THE TECHNIQUE OF STAMMOUS CHLORIDE FUMING

OF CONE 4 GLAZES AND LUSTERS

A THESIS

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR

THE DEGREE OF MASTER OF ARTS IN

IN THE GRADUATE SCHOOL., OF THE

TEXAS WOMAN'S UNIVERSITY

COLLEGE OF

FINE ARTS

BY

CINDY LEE LORD

DENTON, TE~S

HAY, 1974 TABLE OF CONTENTS

ACKNOWLEDGMENTS . . • ! • ...... , . ~ ' .

INTRODUCTION , · • • .. • • • • • • • • • ..• .~ j .' •., ti • - • l

Chapter

I. EXPLANATION OF. .. ,FUMING • • - ·,. . . . .·. · ...... 2

II. LIMITING. THE VARIABLES . .- ...... •..,. . . . 5 ' III •. TECHNIQUE \ OF FUMING ...... 12

IV. EFFECTS OF FUMING ON LUSTERS a ' • • a II ; • a a . 'a i 7

v. CONCLUSIONS • • ...... • • • • • 19

BIBLIOGRAPHY • • ...... ' . . . . . 20

PHOTOGRAPHIC ESSAY ...... ' ...... : . . . . 22 INTRODUCTION

The contemporary artist-potter's search for more varied surfaces for his objects has brought about an interest in fuming with metallic salts. In the past, fuming has been a technique utilized in glassware produc­ tion. Recently potters have employed this technique in conjunction with salt glazing; however, fuming can be used effec tively on a wide variety of glazes.

This study will be concerned with the effect of stannous chloride fuming on medium range cone 4 glazes and on gold and silver luster glazes. These glazes will be used on ceramic . Glazes often tend to visually disintegrate the total form of sculptural piece s.

In this study the luster glazes will be used to enrich certain areas, and the fuming will be employed to unify the total form.

I CHAPTER I

EXPLANATION OF FUMING

How Fuming Imparts Iridescence

"Fuming results from heating a metallic substance,

usually stannous chloride, to the point where it vaporizes

and imparts iridescence to glazed surfaces which are close 1 enough to be affected." Only an extremely thin l ayer of

glaze is subjected to this chemical change. The resulting

iridescence does not have the brilliance or ref l ective

power of a luster glaze. Very subtle r ainbow hues are r e flected from the f umed surface.

Iridescence is based on the principle of thin film interference. The following diagram shows what happens when light be ams interfere:

8 Fig. 1.--Diagram of interference of a light beam

1 Jack Troy , " Fuming in the Salt ," Craft Horizons, June , 1972, pp . 28- 29 , 71.

2 3

Two beams of light (1 and 2) strike the surface of

a fumed glaze. Beam No. 1 is reflected at the upper surface ,

while beam No . 2 enters the film and is deflected from its

path by the film's greater density. Beam No. 2 travels to

the lower surface, is reflected, and returns to the upper

surface, where it rejoins beam No. 1. The distance ABC

minus CD is the phase difference between the two beams.

This phase difference varies with the thickness of the film

and the angle at which the light beam strikes the surface.

If the phase difference between beam No. 1 and beam

No. 2 equals one full wavelength or a multiple thereof, that wavelength or color will be reinforced. Other wavelengths will be weakened or eliminated. With every change of

optical distance a different color appears. Corrunon occur­

rences of the phenomenon of thin film inte rference would be

the rapidly changing colors seen on the surface of soap bubbles or the pure colors seen on the oil slick on a wet street.

