Sometimes called calcium fluoride, fluorspar is used in the formulation of frits, as a low fire opacifier in enamels and in special effect crater glazes. Fluorine gas, a health hazard, is a by product when fired to high temperatures. This crystalline mineral has a lower fluxing temperature than other calcium compounds. It can be used as a substitute for whiting to promote more fusible glazes. Insoluble in water, it should be used at 100 mesh or finer or it can cause mottling in glazes.