While there are some constants in coloring within ceramics (such as blues from cobalt), more often than not, a ceramic artist comes to expect the unexpected in fired glaze colors. Learn how to have more control in your results and more confidence in your firings when it comes to color.
Defining the Terms
Base Glazes: A glaze containing no stains, metallic coloring oxides, gums, suspension agents, or additives.
Encapsulated Stains: Stains enclosed in a zirconium silicate crystal structure to maintain color at high temperatures and in reduction kiln atmospheres.
Flux: A melting agent that promotes fusion in combination with clay and/or glaze materials.
Heat Work: Energy input during a firing, normally represented in terms of temperature and time.
Iridescence: A color change in a glaze or luster surface depending on the angle of illumination. This changeable color effect is most noticeable with luster and fumed glazes.
Metallic Coloring Carbonate: Metals that contribute color to clay body and glaze formulas, which can be found in cobalt carbonate, copper carbonate, manganese carbonate, and nickel carbonate. As a general rule, 1.5 times more metallic carbonate is required to equal its metallic oxide form.
Metallic Coloring Oxide: Metals that contribute color to clay body and glaze formulas, which can be found in cobalt, chrome, copper, nickel, and iron oxides. Metallic coloring oxides can have a larger particle size than their carbonate forms.
Oxidation Firing Cycle: A higher ratio of air to fuel is present in the kiln atmosphere.
Phototrophy: Phototrophy occurs when the glaze changes in the presence of different light sources.
Raku Firing: A low-temperature, fast-firing process where pots are removed from the kiln red hot and either air cooled or placed in a combustible medium to achieve carbon deposits and/or reducing effects on the clay and glaze surfaces.
Reduction Firing Cycle: A higher ratio of fuel to air in the kiln, yielding carbon monoxide, which results in oxygen being taken away from metallic coloring oxides.
Refractory: A material that is resistant to heat. For example, kiln wash, composed of equal parts of silica and kaolin, which are refractory materials resistant to heat.
Slip: A slurry mixture of clay and water, imparting color or opacity to a clay body. However, now the terms slip and engobe are often used interchangeably. Historically, engobes are a suspended mixture of clay, fluxes, and fillers imparting color or texture to a clay body.
It is often said, “The only thing consistent about ceramics is its inconsistency.” It is a system subject to the variable nature of raw materials and techniques used in forming, glazing, and kiln firing. This fact runs counter to the agency potters feel when working with clay and glazes. The ceramic arts have always been a cult of both disappointment and elation. Opening a kiln load of pots can humble and amaze you in the same event. This recurrent truth is prevalent when introducing color to ceramic objects. Statistically, colors are consistent and work well. For example, using cobalt oxide produces a blue glaze.1 Conversely, on occasion a set of underlying variables will take effect, and the intensity of the color or the color itself might change. There is a range of factors that can determine the glaze color as it comes out of the kiln. It is important to know how glaze colors can react under one or multiple conditions.
Percent of Metallic Coloring Oxide or Stain
As in many applications, such as paint, the higher percentage of colorant will increase the intensity of color. As a general rule, which can be subject to other considerations noted below, 0.0625% will result in a tint of color; 5% will yield a half tone of color; and 10–12% will result in a saturated, full-color response.
The oxide form of a metallic color is stronger than the carbonate form. Approximately 1.5 times more carbonate is required to equal the oxide amount. A metallic coloring oxide can have a larger particle size than its carbonate form. For example, cobalt oxide has a larger particle size compared to cobalt carbonate, resulting in blue specking in a blue field. This condition can appear in satin matte or matte glazes. In gloss glazes, the blue specking is present but not noticeable.
Base Glaze Chemistry
There are alkaline-based glazes containing high feldspar content that acts as a primary flux, producing bright, distinct, intense colors. Another group of glazes are alkaline earths, containing materials such as dolomite, talc, whiting, and other secondary fluxes, which can yield semi-opaque or opaque surfaces. When metallic coloring oxides, their carbonate forms, or stains are added to glazes dominated by alkaline earths, a bleached, muted color response is evident, most noticeable in semi-opaque and opaque glazes.