Chemical Reactions Involved in Fuming

Three chemical reactions are involved in vapor deposition onto a glaze. When the stannous chloride is heated, it vaporizes and decomp oses. This f irs t chemical reaction may be stated as follows:

(1) SnC1 heat Sn(vapor) + Cl (vapor) 2 2 4

'rhe chloride vapor is absorbed into the air. The

stannous vapor re a cts with t he oxyge n atoms on t h e glaze

s urf ace to f orm an o xidized layer . The surface l a yer

r e action is:

(2) Sn+ 20 heat Sn0 2

A surface layer of Sn0 is built up on the gla ze 2 until there are no longer surface oxygeu atoms ava ilable to bond with the Sn vapor atoms. When thi s occurs, the

Sn vapor atoms then bond to the Sn atoms in the Sn0 layer 2 to form a tin metal layer :

(3) Sn(bonded to o + Sn(vapor) heat+ 2Sn 2

The layer of tin metal is built up a s the vapor deposition continues. This thin layer of tin metal causes interference e ffects with the light it reflects. CHAPTER II

LIMITING THE VARIABLES

Choice of Body

A cone 4 terracotta clay body suitable for buildin g

sculptura l pieces was selected. The high non- compo­

sition of terracotta contributes to low body shrinkage, and

the coarse grain structure permits even and rapid drying.

The deformation and c r acking corrunon in large forms built with more plastic clay bodies is avoided. With both the

clay body and the glaze maturing at cone 4, there was l ess chance of the glaze crazing. Crazing is the development o f fine cracks in the finished glaze and is caused by tension between the clay body and the glaze.

The following clay r eci pe from Richard Behren's book, Glaze Projects was used:

Terra Cotta Body II Cone 4

Plastic Fireclay 65.0

Grog (substituted for sand) 22.5

Talc 7.5

Potash Feldspar 5.0

100.0%

5 6

Terracotta test were made for glaze exper i­ ments. This clay body lacked the desired plasticity even

after aging. To slightly increase the plasticity, one per cent of was added.

After the glaze recipe was established, the same terracotta clay body was used in building sculptural pieces. Although this was a very rough textured clay, it was possible to use it in throwing v e ry simple forms on the wheel to be intergrated into handbuilt pieces .

Determining the Glaze Composition

R. T. Watkins researched the degree to which different glazes take vapor lusters. Wa tkins selected three different groups of gla zes with v a ria tions in the oxygen ratio, the B 0 to SiO ratio , and the lime and 2 3 2 lead content. A 0.2 equivale nt of Al 2 0 3 was maintained constant in all the glazes. The results of his tests showed that, "glazes having an o xygen ratio of approx­ imate ly 3 t o 1, a medium or low acid conte nt with a B2 o3 to Si0 2 ratio of approximately 1 to 2 .5 a nd high in lead, and low in lime, take the best vapor luster effects ." 2

2 R. T. Wa tkins, "The Degree to Which Different Glaze Compositions take Vapor Lusters," Journal of the American Ceramic Society , V (1922), 28. 7

Charts with unity formulas for cone 10 fritted

glazes are included in his . The following glaze

formula, stated in terms of molecular equivalents, was

selected for use in this study:

K 0 .15 Al 0 • 2 1.5 2 2 3 .15 • 6

.40

PbO .30

The following batch recipes were calculated from

the unity formula:

Glaze I

Colemani te 82.4

Nepheline syenite 22.6

Pearl ash l. 4

Lead carbonate 232.5

Flint 90.0

Glaze II

Potash feldspar 83.6

Borax 114.6

Soda ash 12.7

Lead carbonate 232.5

Flint 36.0

Whiting 40.0

Although Mr. Watkins fired his glaze test tiles to cone 10, it was found that his glaze formula became 8

fluid at cone 4 with no addition of fluxing material.

These two glazes contained a high percentage of alkalines

which resulted in the surface crazing upon cooling. To

reduce this tendency to craze, 15 per cent flint was

added to the glaze batch.

Flint added to the glaze melts during the firing

and becomes part of the non-crystalline glass when cool.

The thermal expansion of the glaze is decreased and the

cooling contraction is lessened. This small addition of

flint reduced the tension between the clay and the glaze.

The ratio between the B2 0 3 to Si0 2 was altered slightly; however, the fuming results were not affected.

