Smalt
Composition and Properties of Smalt
Smalt is a finely ground glass containing cobalt. It is chemically stable, but it contains potassium and thus its stability is lower than glasses containing sodium. The pigment is known to have discolored in oil paintings but not in fresco. The discoloration is due to several factors and it has been thoroughly investigated (1 – 5).
Names
Starch bluePB 32, CI 77365From Anglo-French enamailler, from en– “in” + amailler “to enamel,” variant of Old French esmailler, from esmal “enamel,” from Frankish *smalt, from Proto-Germanic *smaltjan “to smelt”.From Online Etymology Dictionary
Preparation of Smalt
Smalt can be prepared in the laboratory by heating quartz sand, potassium carbonate and a small amount of cobalt (II,III) oxide (Co3O4) for 30 minutes at a temperature of 1150° C.
History of Use
Smalt has been found in European paintings as early as fifteenth century and its use continued until about nineteenth century. The following graph gives the frequency of its use in the paintings of the Schack Collection in the Bavarian State Art Collections in Munich (1).An extensive collection of occurrences of this pigment in paintings from several historical periods can be found in the blog post ‘Pigment: Ground glass‘ by The Eclectic Light Company.Examples of use
Quote
Dyeing was carried to great perfection. Many vegetable and animal coloring matters were known. Mordants were used, and the effects produced were very beautiful. Paints were also prepared, and applied with brushes. The following mineral colors were known at the time of Pliny : white lead, cinnabar, litharge, smalt, verdigris, ochre, lampblack, realgar, orpiment, and stibnite.
Source: A Short History of Chemistry
