This appears to be a double silicate of potash and cobalt, mixed with time, alumina, magnesia, iron oxide, nickel oxide, arsenic, carbonic acid, and water. The intensity of the colour, in Ludwig's opinion, depends on the greater or less proportion of the double silicate. The materials employed in the manufacture are cobalt ore, sand, and potash. The mineral generally used in Saxony is a spews, or arsenide of cobalt and iron. The broken ore is roasted at red heat in a reverberatory furnace, with a very-high stack for carrying away the arsenical end sulphurous fumes. When vapours cease to be disengaged, and the material begins to be pasty, the roasted ' product is removed from the fire, cooled, pulverized, and passed through a silken sieve. This powder is called zaffer. A sand free from iron, mica, talc, and lime is also calcined and thrown into, cold water while still red-hot. It is then powdered, washed with hydrochloric acid, and dried. The potash should contain no lime, sand, or chloride of sodium.
The proportions for the mixture are guided by the quality of the cobalt ore, and by the tone of colour to be produced. The cobalt and sand are first added, and then the potash; the whole is introduced into clay pots having a closable hole in the bottom. The pots are placed in a glass-farnace heated by a wood fire. After 4 to 6 hours of calcination, the material is melted, and forms 3 layers: the uppermost, or dross, is composed of sulphate and arseniate of potash, and chloride of potassium; the lowest consists of ore and unmelted substances; and the middle one is the blue glass. Most of the dross is taken off with hot iron ladles, and the lowest layer of unmelted materials is removed through the bottom hole. After this hole has been closed again, the melted blue glass is ladled out into basins of cold water. The pots are charged again, and a new operation begins. The glass is removed from the water, dried, and pulverized under horizontal stones. The powder is then levigated (floated), in order to obtain various degrees of fineness.
A magnificent blue may be prepared with pure oxide of cobalt. The ore, finely ground, is treated by boiling nitric acid, which makes nitrates of cobalt and iron, and arsenic acid. The liquor is decanted, diluted, and decomposed by a solution of soda carbonate, producing a soluble arseniate of soda, and a precipitate of the carbonates of cobalt and iron, which is collected, carefully washed, dried, and calcined. The resulting cobalt oxide, holding a small proportion of iron oxide, is mixed with sand and potash.
Smalt employed for inside painting has the inconvenience of turning green and black; the difficulty of grinding it fine enough prevents its employment for artistic painting. Its principal use is for giving an azure colour to signs, which are painted with ordinary blue oil-paint, and then dusted over with smalt. It changes less in size than in oil, and is much used in fresco-painting. It dries rapidly. (Riffault.)