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Trying to develop a successful new glaze color can be a frustrating and time-
consuming exercise in trial and error-but it doesn't have to be. By using reliable base glazes and establishing a standard testing procedure, you can maximize your time and results when mixing new glaze colors.
Base GlazesOne reliable method for testing various color combinations in glazes is to use a good-quality base glaze formula as a starting point. A base glaze should be formulated to equal a 100% batch weight and to produce a clear, transparent, glossy surface; a satin, matte, semi-opaque surface; or a matte, opaque surface (see Table 1). If a metallic coloring oxide, metallic coloring carbonate, stain, gum, suspension agent or dye is used, it is added on top of the 100% batch weight and is therefore listed after the total batch weight of the glaze.
Using a base glaze as a 100% total allows one glaze to be roughly compared to another using the 100% notation method. For example, if Glaze A contains 60% frit, a glass former, it will be more fluid and glossy than Glaze B containing 30% frit. While other variables also come into play when comparing different glazes, many potters find the 100% notation method to be a useful tool. Glaze formulas are also expressed in 100% batch weights in most ceramic literature, so it can be helpful to understand this terminology when researching or troubleshooting new glaze formulations.
Testing ProcedureHaving a plan to test color in glazes will greatly lessen the chance of error and will reduce the time and labor required to carry out the testing program. By choosing a base glaze from each of the three general categories-clear, transparent, glossy; satin, matte, semi-opaque; and matte, opaque-a wide range of color variations can be achieved with the same percentage of any coloring oxide or stain. For example, adding a given percentage of cobalt carbonate will produce a different intensity of blue, depending on the opacity and surface texture of the base glaze. In a clear, transparent, glossy glaze, the blue color will be brighter and more intense than in a matte, opaque glaze, which can cause a muting effect. Table 2 shows three sample cone 6 base glaze formulations.
When testing glazes, use a 100- to 300-gram test batch of each base glaze. Add small increments of water to achieve the correct glaze viscosity-excessive amounts of water can cause a thin glaze layer on the test piece. If too much water is added to the glaze batch, it can sometimes be siphoned off after allowing the glaze to settle for a few hours. However, if the glaze contains soluble materials, removing the excess water can change the actual glaze formula.
If color consistency is important, stains can be used (see Figure 3). Stains are manufactured from metallic coloring oxides and various color-enhancing and stabilizing oxides, such as zinc oxide, silica, alumina and calcium. The raw materials are calcined through a process in which they are heated to create new compounds. After cooling, the materials are ground into a fine powder. Stains produce a flat, static color response in glazes and can be used to consistently reproduce a specific shade of color-including colors that are not easily reproducible when using raw metallic coloring oxides.
Glaze TestsWith any test, the goal is to generate as much information as possible from the glaze result. The size of the test pieces should be large enough to allow sufficient surface area to reflect the viscosity of the fired glaze. On small test pieces, the weight of the molten glaze is often not sufficient to cause glaze running. However, the same glaze applied to a larger surface area can drip due to the pulling effect of the weight of the molten glaze. The test piece, not the actual production pot, is the place to determine whether the glaze will run on a vertical surface. The test pieces should be at least 4 to 5 in. in height, with a minimum of 1 in. unglazed on the bottom (see Figure 4).
It is important to make at least three or four samples for each glaze, with a thin and thick glaze layer on each test piece. The test pieces should then be distributed throughout the kiln. Since not every kiln heats evenly, separating the test pieces will reflect any temperature variations in the kiln.
Once a series of successful glaze results have been completed, intermediate testing is essential to ensure the glaze will remain stable and consistent. Mix a pre-production batch of glaze (one or two gallons), glaze a few pots, and place them throughout the kiln. Once several kiln firings have produced acceptable results, the glaze is ready for larger production glazing and firing operations.
Factors Influencing Glaze ColorA number of factors can either completely change the intended glaze color or cause variations in intensity or hue. Before starting any glaze color testing program, consider the following primary causes of color discrepancies:
• The base glaze formula can promote or retard color response in a glaze. For example, glazes high in magnesium, in the form of magnesium carbonate, talc or dolomite, can have a muting effect on a glaze color.
• Kiln atmosphere plays a crucial role in color development. Electric kilns will produce an oxidation atmosphere, while carbon-based fuels, such as wood, natural gas, propane, oil or coal, can produce oxidation, neutral or reduction atmospheres, all of which can change the color and surface texture of the glaze.
• The firing cycle used to reach temperature can affect glaze color and texture. An increase in the firing time to the glaze maturing temperature can cause greater melting of the glaze. A faster firing time to glaze maturity often yields a pale or muted glaze color due to immature glaze development.
• A slow cooling rate can cause devitrification or crystal growth. Micro- or macro-crystals developing on the glaze surface can have a bleaching or spotting effect on glaze color.
• The color of the clay body can alter the glaze color. Lighter clay bodies promote contrasting surfaces for glaze colors, while darker clay body colors can lessen or darken the glaze color response.
• The thickness of the glaze can influence glaze color. A thin application of glaze reveals more of the underlying clay body color.
• The type of glaze application-spraying, dipping or brushing-can influence glaze color due to the irregular or uniform thickness of the glaze.
• The amount of metallic coloring oxide or stain and its particle size can influence the fired color of the glaze. For example, cobalt oxide has a larger particle size than cobalt carbonate and typically yields a blue field with blue specking in a satin or matte glaze.
• Overlapping one glaze with another can introduce a combination of materials into a melt, which can alter the color, texture and viscosity of the glaze.
• Clay slips applied to a clay body can influence the development of the covering glaze color.
In many cases, several of these factors combine to enhance, stabilize or negate color responses from stains and metallic coloring oxides. Knowing how ceramic raw materials function under conditions of temperature and atmosphere can give you a better understanding of how to manipulate color in glazes.
SidebarAs a general rule:
Percentages of Metallic Coloring Oxides
or Stains Used in a Base Glaze
- 0.25-0.50% of metallic coloring oxide or stain will tint a base glaze
- 5-7% of metallic coloring oxide or stain will produce a halftone shade of color
- 10-12% of metallic coloring oxide or stain will produce full color intensity