Glazes are composed of fluxes, stabilizers (melting agents), alumina (controlling viscosity), and silica (glass formers). They will produce clear, semi-opaque, or opaque glazes, depending on the ratios and amounts of fluxes, alumina, and silica. They do not contain colorants or additives. The same metallic coloring oxide, its carbonate form, or stain will reveal a different intensity depending on the light transmission and surface texture of the glaze. For example, cobalt oxide will result in a sharp, distinct blue in a clear base glaze, but lose intensity in a semi-opaque or opaque glaze.
Glaze Temperature
Some stain colors, such as reds, yellows, or oranges, will burn out at stoneware temperature ranges above cone 6 (2232°F (1222°C)). However, encapsulated stains can maintain these colors in oxidation and reduction kiln atmospheres at stoneware temperature ranges. In low-temperature oxidation firings cone 06 or below (1828°F (998°C)), many yellows, pinks, and oranges are still possible from stains and metallic coloring oxides.
Over Firing Excessive glaze vitrification occurs when a glaze is taken past its maturing range. Often this results in darker colors. Due to excessive melting, the glaze surface can appear glossy with an intensifying effect on glaze colors.
Under Firing Glazes that do not reach their maturity can be dull and muted in color with a dry surface texture due to insufficient buildup of glass in the glaze.
Fast Firing Ceramic materials require longer times to reach maturing temperatures. Fast firing cycles can result in immature glaze surfaces with muted colors and dry surface textures.
Firing Cycle Excessively long firing times to maturity can cause colors to run on vertical surfaces or pool in horizontal areas. Muted color can appear more intense due to the over-vitrification of the glaze. Exceptionally long firing cycles expose both clay and glaze to more heat work.
Kiln Atmosphere
A reduction kiln atmosphere (a higher ratio of fuel than air in the firing chamber) creates carbon monoxide. This reaction draws oxygen away from metallic oxides in the glaze, offering the widest diversity of glaze colors as compared to an oxidation atmosphere (more air than fuel is used in combustion). For example, copper oxide and its carbonate form are some of the most reactive coloring agents as to kiln atmosphere. Copper will produce green glazes in oxidation and red glazes in reduction due to the removal of an oxygen molecule in a carbon monoxide atmosphere. Heavy and/or prolonged reduction atmospheres can dull or darken glaze colors. In some instances over reduction can excessively flux or melt and/or darken the color of glazes. An oxidation atmosphere can cause faded or muted glaze colors. Uneven reduction can cause both oxidation and reduction colors on the same pot. This type of mixed atmosphere is most often noted in copper glazes. Reduced areas are red, while oxidized areas are green.
Special Effects (Wood, Soda/Salt, Raku) A wood firing has the potential to deposit ash on unglazed and glazed surfaces. Wood ash is soluble and can contain over 90% alkaline materials, resulting in extreme fluxing of the glaze at temperatures above cone 10 (2345°F (1285°C)).2 Wood ash can alter the glaze color and surface texture depending on the type of wood used in the firing. Salt and soda firing present similar issues due to the fluxing actions of the sodium compounds in salt (sodium chloride), soda ash (sodium carbonate), and baking soda (sodium bicarbonate). Each sodium compound acts as a flux in conjunction with the clay body and glaze. Sodium vapor lands on the exposed clay body, forming a sodium/alumina/silica glaze, which can have an orange-peel-like appearance. Being a flux, sodium vapor landing on the glaze can cause a darker, glossy glaze color.
Raku In raku, after the firing has reached temperature, the pots are immediately taken out of the kiln and deposited into sawdust containing metallic salts of copper, silver, tin, iron, or cobalt, forming a vapor on glazed and unglazed surfaces of the pot. Metallic salts can also be suspended in water and painted directly on the pots before firing. Due to the thin layer of metallic salts, the resulting colors can easily be abraded. Raku firing can produce random results with pots being over or under reduced or over or under fired all of which can affect glaze colors.
