Cobalt glass is a deep blue colored glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 prepared by adding cobalt

Cobalt

Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

 compounds to the molten glass. It is appreciated for its attractive color. It is also used as an optical filter

Filter (optics)

Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....

 in flame tests to filter out the yellow flame caused by the contamination of sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

, and expand the ability to see violet and blue hues. Under fluorescent light, 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

Bristol blue glass

Bristol Blue Glass has been made in Bristol, England since the 17th century.- History :During the late 18th century Richard Champion, a Bristol merchant and potter, making porcelain, was working with a chemist, William Cookworthy...

, as in the distinctive blue bottles of Harvey's Bristol Cream sherry. Cobalt glass changes the color of hydrochloric acid

Hydrochloric acid

Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 in a mixture of multi-colored cations.

For pigment use

Powdered cobalt glass, called smalt

Smalt

Smalt is powdered glass, colored to a deep powder blue hue using cobalt ions derived from cobalt oxide . Smalt is used as a pigment in painting, and for surface decoration of other types of glass and ceramics, and other media...

 (or also simply cobalt glass), has been historically important as a pigment in painting, pottery, and other media. Cobalt glass and smalt are made from silica and are not the same as cobalt blue

Cobalt blue

Cobalt blue is a cool, slightly desaturated blue color, historically made using cobalt salts of alumina. It is used in certain ceramics and painting; the different cobalt pigment smalt, based on silica, is more often used directly in tinted transparent glasses...

, which is colored by cobalt ion is a similar way, but using a melting process with alumina.