Cobalt Carbonate

Cobalt carbonate is the usual source of blue in pottery, and cobalt is the strongest of the colouring oxides. A very small addition turns a clear glaze pale blue, a little more gives a solid medium blue, and a heavy addition gives a very dark blue that is close to black. A true black needs iron and usually manganese as well. The same powder colours slips, engobes and clay bodies, and a slip needs more than a glaze to reach the same depth. Cobalt is sold as this carbonate or as the oxide. The carbonate is the finer of the two, so it disperses more evenly and specks less, and because it contains less cobalt per gram, a recipe written for the oxide needs about half as much again in carbonate. It fires blue in oxidation and in reduction alike. The base glaze changes the shade. Magnesia turns cobalt lavender and then grape purple, a barium matte gives a deep purple blue, a high alkali glaze gives a brilliant ultramarine, and titanium pulls it towards green. Painted under a glaze it bleeds up into the cover glaze as that melts, so fine lines soften. Cobalt is also a strong flux, so a glaze with a lot of it in can run.