Surface Treatments, Colors, and Effects
Different metallic coloring oxides can act as fluxes or refractory agents in a glaze, changing the intensity of glaze colors. This condition is most pronounced when used in high percentages in glazes. For example, red iron oxide can act as a flux or melting agent, resulting in intensified color. Chrome oxide acts as a refractory agent in high percentages and can cause dryer glaze colors.
Overglaze Wash Composed of metallic coloring oxides or stains suspended in water. In some instances a flux is required to facilitate fusion with the underlying glaze. The overglaze color could be muted, altered, or enhanced by the underlying glaze color.
Underglaze Wash Generally applied to greenware or bisque pottery in a watercolor-like consistency. In some instances, a flux is required to facilitate fusion with the underlying clay body. Depending on the stain or oxide used, it can change the color of the covering glaze.
Underglaze, Slip, Engobe The metallic coloring oxide or stain used in the underglaze slip can enhance or mute the covering glaze color. A colored slip depends on the light transmission of the covering glaze, as satin or matte glazes might not reveal the underlying slip color. Slips have several functions as they must fit the underlying clay body as well as the covering glaze and reveal their color through the glaze.
Glaze Thickness Thinner glaze applications tend to reveal more of the underlying clay body color, while thicker applications represent the actual glaze color. This is illustrated when looking at a glaze drip, which is a layer of thicker glaze.
Application Process Dip, spray, brush, pour—each method of glaze application can influence the glaze color, as uniformity of application and thickness of the glaze can alter the intensity of glaze colors.
Overlapping Glazes: Placing one or more glazes on top of each other frequently produces a third glaze color due to the combination of base glazes and/or metallic coloring oxides or stains.
Clay Body Color Just as slips and washes can influence glaze color, the clay body color can intensify or mute it. White clay bodies reflect and enhance a glaze color, while darker clays can make the glaze look less vivid.
Decals Utilize a paper or film medium where the pigment image is immersed in water and slides off onto a fired glaze surface. After drying, the image is then fired to the appropriate temperature. Depending on the underlying glaze color and the image transfer color, one or both colors can change. Decal colors can be altered by an increase or decrease in the recommended firing temperature.
Phototrophy While the majority of glazes will not change colors in the presence of different light sources, some do. In this category are glazes containing rutile, an ore containing iron and titanium dioxide, which will become darker in some lighting conditions. When the light is removed, the glaze returns to its original color.3 Another condition can occur when raku ware is fired with a copper oxide or other metallic oxide wash. The thin coating of fused metallic color can fade in the presence of sunlight. This condition is not reversible.
Fuming is a process of creating a thin layer of metallic coloring oxides or metallic salts, such as bismuth subnitrate or stannous chloride, during the firing process. When heated the vapors randomly land on glazed and unglazed surfaces. Due to the thin layer of the salt deposits, they can easily be abraded during any cleaning operation. Fuming can also be achieved by applying a paint consistency layer of a metallic coloring oxide, such as copper oxide, to a hard brick and placing it within 1⁄8 inch (3.1 cm) of the pot. During the firing, the metallic coloring oxide will vaporize, landing on the pottery’s unglazed and glazed surfaces. For example, a glaze containing tin will blush red when placed next to a hard brick painted with chrome oxide.
Lusters Salts and metal compounds, such as gold, silver, or copper, suspended in an oil medium to assist in application to a fired glaze surface. The oil burns off in the first part of the firing, leaving a thin layer of metals on the clay or glazed surface. Most lusters can be fired between cone 022 (1087°F (580°C)) and cone 017 (1360°F (738°C)). Lusters can produce an iridescent holographic effect depending on the type of luster used. Due to the thin layer of luster on the pottery surface, it can easily be abraded during cleaning operations.
