(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.
In the great mercury works at Idria, in Austria, vermilion is also made from the black sulphide. The latter is made by mixing 84 parts of mercury with 16 parts of finely powdered sulphur in rotating vessels driven by water power, the operation lasting about 3 hours. This quantity of sulphur is larger than is required to form mercuric sulphide; experience has shown that the combination takes place more rapidly when more than the equivalent quantity of sulphur is used. Heat is developed by the reaction and the temperature of the mixture rises to over 30° C. The black sulphide is then sublimed in cast-iron vessels, which are pear-shaped and built 6 together in a furnace; each holds a charge of 315 kilogrammes of black.
Several periods are distinguished during the heating of the mass in the sublimation vessels. The operation is commenced by heating 2 of the vessels first. As soon as the vapours of sulphur, issuing from the neck of the vessels, take fire with explosion, the fire is made to heat the adjacent vessels. When the contents of all the vessels have exploded, the first part of the process known as the “evaporation” is at an end. Earthenware heads are then placed on the vessels and the fire is increased until the excess of sulphur present begins to distil; its vapours take fire in the air with a slight explosion. When this takes place, earthenware receivers are attached which have only a small opening for the escape of uncondensed vapours. The sulphur condenses in these. When sulphur vapours are no longer given off in quantity, the intermediate period (stϋckperiode) is finished and the real sublimation of the vermilion commences. The fire is now considerably increased and the sublimed vermilion collects in the cooler parts of the apparatus. When the sublimation is completely finished, small blue flames, which quickly vanish, appear at the joints of the apparatus. The furnace is allowed to get quite cold, when the various parts of the apparatus are taken apart, the vermilion deposited in the tubes is carefully removed, so that they may be again used, whilst the receivers and head are broken, so that the vermilion they contain can be collected.
The larger pieces form lump vermilion; the fragments of the receivers are cleaned with a wet brush to collect what adheres. The whole process of sublimation from the introduction of the black to the end lasts about 7 hours.
The sublimed vermilion is ground in mills which differ little from ordinary grinding mills. To prevent the formation of dust, water is added and the stones are surrounded by wooden casings. The red paste from the mills, which is now known as vermilion, is then refined.
The refining consists in extracting the excess of sulphur by means of boiling potash solution; 300 kilogrammes of the ground vermilion are stirred up with water in a tub, the water is drawn off and the wet mass brought into an iron pan, in which it is heated with 22·5 kilogrammes of potash lye for about 10 minutes. According to the composition of the crude vermilion the lye has a strength of from 10° to 13° B. The smaller the quantity of sulphur, the weaker is the potash solution; it is, or was, made in Idria in a very primitive manner, by extracting wood ashes. For vermilion of a bright red shade potash solution of 10°, for the dark red of 11°, and for “Chinese vermilion” of 13° is used. The excess of sulphur, together with a trace of mercuric sulphide, dissolves in the potash solution; the sulphur chiefly forms potassium pentasulphide. When the boiling is finished, the vermilion is carefully washed and dried in dishes placed in a heated furnace. During drying, the vermilion agglomerates; finally the lumps are broken and sieved.
Chinese Vermilion is in similar case to Indian ink. Both substances are in common use in Europe, they far surpass in quality our own manufacture, and in neither case do we know the exact method by which they are made. Genuine Chinese vermilion so far surpasses European in brightness that it is bought at five or six times the price. It is said on unauthenticated authority to be made by subliming a mixture of 4 parts of quicksilver with 1 part of sulphur in earthenware pots closed by an iron plate, which is kept constantly wet and serves as a receiver, on which the vermilion deposits. The sublimed masses adhering to the lid are sorted, ground and repeatedly washed with water.
According to Callum’s description of the manufacture of vermilion at Hong Kong, mercury and sulphur are heated in a large iron pan, with continual stirring at about the melting point of sulphur (111° C.), until the whole has changed to a black mass. After cooling, this is mixed with water and mercury, the mixture thoroughly stirred, dried, placed in a hemispherical dish and covered over with broken porcelain. A similar dish is cemented on the top of the first, and the dishes are heated for 16 hours; the vermilion adhering to the porcelain fragments is removed, wet ground and dried.
A vermilion approaching Chinese in beauty is said to be obtained by mixing ordinary vermilion with 1 per cent. of antimony sulphide and again subliming the mixture. The dark grey sublimate produces a reddish brown powder, which is repeatedly boiled with a solution of “liver of sulphur” (potassium polysulphide), washed with water and digested for a long time with hydrochloric acid. The author has repeatedly made vermilion by this process, but could never obtain a product differing appreciably from the original vermilion.