatenblomen, paradiseskorne unde neghelken unde stod tosammende. Rore dat in dat risz unde eigesdodere. Unde giff dat in de schottele‘.

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Translation: To make a blancmange, take 125 g rice (½ verding91) and many almonds. The rice should be washed and the hulls removed, and then dried. Likewise the almonds should be cleaned and a mush be made with wine (?). Take thin milk and put in the rice. Stir vigorously. Take a cleanly plucked chicken breast and a hare [part of] and put them into the rice. Add white ginger, mace, cardamom and cloves to the rice and an egg yolk. Pour everything in a bowl.

11.5.3 Bubenpfulben92 with dip (Fig. 11.20) The recipe can be found in Birlinger, who published in 1865 a commented version of an Alemannic manuscript written in the 15th century CE, and also in Rheinfränkisches Kochbuch (1998) of c. 1445. It reads in Middle High German as follows. Ain bubenpfulben Nim ain kalpszung und das lünglin und süd die gar wol und hack sy dan und speck darunder und schlach ayer daran und saffran und guot wurcz und hāb dinen bleter lang von guotem teig und wol berait und darin bewiltz und bestrich es an der fugen mit ainem dünen ayerteig und bach es in schmalz. Translation: A bubenpfulben [filled pastry in form of a boy’s pillow92]. Take a veal tongue and the lung, and boil them [until tender]. Chop both, add bacon and eggs, and season well with saffron and other spices [pepper]. Wrap the mix into [rolled out] elongated sheets of well-prepared dough, [fold it to form a pillow], seal the seams with a thin egg dough, and bake the pillows in lard.

92 A ‘pfulben’ or ‘pfulmen’ is a pillow positioned on the daybed after the spread is placed over the blankets. These pillows are called Paradekissen in German. Note that the German pfulben and the English pillow have the same the roots. A ‘pfulben’ is also part of the crest of the Swabian town of Pfullingen.

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Figure 11.20. Bubenpfulben (pillow-shaped pastry filled with veal tongue and lung) with ‘Swallenberg’s salse’ (wine-honey-ginger sauce) as a dip, served on a salt-glazed Siegburg plate.

The modern version of this recipe is as follows: Ingredients (for the number of pieces shown in Fig. 11.20): The dough: 3 eggs, 150 g wheat flour, pinch of salt. Additional flour needed to coat the working surface during rolling out the dough. Oil used for deep frying. The filling: 150 g boiled veal lung, 150 g boiled veal tongue, 50 g smoked (fat) bacon, 1 egg, 1 pinch of saffron. Preparation: 1. Boil veal tongue and lung until tender, season well. Mince meat and bacon quickly in a mixer, add the full egg and saffron. 2. For the dough, beat eggs and salt in a mixer. Add flour slowly to make a sticky paste. Coat working surface lavishly with flour and pour dough onto it. Flour top of dough mass. Drive dough mass apart with floured hands under gentle kneading, making sure that always enough flour is underneath the dough to avoid sticking to the surface and also take care that not too much flour will be worked into the dough. If required use a roller pin to form a dough sheet approximately 35x40 cm across and

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2 mm thick. Cut sheet in rectangular pieces of approximately 12x6 cm size Take one piece after another, add 1 tbsp of filling, fold, and press thoroughly together the overlapping sides with a fork so that the filling is tightly enclosed. 3. Heat oil to a 190 °C maximum and brown 1 to 3 ‘bubenpfulben’ at a time, depending on the size of the deep fryer.

Ein gůt salse (Swallenberg’s salse) Nim win vnd honigsaum, setze daz vf daz fiůr vnd laß es sieden vnd tů dar zů gestozzen ingeber me denne pfeffers, stoz knobelauch, doch niht al zů vil vnd mach es starck vnd růrez mit eyner schinen, laz ez sieden, biz daz ez brinnen beginne. Diz sal man ezzen in kaldem wetere vnd heizzet Swallenberges salse. Translation: Take wine and honey, boil the mixture and add crushed ginger more than pepper, crushed garlic (but not too much), and simmer under constant stirring until it thickens. This is recommended to eat in cold weather and is called ‘Swallenberg’s sauce’. Ingredients: 500 ml dry white wine, 5 tbsp liquid honey, 1 medium-sized garlic clove, 1 large piece of ginger, ½ tsp ground black or white pepper, salt to taste. Preparation: Heat wine with honey, skin ginger and grind it into the liquid. Let it simmer for a while. Press garlic into mix, add pepper and salt, and simmer until the liquid is reduced to half its original volume. Remove solid particles by straining.

