Basic Lead Acetate, Pb(C₂H₃O₂)₂.2PbO.—This salt may be regarded as a compound of neutral lead acetate with lead oxide. It is obtained by digesting vinegar with excess of litharge or with metallic lead, and also by treating lead acetate solution with litharge so long as the latter is dissolved. In the method last given, 100 parts of sugar of lead require about 118 parts of litharge to produce a saturated solution of basic acetate. The solution of this compound is alkaline; it turns red litmus paper blue. When exposed to the air, a turbidity is quickly produced owing to the separation of lead carbonate. In one white lead process basic lead acetate is the starting point of the manufacture.
Lead Chloride, PbCl₂, is seldom used in making colours. It may be prepared by stirring powdered litharge in common salt solution until the powder appears white. This, when washed, constitutes basic lead chloride. On adding hydrochloric acid to the washed mass until the liquid remains acid, lead chloride is obtained in the form of crystalline needles, which are very little soluble in cold, but more easily in hot, water.
Like any other soluble lead salt, lead chloride may be used in the precipitation of colours, but is seldom employed on account of its small solubility. Basic lead chloride was, at one time, used as a white pigment, and after melting, by which it is turned yellow, as a yellow pigment; it is no longer in use for these purposes.
Copper Compounds.—These are generally green or blue, and have an extended use in the production of colours. The metallic copper which is used in the preparation of colours is of the ordinary commercial quality. The impurities which it contains are generally so small in quantity that they are without importance for our purpose.
Copper Sulphate (Bluestone, Blue Vitriol), CuSO₄.5H₂O.—This is the commonest of the commercial copper salts, and on that account deserves our especial attention. It forms large sky-blue crystals, which effloresce slightly in the air, possess an unpleasant metallic taste, and are poisonous, like all soluble copper compounds.
Copper sulphate comes into commerce in a very pure form, but some qualities contain zinc sulphate or ferrous sulphate. The presence of zinc may be detected most easily by boiling the solution with excess of caustic soda, when copper oxide separates as a black powder, whilst zinc oxide remains dissolved. When sulphuretted hydrogen is passed through the liquid, a white precipitate of zinc sulphide is formed.
Iron is detected by passing sulphuretted hydrogen through the solution so long as a precipitate is formed, allowing the liquid to stand in a covered vessel, pouring it off from the precipitate, adding nitric acid, boiling and adding a solution of potassium ferrocyanide; a blue precipitate denotes the presence of iron.
Copper sulphate is rarely found which is quite free from iron and zinc. If these impurities are present in but small quantity, the zinc not exceeding 1 per cent. and the iron at most 0·5 per cent., the copper sulphate may be regarded as sufficiently pure for our purposes.
It may be here remarked that copper sulphate obtained from mints is generally of great purity, and hence particularly adapted for colour making.
When copper sulphate and other copper salts are dissolved in water, pale blue flocks of copper carbonate generally separate. This is due to the carbonate of lime contained in the water. An addition of a few drops of sulphuric, nitric or hydrochloric acid suffices to prevent this separation.