How Surface Treatments Can Affect Ceramic Glaze Color Jeff Zamek

While there are some constants in ceramic glaze color (such as blues from cobalt), more often than not, a ceramic artist comes to expect the unexpected in fired glaze colors. 

In this excerpt from the December 2025 issue of Ceramics Monthly, Jeff Zamek explains how some surface treatments impact the ceramic glaze colors in your pots. Check it out if you want to learn how to have more control in your results and more confidence in your firings when it comes to color. –Jennifer Poellot Harnetty, editor

PS. To learn about other factors that influence color in glazes, check out the full article in the December 2025 issue of Ceramics Monthly! Not a subscriber? Subscribe today!

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. 

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.