Cobalt glass

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Cobalt glass, also 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. Cobalt glass plates are used as an optical filter in flame tests to filter out the undesired strong yellow light emitted by traces of sodium, and expand the ability to see violet and blue hues, similar to didymium glass. However, didymium glasses are superior for this purpose as it absorbs less light other than the Sodium D lines. Specialty tasting glasses made of cobalt glass are used by professional olive oil tasters to disguise the colour of the oil being assessed to avoid bias in judging. 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...

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