PPP: Influences on Clay Color

November 29, 2002
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Red earthenware clay containing high percentages of iron and manganese.
When observing a fired piece of clay, whether it’s a functional pot or a ceramic sculpture, we inevitably notice its color. This very simple observation can have a far more complex starting point—the clay body formula. A clay body can be composed of just one clay or any ratio of clays, feldspar, flint, talc and grog (a calcined, inert refractory particle of varying sizes contributing “tooth” [texture], low shrinkage and reduced warping to a clay body). As with glaze colors that provide different aesthetic qualities to the ceramic form, the clay body color also affects the appearance of the finished ware. Ever since the first “primitive” pottery was fashioned before recorded time, potters have always considered the color of the clay once it emerged from the kiln. Archaeological findings from the earliest known pottery indicate that the pots were not glazed, which reflected an even greater reliance on the color of the clay. Several factors can contribute to the fired or final color of a clay body.

White porcelain clay containing low percentages of iron and manganese.

Clay Body Formula

The clay body formula is a critical element in determining the fired color of the clay. Iron oxide is one of the strongest metallic color-producing agents found in clays. It is also very sensitive to changes in kiln atmosphere, which can bring about a color change in the clay body. Iron oxide can be incorporated into a clay body formula by the addition of the raw ore, the synthetic oxide form of iron oxide, or iron-bearing clays. Iron oxide can be found in several naturally occurring forms, including magnetite (Fe3O4), hematite (Fe2O3), limonite (Fe2O3.xH2O), goethite (Fe(OH)3), ferrous oxide (FeO), pyrite (FeS2), ferrous sulfate (FeSO4.7H2O) and siderite (FeCO3), all of which can affect the color of a clay body.1 (See the section on “Kiln Atmosphere” for some specific examples.) Manganese dioxide can also have a profound effect on the color of clay bodies. It is often found in fireclays, ball clays and stoneware clays in varying concentrations and particle sizes. Due to the air floating processing of ball clays and stoneware clays at the mine, large manganese dioxide particles (approximately 48 to 80 mesh) are taken out of the clay, leaving only small pepper-sized specks (below 100 mesh). Manganese dioxide can cause black/brown specking in the fired clay. The degree to which specking is noticed depends in part on the kiln atmosphere, with reducing atmospheres revealing greater specking. Many ceramic suppliers will try to duplicate in oxidation fired electric kiln clay bodies the random look of specking in reduction fired clay bodies by adding granular manganese dioxide 60x mesh to clay (51⁄2% of manganese dioxide 60x mesh will impart approximately 200 specks per/in. of fired clay surface area). However, this effect often looks too uniform to qualify as the typical random specking found in clay bodies fired in a reduction kiln atmosphere. Porcelain or other white firing clay bodies are formulated with high percentages of kaolins, white firing ball clays and white stoneware clays, all of which contain low amounts of iron and manganese. White clay bodies, which often reveal the additions of metallic coloring oxides or stains better than dark bodies, can produce various shades of blue, green, light brown, yellow, orange or almost any shade of color with these additions. For example, cobalt oxide is one of the strongest coloring oxides, and as little as one part will tint 100,000 parts of a white clay. In a few instances, a specific clay body component will have a neutralizing or negative effect on a metallic coloring oxide or stain introduced into the clay body. Mason stain Alpine Rose 6001, for example, which contains chromium oxide (Cr) and tin oxide (Sn), will not cause a pink fired color when added to a low-fire ball clay/talc clay body.2 The talc component of the clay, which contains magnesium, will have a negative effect on this stain-clay body combination. Dark firing clay body formulas, which already contain high percentages of iron bearing clays, can produce darker fired color clay bodies when metallic coloring oxides or stains are added to the clay body. Additions of metallic coloring oxides or stains in a clay body are also subject to the effects of the kiln atmosphere.

Iron bearing stoneware clay fired in a reduction kiln atmosphere.

