Experiments in clay and glaze for decorated pottery

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EXPERIMENTS IN CLAY AND GLAZE FOR DECORATED POTTERY

A Thesis Presented to the Faculty of the Department of Fine Arts University of Southern California

In Partial Fulfillment of the Requirements for the Degree Master of Fine Arts

by John G. Nelson August 1950

UMI Number: EP57894

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T'Aw thesis, w ritten by John G. Nelson under the guidance of h .2 r$ ... Faculty Committee, and approved by a ll its members, has been presented to and accepted by the Council on Graduate Study and Research in p a rtia l f u lf ill­ ment of the requirements fo r the degree of

MASTER OF FINE ARTS ,Qy.

n„„ August 15, 195.0__

7acuity Com mittee)

Chairman

Y.&1C0O)

TABLE OF CONTENTS CHAPTER I.

PAGE INTRODUCTION

. ........................

1

......................

1

Statement of the problem ................

1

Limitations of the problem. . . . . . .

2

Importance of the p r o b l e m ............

R:

The Problem

Definitions of Terms Used

. . . . . . . .

3

Organization of the Following Chapters . .

3

IX. LITERATURE IN THE F I E L D ....................

5

General Ceramic Information . . . . . . . .

6

Information about Materials ..............

7

Books Dealing With C l a y Books Dealing With Glaze Information about Decoration III. THE METHOD OF EXPERIMENTATION The Clay

. . .

........

..........

,

............

$ & 9 11

. . . . . . . . . . . . . . . . .

11

Prepared Clay B o d i e s ..................

11

Constituents of a Clay B o d y ............

12

Calculating the Clay B o d y ..............

14

Testing the Experimental Bodies ........

16

Colored Clay Slips

..............

17

Clay C o l o r a n t s ....................

1#

t.

•• •

111

CHAPTER

PAGE Mixing Colored Clay Slips

19

The Glaze . . .

21

Raw Glazes

..........

• • • • • • • • • • • » • •

21

• » » * » • • » • • • • »

22

Glaze. Fritts

Reasons for Glazing Pottery

23

Glaze Colorants • • * ' • • • • » • » » •

24

Opacifiers

26

• • • * • • • • » • « » » •

Matting Agents

* • • * • * « « » • • •

Decorating Techniques

• • • • • • • » • •

27

Texture in Clay * # ..........

27

Design in Clay

27

• • » • • • » • • • » •

Incised line

23

Colored Clay Slip • • • • • • » • • • •

23

Texture in Glaze

• • • • • • • • • • •

30

Design in Glaze . » » • » » • • • • • »

31

Stanniferous Methods

31

Underglazes

• • • « • • • « * .............

Overglazes and Lusters

I?#

26

. .

33

• * • • • • * »

34

General Information • • * • • • • » • •

34

SURFACE: ENRICHMENT IN THE C U Y Incised line

• • • • • • • • • • • • •

Method of Decoration Results

• • • • • » * • •

37 39 39 39

CHAPTER

PAGE Relief Modeling . . ..................

40

Method of D e c o r a t i o n ........... . • ♦

40

Results .. . . . . . . ................

40

Cuenca

• • • • • • • • • • ......... . . •

Method of Building

41

. . . . . .....

41

Method of D e c o r a t i o n .............

41

Results • • • • • • • • .

............

Slip Trailing with Colored Glaze Method of Building

•• • • •

............

41 43 43

Method of D e c o r a t i o n ............

43

Results . . . .........

43

Colored Slip Painting

................

45

Method of Decoration • • • • • • • • .

45

Results • • • • • • • • • • • • • • • •

45

Sgraffito

........ • • • • .........

Method of Decoration Results . . . . . .

46

................

46

..................

46

............

4$

Texture in Clay

Method of Decoration • • • • • . . » .

lf$

Results

4$

............

Texture in Clay Emphasized by Glaze . . . .

49

Method of Decoration • • • » • • • • •

49

Results •

49

V

CHAPTER

PAGE Texture in Clay Combined with Texture in Glaze . . . . ............

¥.

50

Method of Decoration . . . • • • • • • •

50

R e s u l t s ............ . ................

50

SURFACE.EHRIGHMEHT IN THE GLAZE

. . . . .

51

. . . . . . . . . .

53

Method of Decoration..................

53

Re suits

53

Underglaze Painting

. . . . . . . . . . . . . . . .

Underglaze Crayon

............

5-5

Method

of Building • • • • . . • • • • »

55

Method

of Decoration

55

Results

.......

. • • » • • • • • • • • • • • .

Onglaze Painting with Oxides and Carbonates of Metals .................. Method

of Decoration

Onglaze Painting with One Oxide of Metal on Matt Glaze ............

Results

..............

Method of Decoration..........

Cuerda Seca

57 58 5&

........... . . » • • • • • .

Onglaze Painting with White Glaze on Dark Glaze • ..........

Results

57 57

Results.............. • • • • • • • • • .

Method of Decoration

55

59

» • • •

59

• .............. • • • • • • • •

59

. . .

................

60

CHAPTER

PAGE Method of D e c o r a t i o n ..............

