MgCO3 (m.p. 5072o F/2800o C) is used as a source of MgO in glazes. In glazes (cone 5-10), it can develop a butterfly surface favored for utility ware. At high temperatures (cone 9-16) it has a similar effect as Alumina; it will increase the viscosity of a glaze and hence reduce the fluidity; e.g. crystalline variety, in which Alumina is not desired.