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ferently. This subject requires further investigation before the point can be definitely decided.
MANUFACTURE OF VERMILION.- Vermilion can be made both by dry and wet methods; the former are those mostly used; the latter are employed in some places but not to the extent of the dry methods. The product is not quite equal, although very little inferior to that made by the dry methods. The Chinese have long been renowned as makers of vermilion; although their product is not any finer or more brilliant in tone than that made in Europe. Until lately, the process by which Chinese vermilion was made was not known with certainty, although it was conjectured that the wet method was used, and, consequently, this method is usually described in text-books as " The Chinese method ;” but this is now known to be erroneous, and that Chinese vermilion is made by a process very little different from that used in Europe. The difference in quality almost entirely arises from the greater care the Chinaman takes in making it.
DRY METHODS-1st. Dutch Process. This is the method commonly used for making vermilion. It is conducted in two stages. In the first stage 108 lbs. of mercury are mixed with 15 lbs. of sulphur in a shallow iron pot; this is usually placed over a furnace so that a gentle heat may be applied ; the two bodies gradually combine together to form a black sulphide of mercury or “ethiops as it is called, the union being promoted by a continual stirring with an iron spatula. When the combination is considered by the workman to be complete, the iron pot is emptied of its contents into a store pot and a fresh mixing is made. The “ethiops” contains some free mercury, free sulphur, as well as sulphide; the proportions will vary according to the length of time the operacion has been continued, the heat applied, &c.
The second stage consists in heating black ethiops in a suitable furnace, whereby it is converted into the red vermilion. A number of simple furnaces or fireplaces are built side by side to form a range ; in each of these fireplaces is placed a cylindrical earthenware pot, so arranged that the lower two-thirds of the pot are in, while the upper third is outside the furnace. The pots are fitted with a closely-fitting iron lid, in the centre of which is a small charging hole. The fire in the fireplace is lighted, and, when the pot has been heated to a red heat, a small quantity of the black ethiops obtained in the first stage is charged into the pot; much of the sulphur in the ethiops burns off; when there is no further appearance of sulphur fumes
from the pot more ethiops is added; these additions are continued at intervals for thirty-six hours, the cover being kept on during the whole of the operation; then the pots are allowed to cool down; when cold the cover is removed and the vermilion is found as a crust on the under side of the cover and around the sides of the upper portion of the pot. This crust is carefully removed, the red portions being placed on one side for further treatment, while any black, unchanged portions are mixed with some fresh ethiops to be again heated. The red vermilion is now ground up as fine as possible with water; if not of sufficiently brilliant colour it may be treated either with acids or alkalies as is described below, well washed with water, allowed to settle out of the wash waters, dried at a gentle heat and sent into the market ready for use.
2nd. Chinese Method.--A few years ago a description* of the process used by the Chinese for the preparation of vermilion appeared in several journals, and at the Colonial and Indian Exhibition held in 1886 there was shown in the Hong Kong Court a Model of a Chinese vermilion factory. Like the Dutch method, the Chinese process is in two stages, and is carried out as follows:
An iron pan, measuring 25 inches in diameter and 6 inches deep, is placed over a charcoal fire; into this pan is placed 17} lbs. of sulphur and 371 lbs. of mercury; heat is applied, and the mixture stirred until the materials melt and become amalgamated together; then 374 lbs. more mercury are added, and the heating and stirring continued until the two bodies have become united. The pot is now removed from the fire and water added in sufficient quantity to form a paste, which has a blood-red colour; the first stage of the process is now complete.
Second Stage. — The crude vermilion obtained in the first stage is broken up into small pieces and placed in iron pans measuring 294 inches in diameter and 8inches deep; on the top of the vermilion is placed a number of broken pieces of porcelain plates arranged in the form of a dome; over all is placed the pan used in the first stage, the two pans being luted together with clay, and a few vent holes left in the luting. The pans are placed on a furnace, which is constructed in a simple manner; usually a number are built side by side.
The pans are heated for 18 hours at a dull red heat, after which they are allowed to cool down; when cold, the pans are opened, when
* Oil and Colourman's Journal, 1883, p. 86. Chemical News, 1884, vol. 50, p. 77.
the vermilion is found as a red sublimate on the under side of the porcelain plates and the upper pan; this red mass is collected and transferred to another place for the finishing operation. The crude vermilion which has been scraped off tħe porcelain plates is now ground as finely as possible with water in a mortar; the ground colour is next mixed with water in which alum and glue in the proportions of 1 oz. of each in a gallon of water have been dissolved, and allowed to stand for à day; it settles down and is found as a cake at the bottom of the vessel, which is made of earthenware, and has a capacity of 6 gallons. The top of the cake is of fine quality ; this is separated from the bottom portion, which is re-ground up with the next batch; sometimes the top portion is re-ground. After being washed well with clean water, the finely-ground vermilion is dried and then packed up ready for sale.