Both Glaze I and Glaze II are colorless, clear,

s hiny glazes. Glaze II has a slight yellow tinge caused by the pearl ash. Less crazing and better fuming occurred with thin application of these glazes.

Us e of Colo r e d Engobes

Color was obta ined by applying the transparent glazes over engobes. The iridescent effe cts were more pronounced over engobes containing albany , cobalt oxide and iron oxide. Engobes I and II were published in Jack Troy's article, "Fuming in the Salt Kiln."

Engobes III and IV were developed during the course of this study. 9

Engobe I-Boston Brown Engobe II-Pilfer's Slip (from Daniel Rhodes)

Albany slip 60 Potash feldspar 150

Cornwall stone 25 15 0

Whiting 4 Flint 125

Red iron oxide 5 Fireclay 75

1% cobalt carbonate-

10-30% rutile-honey

Engobe III Engobe IV

China clay 25 Albany slip 100

Ball clay 20 Ball clay 20

Feldspar 30 2% cobalt oxide-blue

F lint 20

Zircopax (Ultrox) 5

Bo rax 5

25 % red i r on oxide-brown

2% cobalt oxide-deep blue

Choice of Me t a llic Salt

The most successful metallic salt used in fuming

is sta nnous or tin chloride. Strontium nitrate, barium

chloride, cobalt nitrate, bismuth nitrate, silver chloride,

silver nitrate, iron chloride, chromi um nitrate and cupric chloride are other chemicals that can be used. These other 10

salts are often combined with stannous chloride to produce

special effects. Glass Technology lists the following

combinations:

" ( 1) For ordinary effects-pure stannous chloride.

(2) Blue shade s-stannous chloride 80 parts,

strontium nitrate 5, barium chloride 15.

(3) Red shades-sta nnous chloride 88, strontium

nitrate 7, barium chloride 5.

(4) Opalescent effects-stannous chloride 90,

bismuth nitrate 10. 11 3

Stannous chloride in c r ystal form was chosen for

the basic fuming ingredient. Stannous chloride is derived b y dissolving tin in hydrochloric acid. A white, crysta l­

line mass results which, when exposed to oxygen in the ai r , is converted into the insoluble oxychloride. Stannous chloride, also called tin salt, is soluble i n water, alk alies, tartaric acid, and alcohol. This chemical be come s ineff ectual for fuming if exposed to air. It should be kept tightly sealed and dry. Caution should be observed in handling stannous chloride f or it can cause skin irritation.

3Hodkin and Cousen, Textbook of Glass Technology, (New York: D. Van Nostrand Company, 1925), p. 521. 11

Se lection of Equipment

Fuming may be accomplished in any type of kiln. A

gas kiln is subject to uneven temperatures, while a more

uniform heating is obtainable in an electric kiln. For this

reason an electric kiln, with interior dimensions of 17" by

18", was selected. An Amaco pyrometer was used during the

first firings; however, visual judgement of temperature

proved to be mor r eliable than a ctual calibration.

Breathing the fumes of the metallic compounds is

extremely hazardous. Fuming should be limited to outdoor or to kilns in well ventilated areas. A kiln with a

chimney draws the poisonous va~ors out the flue and is

tnerefore safer than an electric kiln.

A gas mask may be used for the protection of the e yes and t he mucous membranes o f the n o s e. For this study a war surplus gas mask which c overed the entire face was purchased. Upon the recommendation of the Arlington Fire

Departme nt, the filter was seale d and a twenty foot piece of garden hose was conne cted to the mask for a remote oxygen intake. CHAPTER III

TECHNIQUE OF FUMING

Determining the Temperature

Fuming may be done during the cooling of the glaze firing, or a previously glazed piece may be reheated and fumed. The surface of a previously glazed piece should be perfectly clean.