Chapter 12

English and French white earthenware (creamware, faïence fine) Synopsis The invention of 18th century CE English creamware was roughly contemporaneous with the creation of French faïence fine. French Renaissance precursors are the Saint-Porchaire and Palissy wares. Experiments by Staffordshire potters with aluminium-rich and highly plastic clays gave birth between 1720 and 1740 to a new body called creamware or cream coloured ware, fired at 1100–1200 °C. In 1761 Josiah Wedgwood I at Staffordshire’s Burslem continued to improve the English ceramic bodies by adding kaolin and feldspar to the paste. This new variety, christened Queen’s ware, was much whiter and harder than the common creamware. By 1768 calcined flint was added to the Wedgwood pastes. Transfer printing is the second key contribution of 18th century Britain ceramists to the ceramic world. French white earthenware originated during the years 1730–1750 in Paris and in Lorraine (Eastern France). Claude-Imbert Gérin created in the French capital a new white earthenware body around 1740, and in 1743 started the production of faïence façon d’Angleterre, using non-calcareous clays to which he added crushed fired clay. In Lorraine, Jacques Chambrette II begun experiments with calcareous clays to produce white earthenware in 1745 and achieved in 1748–1749 the so called terre de pipe body. Firing temperatures were between 950 and 1050 °C. According to 18th century CE manuscript sources, Lorraine white earthenware bodies were generally made up of four main constituents: Alrich refractory clay, chalk, frit, and flint. Scientific analyses of Saint-Porchaire, Palissy, English and French white earthenware bodies provided good insights into the production techniques and the complexity of provenancing these objects.

12.1 French Renaissance precursors Whereas the details of the origin of fine and white earthenwares in England and France are still much debated, recent research demonstrates that the invention of 18th century CE English creamware93 was roughly contemporaneous with the creation of French faïence fine (Maire 2008). At present there is general agreement that the antecedents of European white earthenware production can be found in Renaissance France.

93 For the definition of creamware see Massey (2007).

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12.1.1 ‘Saint-Porchaire’ ceramics A technologically outstanding Renaissance ceramic type was created in France during a brief period in the mid-sixteenth century CE (Crépin-Leblond 1997) as confirmed by thermoluminescence dating by Sturman and Barbour (1995). About 80 objects of this socalled ‘Saint-Porchaire’ ware still exist today, mostly in form of candlesticks, cups, ewers and salt cellars that show a white, dense body covered by a transparent lead glaze (Fig. 12.1). Their decorations, moulded elements as well as stamped, inlaid designs, are strongly influenced by French and Italian Renaissance metalwork. The production palette is stylistically divided into three types: (1) black or brown decoration on white ground, (2) decoration with arabesques and interlaces, and (3) decoration with three-dimensional attachments in naturalistic forms including representations of dogs, lizards and snakes (Velde & Bouquillon 2004). This beautiful pottery is rather enigmatic as its production site, production date94 and the name of the master potter(s) involved are still a matter of debate (Crépin-Leblond & Ennès 1997). Monograms and armorial devices associated the ware with King François I (reigned 1515–1547), King Henri II (reigned 1547–1559)95 and the powerful Montmorency family96. The production site of this early high-tech French pottery was first attributed to the

Figure 12.1. Salt cellar decorated with lion heads and festoons, and the coat of arms of Diane de Poitiers. Saint-Porchaire ware, c. 1547–1559. Reg. no. 1189–1864. © Victoria and Albert Museum, London.

94 1540–1560 (Crépin-Leblond 1997), 1510/1525–1570/75 (Poulain 1997). 95 The ware is also called ‘Faïence Henri II’, classified as soft-paste porcelain or lead-glazed fine earthenware. 96 Anne de Montmorency was connetable (constable) to the King.

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LUXEMBOURG BELGIUM

Septfontaines

Paris

Mettlach GERMANY Vaudrevange

Audun-le-Tiche Lunéville Saint-Porchaire

Metz

Bois d’Epense

Sarreguemines

Saintes Montendre

Toul

Nancy Lunéville

Niderviller Saint-Clément

Rambervillers

N

200 km

0

10 20 30 40 50 km

Figure 12.2. Left: Map of France with major white earthenware manufacturing sites indicated. Right: Map of Lorraine (open circles: major towns, solid dots: manufactures).

small village of Saint-Porchaire (Poitou), far away from the Royal Court in Paris (Fig. 12.2). Recent excavations in the Louvre brought to light not only the remnants of Palissy’s atelier, but also some Saint-Porchaire ware (Ennès 1997). Was Palissy the inventor of the latter? Was the production site in the Royal Louvre? Were there one or more workshops? The debate is still ongoing.

12.1.2 Palissy ceramics Bernard Palissy (1510?–1590), one of the most innovative and original natural historians97 and ceramists of the French Renaissance, had a profound impact on the French ceramic world. He started ceramic production around 1540 at his likely birthplace Saintes98, Lot-etGaronne, Aquitaine (Fig. 12.2). King Henri II visited his workshop in 1555 and Connetable Anne de Montmorency ordered at the same time a ceramic grotto, probably for his Château d’Écouen. However, complications arose when Palissy converted to the protestant faith during his Saintes’ years and consequently was imprisoned in 1563 in Bordeaux under the

97 Sometimes Palissy is credited for holding the accurate belief in the organic origin of fossils as extinct life forms and the discovery of marine transgressions. However, his role as a prescient geologist/palaeontologist is much debated (for example Plaziat 2011). 98 Several villages vie for Palissy’s birthplace including Agen and La Chapelle-Biron near Saint-Avit (Audiat 1868). In 1990, in La Chapelle-Biron a society named ‘Les Amis du Musée Bernard Palissy’ was founded and in 1992 the first rooms of the museum were opened to the public.

