A Discussion of Glazes

By Dewayne Imsand

Making a piece of colored pottery is a very complex task. There is much work performed in every step necessary to have a successful outcome. During the early days though, when many farmers had their small “Blue Bird” potteries, the principal glaze was made from salt. At the proper temperature, a small quantity of salt was simply thrown into the kiln. The salt would vaporize and cover all the pieces in the kiln with a shinny clear coating. Toward the end of the 1800’s, colorful glazes were created to enhance the more decorative designs. This is a much more difficult task than it may appear.

A glaze, which is simply a thin coating of glass, begins as a suspension of ground silica, and clays, in water. The silica and the clays join and perform three functions. The first ingredient, silica, is the major ingredient, and it is the glass former. That is the first function, however silica cannot be used alone to make glazes because its melting point is much too high. The clay bisque (a piece of pottery fired without glaze) would melt in the kiln long before the silica would.

What enables the use of silica is the addition of a flux. The flux performs the second function, which is to lower the melting point of the silica. For glazes that are fired at a lower temperature, sodium may be used as a flux, and for higher fired glazes, Calcium carbonate is often used.

Another ingredient that must be added to make an acceptable glaze is alumina (aluminum oxide clay). It performs the third function, which is to control the shrinkage of the glaze. Because the glaze is applied to bisque pieces that will shrink during the glaze firing, the glaze itself must also shrink and match the amount of bisque shrinkage. (Crazing is the result of mismatched shrinkage rates.) The appropriate amount of shrinkage is achieved by the addition of a precise amount of alumina.

There are many substitute sources for each of the glaze ingredients. Also, many substances perform, to varying degrees, the function of more than one component. For example, feldspar provides differing amounts of all three ingredients, silica, flux, and alumina.

In addition to the selection of, and proper proportion of, each ingredient, there is one basic requirement that all glaze ingredients must have, and that is they must be insoluble in water. The reason is that glazes are water suspensions. The only part that the water plays is to transport the glaze ingredients onto the surface of the bisque pieces. If one of the glaze ingredients were soluble, the bisque would adsorb it along with the water, and the ingredient would not properly interact with the other ingredients when it is fired, and it would not properly coat the bisque.

Incidentally, there are many methods used to apply glaze onto the surface of the bisque. A glaze may be applied by dipping, pouring, spraying, brushing, sponging, squeeze bag, or some combination of these techniques. Each technique produces its own special effect.

The glaze ingredients described above result in a base glaze. However, a base glaze may have a finish that is glossy, semi-gloss, or matte, depending on the firing temperature. Firing at a higher temperatures yields a glaze with a glossy finish, and firing at a lower temperature yields a matte finished glaze.

To make a colored glaze, various metal oxides, (or carbonates) are added to the base glaze mixture. Additionally, another ingredient, bentonite (a clay), is typically added. Bentonite promotes a uniform glaze mixture, or slurry, by reducing the settling of the oxides or carbonates in the water. In addition, there is a routine stirring of the slurry to keep the solid particles uniformly suspended in the water.

The various oxides or carbonates that can be added result in many different colors. The resulting colors depend greatly on the percentage of the material used, the kiln temperature, and the type of firing conditions in the kiln. There are two types of firings that potters may use in order to create various colors. They are oxidation firing, and reduction firing. Oxidation firing describes a condition in the kiln where there is ample air (oxygen) flow. Reduction firing is a condition in the kiln in which the flow of air is deficient. Because the airflow in reduction firing is reduced, it results in incomplete burning of the fuel, say natural gas, that is used to heat the kiln. The incomplete burning causes an increase level of specific gases in the kiln, such as carbon monoxide, carbon dioxide, and hydrogen. These gases are very aggressive in wanting to combine with oxygen, and since there is not enough air inside the kiln, they steal it from the glazes. When metallic oxides, for example, give up oxygen they convert to their reduced, or more metallic form. This change is the reason reduction firing produces different colors and visual effects than would be produced when metal oxides are fired with an ample airflow.

The following are examples of a few metallic oxides and the colors that they produce using different percentages of the oxides, and firing types. The variation in the colors that the oxides produce is governed by the kiln temperature that is used. There are many types of metallic oxides that could be used, and considering the differing percentages of them, the two firing types, and the various kiln temperatures, there are very many different possible combinations.

METAL OXIDE
AMOUNT (percent)
OXIDATION FIRING
REDUCTION FIRING
Cobalt oxide
1/2-3%
blue, blue-violet
-
Iron oxide
1/2-2%
-
jade green celadon
Iron oxide
2-3%
-
iron yellows
Iron oxide
3-4%
-
golden-orange
Iron oxide
4-5%
-
brick red (khaki)
Iron oxide
5-6%
-
brown-black
Iron oxide
1/2-6%
amber-greenish browns
-
Manganese dioxide
5-10%
purple
honey browns
Chrome oxide
2-3%
browns, pinks, and reds
greens
Copper oxide
2-3%
turquoise blues, greens
reds, purples
Vanadium pent oxide
5-10%
yellows
grays

In addition to the glaze color, potters may also apply a luster, a china paint (gold is an example), or a decal, and then do another firing to achieve a color, or surface effect that is not possible in glaze firing alone. Lusters are very thin coatings of metallic substances that produce iridescent effects. These lusters, and all over-glaze techniques, are fired at a relative, extremely low temperature, and because of that, they produce brighter colors than can be achieved through a routine glaze firings. However, the low temperature that is used is sufficient to melt the over-glaze, but not the original underlying glaze. The over-glaze, or decal, melts onto the original glaze and fuses.

Some may have thought that it was the designer had the most difficult job, but it is also a “tough” job to develop a particular colored glaze, or to operate the kiln correctly. Also, these jobs are critical ones too. With all of the exacting work involved in making a colorful piece of pottery, it is no wonder that some techniques that potters use are closely guarded. This is especially true with glaze formulas.