Occasionally the flasks are turned about, and after about two hours of this agitation they commence to grow hot, and the contents assume a greenish-brown colour. The lye is robbed of its sulphur by the mercury, and replenishes itself from the excess added.
Complete combination of the mercury and sulphur is accomplished in about three and a half hours, when the colour of the mass becomes dark brown. The next step is to cool the compound, an operation which must proceed very slowly, and should occupy about five hours.
Development of the colour is effected by heat, for which purpose the flasks are placed in a stove room or water bath, and subjected to a temperature which does not fluctuate beyond 113° and 122° F., under the influence of which the red colour appears. The greatest care is necessary in this heating process, as it determines the success or failure of the colour. It lasts several days, during which the flasks should be shaken three or four times daily.
In order to separate the vermilion from the excess of sulphur, water is added to the contents of each bottle, and, after thorough shaking, the whole is turned out into a filter. The clear liquor escapes, and the residual vermilion is mixed with caustic soda lye in stoneware jars, and thus the remaining free sulphur is dissolved out. Subsequently the lye is poured off as completely as possible, and the deposit is repeatedly washed, first by decantation and finally on a filter. The whole operation of filtering and washing cannot be completed in less than two or three days. When this is finished, the drying must be carried on at a very low temperature, till the vermilion can readily be broken and is dry to the touch, when it is put into iron basins and repeatedly stirred, while the temperature is allowed to reach 143° F., but never beyond that. The final desiccation occupies about five hours.
Vermilion made in this way is reputed more permanent and less costly than by the usual methods.
(5) Dutch vermilion has a good name, and one method adopted in Holland is as follows:—A mixture of 2 lb. of mercury and 1 lb. of sulphur is thoroughly ground, and to 100 lb. of the mixture are added 2½ lb. of minium or of granulated lead. About 2 cwt. of the compound is put into each sublimation pot, which is duly heated. When the operation is finished, the pots are allowed to cool for eighteen to twenty hours, when they are broken, and their contents are ground in a mill. The lead remains as a sulphide in the bottom of the pots.
(6) A modification of the Dutch method consists in making an intimate mixture of 54 lb. of mercury squeezed through chamois leather and 7½ lb. of flowers of sulphur, which is then moderately heated on a shallow iron dish, and the resulting black sulphide (“ethiops”) is coarsely broken, ground, and kept in pots. To convert the ethiops into vermilion, the former is put into large clay crucibles in a furnace, and heated to dark-redness, whereupon the mass takes fire. As soon as the flame has subsided, the crucibles are covered with a close-fitting iron plate, and the firing is continued for thirty-six hours. The mass is stirred every half-hour with an iron rod, and fresh additions of ethiops are made at four or five hours’ intervals. The vermilion is sublimed, and condenses on the cool portion of the interior of the crucibles, whence it is collected by breaking the crucibles when cold, and is finally ground and levigated.
(7) Kirchoff’s method requires special care, and consists in grinding 300 lb. of mercury with 68 lb. of flowers of sulphur in a mortar, the sulphur being first moistened with a few drops of caustic potash. The resulting black sulphide of mercury is added to 160 lb. of caustic potash dissolved in very little water, and the whole is heated for half an hour on a sand bath, with occasional addition of water to make up for loss by evaporation. Gradually the mass, under constant agitation, becomes brown and gelatinous, and finally red. Thereupon it is carried to the stove room and still agitated at intervals. After several washings it is drained, and dried very gently.
(8) Weshle mixes finely powdered cinnabar with 1 per cent. of antimony sulphide, and boils the mixture several times with three parts of potassium sulphide in a cast-iron pot. The precipitate is water-washed, digested with hydrochloric acid, washed again, and finally dried.
(9) Jacquelin takes 90 lb. of mercury, 30 of sulphur, 30 of water, and 20 of hydrated potash; the mercury and sulphur are put into a shallow cast-iron dish, dipping into cold water, and the potash solution is added by degrees while the mass is kept in agitation. Then the mixture is heated for an hour at 176° F., the evaporated water being replenished. The vermilion is washed in an excess of boiling water, and again several times in cold water, and finally filtered and dried.