Ceramic Industry

PPP: Adjusting Glazes for Color and Opacity

December 1, 2001
While in the past color in glazes was often found by chance, potters have now come to expect an easy manipulation of ceramic materials to achieve their aesthetic goals. The materials, knowledge and equipment to adjust any glaze have become available to a great number of potters firing in many different types of kilns. Today, with the availability of varied color-producing oxides and the increasing widespread knowledge of how ceramic materials react, many potters can now consistently produce a variety of glaze colors. Glazes can also be modified through the use of opacity-producing materials, which block the transmission of light through the fired glaze. With any number of different opacifying agents, it is possible to change a clear, transparent glaze to an opaque glaze with varying degrees of whiteness.

However, many methods can be used to change the color and opacity of the glaze, and not all of them will produce the desired results. The best option for experimenting with glaze color and opacity is to keep everything simple and take small steps. Instead of using a dozen glazes with different coloring oxides, limit the first series of a test to a few simple glaze formulas, then build on the fired results. Understanding how base glazes are created and how different components of the glaze formula interact can help potters successfully develop a wide range of beautiful, functional glazes for their ware.

Formulating Base Glazes

Base glaze formulas can contain feldspars, frits, clays, dolomite, flint, talc, Gerstley borate, whiting, magnesium carbonate, barium carbonate, zinc oxide, strontium carbonate and any number of other ceramic materials. Base glazes do not contain any metallic coloring oxides, stains or opacity-producing raw materials, or zirconium silicates. Ordinarily, base glaze formulas will produce three broad surface texture classifications: gloss, satin and matte. These three groups can sometimes correspond to three classifications of light transmission: clear/transparent, semi-opaque and opaque. Not all base glazes will fit exactly into these specific groupings, but all will be absent of any coloring ingredients.

Base glaze formulas can be developed at any temperature range for any type of firing conditions, such as oxidation, reduction or neutral kiln atmospheres. They offer a stable starting point for obtaining different opacities, colors and textures in future glazes.

Adding Opacity

A base glaze can be made opaque in several different ways. For the purposes of this article, we have used materials suspended within the glaze to form a semi-opaque or opaque white glaze (see sidebar: Sample Glaze Formulas). Opacity-producing agents are added after the 100% batch weight of the base glaze formula. Depending on the original level of opacity in the base glaze, adding an opacity-producing agent in amounts of approximately 1 to 3% will produce a slightly opaque glaze in a clear base glaze, 5 to 8% will produce a semi-opacity in a clear base glaze, and 10 to 20% will produce complete opacity in a clear base glaze. In semi-opaque and opaque base glaze formulas, the same percentages will produce even greater levels of opacity since the base glaze is somewhat opaque before the opacity-producing agent is added.

The level and quality of the opacity produced in the base glaze will be affected by the base glaze formula (some base glazes are already semi-opaque due to the choice of glaze fluxes and/or the alumina silica ratio in the glaze), the amount of opacity-producing agent added to the base glaze (higher amounts of the agent will decrease the glaze transparency), and the particular agent used in the glaze (i.e., zirconium silicate will produce a flat, uniform white compared to titanium dioxide, which tends toward a frosty, irregular surface white).

One of the most widely used opacity- producing materials in glazes has been Zircopax, a zirconium silicate; however, this product is no longer being produced. Viable substitutions are produced in several grades, which are classified according to particle size (Excelopax, Superpax A, Superpax Plus, Zircopax Plus, Superpax and Zircopax A*). The smaller the particle size, the lower the amount of opacity-producing agent will be required in the glaze formula. The best substitution match will occur when choosing the nearest material listed in particle size. Zircopax Plus and Superpax Plus have slightly higher zirconium contents than the other opacity-producing agents, so smaller amounts should be used to get the same level of opacity when using these materials to replace non-plus opacity producing materials. Zirconium silicate-RZM can also be substituted for Zircopax,** while zirconium spinel is coarser and tends not to be a close match for Zircopax. Zinc zirconium silicate will cause opacity and harden the fired glaze surface.

Another opacity-producing raw material, tin oxide, will yield a sort of “butter fat” (yellowish-white) quality in glazes. Tin oxide produces a softer white than the “refrigerator whites” produced with the zirconium silicates. Titanium dioxide and zirconium oxide can also offer different qualities of opacity in glazes.

