Pigments, Paint and Painting: A practical book for practical men

by George Terry

Another and briefer account of the Chinese vermilion manufacture is given by H. Maccallum, in the Proceedings of the Pharmaceutical Society.

He says there are three vermilion works in Hong Kong, the method of manufacture being exactly the same in each. The largest works consume about 6000 bottles of mercury annually, and it was in this one that the following operations were witnessed:—

First Step.—A large, very thin iron pan, containing a weighed quantity, about 14 lb., of sulphur, is placed over a slow fire, and two-thirds of a bottle of mercury added; as soon as the sulphur begins to melt, the mixture is vigorously stirred with an iron stirrer until it assumes a black pulverulent appearance with some melted sulphur floating on the surface; it is then removed from the fire and the remainder of the bottle of mercury is added, the whole being well stirred. A little water is now poured over the mass, which rapidly cools it; the pan is immediately emptied, when it is again ready for the next batch. The whole operation does not last more than ten minutes. The resulting black powder is not a definite sulphide, as uncombined mercury can be seen throughout the whole mass; besides, the quantity of sulphur used is much in excess of the amount required to form mercuric sulphide.

Second Step.—The black powder obtained in the first step is placed in a semi-hemispherical iron pan, built in with brick, and having a fireplace beneath, covered over with broken pieces of porcelain. These are built up in a loose porous manner, so as to fill another semi-hemispherical iron pan, which is then placed over the fixed one and securely luted with clay, a large stone being placed on the top of it to assist in keeping it in its place. The fire is then lighted and kept up for sixteen hours. The whole is then allowed to cool. When the top pan is removed, the vermilion, together with the greater part of the broken porcelain, is attached to it in a coherent mass, which is easily separated into its component parts. The surfaces of the vermilion which were attached to the porcelain have a brownish red and polished appearance, the broken surfaces being somewhat brighter and crystalline.

Third Step.—The sublimed mass obtained in the second step is pounded in a mortar to a coarse powder, and then ground with water between two stones, somewhat after the manner of grinding corn. The resulting semi-fluid mass is transferred to large vats of water and allowed to settle, the supernatant water is removed, and the sediment is dried at a gentle heat; when dried, it is again powdered, passed through a sieve, and is then fit for the market.—Proc. Pharm. Soc.

(4) Firmenich describes a process which he declares gives better results in the beauty of colour than any other. It consists in using sulphide of potassium, which must be in a state of great purity. Of the various methods for preparing potassium sulphide, Firmenich rejects those in which caustic potash lye is boiled with excess of flowers of sulphur, on account of the simultaneous formation of a hyposulphite or sulphate of potash, which interferes in the preparation of the vermilion.

The process adopted by Firmenich for making pure potassium sulphide is to reduce sulphate of potash by heating with charcoal, and subsequently saturating the lye with sulphur to the necessary degree.

Usually about 20 parts by weight of potassium sulphate and 6 parts by weight of charcoal are reduced to very fine powder and thoroughly incorporated. Placed in a Hessian crucible, the mass is covered, luted, and strongly heated. As considerable ebullition takes place the crucible should be of such a size that the charge only occupies two-thirds of its capacity. After fusion is complete, the mass, which is now potassium sulphide, is allowed to cool; it presents a reddish-brown crystalline appearance, and is very hygroscopic. It is put into a cast-iron pan, with addition of soft water in the proportion of 7 parts of water to every 2 parts of the potassium sulphide; after boiling, it is filtered and on cooling, the undecomposed sulphate of potash collects in crystals attached to the sides of the pan.

The thus purified lye is boiled a second time with flowers of sulphur, added in small doses until saturation is indicated by bubbling and effervescence. The simple (monosulphide) potassium sulphide in this manner takes up four additional atoms of sulphur, and becomes the pentasulphide.

The preparation of the vermilion then proceeds in the following manner:—Into a series of large flasks are put 11 lb. of mercury, 5 lb. of the potassium sulphide lye, and 2¼ lb. of sulphur. The contents are subjected to a moderate heat, and the flasks are then agitated in a curious manner by arranging them in pairs in baskets suspended from strings, over a straw mattress, on which the baskets bump each time they descend.