Smalt

Smalt is a blue pigment made from powdered cobalt-containing glass. It has been used since medieval times in painting, stained glass, and ceramic glazes. The pigment is produced by melting silica with cobalt oxide to create blue glass, which is cooled, broken into chunks, and ground into a fine powder for use as a pigment.

In painting, smalt was employed as a durable blue for shadows and details in tempera, oil, and encaustic works. It was often combined with other blue pigments such as azurite or ultramarine to achieve different tones. When used in glazes or oil paint, the powder can impart a bright, transparent to semi-opaque blue that varies with particle size and medium. In stained glass, smalt also refers to cobalt-blue glass frit or tesserae used to form blue sections in windows.

Smalt has also played a role in ceramic and vitreous glaze work. As a glaze frit, cobalt-containing glass is applied to pottery and fired to produce a vivid blue surface. This use parallels its function in painting, providing a stable cobalt blue that can be fired onto ceramic bodies.

Today smalt is less common than other blue pigments, such as ultramarine, due to cost and the development of synthetic alternatives. It remains of interest to conservators and artists seeking historically informed materials. Handling considerations include the care required when grinding or dusting cobalt-containing pigments, as with other metal oxide pigments.