WET METHODS—1st. Common Method.-In making vermilion by this method 68 lbs. of sulphur and 300 lbs. of mercury are mixed and ground together until they are thoroughly incorporated; they are then added to a solution of 160 lbs. of caustic potash in water, placed in iron pots and heated to a temperature of 45° C., which is maintained for some hours. For the first two hours the water lost by evaporation is made good, but after this no further addition is made, and the mass is kept constantly stirred. After some time the mass, which has at first a blackish appearance, turns brown and then gradually passes into red; when it is considered that the colour is fully developed the mixture is removed from the fire, well washed in water and dried. This process requires careful watching. With care and attention the product is equal in quality to that made by the dry method.
2nd. Firmenich Process.—The process described by Firmenich * consists in taking 10 parts of mercury and agitating them with 2 parts of sulphur and 4} parts of potassium pentasulphide (prepared by heating potassium sulphate with charcoal) and boiling the residue with excess of sulphur for three to four hours, when it takes a brown colour; it is then kept at a temperature of 45° to 50° C. for three to four days, being agitated at intervals during that period; it is next treated with water, then with a weak lye of caustic soda (to free it from excess of sulphur), washed thoroughly and dried.
In these wet processes it is important that care be taken not to heat the mixtures of mercury, sulphur, and alkali to too high a temperature; from 45° to 50° C. is high enough, Time, not
* Chemical News, vol. v., 1862, p. 247.
heat, seems to be the most important element to consider in these processes ; too great a heat turns the vermilion brownish.
The brilliancy or fire, as it is sometimes called, of the vermilion may be increased during manufacture by
1st, Grinding very fine and levigating ;
4th, By treatment at about 50° C. with a mixture of the caustic and sulphide of potash ; and 5th, By treatment with hydrochloric acid.
Any of these, or a combination of them, may be, and are, used for this purpose.
PROPERTIES OF VERMILION.–Vermilion is a very bright scarlet powder. It is the heaviest pigment known, its specific gravity being 8.2, which causes it to settle readily out of paints, &c., in which it is used, and renders its application somewhat troublesome. It is very opaque, and, consequently, has great covering power or body. It is quite insoluble in water, alkalies, and any single acid, but a mixture of nitric and hydrochloric acids dissolves it with the formation of a colourless solution of mercuric chloride; as a rule, very few substances are capable of acting on vermilion.
Heated in a tube out of contact with air, vermilion first turns brown, then sublimes in the form of a red sublimate. Heated in contact with air, vermilion burns with a pale blue, lambent flame, giving off vapours of sulphur dioxide and mercury oxide ; if pure, there will be but a trace of ash left; thus a sample of good vermilion analysed by the author contained
This forms a reliable test for the adulteration of mercury, for any adulterant which may be used will be left behind on heating. The usual adulterants employed are red lead, oxide of iron, red lakes, vermilionettes, &c. The presence of any of these is easily ascertained by the application of the characteristic tests, which will be found described under each particular pigment.
When used as an oil-colour vermilion is permanent; when used as a water-colour it is generally considered to be permanent, but the experiments recently made by Capt. Abney and Dr. Russell throw some doubt on this point; they found that vermilion used as a water-colour turned brown after two years exposure to light
and air, probably owing to an intermolecular change; much appears to depend on the care with which the vermilion has been made.
RED LEAD. This valuable red pigment has been known and used for a very long time.
Pliny, in his writings, describes this body, which in his time was known as minium, under which name it is also frequently referred to in later writings. Pliny also mentions its use for adulterating vermilion. Davy, who had an opportunity of examining the contents of some pots of colour found in the remains of Roman and Greek cities, frequently found red lead among them. How it was made by the ancients is not definitely recorded.
MANUFACTURE OF RED LEAD.—There is only one process for making red lead, which consists of two stages—the
first stage bas for its object the conversion of metallic lead into monoxide of lead; in the second stage this oxide is converted into red lead.
1st Stage, Drossing.—This is conducted in what is called the "drossing oven,” a kind of reverberatory furnace of which Figs.13