Lead antimonite and antimonate are both heavy, white powders which can be used as pigments. They are dearer than white lead, to which they are inferior in covering power, and which they do not exceed in permanence.
Lead Antimonite is obtained by heating 5 parts of finely powdered antimony with 20 parts of sulphuric acid until a dry, white mass of antimony sulphate is left. This is fused with soda ash, the melt is extracted with water, and lead antimonite obtained by precipitating with lead acetate.
Lead Antimonate is formed by introducing in small quantities at a time a mixture of 1 part of finely powdered stibnite (antimony trisulphide) with 5 parts of sodium nitrate into a red-hot crucible, boiling the mass with water and precipitating the solution with lead acetate.
CHAPTER VIII.
ENAMEL WHITE.
Barium sulphate, known as permanent white, enamel white, blanc fixe, barytes white, is the only white pigment which is absolutely unaltered by exposure to the atmosphere. Lead pigments are discoloured in the course of time, and in the end turn black; bismuth white behaves in the same manner; zinc white is much more lasting, but not quite permanent.
Enamel white is really permanent; it deserves the greatest attention from the colour manufacturer, especially as it can be made by a very simple and cheap method. When sulphuric acid or a soluble sulphate is added to the solution of a barium salt, all the barium is at once precipitated in the form of barium sulphate.
When quite pure, barium sulphate forms an extremely soft, brilliantly white powder, which offers complete resistance to the action of the atmosphere, and also of strong acids and alkalis. It is extremely insoluble, and can be precipitated from the most dilute solutions, and is then obtained in so fine a state of division that it cannot be filtered from the liquid; it passes through the closest filter together with the liquid. When the barium solution is heated to boiling before precipitation, the precipitate is somewhat coarser, and can be filtered off without difficulty.
Barium sulphate occurs ready formed in nature as the mineral barytes or heavy spar. Finely ground barytes may be used alone as a pigment, but more commonly is used for reducing white lead, for which purpose it is particularly applicable on account of its high specific gravity. This admixture must be regarded as diminishing the quality of the pigment, because ground barytes has far less covering power than white lead. Artificial barium sulphate is in a state of division which cannot be reached by grinding barytes, consequently it considerably surpasses the latter in covering power.
The raw material for the manufacture of enamel white is either barytes or witherite (barium carbonate); the latter, however, occurs so rarely, in comparison with barytes, that the greater quantity of all barium compounds is obtained from barytes.
If witherite is obtainable in large quantity, enamel white can be prepared from it by dissolving in hydrochloric acid and precipitating the solution of barium chloride so obtained by sulphuric acid. If the witherite is very pure, the process may be simplified by treating the mineral directly with sulphuric acid, and separating the enamel white by a process of levigation from the impurities. In this case it is, however, necessary to add a small quantity of hydrochloric acid to the sulphuric acid, for the latter forms on the surface of the witherite, at the commencement of the reaction, a thin layer of barium sulphate, which is quite sufficient to prevent the further action of the acid on the witherite lying below. The hydrochloric acid forms barium chloride, which is at once decomposed by the sulphuric acid into barium sulphate and free hydrochloric acid; this again dissolves a fresh quantity of witherite, and this process is repeated until the mineral is completely and quickly dissolved.