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.
This compound can also be easily obtained by heating mercury with
sulphur. In a vessel, placed under a chimney with a good draught, which
is necessary to carry away the poisonous mercury vapours, 6 parts of
the metal are heated nearly to boiling and 1 part of sulphur added.
Combination takes place at this temperature with a slight explosion,
and pure mercuric sulphide results when the heating is continued until
the excess of sulphur is driven off.
For the purpose of the manufacture of vermilion, black mercuric
sulphide is most simply made by filling a thick-walled vessel with
equal weights of mercury and sulphur, moistening the mixture with water
and shaking or rotating the vessel for several hours. This can be done
either in a rotating cylinder containing iron balls, or the vessel can
be fastened to any rotating object—to a water-wheel or to the fly-wheel
of a steam-engine. The vessel in which the combination is effected
should of course not be quite full. It has been found that a more jerky
motion than that of rotation effects the combination of the mercury and
sulphur in a shorter time. For example, an opportunity of fastening the
vessel to a saw-mill would be of great advantage.
The mercuric sulphide made by the above methods is a velvety black
mass, which, even when exactly equivalent weights of sulphur and
mercury have been used, is never quite pure. Carbon bisulphide will
always extract a certain quantity of uncombined sulphur. The most
important property of the black sulphide for the present purpose is
that it is changed into the crystalline modification by heating to the
temperature at which it volatilises. The sublimed mercuric sulphide has
the well-known fiery scarlet colour characteristic of vermilion.
=Red Mercuric Sulphide=, or vermilion, exhibits, for a sulphur
compound, considerable resistance to the action of chemical reagents;
dilute mineral acids do not decompose it. Unfortunately, vermilion has
another property which makes it quite unsuitable for the artist’s use:
in the course of time it gradually turns dull and at last is completely
discoloured. This alteration of colour can only be ascribed to a return
of the red crystalline modification into the black non-crystalline.
When a white pigment is tinted by vermilion it should not be a lead
pigment, or in a brief time it will turn black. A white pigment such
as zinc white, which is not acted upon by sulphur compounds, should be
used.