Kiln Atmosphere

The kiln’s firing atmosphere affects all clay bodies to some degree. Reduction Atmosphere. A reduction atmosphere is created when excess hydrocarbon fuel-to-air ratios create carbon monoxide, an “oxygen-hungry” colorless and odorless gas. Carbon monoxide pulls or draws an oxygen component away from specific oxides found in the clay body and glazes. Metallic coloring oxides such as copper, zinc, nickel cobalt, iron and manganese are subject to reduction atmospheres produced in pottery kilns. Alumina, vanadium, titanium, cerium, barium, magnesium, chromium, calcium, sodium, potassium and silica cannot be reduced sufficiently in the typical pottery kiln.3 Hydrocarbon-based fuels, including natural gas, propane, wood, oil or methane, can also contain sulfur dioxide and sulfur trioxide; such fuels can create carbon monoxide and carbon dioxide when heated. These byproducts of combustion, along with the oxygen and nitrogen present in the air, can have subtle effects on clay body color. These effects vary depending on the type and amount of metallic oxide used. The effect of a reducing atmosphere on clays is most noted with clays containing iron. High percentages of iron oxide or iron bearing clays in a clay body formula tend to produce brown/orange colors in reduction atmospheres, and larger particles of iron oxide can cause brown blemishes in reduction fired clay bodies. Small trace amounts of iron found in kaolins used in porcelain clay bodies and white stoneware clay bodies can produce a “blue-white” color in reduction kilns.1 Granular manganese dioxide in larger particle sizes (60x mesh) can cause black/brown specking in reduction fired clay bodies. Other oxides or stains can be introduced into the clay body to produce colors depending on the amount used, the specific metallic coloring oxide/stain, the basic clay body formula, and the reaction of the metallic coloring oxide/stain to reducing atmospheres. For example, when using a porcelain or white firing clay body, it is possible to obtain a pink fired clay color by adding copper carbonate (1⁄2 to 4% based on the dry weight of the clay body). As with other additions of metallic coloring oxides and stains, it is always better to start with a white or off-white clay body, except when a dark brown or black clay color is desired, so the effect of the addition can be seen more clearly. Starting with a clay body that is already brown will enable the potter to add metallic coloring oxides/stains to achieve darker browns or a black fired clay body. Neutral Atmosphere. A neutral kiln atmosphere occurs in a hydrocarbon-fueled kiln when there are equal amounts of air and gas ratios to achieve combustion. Neutral kiln atmospheres can yield different colors in clay bodies and glazes, depending on the base materials in the clay or glaze formula and the addition of metallic coloring oxides or stains. Neutral kiln atmospheres are the middle stage on the continuum of atmosphere firing options from reduction to oxidation that hydrocarbon-based fuels are capable of producing in pottery kilns. Oxidation Atmosphere. Oxidation atmospheres can take place in hydrocarbon-fueled or electric kilns; all that is required is a ratio of air greater than fuel. In electric kilns, an oxidation atmosphere is a given and is always present. Some clay body colors will be altered when fired in oxidation hydrocarbon-fueled kilns as compared to oxidation electric kilns. Changes in clay body color can be attributed to a moisture component in hydrocarbon fuels. Iron oxide in a clay body can produce muted cream colors in an oxidation atmosphere, as opposed to the brown/orange color of the same clay fired in a reduction kiln atmosphere. In oxidation atmospheres, clay bodies containing manganese dioxide in either the powder or granular form are muted in color. Often granular manganese is visible as a light brown speck in the fired clay body. Low iron bearing clays in oxidation kiln atmospheres can exhibit vibrant colors with the addition of metallic coloring oxides or stains. The same clay body formula in a reduction kiln atmosphere shows muted colors due to iron or manganese present in the clay body. Wood/Soda/Salt Atmosphere. The introduction of wood, salt or soda into a kiln can take place in a reduction, neutral or oxidation atmosphere and can cause distinctive color effects on the clay body. Wood ash can combine at high temperatures with the alumina and silica present on unglazed areas of the clay body to produce an alkaline glaze, which can tint the clay body color light tan, brown or gray depending on the ash deposit depth and kiln atmosphere. In reduction kiln atmospheres in salt and soda kilns, the sodium present in sodium chloride or sodium carbonate reacts with alumina and silica combined in the clay body to form a sodium/alumina/silicate glaze. The glaze seals in the black ferrous oxide in the clay body, producing a gray clay body color. The same gray clay body color is also prevalent in wood kiln firings, due to the alkaline ash forming a sealing glaze and preventing the iron present in the clay body from turning to a re-oxidized brown ferric oxide color as oxygen enters the cooling kiln.4

Soda firing produces an “orange peel” pebble effect glaze, which prevents the iron present in the clay body from re-oxidizing (right side of pot).

Firing Temperature

Increasing the firing temperature of a clay body will cause additional glass formation within the clay. Any iron, manganese or metallic coloring oxides found in the clay will cause the clay body to darken in the fired color. The specific oxide and its percentage found in the clay body, along with the kiln atmosphere, will all play a part in the fired clay body color. High iron content earthenware clay bodies fired to cone 012 (1576∞F) will appear light red/brown or dark pink in their fired color. The same clay body will darken considerably when fired higher to cone 01(2043∞F).5 Not only can the absolute, or end point, firing temperature affect clay body color, but the rate of heat increase to the end point temperature can also cause color changes. A relatively fast firing can produce a lighter fired color due to the immaturity of the clay body. Longer firing times promote greater glass formation in the clay, which, in turn, will darken metallic coloring oxides found in the clay. Following this continuum of time/temperature reactions, one indication of an over-fired clay body or a clay body taken past its point of maturity can be its very dark fired color.

Blue slip applied to a black clay body.

