Cobalt glass

Cobalt glass—known as "smalt" when ground as a pigment—is a deep blue colored glass prepared by including a cobalt compound, typically cobalt oxide or cobalt carbonate, in a glass melt. Cobalt is a very intense glass colorant and very little is required to show a noticeable amount of color. Cobalt blue glass is also used as an optical filter in flame tests to filter out the yellow flame caused by the contamination of sodium, and expand the ability to see violet and blue hues.

Cobalt glass such as Bristol blue glass is popular with collectors, and is used in the distinctive blue bottles of Harvey's Bristol Cream sherry. It is appreciated for its attractive color.

For pigment use

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. Cobalt aluminate, also known as "cobalt blue", can be used in a similar way.