The mineral known as heavy spar, or barytes, has been used as a white paint, particularly as an adulterant for white lead. However finely it may be ground, it is always very inferior in body and covering-power to the artificially-prepared barium sulphate - the true blanc fixe. To make this, a cold solution of barium chloride of specific gravity 1.19 is prepared, and to it is gradually added in the cold, and until no further precipitate is formed, dilute sulphuric acid of 1.245 specific gravity. The barium sulphate is washed with cold water until the wash-waters are entirely free from acid; for many purposes to which the product is applicable (fresco and tempera painting) it should be kept under water.
Baryta-white is absolutely unalterable by an impure atmosphere, and is without action upon other pigments. It does not work well in oil, but a mixture of flake-white and baryta-white, in the proportion of 2 to 1, presents the advantage of being very much less affected by sulphuretted hydrogen than flake-white.
The artificial baryta-white may be distinguished from the natural by its much finer state of division, by its greater body, and by the purity of its whiteness. Baryta-white is not adulterated, but its almost absolute insolubility in hydrochloric or nitric acid enables it to be at once distinguished from zinc-white or white lead.
Several mixtures of barium sulphate and zinc sulphide have been introduced as pigments; they are not suitable for the palette of the artist. The reaction by which the majority of them are formed is brought about by mixing together solutions of two soluble salts, barium sulphide (BaS) and zinc sulphate (ZnSO4), when two new salts are precipitated, both insoluble, namely zinc sulphide (ZnS) and barium sulphate (BaSO4).
Other white compounds used in painting are lime, whitening, gypsum and China-clay. These have been considered in the chapter on Painting-Grounds. Amongst white pigments which we need not describe are antimo-nious oxide, antimonious oxychloride, lead sulphite, lead tungstate, lead antimonite, and lead antimoniate. Not only are these compounds difficult to prepare in a satisfactory condition of purity and whiteness, but they are liable to turn yellow or dull in impure air.
It should be stated here that the tests described in the present chapter, and in all the other chapters on pigments, refer only to the dry material, or, at any rate, to pigments mingled with no fluid other than water. If it be desired to operate on paints, this can be done, as a rule, only after the removal of the vehicle with which they have been ground. Oil may be removed by means of benzene or turpentine-spirit, gum by treatment with distilled water.