CRYO

Cryolite is sodium aluminium fluoride, an insoluble source of soda that melts at a very low temperature. It goes into glazes, frits and enamels in small additions.

Cryolite is sodium aluminium fluoride, and it is bought for two things: soda that will not dissolve in the glaze water, and a very low melting temperature. Soda ash and borax dissolve into the glaze water, and once the sodium is in solution it soaks into bisque unevenly, recrystallises in the bucket, and the recipe drifts as the glaze is used up. Cryolite is insoluble, so the soda stays where it is put. It melts around 1010°C, well below where a stoneware glaze matures, and a small addition moves a glaze a long way. Over half its weight is fluorine, and most of that leaves as gas during the firing, late enough to bubble up through a glaze that has begun to seal. Small additions give crackle and larger ones blister and pinhole, which a few crater recipes are built on, though silicon carbide has taken over most of that work. The same gas is hard on kiln elements and on shelves, so cryolite suits occasional firings on well-protected shelves rather than a place in a glaze you fire regularly. It has no effect on kiln atmosphere and will not reduce a glaze in an electric kiln, because reduction needs something to donate electrons and nothing in the material does. An oxide analysis of cryolite adds up to more than the whole material, because reporting a fluoride mineral in oxide form counts oxygen it does not contain, and a recipe calculated straight from those figures overstates what cryolite brings.