A ceramic glaze is made from glass (silicon dioxide) that has been modified so it will melt under high temperatures and adhere to clay. Glazes incorporate many optional ingredients for different effects, but three ingredients are essential: Silica; minerals called fluxes, which are added to silica to lower its melting point; and clay itself (usually kaolin), in a form called alumina, so the glaze will shrink the same percentage as the clay object it is covering.
The essential parts of a glaze are silica, flux and alumina, plus water. Sources of silica include silicon dioxide (quartz, glass, flint) and feldspar, a mineral derived from granite that includes silica, alumina and a touch of flux. Flux sources include borax or gerstley borate, for lower-temperature firing, and whiting, dolomite, bone ash (calcium oxide) and barium carbonate for high-temperature firing. Basic glaze is white.
Glaze ingredients need to be insoluble in water, because water is the vehicle for applying the glaze. The materials form a suspension in water. A bisque-fired piece of clay (fired once, at a lower temperature) is dipped into the glaze, where it absorbs the water but leaves the glaze ingredients on its surface.
After the basic glaze is formulated, minerals are added to achieve different colors. Adding and mixing metallic oxides produces a range of colors, from deep blues and purples to pale yellows. The most common oxides used for ceramic glazes are cobalt, iron, chrome, copper, manganese dioxide and vanadium pentoxide. Depending on the firing temperature -- raku is the highest firing, at Cone 10 -- many other ingredients for achieving unique effects are chosen.
The amount of each ingredient depends on both the kind of clay used to form the object and the temperature and type of firing used. Recipes express ingredients in proportions that total 100, such as Ingredient A: 10, Ingredient B: 25, Ingredient C: 15, Ingredient D: 50. Glaze ingredients are measured on a scale. Wearing a dust mask, the potter mixes the dry ingredients, adding water until the consistency is that of skim milk. The glaze should then sit for 24 hours before the pieces are dipped.
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