Cobalt blue 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.

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.

The earliest known example of cobalt aluminate glass dates to a lump from about 2000BC in ancient Mesopotamia, very possibly intended for use as a pigment; it is then rare until the modern era. About five centuries later cobalt oxide smalt appears as a pigment in Egyptian pottery, and soon after in the Aegean region, and this is the pigment normally known as smalt. In paintings, smalt has a tendency to lose its colour over a long period and is little used today. However used in ceramics for underglaze decoration it keeps its colour well, and is the main blue used in blue and white pottery from a wide range of dates and areas, including Chinese blue and white porcelain from the Yuan and Ming dynasties, Renaissance Italian maiolica and Delftware.

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