How Many Materials are there in a Glaze?

Q: How many materials are there in a glaze?

A: If one examines many glaze recipes, one soon realizes that most of them contain ten ingredients, or less. These, in turn, are used repeatedly in different recipes for a given firing range (low-fire, mid-fire, or high—fire). Each raw material introduces certain “basic oxides” into the glaze mix. Combined into a batch recipe, the materials when fired will yield a certain mix of these oxides. If these are in the correct proportions, the result will be a glass with known properties, i.e, a glaze.

In the high-fire range, cone 8 to cone 11 (1260-1320° C, 2300-2415° F), the recipe usually contains a feldspar, flint, whiting or dolomite, and kaolin or clay. In the mid-fire range, cone 1 to cone 7 (1160-1250° C, 2120-2280° F), the ingredients list will usually be expanded to include raw materials that melt at a lower temperature, such as colemanite/gerstley borate, spodumene/lepidolite, zinc oxide, and certain “frits” (pre-fired, special glasses). In the low-fire range, cone 08 to cone 01 (950-1150° C, 1740-2195° F), gerstley borate or a high-boron frit is usually the main ingredient with the rest being chosen from those already mentioned above. Also, some materials with very low melting points are often used, including lithium carbonate and clays with high iron content (eg, barnard clay).

Why these particular materials? Are there others that could be used? Potters, over decades, have learned by trial an error which low-cost materials will form good glazes on their ware. A feldspar, for instance, is the chief ingredient of high-fire glazes. But not all feldspars are created equal; there can be considerable variation in feldspars mined in different places throughout the world. Furthermore, there are indeed many other glass—forming raw materials available to the glaze-maker; the actual choice of a given set of equivalent materials will vary with cost, with availability, and with a potter’s preference.

by Tom Buck