Mosaic gold consists of tin disulphide, SnS₂, fine scales of a golden yellow colour, which sublime undecomposed at a fairly high temperature and withstand the action of chemical reagents. It has a peculiar greasy nature, and can be easily ground. It is, therefore, much used for bronzing picture frames, as a pigment for painters and for wall papers.

Tin disulphide can be prepared either in the wet or the dry way; in the wet way, by the action of sulphuretted hydrogen on a solution of tin tetrachloride. The yellow precipitate so obtained has no handsome colour. A far finer product is obtained in the dry way. The process is often regarded as accompanied by particular difficulties, but in reality it is quite simple. It is only necessary in preparing this pigment to take care not to raise the temperature above a certain point, otherwise a great portion of the tin disulphide will be decomposed into sulphur and tin monosulphide. To prevent the temperature from rising too high, an addition of ammonium chloride is made. This salt is volatile at a certain temperature; heat which would otherwise raise the temperature above this point is used in volatilising the ammonium chloride. With a little care it is easy to interrupt the operation before all the ammonium chloride has been driven off. The mosaic gold then obtained has a real golden glitter. If the temperature rises too high, grey tin monosulphide is formed, which naturally considerably diminishes the brilliancy of the product. Ammonium chloride may be replaced by mercury or mercury compounds, which are volatile at a temperature below that at which mosaic gold is decomposed. When mercury compounds are used, both on account of their cost and poisonous nature the heating must be conducted in glass retorts in order to recover the mercury. This operation requires great care if loss due to the breakage of the glass vessels at the high temperature is to be avoided. The process in which metallic mercury is used gives the finest product of all, and is to be recommended when a pigment is to be prepared which shall as nearly as possible approximate to the appearance of gold.

In order to obviate the danger and loss associated with the use of glass vessels, manufacturers who make mosaic gold in large quantity should use an iron vessel. This will last a very long time. Such an apparatus consists of an iron pan with a broad rim, upon which is fastened a head which has the form of a retort neck; to this are connected short, wide tubes leading to a chamber in which substances not condensed in the retort neck may deposit, so that the use of this inexpensive apparatus will not only be without danger, but will be accompanied by the recovery of almost the whole of the volatilised substances. The pan is filled with the materials, the head placed on, the joint tightly luted, and the retort neck connected with the chamber by the wide iron tubes.

There are many formulæ for the preparation of mosaic gold. Some of the most important are given, which in every case will yield a good result:—

The tin filings, which must be very fine, are well mixed in a mortar with the sulphur and ammonium chloride. The heating is gradual at first; when the evolution of vapours has ceased the temperature is very slowly increased to a dark red heat. The mosaic gold is found as a yellow mass at the bottom of the vessel, but partly in crystalline scales on the walls and head of the retort.

Other recipes are as follows:—

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.