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Glaze is Off-Color

The fired glaze is not the expected color.

Does the development of the color depend on the chemistry of the glaze?

The development of many colors requires that the host glaze's chemistry be sympathetic. For example, chrome-tin pinks require glazes with 10% CaO and B2O3 must be 1/3 or less the CaO content, certain blues require the presence of BaO. The presence of ZnO is hostile to the development of many colors, as is MgO. Check textbooks or FORESIGHT's Oxide, Material and Property databases for information on possible conflicts and re-formulate glaze if necessary.

Is there enough color in the glaze?

Metal oxide colorants or colorant blends darken glaze color as their proportion is increased. However, at some point maximum color is achieved and further additions will usually begin to give the glaze metallic effects. It is important to note that once metallic effects are noted the glaze has become unstable and will leach metals into liquids it comes in contact with. It may also oxidize in air.

Is the glaze opacity correct?

The brightness of color depends partly on the host glaze's opacity. Opaque glazes give flatter and lighter colors, translucent and transparent glazes produce color with depth.

Is the glaze developing micro-bubbles?

Excessive bubble development in the glaze matrix can alter color considerably. Bubbled transparents become quite cloudy and colors will be subdued, especially if the glaze is not opaque.

Is the glaze developing crystals? Does its color depend on the development of crystals?

Crystals grow in some glazes during cooling of the kiln. Certain glaze chemistries and mineralogies are susceptible to crystal growth (i.e. low alumina). Cooling the kiln slowly during the period when the glaze is freezing allows and promotes crystal growth. Many of the metal oxides freely participate in crystallization and their color can completely change in the process. A high iron fluid glaze, for example, may fire almost black on quick cooling and a muddy yellow on slow cooling.

Does the character of the color depend on an additive?

The character of a glaze often depends on additives to mottle and variegate color (i.e. titanium, rutile). These materials can be variable and the effects they create can be fragile (the application and firing process must be kept very consistent).

Does the glaze depend on a fragile melting mechanism?

Some glazes develop color and character only when they are very fluid. Unfortunately these same glazes are also very prone to crystallization and other problems like running and blistering.

Has it been fired properly?

The mechanism of color development in a glaze may depend on kiln atmosphere (i.e. strong reduction, weak reduction, strong oxidation), or on the speed or curve of both the ramp up and down. On the other hand, over firing a glaze can alter its surface character and therefore its color. Additionally, certain raw colors and stains volatilize above certain temperatures.

Has it been put on the right side of the glaze layer?

The same metal oxide will develop different colors and color depths depending on whether it is mixed into a transparent glaze, or whether it is painted on as decoration under or over the glaze. If it is painted under, glaze thickness, bubble population, and crystal development will also have a considerable effect.

Would a stain be better?

Achieving and maintaining an exact shade of color can be quite difficult with raw coloring oxides, especially if a blend is being used. For example, many people use cobalt, iron and manganese for black. However color shifts are common with this approach and it is usually not obvious which metal oxide should be increased or reduced to stabilize the color. Stain companies have invested considerable time to develop colors that are reliable and stable (often containing things like zircon, alumina, silica in addition to the metal oxides). These companies provide information on how to make the colors work. Stains are more expensive, but the stain company assumes a burden that is often difficult for most companies or potters to handle.

Is the glaze applied in the right thickness?

Many glazes develop deep color only if they are applied thickly enough. Others develop color only when they are thin and can interact with the underlying body.

Does the underlying body effect the color?

Light colored clay bodies foster the development of bright colors, iron bearing bodies muddy and subdue colors, especially when the glaze is thin. Many glazes will develop color of different character on refractory porous bodies compared to vitreous ones.

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