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accusation of being a clandestine Huguenot priest, and his workshop and kilns were destroyed. However, through the intercession of Anne de Montmorency he was released from prison and subsequently established a pottery workshop in Paris in 1567 under royal protection. When Catherine of Médici issued an order to build a ceramic grotto in her gardens in the Tuileries, most probably at the time when the Catherine and Karl IX. visited Saintes in 1565 (Hanschmann 1903), this was presumably the reason why Palissy was allowed to install his workshop to the east of the Tuileries Palace, then under construction, and to carry the royal title ‘Ouvrier de Terre et Inventeur des Figulines Rustiques’, a title he would retain until his death. There he produced ceramic objets d’art for the Royal Court. Threatened again by religious persecution he was forced to leave the capital in 1572, escaping the St. Bartholomew’s Day massacre, and found refuge in Sedan from which he returned to Paris only in 1576. Ten years later he was once more arrested for his religious beliefs, imprisoned in the infamous Bastille and died there in 1590, a victim of his unwavering faith. Palissy specialised in architectural ceramics (tiles, bricks, grotto elements), rustique figulines (shells, plants, animals cast from life), rustique basins and silver ware-imitating ceramics (cups, medals, ornaments, spoons). The artist was the first to apply life castings of animals and plants to ceramics, decorating the basins, ewers and dishes with these vividly coloured (glazed) casts (Fig. 12.3). In her essay on the art of Palissy, Hanna Shell (2004) not only describes in detail how the life casts were made but also celebrates him as the creator of an interpretive framework in which clays emerges as a vital new medium for inquiry into ter-

Figure 12.3. Dish in the manner of Palissy’s rustique figulines. Possibly created either by Bernard Palissy himself, his sons Nicolas and Mathurin, 1565–1585, or by a 17th century CE follower. Reg. no. 5476–1859. © Victoria and Albert Museum, London.

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restrial and organic processes, and thus for the expression of philosophies of nature. His first rustique basin was finished around 1555, and King Henri II purchased it during his visit to the workshop in Saintes. Excavations (1984–1992) discovered Palissy’s workshop in Paris with thousands of ceramic fragments (Dufaÿ et al. 1987). Based on colours Palissy ceramic bodies are divided in three groups (Munier 1949): red clay, beige clay, and extra white clay. The latter was used, for example, to produce medals but was, after firing, very porous (38%, Munier 1949).

12.2 English white earthenware (creamware) In Europe, tobacco smoking became fashionable in the 16th century CE, and to make pipes, white-firing so-called pipe clays were used. During the first decades of the 18th century CE, English potters preferred non-calcareous clays, abundant in England, to create new ceramic types. In contrast, French potters used non-calcareous as well as calcareous whitefiring clays and called the resulting wares terre de pipe or later faïence fine99.

12.2.1 The invention of creamware John Philip and David Elers, two silversmith brothers of German descent migrated around 1690 from the Netherlands to England, and worked there from 1690 to 1693 as John Dwight’s assistants at Fulham. They moved afterwards to Staffordshire (Fig. 12.4, left), and established a pottery at Bradwell, north of Newcastle-under-Lyme. There they created an exceptionally finely made red ware, that is, a novel fine red earthenware type in the tradition of Roman Terra Sigillata (see Chapter 10.3) with its red body covered by an impervious red slip consisting of ferrugineous illitic clay. These slipped teapots, mugs and tea caddies (Fig. 12.4, right) were evidently an imitation of Chinese yixing red stoneware (see Chapter 18) that was used for the preparation and serving of tea. However, the extremely high standard of craftsmanship as well as the penchant of the Elers brothers to apply to their pottery silversmith techniques they had learned much earlier in their homeland such as slip-casting and lathe-turning, led to a non-competitive production environment that eventually forced the brothers to declare bankruptcy as early as 1700 CE while facing stiff competition by their fellow potters. At this time Staffordshire had a thriving and skilled pottery community making full use of abundant clays and easily accessible coal for the firing of the ware100. In those days, coal resources were more important than clay pits for the placement of pottery workshops, because it required 17–20 tons of coal to fire one ton of clay ware101. It was therefore much cheaper to bring the clay to the coal than vice versa. 99 About the somewhat chaotic French terminology see Peiffer (2003). 100 ‘It is estimated that by 1750 there were about 130 potteries (150 according to some authorities) in North Staffordshire, the majority of which would have been making the standard products of the day, including salt glazed stoneware, black glazed wares and red wares.’ (Roberts 2007). 101 By the end of the 18th century 12–15 tons of coals were needed to fire 1,500 pieces of creamware (Dawson 1997, p. 205 quoting Aitkin 1795).