.

R e s u l t s .......... • • ............• •

60

Onglaze Painting with Underglaze Colors •

62

Method of Decoration

........ .. . •

62

Results • • • • • • • • • . . • • • » .

62

Overglaze • • • • • • » • • • • • . • • •

64

Method of Decoration

• • • • • • • • .

64

Results , « • • . • • • • • • • • • • •

64

Crater Texture in Glaze • • • • • • • • •

66

Method of Decoration

. . .

.

Results •..... • ....

• • • • « • » » »

Lava Texture in Glaze • •...... . • ..... Method of Decoration

.

, . .....

Results •. • • • • • • • • • • • • • . VI.

60

SUMMARY ANDCONCLUSIONS......

BIBLIOGRAPHY

66 66 67 67 67

6&

....................

7*

LIST OF TABLES TABLE I. II. III.

PAGE B i a x i a l ..........................

20

Oxide Colors . . . . . .

25

....................

Glaze Formulas . . . . . . . . . . . . . . . .

36

v/iii

LIST OF PLATES PLATE I.

PAGE . . . . . . . . .

33

U*

Slip Trailing with Colored Glaze . . . . . .

42

III.

Methods with Colored Clay Slip . . . . . . .

44

HT#

Textures in Clay • • • • • • • • • . . . • •

47

V.

Underglaze P a i n t i n g . ♦ • • • • • • • * • •

52

..........

54

¥1* III*

Methods of Design in Clay

Under glaze Crayon .. ♦

. . . . . . . . . . . .

5$

VIII.. Onglaze with Under glaze. Colors . . . . . . .

61

.............. • • • • • • • •

63

IX. X#

Stanniferous. Mthods

Overglaze

Textures in Glaze

•

65

CHAPTER I INTRODUCTION Throughout the United States, and especially in Southern California, there are numerous studio potters operating in cramped quarters and on limited funds.

Often,

they tend to shy away from experimentation with their mate­ rials.

They invest in new materials which they can ill

afford or their pottery becomes monotonous because of a poverty of means. I.

THE PROBLEM

Statement of the problem.

It was the purpose of

this investigation (1) to give the small studio potter a simplified method of experimentation in clay and glaze, making possible maximum results with a minimum of materials; (2} to show the variety of decorative techniques possible 1 with one clay body and one glaze fritt by using the results of this simplified method to produce forty-four pieces of 2 pottery decorated in-eighteen different techniques. 1 When several glaze constituents such as white lead, felspar and silica are dry mixed, melted, cooled and re­ ground, a glaze fritt results. 2 By the term decorated is meant the addition of some embellishment to the piece for the visual enjoyment of the beholder.

Limitations of the problem.

This study has been

confined to the techniques and materials used in making decorated pottery.

No attempt was made to deal with the

aesthetic aspects of decoration.

This covers another

field of investigation. Since it would take several lifetimes for a ceramist to utilize every variation and combination of decoration techniques, only a limited number of basic methods have been used in this study.

Obviously, the richness and

variety achieved in this field is dependent upon the in­ genuity of the individual artist. Methods of building the form, i.e., throwing on the potter’s wheel, coil building, etc., lie in another field of study, and have not been covered in this investigation. Importance of the problem.

In most societies, from

prehistoric times to the present, the decorative aspect of pottery has ranked equally in importance with the utili3 tarian aspect. Sometimes, as in the case of the Chinese porcelains, it might be said that it was of first considera­ tion.

3 The reader is referred to the pottery of neolithic Europe; of Crete from 200 B.C. to 1350 B.C.; of Archaic and Classic Greece; and of the pre-Columbian, South American Indian cultures, particularly that of the Nazca from 100 B.C. to 600 A.D.

A survey of prize-winning pottery from such large shows as the National Ceramic Exhibition in Syracuse, New York, and of the pottery featured in department stores and 4 gift shops, indicates that there is a current trend of interest in decorated pottery. There are available to the potter many books and articles dealing with the materials and techniques of his trade.

However, those which concern materials are highly

technical and more applicable to the ceramic engineer than to the studio potter.

No one book, known to the writer,

approaches a complete coverage in the techniques of decora?tion.

An effort was made in this investigation to present

a method of experimentation in decorated ceramics which is not highly technical and to describe a manner of decoration techniques. II.

DEFINITIONS OF TERMS USED

Where a word has been used, the meaning of which was not clear, it has been defined in a footnote. III.

ORGANIZATION OF THE FOLLOWING CHAPTERS

In Chapter II a survey has been made of the litera^ The writer has looked at ceramics in representative Los Angeles department stores and gift shops. While the quality of decoration varies, it is used in some form on more than half of the ceramic ware.

4 ture pertaining to methods of pottery decoration and to the development of clay bodies and glazes. The procedures of experimentation which were used in this study have been described in Chapter III.

The prin­

ciple divisions of this chapter are (1) the clay and (2) the glaze. The materials and methods of decoration used on specific pieces of pottery have been discussed in Chapters IV and V. The results of this study were summarized in Chapter VI.