Special Effects

Potters can also use many special effects either alone or in combination to incorporate additional color properties to their ware. Metallic Oxide/Stain Wash. Metallic coloring oxides such as copper oxide, cobalt oxide, manganese dioxide, iron oxide or chrome oxide and their carbonate forms, including copper carbonate, cobalt carbonate or manganese carbonate, can be combined with water and painted on a raw or bisque fired clay body to impart color to the fired clay. Various fired clay body colors can also be achieved through the use of stains, which, like metallic oxide washes, are suspended in water and applied to the exposed clay body. Washes, whether derived from metallic oxides, metallic carbonates or stains, will not become a glass or glaze when fired as they do not contain sufficient fluxing capability; however, they will impart surface color to the fired clay. Metallic Oxide/Stain Fuming. Fuming with metallic coloring oxides, metallic coloring carbonates or stains can be accomplished by painting a refractory kiln post with a paint consistency mixture of oxide, carbonate or stain. The post is then placed approximately 1⁄16 in. from the clay body or glaze surface. At temperature, the color fumes or vaporizes off of the post and onto the clay surface, leaving a “blush” or vapor trail of color. The pattern and color of the fume is dependent on the shape of the refractory post, the shape of the pot, the fuming agent and the kiln atmosphere. Metallic Salt Fuming. Fuming salts of tin chloride (stannous chloride) and bismuth subnitrate are the most commonly used salts introduced into the kiln during its cooling cycle, at approximately 1292∞F. Strontium nitrate and barium chloride can be added to tin chloride to produce red and blue lusters.6 As the salts volatilize, they can land on exposed clay body surfaces. The pattern and area of coverage depends on the amount of salts introduced (100 grams per 40/cu/ft. of interior kiln space is a workable ratio of salt to kiln space) and the point of entry. The salts can produce a thin, easily abraded, dull pearl-like iridescence on exposed clay body surfaces, and the fuming effects on the fired clay can look very much like oil on a water surface. Clay Slips (Engobes). Slips contain water, clay(s) and other ceramic materials to ensure a compatible fit when brushed, dipped or sprayed on leather hard or bisque ceramic surfaces. The slip can be colored by the addition of metallic coloring oxides, metallic coloring carbonates or stains. While there is not enough glass formulation in the slip to result in a fired glass surface, it can still fuse to the underlying clay body if correctly formulated and can contribute color to a clay body surface; in fact, slips are essentially colored clays. Terra Sigillata. The Latin term terra sigillata, meaning “sealed earth,” is derived from a specific platelet size liquid clay, which is spread thinly on a leather hard pot.6 After the terra sigillata coating has dried, it can be burnished, aligning the clay platelets parallel to the underlying clay body. When the terra sigillata surface is fired it fuses slightly, causing a smooth, satin colored clay surface. Terra sigillata can be formulated into many colors depending on the clay base used and additions of metallic coloring oxides or stains. As with slips and engobes, a white clay base in the terra sigillata will reveal a wide range of color options with the addition of metallic coloring oxides, metallic coloring carbonates and stains. Pit Firing. Pit firing is a “primitive” method of imparting color to a clay body surface. Pots are placed in an earthen pit with combustible materials, and during the firing process, oxidation and reduction atmospheres randomly take place within the pit due to complete and incomplete combustion of the fuel. When excess fuel-to-air ratios take place, the unburned fuel produces carbon in the pit atmosphere. The carbon is then impregnated into the porous unglazed areas of clay body, causing random patterns of black to gray flashing. Glaze Flashing. A color change in a clay body can occur on the edge of a glazed/unglazed fired clay surface. The darker clay flashing along the clay/glaze line is the result of salts in the glaze migrating out into the clay body during the firing. The darker brown vapor line is prevalent when iron is present in the clay body and is fluxed by the soluble salt migration from the glaze. The line of discoloration on the clay body can extend 1⁄4 to 1⁄2 in. beyond where the glaze ends.

Pit-fired pot with carbon flashing pattern on the unglazed clay body.

A Combination of Factors

The individual properties of a clay body formula, kiln atmosphere, firing temperature and various special effects all work together to produce the color of a fired clay body. Each element, depending on use, can have a major or minor influence on the final outcome of a fired piece. By understanding the factors that influence clay body color and knowing how they affect each other, potters can effectively control the use of color in their aesthetic palette.

Black glaze showing brown flashing on the exposed clay surface.

References

1. Lawrence, W.G., Ceramic Science for the Potter, Chilton Book Co., 1972, pp. 35, 119-120. 2. “Mason Color Chart and Reference Guide,” Mason Color Works, Inc., P.O. Box 76, East Liverpool, OH 3920-5076. 3. Hamer, Frank, The Potter’s Dictionary of Materials and Techniques, Watson-Guptill Publications, New York, 1975, pp. 248-249. 4. Zamek, Jeff, “Sodium Carbonate Vapor Firing, Parts 1 and 2,” 1974 research project, New York State College of Ceramics at Alfred University. 5. The Edward Orton Jr. Ceramic Foundation, 6991 Old 3C Highway, Westerville, OH 43082. 6. Rhodes, Daniel, Clay and Glazes for the Potter, Revised and Expanded Third Edition by Robin Hopper, Krause Publications, 2000, pp. 120, 309.

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