A type of slip applied to pottery which produces an impermeable and glassy surface when fired at high temperatures. It is usually produced by coating pottery with powdered glass and reheating them to a temperature where the glass begins to fuse. Glaze is a vitreous substance and, like glass, glaze is made from silica; this substance only melts at a temperature higher than that which would melt the pot, so a flux must be added to make it useable. Silica is present in most pottery, so in these cases only the flux - an oxide of sodium, lead, or potassium - needs to be added, and a colorant if required, usually in the form of a frit crushed and suspended in water. The pot is then fired at a temperature suitable for melting the glaze (somewhere between 900?-1200? C depending on the constituents), which runs into an even layer all over the pot. Known in ancient Egypt where a mixture of fine sand, quartz or crystal dust was used with an alkaline base (soda, potash). Glaze or couverte can be identified in the Persian faiences and Flemish stoneware. In Hellenistic period, lead glaze was invented, in which lead monoxide replaced soda or potash. A large variety of glazes may be used, varying in color, texture, and suitability for different types of pottery.