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Figure 12.4. Left: Map showing the sources of ceramic raw materials used in 18th century Staffordshire (grey area) potteries. BC = ball clay, CC = china clay, CS = Cornish stone, F = flint. Simplified after Dawson (1997). Right: One of the earliest types of English teapots, tea caddies, and chocolate mugs produced by the Elers brothers between 1690 and 1698. The style was inspired by red stoneware pots imported from Yixing, China, the technology by Roman Terra Sigillata. Mus. no. C.4&A-1932. © Victoria and Albert Museum.

Local competition on how to whiten the pastes to emulate the whiteness of porcelain led around 1720 to the discovery of a new body, that is, white salt-glazed stoneware or common ware. This tableware was immensely successful and was produced in Britain until c. 1780. By the 1720s, the firing technique changed due to the introduction of a two-stage firing cycle: unglazed biscuit firing and lead glaze (or glost) firing. Continuous experiments with many other raw materials such as the aluminium-rich and highly plastic Devon or Dorset ball clays (Fig. 12.4) gave birth between 1720 and 1740 to another new body called creamware or cream-coloured ware, fired at 1100–1200 °C. The lead oxide used to glaze the body was contaminated with iron that imparted a creamy tint to the glaze thus giving it its descriptive name. The development of this new body is attributed to the Staffordshire potter Enoch Booth102. Addition of calcined and milled flint, an invention of John Astbury of Shelton or John Heath or John Dwight of Fulham103, improved the solidity and whiteness of the creamware. Since it was soon ascertained that dry grinding of flint in grist or stamp mills caused silicosis among the workers, two patents issued in 1726 and 1732 to Thomas Benson of Newcastle-under-Lyme reduced the inhalation of silica dust by crushing and grinding with addition of water in an edge-runner mill. In England, the recipes for bodies and glazes were no secrets, as it would have been in French manufactures, and thus were known by everybody. This explains why most 18th century CE English fabricants (Astbury, Enoch Booth, Thomas Whieldon, Josiah Wedgwood I) produced identical pastes and glazes. Staffordshire was a thriving potter’s district with six main centres: Burslem, Fenton, Hanley, Longton, Stoke-on-Trent and Tunstall, collectively known as ‘The Potteries’ (see Chapter 16). In 1763 Burslem had about 150 pottery workshops with nearly 7,000 employees.

102 The oldest dated creamware (1743) shows the mark of Enoch Booth. 103 He recorded in his notebook in 1698 that ‘when giving the ingredients for a white body, calcined beaten and sifted flints will doe instead of white sand and rather whiter but ye charge and trouble is more’ (Copeland 1972).

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12.2.2 Josiah Wedgwood I in Burslem (Staffordshire) Josiah Wedgwood I (1730–1795) originated from a potter’s family in the Staffordshire village of Burslem. In 1754 he joined the pottery of Thomas Whieldon and experimented during five years with many paste mixtures to improve the properties of the creamware and to create coloured glazes. He founded, in 1759 in Burslem, his own manufacture called Ivy-house and produced by 1761 an improved creamware body by adding kaolin and feldspar to the paste. The new variety was much whiter and harder as the usual creamware. In 1765, a tea service for Queen Charlotte, wife of George III, triggered the commercial success of his new ceramic body, christened Queen’s ware in 1765 (Fig. 12.5). Wedgwood copied many formal elements from porcelain, as evidenced by the Rococo style plate shown in Fig. 12.5, a precise copy of a 1750 model made by Jean-Jacques Duplessis for Vincennes (Blaettler 1995). Wedgwood built in 1769 Etruria, a large industrial-size factory. Other new successful bodies were black basalt (black stoneware, in production since 1769), the famous jasperware (blue bisque stoneware, early 1770s) and caneware (brown stoneware, 1779). Royal customers were to follow: King George III ordered a creamware service in 1770, and Empress Catherine II the Great, the Russian Tsarina, ordered in 1769 the Husk Service with more than 1,500 pieces, the hugest creamware service ever made. In 1773 followed the spectacular commission for the Frog Service, a 50-person table and dessert service of 952 items, marked with a frog’s emblem104 and decorated with 1,222 handpainted views of Brit-

Figure 12.5. Tureen with supporting plate. Wedgwood Queen’s ware, Etruria manufactory, c. 1770. Tureen H: 20.5 cm, L: 36 cm. Plate L: 43 cm. Photo: Jacques Pugin. © Musée Ariana, Geneva, Switzerland.

104 The service was intended for Catharina’s Neo-Gothic castle-palace of Kekerekeksinen, built in a frog-inhabited marsh outside St. Petersburg. The Finnish place name simulates the sound of croaking, similar to the ‘brekeke-kex’ chorus in Aristophanes’ play ‘The Frogs’.

