The Technique of Stammous Chloride Fuming of Cone 4 Glazes and Lusters in the Graduate School of the Texas Woman's University Fi
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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 CERAMICS 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 ceramic 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 sculptures. 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 Kiln," 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 Clay Body A cone 4 terracotta clay body suitable for buildin g sculptura l pieces was selected. The high non-plastic 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 tiles 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 bentonite 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 paper. 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 slip, 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 Ball clay 15 0 Whiting 4 Flint 125 Red iron oxide 5 Fireclay 75 1% cobalt carbonate-blue 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.