=Payen’s Mountain Blue= is a mixture of copper hydroxide with varying quantities of calcium carbonate. It is prepared by adding calcium chloride to a solution of copper sulphate and stirring in dilute milk of lime. A green precipitate of basic copper chloride is formed; potassium carbonate solution mixed with milk of lime is then added. The latter mixture contains caustic potash, which decomposes the copper chloride, copper hydroxide and calcium carbonate being simultaneously precipitated. Different shades may be obtained; by increasing the quantity of lime the product is paler. This pigment commonly comes into the market in the paste form. When it is dried the operation must be conducted at a low temperature; copper hydroxide is converted into copper oxide at relatively low temperatures. =Oil Blue.=—The pigment known under this name consists of copper sulphide (CuS). It can be made in several ways. The simplest process, which also gives a good product, is here described. Sulphur is boiled in a glass flask with a long neck. When the heavy reddish brown vapours begin to fill the neck, copper turnings or wire free from oxide are thrown in. The copper burns in the sulphur vapour with a red flame to cupric sulphide. When it has once commenced to burn, the flask requires little heating, so much heat is produced by the combustion of the copper that the sulphur continues to boil. The introduction of copper is continued until only a small excess of sulphur is left, the flask is then closed and allowed to cool. When the contents have reached the temperature of the air the flask is broken, and the mass boiled with caustic potash solution in order to remove excess of sulphur. The product obtained by this process has a handsome violet blue colour, which must be shown by the whole mass. If portions taken out of the flask are black, an excess of copper has been used. In order to improve the colour of a faulty batch, it is mixed with sulphur and quickly heated to the boiling point of sulphur, air being excluded. The excess of sulphur is then removed by boiling with caustic potash solution. Oil blue is a handsome but not very durable pigment. When used in varnish, which protects it from the action of air, it is fairly permanent. =Copper Hydroxide.=—By precipitating carefully a solution of a copper salt with caustic soda, a blue precipitate of copper hydroxide is obtained, which, though not distinguished by any particular beauty, is adapted for use as a distemper colour and for similar purposes. The operation is conducted at ordinary temperatures; on account of its voluminous nature the pale blue precipitate requires a long time for washing. Hot solutions must not be used; at a temperature near the boiling point of water copper hydroxide is completely decomposed into black copper oxide. CHAPTER XXVI. BLUE COBALT PIGMENTS.