Example of a gloss black c/6 oxidation (2232¿F) glaze on a coffee cup.

Adding Color

Color development in glazes depends on several factors, including the composition of the base glaze, the firing conditions, the application method, and the oxide or stain content.

Base Glaze Formulas. Base glaze formulas high in alkaline content, such as feldspar, will produce an intense blue color with the addition of cobalt oxide.

Firing Conditions. Under-fired, immature glazes can look pale and washed out, while over-fired glazes often exhibit a shift in the intensity of the color. The firing atmosphere can also alter glaze color—for instance, a reduction atmosphere can promote red color in glazes that contain copper oxide.

Application Method. The glaze application method (spraying, dipping or brushing) can influence the glaze color, as well as the thickness or thinness of the glaze application on the ware. For example, an uneven glaze layer can allow the underlying body color to dominate the fired glaze color.

Metallic Coloring Oxides/Carbonates. Generally, 1 to 2% of metallic coloring oxide/carbonate will yield a slight tint to a base glaze, 5% will produce a medium color response, and 10 to 15% will yield a darker, full color response. For our example, we have used cobalt oxide, but any number of coloring oxides/carbonates or stains could be used alone or in combination, including cobalt carbonate, cobalt sulfate, chrome oxide, copper oxide black, copper oxide red, copper carbonate, copper sulfate, crocus martis, powdered illmenite, iron chromate, iron oxide red, iron oxide black, iron oxide yellow, iron sulfate, powdered manganese dioxide, manganese carbonate, nickel oxide black, nickel oxide green, nickel carbonate, potassium bichromate, yellow ochre, raw sienna, rutile dark, rutile light, burnt umber, or vanadium pentoxide.

Using 5% cobalt in each type of base glaze will produce gloss blue, satin blue and matte blue glazes. However, the intensity and shade of blue will be different, depending on the particular base glaze.

Stains. A stain can be composed of a metallic coloring oxide or combinations of oxides, stabilizers and opacity-producing agents. The mixture is calcined or fired to high temperature, then pulverized into a fine powder.

The advantage of stains over metallic coloring oxides is their reliable color reproduction and their ability to produce specific shades of color not achievable through raw metallic coloring oxides. Many individual colors and shades of stains exist and can be obtained from companies such as Mason Color Works and Drakenfield. Each company produces a chart with a listing of their available stain colors. As with metallic oxides, a 1 to 2% addition of stain will yield a slight tint to a base glaze, a 5% addition of stain will produce a medium color response, and a 10 to 15% addition of stain will yield a darker, full color response. The stain color can be altered by the base glaze, kiln atmosphere, firing temperature, glaze application thickness, clay body formula and kiln firing cycle. Any stain color selected for use in a glaze should be tested to ensure that it provides the desired result.

Table 1.

Formulating and Testing Glazes

When formulating and testing glazes, a simple comparative system of glaze notation is essential to understand how glazes work (see Table 1, p. 22). This type of system will save time, but, more importantly, it will gather diverse bits of information and place them in an understandable format that is easy to analyze. As a standardized notation method, base glazes are calculated to a 100% batch weight. Coloring oxides, stains, opacifiers, gums, suspension agents, dyes and other additives are always listed after the 100% batch weight of the glaze. Many glaze formulas list bentonite 1% or 2% as a glaze suspension agent. Bentonite should also be noted after the 100% batch weight. The potter can then compare the ingredients of one glaze with another using the same standardized percentage system. For example, if glaze A has 20% feldspar and has a dry matte surface texture when fired, while glaze B with 60% feldspar has a gloss, transparent surface, some inference can be made that the higher amount of feldspar in glaze B is contributing to additional melting, causing a gloss surface. Even though other variables can influence this outcome, such comparisons offer valuable insights into how additions of feldspar stimulate glaze development. Using the 100% batch method of listing glazes also allows the potter to compare their glazes with others who use the same universal percentage system of glaze notation.

For this series of adjusting glaze color and opacity, Cone 6 (c/6) (2232∞F) glazes have been chosen. However, the same procedures of adding coloring oxides, stains and opacity producing agents can be applied to any temperature range glaze.

