Glazes strengthen the surface of the clay and make pottery waterproof.
Glazes are applied to clay when it is leather-hard or bisque fired. Glazes strengthen the surface of the clay and make pottery waterproof. They are made of a combination of silica and minerals. As the pottery is fired, the glaze floods and rehardens on the surface of the clay. Glazes have to be matched to the clay according to its maturation temperature. Many clays and glazes are rated by a cone system. This is a way to measure heat work (similar to temperature).