Cobalt glass is a deep blue colored glass prepared by adding cobalt compounds to the molten glass. It is appreciated for its attractive color and 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, under fluorescent light, the cobalt glass has twice the effect.
Some people collect objects made from cobalt glass, amassing large collections. One popular example of cobalt glass is Bristol blue glass, as in the distinctive blue bottles of Harvey's Bristol Cream sherry. Cobalt glass changes the color of hydrochloric acid in a mixture of multi-colored cations.
