BACA

Barium carbonate is the source of barium oxide in a glaze, behind its satin and dry matte surfaces and the barium blues. In a clay body, a small addition prevents soluble salts scumming the surface as ware dries and fires.

Barium carbonate is the source of barium oxide in a glaze. As the glaze cools, barium oxide grows fine crystals through the melt, which produces the satin-to-dry matte surfaces the material is known for; barium matte is a named category in its own right. Barium also has the strongest colour response of the alkaline earths, ahead of strontium and calcium, and its clearest effect is with copper, which gives the intense turquoise known as barium blue. It is manufactured rather than mined, which is part of why it comes at a consistent purity. It is the most thermally stable of the alkaline earth carbonates, and holds its carbon dioxide until stoneware temperature — late enough to pinhole or blister a glaze that has already sealed over. In a clay body it does a different job: a small addition fixes soluble sulphates as insoluble barium sulphate, so they cannot migrate to the surface and scum the ware as it dries and fires, and it does the same for the sulphates that interfere with deflocculating a casting slip. Strontium carbonate is the standard substitute.