Smalt is a a ground blue potassium glass containing cobalt used from the 15th to 18th centuries.
Its origins probably lie in the ancient pigment Egyptian Blue; both pigments are made from glass that has been coloured blue. However, Egyptian Blue contains copper, whereas Smalt’s colour comes from cobalt.
Widely used from the 16th century, Smalt is manufac- tured by thoroughly mixing cobalt oxide and potas- sium carbonate with molten glass. Once thoroughly combined, the hot glass is added to water where it shatters into small pieces. The pieces are then ground to yield a coarse pigment.
As smalt is a glass, its particles are transparent, and its covering power as a pigment for painting is low. To retain its strong, deep blue colour, it must not be ground too ne; artists would have known that it should be ground carefully by hand, and not for too long. The resulting gritty pigment makes it a dif cult material to paint with.
Smalt was popular because of its low cost. The best grade of smalt had a purplish cast, commending it as a substitute for expensive genuine ultramarine. But this lustre sadly diminishes when mixed with oil as the yel- lowing of the binder reduces its colour.