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.