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Figure 12.6. Serving plate for meat and vegetable dishes of the Frog Service. View of the lake at West Wycombe, Buckinghamshire. On-glaze colour painting. Wedgwood Queen’s ware, made at the Etruria factory, Staffordshire and painted at Wedgwood’s decorating studio in Chelsea, London, 1773–1774. Reg. no. C.74–1931. © Victoria and Albert Museum, London.

ish buildings, gardens and natural wonders (Fig. 12.6). Thirty artists of Wedgwood’s painting studio in Chelsea, London, finished the order by 1774. After Josiah Wedgwood I invented a pyrometer to measure the temperatures inside a kiln, the Royal Society of London elected him a member. Creamware was suitable for manufacturing fashionable, elegant tableware (Fig. 12.7). In turn this pottery, prized all over Europe and beyond, propelled Britain from the backwater of ceramic production into the limelight of attention that was later reinforced by the invention and development of new stoneware types (ironstone china, feldspar porcelain, stone china) and, eventually bone china (see, for example Freestone 1999b; Chapter 16). Pearlware or china glaze ware was introduced around 1775 in a successful attempt to improve the whiteness of creamware and thus to make it appear more like true porcelain. Cobalt oxide was added to the glaze mix to optically mask the yellow iron tone thus yielding earthenware with a distinct grey-blue finish. Since this glaze was ideal for blue transfer printed earthenware (see below), potters continued to use it long after a clear, colourless glaze was achieved in the early years of the 19th century CE.

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Figure 12.7. Queen’s ware dessert plate, pressed and turned over a mould, painted in onglaze purple colours, Josiah Wedgwood’s factory, c. 1770. The moulded and painted decoration is very similar to the features of the ‘Husk Service’ supplied to Catherine the Great in 1770. Reg. no. 968–1853. © Victoria and Albert Museum, London.

12.2.3 Decoration by transfer-printing ‘Transfer-printing is a method of decorating pottery by using an inked, engraved copperplate to make a print on paper that, while still wet, is pressed against a glazed pottery surface, leaving behind an impression, or transfer, of the engraving’ (Encyclopaedia Britannica105). However, John Sadler and Guy Green, the inventors of the transfer printing around 1750 used at the beginning a different technique106. The copper engraving was coated with oil. Then a gelatine (animal glue) sheet was pressed against the copper plate, onto which the oiled engraving was transferred. The sheet was thereafter pressed on the glaze of the glostfired object and withdrawn. The object was powdered with colour pigments which would only stick on the transferred oily motif. Gelatine was rapidly replaced by paper and cloth. Transfer-printing, this second key contribution of 18th century Britain ceramists to the ceramic world, was greatly successful since (i) it enabled less skilled workers to decorate pottery, (ii) high-quality decorations could be achieved at relatively low cost, and (iii) the decoration patterns could rapidly be adapted to new fashions. The technique reached Germany in c. 1770, Switzerland in c. 1775, and France in c. 1790.

105 www.britannica.com/EBchecked/topic/602539/transfer-printing (accessed May 11, 2012). 106 Mallet (2011) traced the earliest occurrence of transfer-printing on ceramics to faiences made during the late 17th CE in Turin.

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Figure 12.8. Left: Queen’s ware service, onglaze transfer-printed with purple pigments. Josiah Wedgwood and Sons Ltd, Etruria, printed in Liverpool by Guy Green, c. 1775. Reg. no. 414:1157/ B&C-1885. © Victoria and Albert Museum, London. Right: Dessert plate (?). Creamware decorated with the underglaze transfer-printed blue Willow Pattern and the inscription ‘THOMASINE WILLEY 1818’. It was made for the Willey family of Cornwall, perhaps involved in the supply of cobalt pigment to Staffordshire. Possibly from the Spode Ceramic Works, Stoke-on-Trent, Staffordshire. Reg. no. C.231-1934. © Victoria and Albert Museum, London.

Creamware from Josiah Wedgwood’s and other factories were decorated with the overglaze transfer-printing technique from September 1761107 onwards by John Sadler and Guy Green in Liverpool (Fig. 12.8, left). Colours included temperature-sensitive black, red, brown, purple etc. that were applied as overglaze pigments to glazed wares which already had endured two firings, and fired a third time at a lower temperature to fix the colour to the glaze. Josiah Spode I is credited with inventing transfer printing of underglaze blue decoration of earthenware bodies from 1784 onwards (Fig. 12.8, right). His early patterns were copied from Chinese porcelain imports. The predominant blue colour of Staffordshire printed ware was supplied by the mines of the Saxon Erzgebirge until 1816. Afterwards, Cornish cobalt was used. Cobalt blue could be applied to biscuit ware and ‘hardened on’ in a muffle oven as an underglaze decoration (see Chapter 13). The painted ware was then dipped in a lead glaze and developed a strong blue colour during glost firing. This colouration was more economical to apply and durable in service, and hence became a widely used way of decoration. The underglaze transfer printing technique was later improved by Josiah I’s son, Josiah Spode II who in 1822 moved from blue to green, brown, manganese purple, grey and black colours. In 1824, two-colour underglaze transfer printing was achieved. First, print outline patterns were transferred to the biscuit-fired body and subsequently painted in or between the lines in a second colour. After glost firing, overglaze patterns could be added

107 Roberts (2007, p. 53).

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by enamelling with additional colours and gilded decorations applied. By the middle of the 19th century CE, some manufacturers had their own engraving and printing departments, but independent engravers also supplied designs. As many designs were pirated and copper plates often changed hands, it is frequently impossible to attribute specific designs accurately to individual manufacturers.

