Ceramic Glaze is an impervious layer of a vitreous substance which has been fused to a ceramic body through firing. Glaze can serve to color, decorate or waterproof an item. All glazes contain a variety of Silica, Alumina, and Flux. It may also contain opacifiers, suspenders and colorants. When heated Silica creates glass while the Alumina stiffens the glaze preventing it from running of the clay and the flux allows the glaze to melt at a lower temperature.
Some potters prefer to mix their own glazes that work well with their particular clay body. Commercial glazes are available to simplify the process.