CHAPTER II LITERATURE IN THE FIELD It had been the intention of the writer to review the historical development of the various methods of sur­ face enrichment on pottery.

However, it was found that

an excellent coverage of the subject was made by Jean Goodwin in her Master of Fine Arts Thesis entitled "A Ceramic Mural for the Lobby of the Science Building."

1

Her thesis was concerned with the decoration of tiles, particularly as applied to ceramic murals, but the methods of decorating tiles are so similar to those used on pottery that it seemed unnecessary .to duplicate the information in this writing.

Instead, literature in the field which was

valuable as source material for this particular problem and which would be useful to the studio potter has been re­ viewed .

The reader is referred to Chapter III of the thesis "A Ceramic Mural for the Lobby of the Science Building" which was written in 19^7 by Jean Goodwin in par­ tial fulfillment of the requirements for the degree of Master of Fine Arts. A copy is available at the University of Southern California Library.

6 The field is so over-run with books and articles dealing with the various facets of ceramics that it was difficult to make a selection.

The opinion and advice of

one of the nations foremost potters, Dr. Glen Lukens, were readily accessible because of his presence on the University of Southern California campus as professor of ceramics. He recommended a reading list based upon his experience in the field from which the books referred to in this chapter were selected.

All of these books contain information which

is invaluable to the potter in understanding the materials of his craft, but no one source answers all of his problems. It is necessary to read selectively, retaining that which is pertinent and discarding that which is not. I.

GENERAL CERAMIC INFORMATION

The books in this category provide an over-all pic­ ture of pottery making.

They present general information

about all phases of the craft, but offer very little of a specific nature.

They are particularly helpful to the

teacher and the beginning student. One of the best books in this group is MMaking 2 P0tterytf by Walter A. de Sager. It is meant to be used as

2 For information regarding the name of the publisher, date and place of publication of books mentioned in this chapter, the reader is referred to the bibliography.

7 a text book, and contains a short history of ceramics, directions for the handling and conditioning of clay, methods of throwing and hand-building, methods of decora- * ting and the principles of glaze making and application. Other recommended books in this category are:

"How to

Make Pottery and Other Ceramics” by Muriel Pargh Turnoff, "Pottery Made Easy" by John Wolfe Dougherty, "Practical Pottery" by R. Horace Jenkins and "Pottery, Its Crafts­ manship and Its Appreciation" by Edmund de Forest Curtis. Books offering general ceramic information are of interest to the studio potter, but are not adequate to his needs and must be supplemented with literature containing more specific information with regard to materials and methods of decoration. -II.

INFORMATION ABOUT MATERIALS

Books on materials can be conveniently divided into those dealing with clays and those dealing with glazes. In some cases, one source covers both subjects.

It should

be pointed out that catalogs published by manufacturers and retailers of ceramic materials often contain "boileddown" information about specific materials.

Other valu­

able sources are the various ceramic publications such as "The Ceramic Data Book", "The Ceramic Age", and "The

g C'eramic Journal” .

These are technical publications and

should be used as reference material in solving special clay and glaze problems. Books dealing with clay.

Three books were found

to be particularly well suited for presenting the study of clay to the studio potter. The California State Mining Bureau published “The Clay Resources and the Ceramic Industry of California” in 1928.

This book gives the sources of California clays

and sescribes their physical characteristics.

“Ceramics —

Clay Technology” by Hewitt Wilson treats specific clay problems and the general properties of clay.

Dr. C. F.

Binns, in his book, ”The Manual of Practical Potting”, explains the calculation of clay bodies.

Although the

book was written nearly thirty years ago, it still contains a sound basis for the study of clay bodies.

Information

from these three volumes was used in formulating the clay body used in this study. Books dealing with glaze.

Among the books written

on the subject of glazes, “Outlines of Essentials of Glaze Composition” by C. W. Parmelee and "Literature Abstracts of Ceramic Glazes” by J. H. Koenig and W. H. Earhart were found to be illuminating.

However, a knowledge of ceramic

9 chemistry and engineering is essential for a complete understanding of the contents.

Without this technical

knowledge, the studio potter can only find clues to glaze construction which are helpful in guiding his experiments. He will find a condensed chemical dictionary an aid to the comprehension of glaze study. *The Manual of Practical Potting* by Dr. Binns and *A Potters Book* by Bernard Leach also devote space to glaze composition.

Here, the reading must be tempered

with the realization that the books were written in England, and in the case of *A Potters Book*, about Japanese glazes. III.

INFORMATION ABOUT DECORATION

The literature in this area of ceramics is scattered throughout the many books and articles written about pottery. It is necessary for the student of decorative techniques to pick up information whenever it is to be found.

*Pottery

Decorating* by R. Hainbach is the only book known to the writer which deals specifically with decorative techniques, and most of the techniques described in this book are more applicable to the needs of the large pottery manufacturer than to those of the studio potter.

Such a book as *20th

Century Ceramics* by Gordon Forsyth is filled with illustra­ tions of decorated pottery, but gives little or no informa-

tion about methods and techniques. While it is possible to find well organized litera­ ture in other areas of the ceramic field, in this one the material is nowhere organized in a single volume or group of volumes. In the following chapter, specific information has been given about the materials and methods of decoration used in this study.