12.3 French white earthenware (faïence fine) French white earthenware originated, according to the few general studies of this ceramic type (eg Guillemé-Brulon 1995, Garric 2006, Maire 2008), during the years 1730–1750 in central (Rue de Charenton, Paris) and eastern (Lunéville, Lorraine) France. Earlier, unsuccessful attempts to produce white earthenware are related in the mid-17th century CE Caussy manuscript (De la Hubaudière & Soudée-Lacombe 2007). Since 1743–1759 and until the French Revolution (1789) only five manufactures produced white earthenware continuously, either exclusively (Rue de Charenton/Pont-aux-Choux in Paris) or alongside with traditional tin-glazed faience (Épinal, Lunéville and Saint-Clément in Lorraine, and Montereau in the Paris region).

12.3.1 Claude-Imbert Gérin in Paris On July 30, 1743, Claude-Imbert Gérin, three years after having established the porcelain factory at Vincennes (see Chapter 14.1.4), was granted a 10-year Royal privilege with the exclusive right to produce ‘faïence façon d’Angleterre’ (faience England-style) in a six league radius (c. 24 km) around Paris, and left together with the Dubois brothers Vincennes for Paris (Hosotte-Reynaud 1967). His pottery mark was the Royal fleur de lys. The trio settled in the faience manufacture of Edme Serrurier108 at the Rue de Charenton (Faubourg St. Antoine, Paris), producing a pottery ‘dont la composition se fait avec une terre différente de celle qui s’employe dans la fayance ordinaire’ (the composition of which is made with a clay different from the one used to make ordinary fayence). Gérin created the new white earthenware body probably between 1740 and 1743 when working at Vincennes to compose good recipes for crucibles in which glazes could be melted. According to Hellot the new paste was made of 66% unfired and 33% calcined non-calcareous Moret clay109. André Pierre Mignon, a wood merchant, joined the directory. Although the enterprise was very successful as testified by the large number of 250 employees in 1745 (De Plinval de Guillebon 1995, p. 58), the brothers Gilles and Robert Dubois left the Rue de Charenton in the same year. As for Gérin, he returned to Vincennes on July 4, 1746. As much as 8,144 pieces were produced in 1747. The Manufacture Royale des Faïences à l’imitation de celles 108 He was also pottery merchant (De la Hubaudière & Soudée Lacombe 2003). 109 The clay deposit of the Hill of Moret is near the river between Moret and Montereau at a distance of about 40 km south of Paris (Smith 2007, quoting Hellot, Dawson 2007) or more precisely from the field Tortu of the farm Froide Fontaine close to Montereau (De Plinval de Guillebon 1995, p. 60).

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Figure 12.9. Left: Bust of Louis XV, white earthenware, Rue de Charenton or Pont-aux-Choux pottery factory, c. 1747–1760. Reg. no. CRIC.92&A-1951. © Victoria and Albert Museum, London. Right: Gravy (meat juice) pot with cover, white earthenware. Moulded decoration with rice grain or barley corn patterns. Probably Pont-aux-Choux pottery factory, c. 1760–1780. Reg. no. C.127&A-1945. © Victoria and Albert Museum, London.

de l’Angleterre moved in 1749 to a place opposite the Seine bridge Pont-aux-Choux. The establishment flourished as shown, for example by the 1765 output of c. 58,000 pieces. Production continued until 1788. After closing, the manufactory had a stockpile of 120,000 unsold pieces and a gigantic archive of 3,550 moulds. The Rue de Charenton/Pont-aux-Choux factory produced a wide variety of shapes from its beginnings around 1743 to the time of the French Revolution. There were figurines (Fig. 12.9, left) as well as decorative objects or tableware (Fig. 12.9, right). The latter shows the famous rice grain or barley corn moulded motif, created in 1750–1756 at Pont-aux-Choux and probably derived from English salt-glazed wares (Maire 2003). The Rue de Charenton factory adopted perfectly the Rococo style of Louis XV. Due to the high plasticity of the paste, even parts resembling metal objects could be moulded. The extravagant undulations recalled the water wave motif (Le Duc 1993). The economic success of this pottery factory – Pierre Mignon died a rich man in 1788 – evidently alerted the competition (Maire 2008). The next manufacture was created by François Mazois in 1745 at Plessis-Chenet, a distance of eight leagues from Paris, beyond the six league limit granted to Gérin. Mazois was a financier and relied for the daily business on Jacques Chapelle who had learned the secret of the paste composition when working with Serrurier, Gérin and Mignon at Rue de Charenton. Mazois and his new associate Jean Hill moved to Montereau in 1748, also at a safe distance from Paris. However, the monopoly of Pont-aux-Choux continued to be heavily defended by the patrons and Mazois faced many

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troubles, as did Beaufils, another white earthenware potter of Paris, who was heavily fined for encroaching on Gérin’s monopoly. Montereau managed to survive by applying a different technique, i. e. salt-glazed pottery, and a single firing cycle110. Other manufactures were established at Châteaudun, Lyon, Neuvy-sur-Allier, Orléans, Paris, Rouen, Saint-Omer, Sceaux and Tours (Maire 2008).

