Ceramic glaze raw materials generally include silica, which forms glass when fired. Metal oxides, such as sodium, potassium and calcium, act as a "flux" to lower the melting temperature. Alumina, often derived from clay, stiffens the molten glaze to prevent it from running off the piece. Colour comes from iron oxide, copper carbonate or cobalt carbonate. Tin oxide or zirconium oxide make the glaze opaque.