The temperature at which to fume is critical. A chalky, cloudy haze occurs on the glazed ware if fuming takes place much above or below 700 degrees Centigrade, If clouding occurs, the piece is ruine d, Reglazing is not possible, for bubbling occurs betwee n the fumed surface and the new glaze surfa ce during the firing.

It is best to visually observe the kiln's interi or and become acquainted with the color variations which occur during the cooling cycle. The best results are obtained with the f uming salt being introduce d when darkness fi rs t invades the kiln • . If the outlines of the fired objects are still discernable and the kiln walls still glow with redness, it is too ear ly for fuming.

12 13

Me thods of Fuming

There are various methods of fuming in an electric

k iln. The pot, when cooled to a low red heat, may be t aken

out of the kiln with tongs and sprayed with the chloride

dissolved in alcohol.

Instead of spraying, the pot may be held over

chloride fumes. For this method, a tin can containing a

mixture of chloride and alcohol is placed over a small

burner. The he ated chloride will vaporize, and the pot

can be rotated above the fumes. I

iin c.on con~a,r.,n~ chloride and ci\cono\

Fig. 2.--Diagram of tin can fuming

After using either of the above methods , the pot is returned to the kiln and allowed to cool slowly. Quartz inversion, during which pie ces a re likely to crack, occurs 14

at 300 degrees Centigrade. The pieces should be fumed and

place d back into the sealed kiln before this temperature i s

reached.

The potter has more control over the placement of

the fumes when the piece is removed f rom the kiln. With

r e moval, howe ver, the size of the piece must be limited,

and there is a risk of breakage if the piece is sprayed

with the alcohol and chloride solution.

The pipe fuming method involves making a wand of

a pipe with holes drilled near the plugged end. The pipe

is heated, the chloride crystals inserted, and the p i pe i s

moved around inside the kiln through a por t. The way the

kiln is stacked will de termine on which pieces the fuming

is concentrated.

) Cop on end GI;::_•·'.!_.~·:~·~:~·~: ______II;nsert ch \ol'"id e. +his end

Fig. 3.--Diagram of pipe for fuming

After experimenting with several procedures, it

was found that the best method of introducing fuming int o

an electric kiln was with a c e ramic cup and iron tongs.

The cup was heated along with the pieces to be fumed in the kiln. When fuming temperature was reached, one fourth o f 15

a teaspo on of the stannous chloride crystals were placed

into the cup and moved a r ound inside the kiln wi t. h the

tongs. This method gives complete control over the place-

ment and amount of fuming. Wh en the desired effect is

obtained, the cup is removed and the kiln is sealed. Eye

and respiratory hazards are the grea test drawbacks with

this procedure, and extreme caut i on must be taken. The

fuming was done outdoors and a gas mask was used.

K,\n wa.\ls

Fig. 4.--Diag ram of fuming with cup and tongs

Failures in Fuming

In fuming t here is a l ways the risk of a cloudy haze

f orming on the ware . With e xperience and some los ses, the uotter learns how to a void this scumming. This undesirable haze occurs when: (1) the kiln is inadequately ventilated, 16 thus allowing the fumes to remain in the kiln; (2) the fuming occurs at too hot a temperature; (3) the ~urning occurs a t too low a temperature; (4) the fuming is too heavy; (5) the fum­ ing salts are deposited directly on the surface of the ware. CHAPTER IV

EFFECTS OF FUMING ON LUSTERS

;?\rrne a ling of Lusters

Duane Littell's thesis contains a small section on

the use of luster glazes and fuming in combination with salt

glazing. Similar experiments usin g the cone 4 glazes and

commercial gold and silver lusters produced some outstanding

results.

Cleanliness and neatness are imperative when apply­ ing lusters, for lusters are easily contaminated. Benzol, lacquer thinner or alcohol may be used for cleaning the surface to be decorated and for cleaning the applicator.

After applying the luster, the piece should be slowly heated in a well ventilated kiln. The kiln door may remain open during the early stages of firing. Too rapid heating ot the kiln will cause a crust to form on the luster, which, in turn, will prevent the burning out of volatile oils. When all smoking has ceased, the kiln may be closed and fired to cone 019. At this temperature the luster will be annealed to the glaze surf ace.

