METALLIC PIGMENTS.
Metallic pigments do not always consist of metals; the name is also
applied to compounds which possess a pronounced metallic lustre; mosaic
gold, the preparation of which has been previously described, is an
example of such a pigment. Another variety of metallic pigment is made
by heating finely-powdered alloys; a layer of oxide is thus produced
upon the surface of the metallic particles, this layer produces a
shade of the colour of the metal or alloy. Metallic pigments are only
unalterable when they are composed of metals which are not changed
by the action of the air. As a matter of fact, this property is not
possessed by any of the metals used for this purpose; even gold and
silver are blackened by the action of the sulphuretted hydrogen in the
air. This alteration proceeds, however, very slowly when the metal is
enveloped by a layer of a binding medium, which is always the case when
the metallic powder is used for painting. There are many manuscripts,
the initial letters of which have been coloured by gold or silver,
in which the metals retain their peculiar lustre after the lapse of
centuries.
Metallic paints made from yellow alloys have never much durability:
they always contain copper, which readily alters; articles painted with
imitation gold soon lose their lustre, and in the course of time become
green.
=Shell-Gold.=—This very expensive artists’ paint is made by rubbing
gold-beaters’ refuse with gum solution upon a stone slab until a
completely homogeneous mixture results, which is then rapidly thickened
over the fire and generally allowed to dry in small shells. The
preparation so made is known as shell-gold: it is sold at very high
prices.
The operation of grinding the gold is very lengthy; it considerably
increases the cost of the already expensive material. The process may
be considerably shortened if the gold is obtained in a very finely
divided state by a chemical operation. For this process coins or broken
jewellery is heated in hydrochloric acid, and nitric acid gradually
added. The gold dissolves in the mixture. If it was alloyed with silver
a white precipitate of silver chloride is formed, which is filtered off
after largely diluting with distilled water. The solution is boiled
for some time to remove excess of nitric acid. A solution of ferrous
sulphate is then added; the liquid at once becomes bluish black, and
in a short time deposits a brown precipitate composed of chemically
pure gold, which is so finely divided that it is almost without the
characteristic glitter of gold. The precipitate is filtered off, dried
and preserved in stoppered bottles. In order to prepare shell-gold
from this gold powder it is simply necessary to grind it with thick
gum solution in a porcelain mortar; under the pressure of the pestle
the gold rapidly acquires its natural glitter. The grinding need only
be continued until the gold and gum solution are uniformly mixed.
The mixture so made should be at once filled into the shells, since
on standing the gold would soon separate from the gum solution in
consequence of its high specific gravity.
=Shell-Silver.=—Genuine shell-silver can be made by rubbing silver leaf
with gum solution upon a slab exactly as genuine shell-gold. In this
case also the labour may be considerably lessened by converting the
silver into a state of fine division by a chemical process. Silver is
dissolved in nitric acid, which must be quite free from hydrochloric
acid, or insoluble silver chloride would be formed. In the operation
brown suffocating fumes are evolved which attack the respiratory
organs; it should therefore be conducted in the open air or under a
flue with a good draught. The silver solution, which is generally
coloured blue by admixed copper, is diluted with a large quantity of
distilled water. A sheet of copper is then dipped into the liquid and
rapidly moved about in it. The silver then separates as a dark grey
powder; after washing it is chemically pure, and when ground with gum
solution it produces genuine shell-silver.
Shell-silver and shell-gold are rarely used in painting on account of
their cost; they are chiefly used for illuminated manuscripts. When
these metallic pigments are to be used in oil painting, in place of
gum they must be ground with a liquid which mixes with boiled oil or
essential oils. For this purpose copaiba balsam is to be recommended.
=Imitation Silver.=—The imitation silver pigments are made from an
alloy of tin and bismuth, or from a tin amalgam. The latter is most
easily made by melting tin in a porcelain dish and adding one quarter
of its weight of mercury; the mixture is then well stirred, and
allowed to cool. It quickly solidifies to a crystalline mass, which is
tolerably brittle and can be powdered without difficulty. To obtain a
substance of a true silvery appearance the amalgam must be converted to
a powder of a certain degree of fineness. If the powdering is carried
too far the product loses a great part of its metallic lustre, and
acquires a dull grey colour.
The bismuth alloy is made by fusing 100 parts of tin, adding 100
parts of bismuth, and then 10 parts of mercury. It is not absolutely
necessary to add mercury, but the addition has the advantage that the
solid alloy is far more easily powdered. The imitation silver made by
this process has a white metallic colour approaching that of genuine
silver, but not equal to it, especially in lustre. It is, however,
largely used in the industries on account of its low price; for
example, for paper hangings.