PPP: Experimenting with Color

By using reliable base glazes and establishing standard testing procedures, you can maximize your time and results when mixing new glaze colors.

Photo courtesy of Ferro Corp., Cleveland, Ohio.
At some point, every potter considers expanding his or her palette of glaze colors. Even today's most reliable, exciting colors can become boring and static tomorrow. In some cases, a color change is dictated by a decline in sales-maybe the midnight blue glaze that was selling so well six months ago is no longer moving off distributors' shelves. In other cases, the change might be primarily for aesthetic reasons, such as when a new glaze color is needed to complement the artistic considerations of a new design. In fact, there are probably just as many reasons for investigating color in glazes as there are for potters making pots.

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 Glazes

One 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.

Choosing one or more base glazes for color development allows the potter to start from a known entity-namely, a glaze that is reliable and functionally correct in its firing characteristics. Different metallic coloring oxides or stains in varying percentages can then be added to expand the glaze results while working on a steady foundation of base glaze reliability. Using a limited selection of base glazes reduces the variable factors associated with different glaze materials, which can affect color development in the glaze. For example, glazes that contain tin oxide can produce a pink color with additions of chrome oxide, while glazes with high levels of zinc oxide can produce an intense blue with the addition of cobalt oxide or cobalt carbonate. Tracking these variable factors can be difficult when trying to interpret the results of a glaze test. It is always best to start with a few base glazes and then expand the scope of testing once reliable results are accurately interpreted in the fired test pieces.

Figure 1. The effects of cobalt carbonate added to a base glaze in (from left) 1/2%, 1% and 10% increments.

Testing Procedure

Having 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.

Figure 2. Using rutile and cobalt oxide in a glaze produces depth and variation of color.
Once the base glazes have been chosen, add a percentage of the coloring oxide or stain to each glaze to achieve the test glaze (see Figure 1). Metallic coloring oxides or their carbonate forms can be used to produce depth of color and variations in shades of color to give the glaze diffusion and variability (see Figure 2). They can also be used in combinations to produce many different colors. However, depending on their processing source, metallic coloring oxides can differ in their particle size, trace material content and percentage of oxide present in the individual sample, which can cause possible inconsistencies in color.

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.

Figure 3. When used alone in a glaze, blue stain produces a static color.

Glaze Tests

With 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.

Figure 4. A vertical glazed test piece 5 in. high by 2 in. wide.

Factors Influencing Glaze Color

A 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.

Percentages of Metallic Coloring Oxides or Stains Used in a Base Glaze

As a general rule:

  • 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

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