Smalt

Cobalt glass—known as "smalt" when ground as a pigment—is a deep blue coloured glass prepared by including a cobalt compound, typically cobalt oxide or cobalt carbonate, in a glass melt. Cobalt is a very intense colouring agent and very little is required to show a noticeable amount of colour. Moderately ground cobalt glass (potassium cobalt silicate)—called "smalt"—has been historically important as a pigment in glassmaking, painting, pottery, for surface decoration of other types of glass and ceramics, and other media. The long history of its manufacture and use has been described comprehensively. Cobalt aluminate, also known as "cobalt blue", can be used in a similar way.