The goal is to find a successful clear, satin or matte glaze. Start by choosing two or three base glaze formulas within the same category, such as clear glazes. Then apply the glazes to vertical test tiles (vertical testing of glazes will determine if they will run or drip) using an old kiln shelf underneath. This should be the standard procedure until you have become familiar with the glaze characteristics in your kiln. Make the test tiles from the same clay body that you plan to use in the future, as different clay bodies can alter the fired glaze. It is also a good practice to place several test tiles of the same glaze throughout the kiln. Not all kilns heat equally, and in reduction kilns, the atmosphere is sometimes distributed unevenly throughout the kiln. Placing several test tiles throughout the kiln will better indicate how the glaze reacts at different temperatures and atmosphere conditions during the firing process. To broaden the types of base glaze formulas, follow the procedures for testing satin and matte glazes.

At the end of testing, the objective should be a gloss, satin and matte base glaze that works satisfactorily on your clay body.

Achieving Successful Glazes

One of the most common situations potters encounter involves using many different glaze formulas to achieve various colors and textures on their ware. However, the combination of many glaze formulas sometimes produces too many raw materials combinations to “track” in a logical cause and effect manner. When looking at a table full of glaze tests, the potter can easily become frustrated due to the overwhelming amount of results, some of which look like they might be good glazes, while other glaze tests are impossible to interpret. Often, the desired multiple fired glaze effects are not worth the effort in stocking raw materials, weighing out formulas and mixing glazes. There is also the possibility of glaze incompatibility when applying overlapping glazes with different formulas on the same piece of ware.

It is quite natural to acquire different glaze formulas from many sources, but experimenting with these formulas can be a time-consuming and frustrating project if the potter does not have a basic knowledge regarding how coloring oxides, stains and opacity producing agents can change a glaze. A simpler approach is to use only a few base glaze formulas (glaze formulas that are either clear, satin or matte) and then add various opacity producing agents, metallic coloring oxides and/or stains to achieve varied fired results. Additions to a few reliable base glazes will likely produce dependable colors and/or opacity in the new glaze variations.

The base glaze formulas are designed to work at the specified temperature range in oxidation or reduction kiln atmospheres. Some base formulas might produce slight variations in opacity and texture, depending on the individual kiln firing cycle, kiln size, glaze application thickness, method of glaze application (brush, dipping, pouring, spraying), clay body formula and kiln atmosphere. Glaze results can also be altered due to raw material inconsistencies, kiln firing variables, and common errors in weighing out raw materials.

Coloring oxides, stains and/or opacity producing agents can then be added to the base glaze formulas that work best on your clay body. When test firing the glaze, keep in mind that a small test kiln will heat up and cool down faster than a larger production kiln. High-volume production kilns have increased thermal mass due to their size, and will contain a greater number of bricks, shelves, posts and pots. They will take longer to heat and cool, which produces a more comprehensive melt in clay bodies and glazes. While small test kilns can give an approximation of a fired glaze (greater accuracy in predicting glazes occurs with gloss glazes as opposed to satin or matte glazes in small test kilns), the most accurate results are obtained by testing the glaze in the same kiln in which it will eventually be used.

The actual glaze formulas are not as important as the ability to manipulate the formula based on how it fires in your own kiln and on your clay body. Using this simple method of adding coloring oxides, stains or opacity-producing agents, any temperature range of base glaze can be adjusted for color and/or opacity. The knowledge gained from understanding the effects of coloring oxides, stains or opacity-producing agents in glazes is valuable information that can be used in future glaze testing. Once a stable, consistent glaze has been developed, many color and opacity variations can be developed from reliable base glaze formulas.

*These Zircopax substitutes are supplied by Johnson Matthey Structural Ceramics, 11400 New Berlin Road, Jacksonville, FL 32226; (904) 751 2828.
**Zirconium silicate-RZM is supplied by Clayworld, P.O. Box 100, Hickory, KY 42051; (270) 247-3849.


