Barium Carbonate

Barium carbonate supplies barium oxide to a glaze. As the glaze cools, the barium grows fine crystals through the melt, and those crystals give the satin to dry matte surfaces that barium mattes are named for. Barium also has the strongest colour response of the alkaline earths. With copper it gives the intense turquoise known as barium blue. It is manufactured, not mined, so it comes at a consistent purity. It holds its carbon dioxide until stoneware temperatures in the cone 5-6 range, late enough to pinhole or blister a glaze that has already sealed over. In a clay body a small addition locks soluble sulphates up as insoluble barium sulphate so they cannot migrate to the surface and scum the ware, and it does the same job in a casting slip, where those sulphates interfere with deflocculation. Strontium carbonate is the usual substitute.