The residuum of cobalt, after the sulphur, arsenic, and other volatile matters of this mineral, have been expelled by calcination. The ores of cobalt are roasted in verberatory furnaces, provided with chambers to receive the arsenic: the product of zaffre is usually about 68 per cent. of that of the ore. The ores that contain much nickel are not fit for the preparation of zaffre, as the oxide of nickel would injure the beauty of the blue colour, or smalts, for the making of which zaffre is manufactured. Inferior kinds of zaffre are made by mixing this oxide, previously stamped and sifted to a fine powder, along with calcined flints or quartz, also ground in various proportions according to the use for which it is intended, moistening the whole with water, and packing it tight in casks, where it hardens to a stone. A very fine zaffre, or China blue, is obtained from the arsenical and grey cobalt ore, found in Cornwall, by boiling the powdered ore in nitric acid, which converts the arsenic into arsenical acid, and unites it with the different metals contained in the ore.
The solution being diluted with a large quantity of water, purified pearl-ash water is then added in small portions to the diluted solution; and on the addition of each portion the liquid is well stirred, left to settle, and the clear part poured off. This is repeated until the solution becomes of a rose colour, which shows that it contains only the arseniate of cobalt. The pearl-ash water is then added in larger quantity than is necessary to throw down all it contains, and the solution is boiled for a few minutes. Being then left to settle, the liquid is filtered, the oxide of cobalt left on the filter, washed with boiling water, and dried. This oxide is then melted with feldspar and a little potash, and thus yields a beautiful zaffre for painting porcelain. Another method is to grind the ore, mix it with two or three times its weight of China ware, grossly powdered, and heat it very strongly. The whole is then put into three or four parts of nitric acid, diluted with an equal weight of water. The clear solution is poured off, evaporated gently to a syrupy consistence, diluted afresh with water, left to settle, poured off clear from the arsenic that is separated, and then the pearl-ash water is added by small portions, and the operation finished as in the former process.
Zaffreis used for making smalts, and for painting on the best kinds of pottery. The common zaffre is cheap, but the best sells for two guineas the pound in the potteries. Zaffre is likewise used in the manufacture of cobalt.