172	painting materials
mercuric sulphide could be made by treating the black sulphide with alkali
sulphides (see Kopp, p. 187). Production of vermilion by this method began in the
late XVIII century in Germany (see Rose, p. in). The mercury and sulphur are
ground together in the presence of water and, toward the end of the grinding
operation, a warm solution of caustic potash is added to complete the transforma-
tion. After being stirred for some time, the black mercuric sulphide develops the
desired vermilion color. In an improved method, potassium pentasulphide is used
in place of caustic potash (see Beam, p. 119). Vermilion prepared in this way must
be washed and dried to be rid of the soluble sulphur compounds.
Chemically and physically, artificial cinnabar does not differ from the natural.
There are no optical differences between them, and it is often quite impossible to
tell the origin of the vermilion in a paint film. If it is coarse and if the particles
appear to be broken fragments rather than single small crystals and if there are
inclusions of impurities in the broken fragments, then it may be natural in origin.
Impurities in vermilion are no satisfactory criterion of origin, however, since
very pure, natural cinnabar frequently occurs in nature. Artificial vermilion,
particularly that made by the wet process, is very finely divided and homo-
geneous; differences in the color of different samples are caused mainly by
differences in particle size. The sublimed product from the dry process (e.g.,
Chinese vermilion) is usually more coarsely crystalline and it has a bluish, carmine-
red color. When it is finely ground, the color approaches the orange of wet-process
vermilion.
Vermilion is one of the heaviest pigments (sp. gr. = 8.2). It has excellent body
and hiding power. It is highly refracting and birefracting (cl* — 3-H> &Li — 2.81).
Under the microscope, the particles are translucent, deep orange-red by trans-
mitted light. By reflected light at high magnification, the red particles have a
certain waxy lustre which seems to be quite characteristic. Many fragments show
perfect cleavage.
Vermilion, on the whole, has been a permanent pigment. It is frequently seen
on Roman wall paintings, quite unchanged. On many Flemish paintings of the XV
century it appears to have retained all of its original brilliance. It is not permanent
under all conditions, however, and one peculiar property is that specimens of it
are frequently observed to darken when exposed to direct sunlight* This occurs
more often where the pigment is used with tempera or water color mediums than
with oil. Darkening appears to be a purely physical change, and is thought to be
caused by the formation of a metastable black modification of mercuric sulphide
(see De Wild, p. 67, and Church, p. 169). Wet-process vermilion is more often
observed to darken than the dry-process or the natural vermilion; impurities seem
to have some part in the change. The tendency to discolor has caused vermilion
to be replaced on the modern artist's palette by cadmium red. When heated,
vermilion sublimes at about 580° C. (see Mellor, IV, 944). At higher temperatures
it burns with a bluish flame and leaves no appreciable residue. It is insoluble in