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.
(b) Wet Methods.
The manufacture of vermilion in the wet way is founded on the conversion of metallic mercury or its compounds into mercuric sulphide by heating with alkaline sulphides, such as ammonium sulphide and liver of sulphur. The product always contains uncombined sulphur, which is eliminated by treatment with alkalis.