This compound can also be easily obtained by heating mercury with sulphur. In a vessel, placed under a chimney with a good draught, which is necessary to carry away the poisonous mercury vapours, 6 parts of the metal are heated nearly to boiling and 1 part of sulphur added. Combination takes place at this temperature with a slight explosion, and pure mercuric sulphide results when the heating is continued until the excess of sulphur is driven off.
For the purpose of the manufacture of vermilion, black mercuric sulphide is most simply made by filling a thick-walled vessel with equal weights of mercury and sulphur, moistening the mixture with water and shaking or rotating the vessel for several hours. This can be done either in a rotating cylinder containing iron balls, or the vessel can be fastened to any rotating object—to a water-wheel or to the fly-wheel of a steam-engine. The vessel in which the combination is effected should of course not be quite full. It has been found that a more jerky motion than that of rotation effects the combination of the mercury and sulphur in a shorter time. For example, an opportunity of fastening the vessel to a saw-mill would be of great advantage.
The mercuric sulphide made by the above methods is a velvety black mass, which, even when exactly equivalent weights of sulphur and mercury have been used, is never quite pure. Carbon bisulphide will always extract a certain quantity of uncombined sulphur. The most important property of the black sulphide for the present purpose is that it is changed into the crystalline modification by heating to the temperature at which it volatilises. The sublimed mercuric sulphide has the well-known fiery scarlet colour characteristic of vermilion.
Red Mercuric Sulphide, or vermilion, exhibits, for a sulphur compound, considerable resistance to the action of chemical reagents; dilute mineral acids do not decompose it. Unfortunately, vermilion has another property which makes it quite unsuitable for the artist’s use: in the course of time it gradually turns dull and at last is completely discoloured. This alteration of colour can only be ascribed to a return of the red crystalline modification into the black non-crystalline. When a white pigment is tinted by vermilion it should not be a lead pigment, or in a brief time it will turn black. A white pigment such as zinc white, which is not acted upon by sulphur compounds, should be used.
CHAPTER XVII.
THE MANUFACTURE OF VERMILION.
The red modification of mercuric sulphide can be prepared in the wet or the dry way. The latter was formerly in general use, but at present the wet method is more generally employed, as it more easily and certainly produces a handsome pigment. Each method has its advantages, and both will be described.
(a) Dry Method.
The numerous prescriptions which have been given for the manufacture of vermilion by the dry method are all founded on endeavours to convert black mercuric sulphide into the red form. Many of these prescriptions contain directions for the soundness of which no reason can be discovered, and it is not going too far to say that none exists, and that operations described as essential for the success of the process have been inserted merely to give the recipe the appearance of novelty. It cannot be denied that certain manipulations impart a greater brilliancy to the product, although it is impossible to assign a physical or chemical reason; but the manufacturer will quickly be able to differentiate the valuable from the worthless in these processes. Two conditions have the greatest influence on the beauty of the pigment—the temperature at which the black sulphide is sublimed, and the complete freedom of the vermilion from excess of sulphur. Of less importance is the repeated grinding of the vermilion; the oftener it is ground, the smaller the crystals become and the paler the shade.
The operation of grinding the pigment in ordinary mills is known practically as “preparing,” the extraction of excess of sulphur by boiling with alkaline liquids as “refining,” the product.
The usual process in Holland, especially in the Amsterdam works, is as follows: The black sulphide is made by heating 270 parts of mercury with 37·5 parts of sulphur in copper pans—the fire is so regulated that the temperature is not high enough to ignite the sulphur. If properly prepared the product has now a pure black colour. It is immediately finely ground and preserved in earthenware bottles containing only 0·7 kilogramme each. It is sublimed from hemispherical vessels, provided with iron covers carefully joined to the rim of the sublimation vessel by a suitable fire-resisting cement. Generally 3 such vessels are contained in one furnace. The operation is commenced by heating them until the bottom shows a dark red heat. The temperature should now be increased to such an extent that, when the contents of one of the above-mentioned small bottles are poured into the vessel, a small flame only appears; if on the contrary the contents burn explosively, the temperature is too high, and further quantities of the black sulphide must not be added until the vessels have cooled down to a certain extent. If, on introducing the first quantity of mercuric sulphide, a flame appears unaccompanied by an explosion, the contents of several of the bottles may be introduced; the openings through which this addition is made are immediately closed by a well-fitting iron plate. From time to time this cover is raised and fresh quantities of mercuric sulphide added. The operation lasts about 36 hours, when double the quantity of mercuric sulphide, made from the above mixture of mercury and sulphur, will have been introduced into the 3 vessels. For the complete success of the process, the accurate regulation of the temperature to which the vessels are exposed is particularly important; in practice, the temperature is judged by the height of the flame which issues on removing the iron plates: if this reaches to 1 metre the fire is too fierce, but if the flame is very small the fire must be increased. Towards the end of the sublimation the mass in the vessels is stirred about every 15 minutes. As soon as the sublimation is finished the fire is extinguished. The vessels are broken when completely cold; the vermilion is then found in the upper portion as a sublimate of fibrous character. The vermilion made by this process requires simply wet grinding under ordinary mills and drying to be ready for market.