CHAPTER III THE METHOD OF EXPERIMENTATION In this study, the method of experimentation was (1) to determine in advance what characteristics were de­ sired of the end product; (2) to select raw materials which in combination might lead to this result; and (3) to com­ bine materials in various proportions, always keeping care­ ful records of the experiments and their results.

If the

results were not satisfactory, a new proportion of the materials was tried until satisfactory results were achieved. For reasons of economy, the clay and glaze materials used in this experiment were selected from a number of ex­ cellent ones available in this area.

Whenever possible,

potters should utilize materials which are available to them locally in order to avoid high shipping costs. I.

THE CLAY

Prepared clay bodies.

The studio potter can save

time, space and money by selecting a prepared clay body which suits his requirements, rather than to stock raw clay mate­ rials.

Many good prepared clay bodies are on the market.

It is suggested that these prepared clay bodies be purchased

12

in the dry state and mixed in the studio in order to avoid paying clay prices for water. However, since: these prepared clay bodies might not always be available, it is well for the ceramist to under­ stand the qualities of the various raw constituents which make up a clay body.

It is rare that a single earthen

material has all of the qualities desired of a clay body. For this reason, the writer has calculated a clay body which has been used in all the pieces made for this study. Constituents of a clay body.

The usual ingredients

of a clay body are ball clay, china clay, silica and cornish stone.

According to the rtManual of Practical Potting” by

C. F. Binns, a standard formula is: Ball clay China clay Silica Cornish stone

25 parts by weight 25 parts by weight 30 parts by weight 20 parts by weight 1 Ball clay is an extremely plastic substance, but it is too sticky to work easily.

It shrinks and also warps

in drying and firing.

It is usually buff burning, slow 2 in drying and low in porosity when fired.

^ Plasticity is that characteristic of a clay body which allows it to be shaped by pressure while in the wet state and to retain that shape through drying and firing. 2 G. F. Binns, The Manual of Practical Potting, 5th revised edition, (Mew York: D. Van Nostrand Co. Inc., 1922), p. 17.

13 China clay is only slightly plastic.

It has

little strength in the wet state and has only a small amount of shrinkage in drying and firing. drying.

It is quick-

In the fired state it is white in color and high

in porosity. 3 ball clay.

It decreases the warpage tendency of the

Flint (silica) is a non-plastic material which has a very low drying and firing contraction.

Highly flinted

bodies facilitate drying and firing because of high poros­ ity, but the resultant decrease in tensile strength is apt to cause an excess of losses in the clay state. 4 5 6 glazed, they are apt to shiver and dunt.

When

Cornish stone is a flux which, when fired alone, 5 Ibid.. p. 20. ^ Shivering results from a lack of harmony between the glaze and the body. If the glaze has less cooling shrinkage than the body, it tends to be forced off the body in a sheet. 5 Bunting is a form of shivering wherein a weakness in the body permits the glaze to tear away portions of the body. 5 Binns, op. cit., p. 21. 7 A flux is the constituent of a mixture which melts at the lowest temperature.

14 softens without actually melting.

It decreases the ten­

sile strength and increases the porosity of unfired clay, but in the fired clay it decreases the porosity and increases the tensile strength Calculating the clay body.

The writer desired a

clay body which was sufficiently plastic to allow for throwing on the potterrs wheel; one which would allow for pressing in a mold; one that had sufficient strength in the wet state to permit necessary handling without breakage; 9 one that would dry and fire at cone 05 with a minimum of shrinkage and warpage. color be a warm buff.

It was desired that the fired These results were satisfactorily

achieved with the following formula: Alberhill Blue Clay Lincoln Fire Clay Talc (Sierra 99) Bone Ash (synthetic) Cornish stone Wheat straw ash

25 25 25 15 10 2

parts parts parts parts parts cups

by by by by by

weight weight weight weight weight

Alberhill Blue is a plastic clay mined by the Alberhill Olay Company in California.

It is too sticky to

work easily and has a high amount of shrinkage in drying

^ Binns, op. cit., p. 21. 9 Pyrometric cones are used in the kiln to indicate the temperature.

15 and firing.

It is buff burning and dense when fired.

Lincoln fire clay is also a California clay.

It

is a low grade ball clay and has very little plasticity. This reduces the strength in the wet state, but increases the strength of the fired clay.

It is buff burning and

high in porosity.^ Sierra Talc 99 is one of several varieties of talc which are mined at Silver Lake, Nevada. plastic material.

It is a non­

When added to a clay body it has the

characteristic of increasing the hardness of the bisque at low temperatures.

Because it has a coefficient of expan­

sion and contraction of nearly zero, it inhibits crazing and shivering of glaze.^ The characteristics of cornish stone were described on page ten. Bone ash is ordinarily used as a flux in translucent bodies.

However, in the body used in this study, it was

used as a tempering agent.