12.3.2 Jaques Chambrette II in Lunéville (Lorraine) Around 1710, Jacques Chambrette I moved from Dijon into the Duchy of Lorraine and founded a faience manufacture at Champigneulles. His son Jacques Chambrette II (c. 1705– 1758) became faience merchant in Lunéville (Fig. 12.2, right) and established there in c. 1730 a faience and porcelain manufacture111 which employed in 1748 as many as fifty people (Maire 2008, p. 95). Early faience wares produced at Lunéville were probably in imitation of those made at Strasbourg, Sceaux and Niderviller (Boger 1971). Experiments to produce white earthenware started in 1745 and resulted in 1748–1749 in a terre de pipe body (Maire 2008, p. 94–95)112. In 1749 Stanislas Leszczynski (1677–1766), former King of Poland and since 1737 Duke of Lorraine, witnessed, together with the anglophile Voltaire, a fire resistance demonstration of this new ware (Durival 1753). His former Majesty was so impressed that a privilege was issued on December 13 of the same year to establish a manufacture of terre de pipe at Lunéville (Grandjean 1983, p. 12). Rapidly his manufacture became very successful, supplanting in a short time Dutch and English white earthenware imports to Lorraine, and commenced to export its products to Germany, Italy and even Poland (Fig. 12.10). Jaques Chambrette II opened a third manufacture in Saint-Clément in January 1758, one month before his death on February 17, 1758. Although a 1759 census of the terre de pipe manufacture provides precise numbers of the moulds in stock, there is poor information on stylistic descriptions of the decorations and paintings (Grandjean 1983). This and the absence of marks make it difficult to distinguish Chambrette’s white earthenware from Lunéville or Saint-Clément wares. The Saint-Clément products were famous for their gold decoration (Fig. 12.11). Both factories coated the white calcareous bodies with the traditional tin-opacified lead glazes, but also with transparent lead glazes. The attribution problem is very complex since white earthenware was produced at many other sites in Lorraine (Audun-le-Tiche c. 1760?, Pexonne 1760?, Rambersviller 1762, Moyen 1763, Toul-Bellevue 1773, Niderviller 1778, Épinal c. 1780?, Domévre 1780, Septfontaines

110 Thomas Bentley, associate of Josiah I Wedgwood, visited Paris in 1776 and wrote about Montereau: ‘Saw a shop of Queensware in rue St. Jacques. And bought two small compotiers for 24 sous. The models and glaze in general are very indifferent, and the workmanship bad; Plates 4 livres 10 sous the douzain. This ware is manufactured at Montremi sur le route d’Auxerre’. (Bentley 1977). 111 The founding year could have been 1723–24 but porcelain was most probably never made there (Maire 2008, p. 96). 112 These ceramic bodies were also named Terre de Lorraine (Céramique Lorraine 1990).

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Figure 12.10. Teapot attributed to Lunéville (Sample TBL 1, Maggetti et al. 2011b). Lead-glazed calcareous white earthenware (terre de pipe). Late 18th century CE. H: 12 cm. Private collection. Photo: Maggetti.

Figure 12.11. Left: Watch stand attributed to Saint-Clément (Sample TBL 21, Maggetti et al. 2011b). Tin-glazed calcareous white earthenware (terre de pipe) with on-glaze blue painting (allegory of astronomy and marble imitation) and gilding. Late 18th century CE. Height: 39 cm. Width: 18 cm. Castle museum of Lunéville. Object destroyed in the blaze of January 2, 2003. Photo: Martine Beck-Coppola. Right: Figure group ‘The blind Belisarius led by a boy’. From a model of Paul-Louis Cyfflé, based on a book illustration by H.-F. Gravelot (Noël 1959). ‘TERRE DE LORRAINE’ (L.T.D.) impressed in a rectangular cartouche underneath the figure group. Cyfflé’s factory in Lunéville, 1766–1780. Reg. no. CIRC.108&A-1951. © Victoria and Albert Museum, London.

