Cobalt Carbonate is a pinkish tan powder used to color slips and glazes. When used up to 1.5%, the powder will create shades of blue. Higher amounts of the powder will create a black. While cobalt carbonate disperses better into a slurry than cobalt oxide, the carbonate will produce carbon dioxide gas in the firing, meaning it can cause blistering and pinholes in the glaze it was added to. Cobalt Carbonate is often used instead of cobalt oxide because it is able to disperse itself in glazes better, yielding a more solid color rather than a patchy or speckled color that often shows itself when cobalt oxide is used.
Information gathered from Digitalfire Ceramic Materials Database and The Potter's Dictionary of Materials and Techniques by Frank Hamer.
