Cobalt Oxide

Cobalt oxide is a black powder that fires blue in glazes, slips and clay bodies. Cobalt is sold as this oxide or as the carbonate, and the oxide is the more concentrated of the two. A very small addition gives a medium blue in a clear glaze, and a heavier one gives a deep blue that is close to black. A true black needs iron and usually manganese as well. It contains about half again as much cobalt per gram as the carbonate, so a recipe written for carbonate needs roughly two thirds as much oxide. It is also the coarser of the two and does not disperse evenly on its own, so a glaze with cobalt oxide in it should be sieved. Unsieved, the colour shows as blue specks and mottling. The shade depends heavily on the base glaze, just as it does with the carbonate. Blended with iron oxide, and often manganese, it is one of the standard routes to a black, though published black recipes vary a great deal in how much cobalt they use.