SIDEBAR: Sample Glaze Formulas

Cone 6 Clear Gloss Base Glazes

Zam #4 Clear, Gloss
Nepheline syenite 270x = 20%
Whiting = 20%
EPK (kaolin) = 20%
Flint 325x = 20%
Ferro frit #3124 = 20%

Zam #5 Clear, Gloss
Ferro frit #3195 = 60%
Flint 325x = 22%
EPK (kaolin) = 12%
Whiting = 6%
Bentonite = 2%

Zam #6 Clear, Gloss
Nepheline syenite 270x = 41%
Zinc oxide = 6%
Whiting = 16%
Barium carbonate = 7%
Flint 325x = 30%

Cone 6 Satin Base Glazes

Zam #8 Satin
Whiting = 15%
Nepheline syenite 270x = 40%
Flint 325x = 38%
EPK (kaolin) = 7%

Zam #11 Satin
Nepheline syenite = 29%
Magnesium carbonate = 11%
Zinc oxide = 3%
Gillespie borate* = 14%
Flint 325x = 35%
EPK (kaolin) = 8%

Zam #12 Satin
Custer feldspar = 25%
Dolomite = 16%
Whiting = 3%
Zinc oxide = 3%
EPK (kaolin) = 18%
Flint 325x = 35%

Cone 6 Matte Base Glazes

Zam #13 Matte
Nepheline syenite 270x = 60%
Whiting = 15%
EPK (kaolin) = 10%
Flint 325x = 15%

Zam #14 Matte
Nepheline syenite 270x = 60%
Dolomite = 15%
Zamek Ball Clay** = 10%
Flint 325x = 15%

Zam #15 Matte
Nepheline syenite 270x = 45%
Whiting = 18%
EPK (kaolin) = 20%
Flint 325x = 5%
Zinc oxide = 12%

Cone 6 Base Glazes (Gloss, Satin and Matte) with the Same Metallic Coloring Oxide

Zam #14 Clear Gloss Blue
Nepheline syenite 270x = 20%
Whiting = 20%
EPK (kaolin) = 20%
Flint 325x = 20%
Ferro frit #3124 = 20%
Cobalt oxide = 5%

Zam #8 Satin Blue
Whiting = 15%
Nepheline syenite 270x = 40
Flint 325x = 38%
EPK (kaolin) = 7%
Cobalt oxide = 5%

Zam #13 Matte Blue
Nepheline syenite 270x = 60%
Dolomite = 15%
EPK (kaolin) = 10%
Flint 325x = 15%
Cobalt oxide = 5%

Cone 6 Gloss Base Glazes with Metallic Oxides, Stains and Opacity-Producing Agents

Zam #4 Gloss Blue
Nepheline syenite 270x = 20%
Whiting = 20%
EPK (kaolin) = 20%
Flint 325x = 20%
Ferro frit #3124 = 20%
Cobalt oxide = 5%

Zam #4 Gloss Black
Whiting = 15%
Nepheline syenite 270x = 40%
Flint 325x = 38%
EPK (kaolin) = 7%
Cobalt oxide = 5%

Zam #4 Gloss White
Nepheline syenite 270x = 60%
Dolomite = 15%
EPK (kaolin) = 10%
Flint 325x = 15%
Cobalt oxide = 5%

Cone 6 Gloss, Satin, Matte Base Glazes with Stains, Opacity Producing Agents and Metallic Color-Producing Oxides

Zam #4 Gloss Black
Nepheline syenite 270x = 20%
Whiting = 20%
EPK (kaolin) = 20%
Flint 325x = 20%
Ferro frit #3124 = 20%
Mason Stain Black #6600 = 10%

Zam #8 Satin White
Whiting = 15%
Nepheline syenite 270x = 40%
Flint 325x = 38%
EPK (kaolin) = 7%
Superpax = 5%

Zam #13 Matte Blue
Nepheline syenite 270x = 60%
Dolomite = 15%
EPK (kaolin) = 10%
Flint 325x = 15%
Cobalt oxide = 5%

*Gillespie borate is a substitute for Gerstley borate in many glaze formulas. It is sold by Hammill & Gillespie, 154 S. Livingston Ave., P.O. Box 104, Livingston, NJ 07039; (973) 994-3650.
**Zamek Ball Clay is distributed by Old Hickory Clay Co., P.O. Box 66, Hickory, KY 42051-0066; (270) 247-3042.