12

It has the effect of grog in

the unfired clay, but fuses with the clay in the bisque

The characteristics of Alberhill Blue and Lincoln Fire Clay were found by experimentation. 11 Ceramic Talcs, a catalog published by the Sierra Talc and Clay Company. 12 Grog is ground, fired clay which is used in a clay body to control shrinkage.

firing."1*3 14 Wheat-straw ash was used to add potash to the clay body.

It tends to increase the plastic quality of the wet

clay and acts as a catalyst to the action of the talc. All of the clays used were low in potash. Testing the experimental bodies.

Testing for

plasticity was done by making a piece of pottery by the pressing and throwing methods.

If the clay manipulated to

the satisfaction of the writer and maintained its shape in drying and firing without cracking or warping, it was con­ sidered successful.

The test for shrinkage was done by

making a coil of the clay body about one-half inch in di­ ameter and five inches long. dry and after firing.

This coil was measured when

If the coil shrank no more than

2.5$ in drying and another 2.5$ in firing, it was considered satisfactory.

The rule of thumb method of testing for

hardness by scratching with a copper coin, a silver coin and a knife blade were used.

The clay was considered

sufficiently hard if it could not be scratched easily with a knife blade.

The color of the fired clay is relative to

—

Bisque is fired clay. ^ Wheat-straw ash can be obtained by burning wheat straw and forcing the resulting ashes through a 60-mesh screen.

the desire of the individual ceramist.

There are more

scientific methods of testing the various characteristics of a clay body, but the above methods are adequate for the studio potter. 15 Colored clay slips.

Colored clay slips are an

important part of the potter’s materials.

In order that

the percentage of expansion and contraction be the same as that of the clay body, it is a good procedure to make the colored slips from the same clay mixture as is used in l6 the body. Slip should always be applied to the piece while 17 it is no more than leather hard. Colored slips serve the potter in two ways: (l) To use in painting a decorative pattern on the ware and (2) to IS use as an engobe. Colorants used to color slips and clays are often expensive.

For this reason when a bisque color

other than buff was desired, the engobe method was used.

^

Slip is clay made liquid by the addition of water.

16 Bernard Leach, A Potter’s Book, (New York; TransAtlantic Arts Incorporation, 1946], p. 5&. 17 Leather-hard is that stage in the drying process when the clay is still damp but has set to the point where the shape cannot be changed without rupture. ^ Engobe is slip applied to an entire piece for the purpose of masking an undesirable bisque color.

gngobes may be sprayed, dipped, or painted on the ware. It is difficult to obtain an even application of engobe by the painting method. Clav colorants.

Oxides and carbonates of metals,

body stains and underglaze colors can be used as coloring agents in slips. Of the oxides and carbonates, red iron oxide was found to be the most successful.

It produces a rich, brick

red in a proportion of one part colorant to twelve parts of clay, dry measure.

Manganese dioxide produces a brownish-

black color when mixed in a proportion of one part colorant to ten parts clay.

It tends to combine with certain ele­

ments of the clay and produce gases which cause blisters in the glaze.

A small percentage of barium carbonate in the

slip counteracts this.

The other oxides and carbonates of

metals have an even stronger tendency to produce blistering in the glazes and should not be used as slip colorants. If too high a percentage of colorant of a refractory nature is used it reduces the shrinkage of the slip, causing it to shrink away from the body. Underglaze colors can be used successfully in slip in a proportion of one part underglaze to four parts clay. With the exception of yellow, these underglaze colors pro­

19 duce a strong, brilliant color.

In this study, the

colors were somewhat muted in tone caused by the buff color of the clay. Body stains, while more economical than underglazes, come in a limited range of colors.

For this reason, and 19 because they can only be used as clay colorants, the slight difference in cost was considered neglible, and body stains were not used in this study. Since the clay in this study was buff in color, no white slip was possible.

If a white slip were desired, it

would be necessary to find a white burning clay which had the same coefficient of expansion and contraction as the clay body.

If a greater brillance were desired in the other

slip colors, this white burning slip would be used for the clay base of all the colored clay slips. If color variants are desired, a biaxial could be run between two colors of slip, or between white slip and a colored slip.

The procedure for running a biaxial has been

explained in Table I on the following page. Mixing colored clav slips. 19

Care should be taken in

‘ Ceramics, a catalog published by L. H. Butcher Co., 6th edition, p. 16.

TABLE. I BIAXIAL

Color "A"

1 unit

2 units

3 units

4 units

5 units

6 units

Color "B"

6 units

5 units

4 units

3 units

2 units

1 unit

Color Variant

Point 1

Point 2

Point 3

Point 4

Point 5

Point 6

Note:

1.

A six-point biaxial is indicated in the diagram above. If a finer gradation of color is desired, a biaxial with more points can be run.

2.

When slip colors are used in a biaxial, care should be taken that the density of both slips be the same in order that the amount of each colorant can be correctly estimated.

S.. A teaspoon can be used as the unit of measure. 4*

After the colors are mixed, the slip is applied to a leather-hard clay tester.

5.

The proportion should be indicated on the back of the tester, i. e., nku-l for point 1, etc. ”Btt-6

mixing the dry ingredients thoroughly before adding water. After adding water, the slip should be strained several times through a 60-mesh screen to assure a complete amalga­ mation of the clay and colorant.

