Cobalt blue is a cool, slightly desaturated blue color, historically made using cobalt salts of alumina. It is used in certain ceramics (historically, especially Chinese porcelain) and painting; the different cobalt pigment smalt, based on silica, is more often used directly in tinted transparent glasses. The pigment is extraordinarily stable. Chemically, cobalt blue is a cobalt(II) oxide-aluminium oxide, or cobalt(II) aluminate, CoAl2O4. The compound is made by sintering the stoichiometric mixture of finely ground CoO and Al2O3 (alumina) at 1200°C.