17 18

Fuming of Lusters

After the luster is annealed, the kiln is heated to fuming temperature, and the stannous chloride is then intro­ duced. Silve r luster when fumed changes to a mottled blue with tinges of red. Gold luster when fumed turns a green to blue-green or a reddish-gold to pink.

Fumed lusters may be refired to cone 019 without altering the fumed color effects; thus, gold and silver lusters may be applied next to fumed gold and silver lus- ters. Several distinct color variations may be produced in t h is manner. CONCLUSIONS

The technique of fuming offers the artist-potter a method of further enhancing the surface of h e r work. The equipment and chemicals necessary for fuming is minimal, and the results are possible to obtain on a wide range of glazes.

Fuming adds a certain vitality, a visual e x citement to an object. Fumed surfaces are their most provocative outdoors or near a window where the play of sunshine brings a changing array of rainbow hues.

The colors res ulting when luste rs a r e fumed are very unusual and are not obtainable with any othe r method.

The lusters, with their very rich s u rfaces, shou ld be used s paringly, for the y e a sily become garish a nd overwhelming.

This study was concerned with the effects of only one fuming salt. Future plans include fuming experiments with other salts, combinations of other salts and different glazes and different lusters.

19 BIBLIOGRAPHY

PUBLISHED MATERIALS

Hodkin and Cousen, ~tbook of Glass Technology. New York:

D. Van Nostrand Company, 1925.

Reitz, Donald. 11 Salt Glaze Process." Salt Gl a ze Ceramics ,

New York: American Crafts Council Research and

Education Department and the Museum of Contemporary

Crafts, 1972.

Rhodes , Daniel. Clay and Glazes for the Potter. Philadelphia

and New York: Chilton Books, 1 95 7.

Simon, Hilda. The Splendor of Iridescence. New York: Dodd,

Mead and Company, 1971.

'Troy, Jack. "Fuming in the Salt Kiln." Craft Horizons.

June, 1972, pp. 28-29, 71.

Watkins, R. T. "The Degree to Which Different Glaze

Cornposi tions take Vapor Lusters." Journal of the

American Ceramic Society, V (1922), 28-33.

20 21

UNPUBLISHED MATERIAL

Littell, Duane. "Salt Glazing and Related Techniques."

Unpublished M.F.A. thesis, College of Ceramics,

Alfred University, 1971.

Troy, Jack. letter. Juniata College, Huntingdon,

Pennsylvania, April 12, 1973.

Vergano, P. J. letter. Owens-Illinois Technical Center,

Toledo, Ohio, November 8, 1973. 22

PLATE I

Stoneware clay body Cone 4 glaze, lusters, fumed 24" X 15" 23

PLATE II

Stoneware clay body Cone 8 glaze 15-1/2" X 11" ' 24

Group of stoneware clay pots Cone 4 glazes Tallest pot 13" 25

PLATE IV

Stoneware clay body Cone 8 glaze 13-1/2" X 12" 26

PLATE V

Stoneware clay body cone 4 glaze, lusters, fumed 21" X 8" 27

Stoneware clay body Cone 8 glaze 9" X 13-1/2" 28 ~------PLATE VII

Stoneware clay body Cone 4 glaze, lusters, fumed 20" X 15-1/2" 29

PLATE VIII

Stoneware clay body Cone 4 glaze 11" X 13" 30

PLATE IX

Stoneware oiay boy cone 4 giaze 23" X i2" 31

PLATE X

Stoneware clay body Cone 8 glaze 21-1/2" X 11 11 32

PLATE XI

Stoneware clay body Cone 4 glaze, lusters, fumed 22-1/2" X 10" 33

PLATE XII

Stoneware clay body Colored engobes, salt gl a zed 36-1/2" X 9'-1/2"