COCA

Cobalt carbonate is the usual source of blue in pottery, and cobalt is the strongest of the colouring oxides — a very small addition already reads as blue. It colours glazes, slips, engobes and clay bodies.

Cobalt carbonate is where most blues in pottery come from, 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 heavy additions go to a very dark, near-black blue — though a true black needs iron, and usually manganese, alongside it. The same powder colours slips, engobes and clay bodies, and a slip needs more of it than a glaze to reach the same depth. Cobalt reaches a recipe either 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 carries less cobalt per gram a recipe written for the oxide needs about half as much again in carbonate. It fires blue whether the kiln is in oxidation or reduction. What does change the colour is the glaze around it: magnesia turns cobalt lavender and then grape purple, a barium matte gives a deep purple-blue, a high-alkali glaze takes it to a brilliant ultramarine, and titania pulls it toward green. Painted under a glaze it bleeds up into the cover glaze as that melts, so fine lines soften. Cobalt is a strong flux as well as a colourant, so a glaze carrying a lot of it can run.