(2) Precipitation of copper sulphate by excess of thin milk of lime in the cold, followed by washing and drying, will give a lime blue which will dry without turning black. Or 100 lb. of the copper sulphate may be treated with a milk of lime prepared from 30 lb. quicklime and addition of 12½ lb. salammoniac. When the liquor has become colourless, the pigment is prepared from the precipitate; but the lime should be ground after slaking, and the milk of lime left to stand for some days, before use. The salammoniac seems to be essential to the production of a pure full blue. Milk of lime poured drop by drop into the ammoniacal copper solution gives a precipitate which redissolves on agitation, and remains long in solution under heat, but finally throws down a permanent precipitate, while the liquor on standing gives beautiful blue crystals. From experiments it appears that out of seven atoms of copper sulphate in the liquor, five are precipitated by milk of lime and the last two are decomposed by ammonia. A greater proportion of lime will produce a precipitate holding a certain quantity of less valuable pigment. A smaller proportion of lime yields a finer coloured and more crystalline pigment, because it crystallises partly in the excess of solution, so that by incomplete decomposition a smaller yield of superior pigment is obtained. The proportions necessary for formation of the colour are 7 equivalents of copper sulphate, 5 of lime, and 2 of ammonia, and if the 2 equivalents of ammonia be replaced by 2 of lime and 2 of salammoniac, the proportions furnishing the best colour will be 100 lb. copper sulphate, 24 lb. lime, and 22½ lb. salammoniac. Both caustic soda and caustic potash produce a fine blue precipitate in a solution of ammoniacal copper sulphate with excess of ammonia, but the liquor decolorises only on evaporation of the ammonia. The precipitate becomes lighter-hued the more it is washed, and consists of hydrated oxide of copper with a little carbonic acid; it does not turn brown even when heated in presence of excess of potash or soda. Moreover the presence of ammonia renders the hydrated oxide of copper much more permanent. The composition of this pigment is given by Gentele as 33½ per cent. copper oxide, 23½ sulphuric acid, 16 lime, and the remainder water, &c. MOUNTAIN BLUE OR AZURITE.--This natural blue pigment consists essentially of a basic carbonate of copper, and is found in quartz rocks in England, France, Bohemia, Hesse, Saxony, the Tyrol, and Siberia. It affords a rich sky-blue paint of a permanent character, but being comparatively costly is not largely employed. Its composition is about 69 per cent. copper oxide, 25½ carbonic acid, and 5 water. The only preparation needed is exceedingly fine grinding.