Cryolite

Cryolite (Na3×AlF6) is a strong fluxing agent with a very low melting point. This powdered material is derived from granite. Cryolite is used in frits and glazes, and is a source of aluminum and insoluble sodium. It is occasionally used as an opacifier for enamels, or to produce crackle glazes. In large amounts, it can become volatile with silica and cause blistering.

In glazes, cryolite is used as an additive that helps to create artificial reduction in electric firings. This is achieved through cryolite's lack of oxygen -- a valuable and specific property that can be used to great effect.