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.
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.
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.
Apart from the danger to the health of the workmen caused by the poisonous mercury vapours, which are present in rooms where mercury or its compounds are heated in apparatus even of the best construction, and which is not to be feared in a wet process, the vermilion prepared in the latter manner has a finer and brighter shade than that prepared by a dry process. The greater cost of the wet process is covered by the higher value of the product.
The starting point of the wet method may be either metallic mercury, black mercuric sulphide or another mercury salt. When metallic mercury is used, according to an old process, 100 parts are ground with 23 parts of flowers of sulphur and a little caustic potash solution to a homogeneous mass, which is then heated with a solution of 53 parts of caustic potash in an equal quantity of water, the evaporated water being continually replaced, until the colour changes from brownish-red to the fiery red of vermilion. When the colour approaches the desired shade a careful watch must be kept, and immediately the proper shade is obtained the heating must be stopped. If it is continued beyond this point for but a short time, the vermilion at once loses its fire and cannot be again brightened. An interruption in the heating is equally harmful, a dull shade being produced. When the proper shade has appeared, the contents of the vessel are poured into a large quantity of water. The vermilion is then washed with dilute caustic potash and afterwards with water until the alkaline reaction disappears. It is finally dried. The alkaline solutions obtained in this process contain considerable quantities of mercury in solution. They are collected, and when a sufficient quantity has accumulated the mercury is extracted.
Barff recommends the following method: Mercury is rubbed with ⅙ to ⅕ of its weight of sulphur until a uniform grey powder results. This is heated in a porcelain dish with caustic potash solution (133 parts of potash in 150 parts of water) at 45° C. until the powder has become bright red. Heating above 45° C. is to be avoided; it would impart a brown tinge to the vermilion.
A convenient process for making vermilion in the wet way is to first produce the black sulphide in the dry way and then treat this with alkalis. Brunner’s method is founded on this procedure: 100 parts of mercury and 38 parts of sulphur are used to prepare the black sulphide. This is heated with a solution of 25 parts of caustic potash in 150 parts of water. The operation is best conducted in a vessel placed in a water bath, the temperature of which does not exceed 45° C. After heating 7 to 8 hours, the mass begins to turn red, when the change proceeds more quickly, and the greatest care must be taken not to exceed the point at which the colour has reached its greatest brilliance. As soon as the desired shade has appeared, the temperature of the water bath is lowered, but it is kept warm for several hours. Caustic solutions of different strengths produce different shades. Thus, in order always to obtain the same product from the same quantities, it is necessary to replace the evaporated water at short intervals. In this process some quantity of mercury remains uncombined; it is separated from the vermilion by a process of levigation in the washing.
Firmenich’s Method.—Many wet methods for the manufacture of vermilion differ but little from the preceding. In Firmenich’s method the production of the black sulphide is united with the formation of vermilion in one operation.
A solution of potassium sulphide of a certain strength is made by igniting 20 parts of potassium sulphate with 6 parts of coal, boiling the mixture with 1·5 time the quantity of rain water, cooling the solution, separating from the potassium sulphate which crystallises out, and boiling the solution with sulphur so long as it is dissolved. A mixture of 4·5 parts of this solution, 100 parts of mercury, and 2 parts of sulphur is placed in flasks, which are subjected for several hours to a shaking or rocking motion. The liquid becomes greenish, and its temperature rises in consequence of the combination of mercury with a portion of the sulphur of the potassium sulphide. The latter immediately dissolves the free sulphur present, and again gives it up to the mercury, which in the course of a few hours is completely converted into the black sulphide. The shaking is discontinued when the contents of the flasks have acquired a deep brownish-red colour. The flasks are then placed in a room heated to 45° C., and their contents repeatedly shaken up. At this temperature the transformation of the black into the red mercuric sulphide is completed. The deposit in the flasks acquires more and more a scarlet shade. As soon as the colour is found to have reached its greatest intensity the liquid is carefully poured off, and the vermilion treated with caustic soda solution in order to dissolve free sulphur; it is then very carefully washed.
In this process, the temperature at which the operation commences is important. The cooler the mixture which is placed in the flasks before shaking, the paler will be the colour of the vermilion obtained. It is to be supposed that the reaction, which is but slow in the cold, forms in consequence of its slowness a black sulphide of such character that in its subsequent transformation into the red crystalline sulphide very small crystals are produced.
