| Tin dioxide | 80 | parts |
| Sulphur | 60 | ” |
| Ammonium chloride | 30 | ” |
| Tin filings | 45 | ” |
| Sulphur | 35 | ” |
| Ammonium chloride | 25 | ” |
In all these cases the chief endeavour should be not to raise the temperature too high; a dark red heat is sufficient to give a perfectly satisfactory result.
When metallic mercury is used, it is employed in the form of an amalgam with tin, which is then in such a finely divided condition that it readily enters into chemical combination with the sulphur. The amalgam is most simply obtained by heating 1 part of mercury almost to boiling, and stirring 2 parts of tin filings into the hot metal; 18 parts of this amalgam mixed with 7 parts of sulphur and 6 of ammonium chloride are heated together.
The mosaic gold made after any of these methods may be used for gilding gold frames or as a painter’s pigment. Much so-called gold paint consists of mosaic gold ground with a thick gum solution.
Chrysean.—Wallach found that when a current of sulphuretted hydrogen was passed through a saturated solution of potassium cyanide a precipitate was formed, which had the formula C₄H₅N₃S₃. This substance, chrysean, is similar in appearance to mosaic gold; its technical employment is prevented by its extremely poisonous nature and its high cost as compared with mosaic gold.
CHAPTER XVI.
RED MINERAL PIGMENTS.
Vermilion.
This beautiful scarlet red pigment, which has been used for so long a time, consists of mercuric sulphide, HgS. The same compound occurs ready formed in nature as cinnabar; picked pieces of this mineral come into the market under the name of mountain vermilion (Bergzinnober). A far larger quantity of vermilion is made artificially.
Mercuric sulphide exists in two forms—as a black non-crystalline powder and in the crystalline form, which is used as a pigment. Each modification may be transformed into the other by suitable treatment, and each may pass into the other spontaneously under certain conditions. In the manufacture of vermilion, the black form of mercuric sulphide plays an important part; it is, therefore, necessary to give an account of the chemical behaviour of the two modifications before proceeding to an account of the method by which vermilion is made.
Black Mercuric Sulphide may be obtained either by the direct union of metallic mercury with sulphur, or by precipitating the solution of a mercuric salt with sulphuretted hydrogen. It is most simply formed by rubbing together equal parts of sulphur and mercury moistened with water, until the mixture is uniformly black. It is, however, difficult in this way to convert all the mercury into sulphide. A better result is obtained when the mixture is moistened by ammonium sulphide instead of water. In this case the time required for the operation is shortened by warming the vessel. If the mortar is placed in hot water it is generally sufficient to grind for about two hours to bring about the combination of the mercury and sulphur.