BROWN ORGANIC PIGMENTS.
=Asphaltum= is found in large deposits in several regions of the earth.
It is a compound of carbon and hydrogen, and varies in appearance from
cobblers’ wax to tar. Asphaltum is of organic origin and is closely
related to the mineral oils. Among the pigments usually employed by
artists asphaltum plays an important part; it produces very warm
shades between brown and deep black. The preparation of asphaltum for
artists’ use is very simple. Good uniform lumps, free from sand and
other impurities, are coarsely powdered and mixed with a solvent in a
well-stoppered flask. Asphaltum readily dissolves in essential oils,
and also, though with more difficulty, in fatty oils. It dissolves
slowly at the ordinary temperature, but quickly on warming. On account
of the inflammability of the essential oils generally used, certain
precautions must be taken. The asphaltum is mixed with turpentine
in a large flask which is well closed and heated by hot water. In a
short time a viscous mass is formed which is mixed with the solvent by
shaking.
Asphaltum may also be prepared, without dissolving, by powdering and
grinding with oil exactly as a mineral pigment.
=Sepia.=—This brown pigment has a warm hue which is not readily
surpassed. It is an animal product; the cuttle-fish, a species of
cephalopod which abounds in all warm seas, produces a peculiar
colouring matter which is stored in a sac, commonly called the ink-bag.
It is used by the animal for protection, when pursued it ejects the
contents of the bag, and thus makes the surrounding water so dark that
it is enabled to escape.
Sepia is made almost exclusively in Italy. The contents of the ink-bag
are quickly dried, and then rubbed with strong caustic lye to a thick
pulp. More caustic lye is then added, and the mixture heated almost to
boiling; from the filtered solution sulphuric acid precipitates the
pure colouring matter.
As one of the most handsome brown pigments, sepia is largely used, but
unfortunately it can only be used as a water colour. On account of its
cost, sepia is frequently imitated. Vegetable substances are charred,
extracted, and the extract concentrated until it solidifies on cooling;
it is then finely powdered, made into a paste with gum Arabic or
tragacanth, and formed into cakes. All such imitations are, however, so
imperfect that they are at once recognised. On comparison with genuine
sepia, none of them is found to possess the warm shade peculiar to
sepia.
Brown colouring matters can be obtained by heating all soft portions of
plants. The products have a deep brown shade in consequence of the high
proportion of carbon they contain. When the young twigs of soft woods
are exposed in closed cylinders to a temperature of about 300° to 400°
C., and the residue powdered, colours ranging from a dark rust brown to
almost pure black are produced. The higher the temperature employed the
more nearly the shade approaches to black.
Parts of plants which contain sugar or similar compounds become deep
brown at comparatively a low temperature. The colour, which varies from
brown to hyacinth red, is due to caramel. This substance is produced
when coffee, beetroot, or chicory root, all of which contain a large
proportion of sugar, are heated. Such pigments are little used; they
cannot be ground with oil, and in water can rarely be mixed with other
colours.