The Gautier-Bouchart method for the preparation of vermilion from mercury and ammonium sulphide is applied on the large scale as follows: 1,000 parts of mercury are shaken for 7 hours with 200 parts of flowers of sulphur and 400 parts of ammonium sulphide saturated with sulphur; the dark-coloured mixture is exposed for several days to a temperature of 60° C., when the colour changes to red. In addition to the usual washings, the mass is further treated with nitric acid. The purpose of this operation is to oxidise all the free sulphur to sulphuric acid.
For a colour works in which, in addition to vermilion, are made other pigments, especially such as are sensitive to the action of sulphuretted hydrogen, this process, which produces vermilion of a good shade, although not particularly stable, is not to be recommended on account of the evolution of sulphuretted hydrogen, which cannot be completely prevented even by the greatest care.
Liebig’s Process.—The starting point in the manufacture of vermilion by this process is the compound known as white precipitate (see below). It is only necessary to heat white precipitate with a solution of ammonium sulphide saturated with sulphur, at 40° to 50° C., for a long time. The operation may be conducted in well-closed flasks in a place the temperature of which is, as nearly as possible, 45° to 50° C., such as the flues leading from a furnace in constant use. The change of colour is gradual; this is an advantage, since it is easier to obtain the correct shade. By treating the product with weak caustic potash solution it may be made still brighter. After washing and drying at a gentle heat the pigment is finished.
This method is especially adapted for manufacturers not exclusively engaged in making vermilion. No special arrangement of apparatus is necessary; the apparatus for making ammonium sulphide and a number of glass flasks are the only essentials. There is another considerable advantage, that the product cannot be completely spoiled. The process of formation of the vermilion is tolerably slow; a careful observation of the progress of the earlier operations is sufficient to determine the time required. In order to be able to do this with certainty, it is necessary to work always under exactly the same conditions; the same quantity of white precipitate must always be used and the solution of ammonium sulphide must always have the same strength.
Mercuric Ammonium Chloride, Infusible White Precipitate, HgClNH₂.—This compound, which is required in the last-mentioned process for the manufacture of vermilion, is most cheaply made in the following way: to a solution of 1 part of common salt in 32 parts of water, 2 parts of dry mercuric sulphate are added in small quantities, whilst thoroughly stirring. It is absolutely necessary to work in this way, because mercuric sulphate is decomposed by water into free sulphuric acid and a basic salt, which is much more slowly converted into white precipitate than the neutral salt. The liquid now contains mercuric chloride; on the addition of ammonia to alkaline reaction it gives a heavy white precipitate. The liquid is poured off and the precipitate washed with water containing a little ammonia, until the washings give only a slight turbidity with barium chloride. This process may also be commenced with mercuric chloride, but the above method is cheaper. White precipitate should volatilise without fusing when heated on platinum foil and should keep its white colour when treated with ammonia.
Electrolytic Process.—In a wooden vessel 1 metre in diameter and 2 metres deep are placed, at the sides, dishes 15 centimetres wide, containing a layer of mercury 1 centimetre deep. These dishes are connected with the positive pole of a dynamo. The negative pole is connected to an iron plate, electrolytically coated by copper, placed at the bottom of the vessel, which is filled with a solution containing 8 per cent. of ammonium nitrate and 8 per cent. of sodium nitrate. A regular current of sulphur dioxide is introduced through a perforated coil. The excess of gas escapes by a tube in the cover. When the current is passed a precipitate of red mercuric sulphide is at once formed. Attempts have been made to dispense with the current of sulphur dioxide. The bath then contains 100 litres of water and 8 kilogrammes each of ammonium nitrate, sodium nitrate, sodium sulphide and sulphur. Under these conditions it is only necessary to add sulphur and mercury in order to obtain, at the end of the operation, vermilion which is in no way inferior to that made by the first process.
Vermilion is frequently grossly adulterated by cheaper pigments. Substances are often found under the name of vermilion which contain no trace of mercury, but consist of bright orange lead mixed with a few per cent. of ferric oxide, and having a deceptive similarity in colour to the best vermilion.
Mercuric Iodide.—When corrosive sublimate solution is precipitated with exactly the necessary quantity of potassium iodide, mercuric iodide is obtained as a scarlet precipitate which surpasses in beauty even the best samples of vermilion. Unfortunately, this substance cannot be used as an artists’ colour. Exposure to light soon turns it brown and finally black. It appears to be unaltered in the dark; the author possesses a sample which has been so kept for 30 years without losing its shade in any way.