Zinc-White

Synonyms:

Zinc-White, Chinese White, Blanc De Zinc, Zinkweiss

The substitution of carbonate of zinc for white lead seems to have been first suggested by Courtois of Dijon in 1787. After several unsuccessful attempts to introduce either the carbonate or the oxide as an oil paint, the latter began to be used about 1849-50, shortly after Leclairehad shown how to prepare an oil suitable for making the paint dry. We believe that it had been frequently employed as a water-colour many years before 1849. So early as 1834 Messrs. Winsor and Newton prepared a peculiarly dense form of this pigment under the name of Chinese white.

For the preparation of the best zinc-white it is essential that the zinc be pure; especially should it be as free as possible from the metal cadmium. The zinc is heated to the distilling-point in crucibles or retorts set in a furnace; the vapour, meeting with air, burns into the white oxide, which condenses in a series of chambers. The contents of these chambers vary somewhat in purity of tint; the presence of some metallic zinc generally imparts a greyish hue to the zinc oxide nearest the crucibles or retorts. By selecting the densest and whitest product, and then submitting this to powerful mechanical compression when red-hot, an excellent pigment having a dense body is obtained. Zinc-white prepared in the wet way, as by the action of lime-water upon zinc chloride, is inferior in substance to that made as above described, while that obtained directly from blende is of bad colour.

As an oil-paint, zinc-white is a bad dryer. Instead of being ground in raw poppy or linseed oil, an oil rendered highly siccative by borate of manganese should be employed. In spite of its unquestionable merits, zinc-white in oil cannot be recommended as a complete substitute for flake-white. When used freely, it often shows a tendency to crack and scale, besides becoming with age more translucent, or rather, less opaque. For water-colour painting, tempera, and for fresco, zinc-white is practically perfect, being unchangeable in hue or opacity under the most adverse influences. Paper washed with zinc-white, either alone or tinted with a coloured pigment, affords a good ground for silver-point, platinum-point, or pencil drawings. There is a peculiar 'tooth' in the zinc-white which freely brings off the metal or graphite from the pencil, and serves to fix it on the prepared surface.

The purity of zinc-white is easily tested. Heated in a tube, it should yield no volatile product, and should suffer no permanent change of hue. It should dissolve completely without effervescence in boiling dilute nitric or hydrochloric acid. If, on heating, it acquires a permanent yellowish hue, giving off moisture at the same time, white lead is probably present. If it does not dissolve completely in acid, it probably contains barium sulphate; if effervescence occurs during solution, either whitening, or white lead, or zinc carbonate is present. Zinc carbonate, however prepared, is inferior in whiteness and body to the oxide.

Zinc sulphide has been prepared as a paint; its liability to evolve sulphuretted hydrogen renders its use as an artists' pigment dangerous, for there are several other colours upon which it would exert a deleterious action. It has very considerable body.