<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 contamination with sodium, and expand the ability to see violet and blue hues . </P> <P> Cobalt glass such as Bristol blue glass is appreciated for its attractive color and is popular with collectors . It is used in the distinctive blue bottles of Harvey's Bristol Cream sherry and Tŷ Nant mineral water . </P> <P> 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 . </P>

What is the purpose of cobalt blue glass