MOSAIC GOLD.
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:—
Tin filings 40 parts
Sulphur 35 ”
Ammonium chloride 25 ”
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:—
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.