STCA

Strontium carbonate is an active flux at middle and high temperatures, where it thins the melt and smooths the surface. About seven-tenths of its weight is strontium oxide, and the remainder leaves as gas.

Strontium carbonate does its work from the middle of the range upward, where it lowers the viscosity of the melt: a glaze carrying it runs smoother and glossier, not drier. It is not itself a matting agent — a glaze with enough of it in can come out entirely glass, with no crystals at all, and where a strontium glaze is matte something else in the recipe is doing it. A glaze commonly carries somewhere between a twentieth and a fifth of its weight in it. The carbonate has to break down before any of that happens, and that comes late enough in a firing to pinhole a glaze that has already sealed.