Alumina is added to thicken the glaze to prevent it from running off the pots during the firing, similar to flour mixed into gravy.
Glazes are mixtures of three main ingredients: silica, flux and alumina (the main component of clay). Silica is the part of the glaze that turns to glass, also known as glass former. Because silica requires temperatures above 3000ºF to melt, it is necessary to add flux to lower its melting point. Many different materials can be used as flux, including feldspar, wood ash, calcium carbonate and various other compounds. Alumina is added to thicken the glaze to prevent it from running off the pots during the firing, similar to flour mixed into gravy. Silica, flux and alumina mixed in the proper proportion will make a gloss glaze. Changes in the relative proportions of these components can also produce semi-matte, satin-matte, or a dry stony-matte finish. In mixing a glaze, there has to be enough flux present to absorb the silica/alumina, and there has to be enough silica/alumina present to take in the flux. Firing temperature also influences the melt. A glaze that is formulated to be a matte glaze at cone 6 (2200ºF), when fired to cone 10 (2350ºF), becomes a gloss glaze. 1