Slips should be about the

consistency of thick cream. II. Raw glazes.

THE GLAZE

Glazes can be satisfactorily compounded

20 in the studio from raw materials.

When this is done, the

potter should understand what types of materials are essen­ tial to a glaze. number.

These essential materials are three in

They are (l) the flux, that ingredient which melts

and flows; (2) the stiffener which retards the flowing quality of the flux, keeping the glaze from running off the piece; and (3) the fitting agent which has the quality of • elasticity, enabling the glaze to adjust itself to the clay body to which it is applied.

A combination of these three

types of materials in the proper proportion, fired at the proper temperature produces a glaze which is colorless, transparent, glossy, and one which will neither shiver nor craze. 20

Raw materials refer to materials which have not been fritted.

22

Fluxes in glazes are divided into three principle categories. borates.

These are (1} lead, (2) alkalies, and (3) The flux, typically, represents from sixty to

eighty percent by weight of a glaze.

Raw lead has the dis­

advantage of being dangerous to the worker because it is highly toxic if introduced internally, either orally, via the lungs or the blood stream.

Most of the alkalin fluxes,

such as borax have the disadvantage of being water soluble. This causes a certain amount of the flux to be absorbed by the bisque when applied, creating some slight deficiency of flux in the glaze.

When using alkalin glazes, the potter

should never drain water from the glaze slips as this also causes the glaze to be unbalanced.

In spite of these dis­

advantages, with proper care, raw glazes can be used satis­ factorily. China clay is commonly used as a stiffener in glazes. Silica is used as the fitting agent.

A deficiency of

silica will cause a glaze to craze, while a surplus of sili­ ca will cause a glaze to shiver.

The percentage of fitter

and stiffener used in a glaze must be determined by the potter so that the glaze performs satisfactorily on a par­ ticular clay body and at a given kiln temperature. (Haze fritts.

Glaze fritts have several advantages

23 for the ceramist*

The fritting of the flux with other in­

gredients renders the lead non-toxic and the alkalin mate­ rials non-soluble in water.

In addition, since fritts

are always composed of two or more glaze ingredients, the process of measuring the constituents of the glaze is sim­ plified. Fritting of glazes entails the use of special equip­ ment and it is not expedient for the studio potter to attempt making his own fritts..

Many good commercial fritts

are on the market; leaded and alkalin, transparent and opaque. In this study, Ferro Enamel Company’s fritt 3495 was arbitraily selected.

It contains approximately thirty

percent lead and is transparent.

It fits the clay body used

in this study with a minimum of adjustment.

The addition

of 20 parts of china clay and 10 parts of silica to 70 parts of fritt 3495 produced an excellent transparent glaze which fires at cone o4# Reasons for glazing pottery. for glazing pottery.

There are two reasons

The first is one of utility; that is,

to make the piece impervious to moisture, easy to clean and pleasant to the touch. decorative purposes.

The second is for ornamental or In the latter case, new qualities may

be required of the glaze. ty and (3) mattness.

These are (l) color, (2) opaci­

These and other glaze effects can be

achieved by the addition of other materials to the glaze. Glaze colorants.

As in the case of colored clay

slips, oxides and carbonates of metals and underglaze colors are used as colorants in glazre.

The diagram on the follow­

ing page indicates what colors can be obtained from the various oxides and carbonates of metals and the average per­ centage of colorant used.

Larger proportions of the color­

ants can be used as long as. the glaze continues to function properly.

Some of the oxides in too high a concentration

tend to cause bubbles in the glaze.

Some of them are also

powerful fluxes and this must be taken into consideration when large amounts are used.

Besides the colors indicated 23 in Table II, reduction firing produces still different

colors.

For instance, a copper glaze, when reduced, is

red rather than green. Underglaze colors vary in intensity and therefore in the percentage of colorant needed.In experimenting new underglaze color, it is a good

with a

procedure to start with

five percent of colorant and to add or substract colorant in order to produce the desired color.

The range of colors

^ A reducing atmosphere indicates the absence of oxygen in the kiln. This produces chemical and color changes in oxides and carbonates of metals.

TABLE II 21 OXIDE COLORS

Colorant

Per­ cent

Leaded

Boracic Leadless

Alkalin Leadless

Copper oxide Copper carbonate Rea iron oxide Cobalt oxide Cobalt carbonate Manganese dioxide Manganese carbonate Mickle oxide Chrome oxide22 Lead chrornate

1-6 1-6 1-5 1-5 1-5 1-5 1-5 .10-.50 .10-*40 1-5

green green yellow brown blue blue claret brown violet straw brown yellow green orange yellow

blue green blue green brown blue blue claret brown violet brown green yellow green

turquoise turquoise brown blue blue violet violet brown green yellow green

21 22

red*

Binns, op. cit.. p. 79* High percentages of chrome oxide in leaded glazes will produce opaque

26 obtainable with underglazes is wide. The biaxial can be used in glazes, as in slips, for intensity of color and for variants; between two colors* Opacifiers.

Tin oxide was the original material

used as an opacifier in glazes. opaque glaze.

