A glaze is a vitreous substance fused on to the surface of pottery to form a hard, impervious coating. This coating is typically applied after an initial bisque firing occurs, although certain glazes can be single-fired.  After a bisqued (or once-fired) piece of pottery is covered with glaze either by dipping, brushing, or spraying, the piece is then fired again, to a temperature appropriate for both the claybody and glaze being used.