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.

Enamel white is, however, generally prepared from barytes, which is ground into a very fine powder and converted into barium sulphide by heating with coal (see pages 41 and 42). Hydrochloric acid acting on the sulphide produces barium chloride and sulphuretted hydrogen.

The covering power of a pigment is greater the finer its state of division, so that it would appear advisable to precipitate a weak solution of barium chloride by sulphuric acid at the ordinary temperature. When the barium sulphate has been completely precipitated, a solution of pure hydrochloric acid remains, which ought to be utilised; but when very dilute barium chloride solution is used, the hydrochloric acid is so dilute as to be useless. The barium chloride is, therefore, given in practice such a strength that it has a specific gravity of about 1·198; when the barium sulphate has been precipitated from this solution the residual hydrochloric acid has a specific gravity of 1·043.

Water of considerable purity must be used to dissolve the barium chloride. Experience has shown that water which contains appreciable quantities of organic matter does not give a pure white product. The presence of sulphate of lime in the water, which precipitates barium sulphate, need not be regarded, because the barium sulphate is so finely divided that it remains suspended in the liquid, and is carried down on precipitation of the enamel white by sulphuric acid. Carbonate of lime in the water causes the separation of barium carbonate; this may be avoided by slightly acidifying the barium chloride solution, thus converting the calcium carbonate into chloride.

According to C. A. F. Meissner, artificial barytes, suitable for use in oil paints, is obtained by precipitating barium salts by soluble sulphates in place of sulphuric acid, then quickly heating the washed and dried precipitate in a muffle to a red heat and throwing into cold water.

As has been already stated, enamel white is the most permanent pigment that exists; it appears destined in course of time to replace white lead and all other white pigments, especially as its cost is generally lower than that of the other white pigments. It costs, for example, only half as much as white lead. At present, the principal uses of enamel white are found in paper staining; it is not used to any extent in oil paints. On account of its permanence, it should be used in the place of white lead and white zinc. It also appears particularly suitable for obtaining pale shades; it can be mixed with any other pigment in any quantity without altering it in the least. This is, of course, only true when the enamel white is completely pure, and when it has been freed from every trace of hydrochloric acid by careful washing.

Lithopone.

A white pigment is obtained, according to Orr’s process, by lixiviating crude barium sulphide, obtained by igniting barytes with coal, with water and dividing the solution into two parts. Zinc chloride is added to the first portion, then zinc sulphate, and finally the second portion of barium sulphide solution. The white precipitate obtained by this process contains one equivalent of barium sulphate to two equivalents of zinc sulphide. It is collected, quickly dried, heated in retorts to redness, and, whilst still hot, thrown into cold water, by which its density and therefore covering power are increased. The pigment is finally washed and ground. It is a good white, but when mixed with lead pigments discolours them by reason of the sulphide it contains.