It produces a very white,

At the present time, tin oxide is expensive

and not always available.

There are several other mate­

rials which are equally satisfactory as glaze opacifiers. Some of these are ultrox, zircopax and zirconium spinel. The latter material was used as the opacifier in all the glazes used in this study. Matting agents. in two ways.

Mattness can be achieved in glazes

True matts are achieved by an excess of china

clay, barium carbonate or titonox in the glaze, causing silicate crystals to form on the surface of the glaze. Under-firing a glaze will also cause a matt effect.

China

clay, barium carbonate and titonox were all used in this study for mattness because the quality of the matt was different in each case. The biaxial is helpful when run between a very gloss glaze and a very matt glaze to determine the desired degree of mattness.

27

III.

DECORATING TECHNIQUES

The various techniques of decorating pottery can be conveniently divided into two categories; those which are concerned primarily with the clay and those which are con­ cerned primarily with the glaze. Each of these can be 2/j. divided into Cl) texture and (2) design. Texture in clay.

Texture is added to clay by im­

pressing the wet clay with various objects such as the fin­ ger nail, woven fabric or cord.

Clay can be polished to a

smooth, shiny surface or scratched to a rough surface. Textures can be applied to an entire piece or to only a portion of a piece.

They may be the only enrichment of the

surface, or they may be made part of a design. range from rough to smooth; from shiny to dull.

Textures Contrast­

ing textures can be used effectively on a single pot. Design in clay. number of ways.

Design can be applied to clay in a

Those which were considered to be basic

methods have been reviewed in the following paragraphs en~24

Texture differs from design in that it deals with the surface quality of a piece without reference to color or line.

23 titled incised line, low relief, and colored clay slip. Incised line.

This is a linear technique.

It is

done by cutting a narrow trough with slightly beveled edges- in such a way as to outline a pattern.

This is done

when the clay is leather hard, using a knife or modeling tool.

When a colored transparent glaze is applied, the

glaze collects in the line, creating a dark line on a light ground.

When opaque glaze is used, the line depends upon

light and shadow for its effect.

Shading can be achieved

by varying the depth of the line.

Textured areas can be

used effectively with incised pattern. A particularly good method for floor tiles is the cuenca method. in the clay.

It is done by cutting a broad, shallow line After the bisque firing, colored glazes are

placed inside the line, using a rubber syringe or brush. This creates a glazed line against a clay ground. surface is then stained and waxed.

The clay

Because glazed surfaces

are more easily marred than clay surfaces, in floor tiles it is well for the glaze to be lower than the clay so that it does not come into contact with the tread of feet. Colored clay slip.

Of the many ways that colored

clay slips can be used to decorate pottery, the three prin­ ciple methods are sgraffito, slip painting and slip trailing.

29 Sgraffito is essentially a scratched design.

The

clay body is covered with a thin coat of engobe of a con­ trasting color.

This must be done while the piece is no

drier than leather-hard.

After the engobe has set, the

design is scratched through the engobe exposing the body. The process lends itself to linear design, mass areas and 25 textured areas. It is also possible to apply several coats of various colored slips, the design being scratched to the various levels, exposing different colors. Slip painting simply is the application of colored slips with a brush, creating a pattern on the piece. or several colors of slip can be used. applied to leather-hard clay.

One

The slip should be

Slip painting and sgraffito

are sometimes used together on a single piece. Stencils can be used when repeated designs are de­ sired.

In this case, the design is cut from stencil paper,

the stencil placed on the piece and colored slip applied with a sponge.

Several colors and several stenciled designs can

be used on a single piece and it may be desirable to do some finish work with a brush.

Julia Hamlin Duncan and Victor D TAmico, How to

Make Pottery and Ceramic Sculpture, (New York: The Museum of Modern Art, 1947), p. 64.

30

Slip trailing is a method of applying a raised, linear pattern to a piece with slip squeezed from a rubber syringe.

Slip of the same color as the body or in con­

trast to it can be used.

A single glaze may be added to

26 the piece, or colored glazes can be applied to the sectiles created by the slip line.

The line will keep the glazes

from running together when fired.

A similar effect can be

achieved by applying colored slips to the sectiles and cover­ ing the entire piece with a clear glaze. All slips must be added when the piece is no more than leather-hard.

It is usually desirable to allow a piece

to become firm, in order that it not be warped by handling. Texture in glaze.

As in clay, certain surface tex­

tures can be achieved with glaze without reference to line or color areas, and various textures can be combined effec­ tively.

Glazes with cool surface craters give a rough

texture.

Crackle glazes and rutiles give other textural

effects, although these are visual rather than tactile. Textures in glazes vary from the very rough cratered glazes through the various degrees of mattne'ss to smooth, glossy 26

"

A sectile is an area of a design which is inclosed by a line.

31 glazes. Design in glaze.

Underglaze, overglaze and the

stanniferous methods are basic to applying design to pottery in glaze.

Endless variations in decoration are

possible from these three processes. Stanniferous methods.

The word "stanniferous" is

derived from the chemical word for tin, "stannum".

