A fairly concentrated solution of copper sulphate is warmed in a pan to about 30° C.; weak caustic potash solution is added until the liquid is slightly alkaline. A green precipitate separates, which consists of copper sulphate and hydroxide. In order to remove the copper sulphate from the precipitate, and impart to it the correct bluish-green colour, it is left on a strainer until it has acquired a pasty consistency, when it is brought into a tub and mixed with weak caustic potash solution. Care must be taken that this potash solution is not too concentrated, or it will withdraw water from the copper hydroxide and produce black copper oxide. In order to prevent this result, which would render the whole mass useless, a portion of the precipitate is tested with the caustic potash solution. If it is blackened, the solution must be diluted with the necessary quantity of water before it is used. Instead of diluting with water, potassium carbonate solution is often added. The addition of the caustic liquor is continued until the colour of a test portion of the precipitate is not deepened by further additions. When this point is reached the colour must be at once withdrawn from the action of the alkali. It is brought into a large tub filled with water, in which it is most carefully washed with many waters. Basic copper chloride may also be used as the raw material for this pigment. It is obtained by treating scrap copper with 60 per cent. of common salt and 30 per cent. of sulphuric acid. The pale green basic chloride is then treated with caustic potash solution. This method is now rarely used; copper sulphate is considerably lower in price than formerly and copper hydroxide is prepared from it with the least trouble. =Neuberg Blue= is a mixture of copper blue with Chinese blue. The larger the quantity of the latter component the deeper the colour. Neuberg blue can be more easily ground with oil than pure copper blue. It should not be used as a distemper colour, for the Chinese blue is decomposed by the action of lime, and the colour will quickly be spoiled by the separated ferric oxide. =Lime Blue= is ordinary copper blue, mixed with varying quantities of gypsum. It is paler than pure copper blue; different shades are obtained by increasing the quantity of gypsum. This pigment is made by dissolving 15 parts of copper sulphate in 1,300 parts of water, adding a strong solution of 121 parts of ammonium chloride, and then milk of lime prepared from 30 parts of quicklime. The precipitate is well washed, strained, ground, and finally dried. Lime blue is brought into the market in several forms; in cubes obtained by cutting up the stiff paste and then completely drying, and in irregular lumps or a coarse powder. The pigments containing copper hydroxide as their colouring principle are used less as artists’ colours than for distempering.