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Table 12.1. White earthenware body recipes (no. 1–3,5: Calame 2009, p. 54, no. 4: Maire 2008, p. 23). Manufacture and units 1) Lunéville, Jacques II Chambrette, 1748-49 CE, lb 2) Lunéville, Jacques II Chambrette, 1748-49 CE, lb 3) Lunéville, 19th century CE, kg

Clay

Chalk

Frit

36

16

8

36

16

14

275

124

62

100 12

4) Saint-Clément, 1764 CE, no unit given

20

12

13

5) Saint-Clément, 19th century CE, kg

45

18

19

Flint

Grog 6 50 5

1766, Sarreguemines 1790, Longwy 1798)113. All in all, there were 23 Lorraine manufactures likely to have produced white earthenware in the 18th century CE (Peiffer 2007). A special case is the Terre de Lorraine manufacture of Paul-Louis Cyfflé in Lunéville (Noël 1959, 1961, Calame 2009). This artist was specialised in the production of unglazed bisque figurines (Fig. 12.11, right). According to 18th century CE manuscript sources, Lorraine white earthenware bodies were generally made up of four main constituents (Maggetti et al. 2011b): Al-rich refractory clay, chalk, frit, and flint. Grog can be a supplementary addition. However, as shown in Table 12.1 the oldest Lorraine recipe from 1748–49 (Jacques II Chambrette’s recipe) does not contain any flint. This ingredient was added to the paste most probably only after 1760 (Maggetti et al. 2011b). The Lorraine white bodies belong to the group of calcareous white earthenware (see Chapter 2). Their clay, a sort of ball clay, originated in the German Westerwald region near Koblenz on the right bank of the Rhine River, and was dispatched from Cologne. Chambrette’s frit was a mix of crushed SiO2-rich pebbles from the Meurthe River (100 lb) and 30 lb of milled lead sulphide (galena, PbS). Lunéville’s 19th century CE frit no. 3 (Table 12.1) used 100 kg flint or sand from Badonviller and 9 lb of lead oxide (minium, Pb3O4). The frit of the 19th century recipe no. 5 (Saint-Clément, Table 12.1) was made up of 100 kg flint + 10 kg salt + 10 kg minium.

12.3.3 New ceramic bodies of the 19th century CE Many new white earthenware factories blossomed after the French revolution and many ‘old’ faience manufactories produced white earthenware alongside with classical faience. The beginning of the 19th century CE saw the emergence of a new body called cailloutage. This pottery type, like the English creamware, was produced by adding calcined and milled flint to non-calcareous kaolinitic clay. After 1830, feldspar and/or kaolinite were added, leading to bodies called china, granit, porcelaine opaque (opaque porcelain) or demi-porcelaine (half-porcelain). Despite their fanciful designations, these bodies are still earthenware and definitely not porcelain, as evidenced by their high porosity (8–15%, GuilleméBrulon 1995, p. 16). 113 For dates see Faïences de Lorraine (1997).

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12.4 Scientific analyses of English and French white earthenware 12.4.1 Saint-Porchaire Chemical analyses (Table 12.2) of Saint-Porchaire bodies114 show high alumina (35– 42 mass%), medium silica (54–61 mass%) and very low amounts of fluxes (Na2O, K2O, MgO, CaO, FeO, TiO2). They were therefore made with refractory, almost pure kaolinitic clay, chemically comparable to kaolinitic clays from Northern Aquitaine (clay Montendre in Table 12.2). However, the bodies contain much more silica and less alumina than expected by firing pure kaolinite (Table 12.2). Therefore, some quartz and other impurities (feldspar) must be present. The match between the ceramic bodies and the clays in terms of the other elements proves that no fluxes were added during the paste preparation. The microstructures are comparable to those of high-aluminous Aquitanian clays, experimentally fired at 1000–1100 °C (Tite 1996). This temperature range was therefore assumed to have been the original firing temperature of the ‘Saint-Porchaire’ ware. As these temperatures are somewhat higher than those obtainable in a normal French faience kiln (max. 1050 °C, Maggetti 2007), it was suggested that the original clay contained smectite, which would lower the sintering temperatures (Velde & Bouquillon 2004). In terms of their composition and microstructure, Saint-Porchaire ceramics can be considered underfired hard-paste porcelain.

Table 12.2. Selected compositions (mass%) of French Renaissance white bodies and clays. The theoretical composition of fired kaolinite was calculated from the chemical formula Al2O3·2SiO2·2H2O. Saint-Porchaire ware and Montendre clay: Electron probe microanalyses by Sturman and Barbour (1995, 1996) and Perrin (1997) and SEM-EDS analyses by Tite (1996). Palissy extra white: Wet chemical analyses (Munier 1949), SEM-EDS analyses by Tite (1996) and Bouquillon et al. (2005), and electron probe microanalyses by Perrin (1997). Oxide

St. Porchaire

St. Porchaire

Kaolinite

Kaolinitic Clay

Palissy

Body

Dark inlay

Montendre

Extra white

SiO2

54.5–61.4

52.7–59.6

45.89

53.5–54.3

54.5–66.0

Al2O3

35.0–41.9

24.9–38.0

54.11

41.1–42.4

31.0–42.0

CaO

0.3–0.7

0.4–0.5

0.2–0.4

0.2–1.2

MgO

0.3–0.6

0.4