Terra Sigillata Translated as sealed earth, is a method of suspending a specific layer of clay in a water solution. It is then drawn off and applied in a thin application to low-fire pottery, imparting a smooth, colored-clay coating when burnished. The color of terra sigillata depends in part on the color of its clay and covering glaze, if any.
Phase Separation During the firing, two different chemistries of glaze transition between transparent and opaque, which separate into two mutually immiscible liquids. This reaction can alter the colors of the glaze.
Glaze Etching The glazed pot is dipped into a solution of muriatic acid or sodium bisulfate, which causes micro pits in the glaze surface. The acid attacks and scatters light, causing a matte surface. Then, when applying water to the glazed pot, the pits are filled in, leveling out the glaze surface and making it reflective again. When dry, the etched matte surface returns. Glaze color can be affected by etching.
As stated, accurate color representation is achieved in most situations. However, when it does not take place, look to one or more of the factors listed.
the author Jeff Zamek started his career 48 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
1 Cobalt oxide is one of the strongest metallic coloring oxides. It will produce a blue color in oxidation and reduction kiln atmospheres. One part of cobalt oxide will tint 100,000 parts of white glaze. 2 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 186. 3 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 264.
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While there are some constants in coloring within ceramics (such as blues from cobalt), more often than not, a ceramic artist comes to expect the unexpected in fired glaze colors. Learn how to have more control in your results and more confidence in your firings when it comes to color.
Defining the Terms
Base Glazes: A glaze containing no stains, metallic coloring oxides, gums, suspension agents, or additives.
Encapsulated Stains: Stains enclosed in a zirconium silicate crystal structure to maintain color at high temperatures and in reduction kiln atmospheres.
Flux: A melting agent that promotes fusion in combination with clay and/or glaze materials.
Heat Work: Energy input during a firing, normally represented in terms of temperature and time.
Iridescence: A color change in a glaze or luster surface depending on the angle of illumination. This changeable color effect is most noticeable with luster and fumed glazes.
Metallic Coloring Carbonate: Metals that contribute color to clay body and glaze formulas, which can be found in cobalt carbonate, copper carbonate, manganese carbonate, and nickel carbonate. As a general rule, 1.5 times more metallic carbonate is required to equal its metallic oxide form.
Metallic Coloring Oxide: Metals that contribute color to clay body and glaze formulas, which can be found in cobalt, chrome, copper, nickel, and iron oxides. Metallic coloring oxides can have a larger particle size than their carbonate forms.
Oxidation Firing Cycle: A higher ratio of air to fuel is present in the kiln atmosphere.
Phototrophy: Phototrophy occurs when the glaze changes in the presence of different light sources.
Raku Firing: A low-temperature, fast-firing process where pots are removed from the kiln red hot and either air cooled or placed in a combustible medium to achieve carbon deposits and/or reducing effects on the clay and glaze surfaces.
Reduction Firing Cycle: A higher ratio of fuel to air in the kiln, yielding carbon monoxide, which results in oxygen being taken away from metallic coloring oxides.
Refractory: A material that is resistant to heat. For example, kiln wash, composed of equal parts of silica and kaolin, which are refractory materials resistant to heat.
Slip: A slurry mixture of clay and water, imparting color or opacity to a clay body. However, now the terms slip and engobe are often used interchangeably. Historically, engobes are a suspended mixture of clay, fluxes, and fillers imparting color or texture to a clay body.
It is often said, “The only thing consistent about ceramics is its inconsistency.” It is a system subject to the variable nature of raw materials and techniques used in forming, glazing, and kiln firing. This fact runs counter to the agency potters feel when working with clay and glazes. The ceramic arts have always been a cult of both disappointment and elation. Opening a kiln load of pots can humble and amaze you in the same event. This recurrent truth is prevalent when introducing color to ceramic objects. Statistically, colors are consistent and work well. For example, using cobalt oxide produces a blue glaze.1 Conversely, on occasion a set of underlying variables will take effect, and the intensity of the color or the color itself might change. There is a range of factors that can determine the glaze color as it comes out of the kiln. It is important to know how glaze colors can react under one or multiple conditions.
