blue cobalt - online puzzles

Cobalt blue is a blue pigment made by sintering cobalt (II) oxide with alumina at 1200 °C. Chemically, cobalt blue pigment is cobalt (II) oxide-aluminium oxide, or cobalt (II) aluminate, CoAl2O4. Cobalt blue is lighter and less intense than the ( iron -cyanide based) pigment Prussian blue. It is extremely stable and has historically been used as a coloring agent in ceramics (especially Chinese porcelain ), jewelry, and paint. Transparent glasses are tinted with the silica-based cobalt pigment smalt.