Kremer Pigmente GmbH & Co. KG     Kremer Pigments Inc., USA  

Smalt was the earliest of the cobalt pigments. It is artificial, in the nature of glass, a potash silicate strongly coloured with cobalt oxide and reduced to a powder.

The origin is obscure. For years there has been much debate concerning wether or not cobalt was used by the Egyptians and other ancient cultures to colour glass. Recent studies have shown that cobalt was definately present along with copper in much Egyptian blue glass, however only in small amounts. There is no evidence as yet, however, that any powdered cobalt glass was ever used as a painter's pigment in ancient times.

When cobalt was first employed in Europe for glass making is not known, but probably the Venetian glass makers knew of its properties. Beckmann and Meltzer (1956) credit the discovery to Christoph Schürer, a Bohemian glassmaker, in about 1540-1560, but it was certainly available before the sixteenth century, as it has been detected at least two European painting s from the fifteenth century (Dieric Bouts The Entombment and Michael Pacher The Early Fathers).

About the middle of the fifteenth century, certain cobalt minerals such as cobaltite (CoAsS) and smaltite (CoAs₂) were discovered on the borders of Saxony and Bohemia. At the time, the nature of these minerals was not known and, since they gave the miners a certain amount of trouble, they were called "cobalt" for spirits or "kobolds" which were thought to haunt the mines. The Bohemian glass makers soon found, however, that tehse strange minerals had the property of colouring glass blue.

We produce smalt according to a recipe from the 19^th century, by roasting a powder mixture of cobalt salts. At about 1150° C the powder melts forms a glass mass of a deep bluish black colour. The glass melt is deterred and is ground into pulver.

Smalt, since it is a glass and is transparent, has very poor hiding power and, for this reason, it has to be used coarsly ground. Like other glasses, smalt is considered to be stable. It is better used in aqueous media and lime (fresco); in oil-media only a dull colour is obtained because the refractive index of smalt is so close to that of dried oil. Smalt in oil-media is often observed to have partially or completely discoloured, although when lead white is mixed with the paint film, some protection against the loss of colour is sometimes afforded.

Smalt, in common with all other cobalt pigments, exerts a powerful siccative effect on oil films. This property of smalt was appreciated quite early. Examples of its use as a siccative are found in at least one recipe from the seventeenth century.