Strict­

ly speaking, the work "stanniferous" applies only to those glazes which are made opaque by the use of tin.

Since the

sixteenth century when only tin was used as an opacifier, other materials have been discovered which are as efficient opacifing agents as tin and less expensive.

The term

stanniferous has been used to include all methods of deco­ ration which entail the use of opaque glazes, no matter what the opacifing agent. Majolica or on-glaze painting, and dry line or cuerda seca are the two basic stanniferous methods.

The

cuenca method and the method in which colored glazes are separated by a slip trailed line might also be considered stanniferous methods, but because their execution involves manipulation of the clay, they were described in the sec­ tion on designs in clay. On-glaze painting is the method of painting a design

32 on the unfired glaze with oxides of metal or with underglaze colors.

The piece is first covered with opaque, white or

light colored glaze*

When the glaze is dry, the oxides or

underglaze colors are mixed with water and a design is ap­ plied to the glaze. glaze.

In firing, the colors fuse with the

Underglaze offers a wider range of'colors than the

oxides.

In applying the colors, care must be taken not to

apply too heavy a coat of color.

Too much of the oxides will

cause bubbling and a dispersion of the color.

W e n under­

glazes are applied too heavily, they do not completely pene-.. trate with glaze, causing a rough, unattractive surface. A deviation of this method is to apply a design with white glaze to a dark-colored, opaque glaze, making a light pattern on a dark ground. When painting a design on the bisque with colored, opaque glazes, it is necessary to separate the colored glazes with a line which will repel the molten glaze in order that the various colors maintain a clean separation.

A mixture

of manganese and light*motor 'oil or oil of< layander is used for this line.

Unless a thin coat of transparent glaze is

applied to the piece, over the glaze pattern, this line will be dry, or unglazed, thus the name dry line or cuerda seca. The method is to paint a linear pattern on the bisque ware with the manganese dioxide and oil mixture, outlining color

33 areas.

When the line is dry, colored opaque glazes are

applied to the sectiles created by the line.

If the

glazes are applied too heavily to vertical or near-vertical surfaces, the weight of the molten glaze will pull it over the line.

It is safest to use this method on surfaces

which are more or less horizontal, and to apply the glazes as smoothly as possible about the thickness of a knife blade. In all of the stanniferous methods, matt, semi-matt or glossy glazes can be used. Underglazes.

It has been seen that underglaze

colors are useful colorants for clay slips and glazes, and that they can be used in on-glaze painting.

In addition,

underglazes can be, and most commonly are, painted directly on the bisque and covered with a transparent glaze. Water is used to make the colors liquid.

Underglaze must

not be applied too heavily, or the glaze will not adhere to the piece. Underglazes are also available in underglaze crayons and are applied to the bisque in the manner of crayon or pastel.

If desired, these crayons can be made by mixing

the underglaze colors with china clay in about the same proportion as in making colored clay slips.

This mixture

34

is made into a dough consistency by adding a solution of gum arabic, rolled into a crayon shape and dried.

The gum

can be burned out of the crayons by firing to cone 022. Overglazes and lusters.

Unlike the underglaze

colors, overglaze colors are low fusion glazes.

They are

applied to the fired glaze and the piece refired to about 01$.

The most brilliant colors are obtainable with over­

glaze, particularly the reds.

Overglaze colors are usu­

ally applied to a white, opaque glaze, although light col­ ored glazes are also suitable.

The method is to mix the

colors on a palette with a knife in a mixture of one part oil of capaiba to three parts fat oil of turpentine.

Care

should be taken to work all of the bubbles out of the over­ glazes before applying them to the piece.

The colors can

be applied thinly like water color or more thickly like tempera.

The colors change very little if properly fired.

Lusters are metallic resinates in a combination of bismuth. per.

The most common are liquid gold, silver and cop­

These colors are also applied to the fired glaze and

require a separate firing.

They can be used successfully

on gloss, semi-matt or matt glazes; white or colored.

General information.

All of the methods described

35

above require a certain amount of practice in order to perfect the techniques*

Each method has its own peculi

arities and these can only be learned and mastered by working with the material itself.

Insofar as possible,

however, the writer has pointed out common errors and things to avoid. The following two chapters have been illustrated with photographs of pieces produced and decorated by the writer.

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Turoff, Muriel Pargh, Pottery and Other Ceramic Ware. New York: Crown Publishers/ 1949.w 145 pp. Wilson, Hewitt, Ceramic Clay Technology. New York: Hill Book c “ T 9 2 7 7 ^ 9 ^ T p 7 ” B.

McGraw-

CATALOGS

Ceramic Talcs for Low Temperature Whitewares, a catalog. Los Angeles: S’ferra Talc and’ Clay “Company, 194$. Ceramics, a catalog, sixth edition. Butcher Co., 1949. 64 pp. C.

Los Angeles:

L. H.

UNPUBLISHED MATERIALS

Goodwin, Je&n, tfA Ceramic Mural for the Lobby of the Science Building” . Unpublished Master of Fine Arts Thesis, The University of Southern California, Los Angeles, 1937. 66 pp.

o f S o u th e r n C a lifo rn ia Literary