Percent of Metallic Coloring Oxide or Stain
As in many applications, such as paint, the higher percentage of colorant will increase the intensity of color. As a general rule, which can be subject to other considerations noted below, 0.0625% will result in a tint of color; 5% will yield a half tone of color; and 10–12% will result in a saturated, full-color response.
The oxide form of a metallic color is stronger than the carbonate form. Approximately 1.5 times more carbonate is required to equal the oxide amount. A metallic coloring oxide can have a larger particle size than its carbonate form. For example, cobalt oxide has a larger particle size compared to cobalt carbonate, resulting in blue specking in a blue field. This condition can appear in satin matte or matte glazes. In gloss glazes, the blue specking is present but not noticeable.
Base Glaze Chemistry
There are alkaline-based glazes containing high feldspar content that acts as a primary flux, producing bright, distinct, intense colors. Another group of glazes are alkaline earths, containing materials such as dolomite, talc, whiting, and other secondary fluxes, which can yield semi-opaque or opaque surfaces. When metallic coloring oxides, their carbonate forms, or stains are added to glazes dominated by alkaline earths, a bleached, muted color response is evident, most noticeable in semi-opaque and opaque glazes.
Glazes are composed of fluxes, stabilizers (melting agents), alumina (controlling viscosity), and silica (glass formers). They will produce clear, semi-opaque, or opaque glazes, depending on the ratios and amounts of fluxes, alumina, and silica. They do not contain colorants or additives. The same metallic coloring oxide, its carbonate form, or stain will reveal a different intensity depending on the light transmission and surface texture of the glaze. For example, cobalt oxide will result in a sharp, distinct blue in a clear base glaze, but lose intensity in a semi-opaque or opaque glaze.
Glaze Temperature
Some stain colors, such as reds, yellows, or oranges, will burn out at stoneware temperature ranges above cone 6 (2232°F (1222°C)). However, encapsulated stains can maintain these colors in oxidation and reduction kiln atmospheres at stoneware temperature ranges. In low-temperature oxidation firings cone 06 or below (1828°F (998°C)), many yellows, pinks, and oranges are still possible from stains and metallic coloring oxides.
Kiln Atmosphere
A reduction kiln atmosphere (a higher ratio of fuel than air in the firing chamber) creates carbon monoxide. This reaction draws oxygen away from metallic oxides in the glaze, offering the widest diversity of glaze colors as compared to an oxidation atmosphere (more air than fuel is used in combustion). For example, copper oxide and its carbonate form are some of the most reactive coloring agents as to kiln atmosphere. Copper will produce green glazes in oxidation and red glazes in reduction due to the removal of an oxygen molecule in a carbon monoxide atmosphere. Heavy and/or prolonged reduction atmospheres can dull or darken glaze colors. In some instances over reduction can excessively flux or melt and/or darken the color of glazes. An oxidation atmosphere can cause faded or muted glaze colors. Uneven reduction can cause both oxidation and reduction colors on the same pot. This type of mixed atmosphere is most often noted in copper glazes. Reduced areas are red, while oxidized areas are green.
Surface Treatments, Colors, and Effects
Different metallic coloring oxides can act as fluxes or refractory agents in a glaze, changing the intensity of glaze colors. This condition is most pronounced when used in high percentages in glazes. For example, red iron oxide can act as a flux or melting agent, resulting in intensified color. Chrome oxide acts as a refractory agent in high percentages and can cause dryer glaze colors.
As stated, accurate color representation is achieved in most situations. However, when it does not take place, look to one or more of the factors listed.
the author Jeff Zamek started his career 48 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
1 Cobalt oxide is one of the strongest metallic coloring oxides. It will produce a blue color in oxidation and reduction kiln atmospheres. One part of cobalt oxide will tint 100,000 parts of white glaze.
2 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 186.
3 Hammer, Frank and Janet. The Potter’s Dictionary, Fifth Edition. A&C Black, University of Pennsylvania Press, 2004, page 264.
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