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.
CHAPTER LXV.
SAP COLOURS.
Under the designation of ”sap colours” several pigments are brought into the market in the condition in which they are ready for immediate use in printing. The term is practically restricted to lake pigments which form a transparent layer when dry, thus the sap colours may be defined as dissolved lakes mixed in a viscous medium, such as thick gum solution. In general the sap colours are not much used, since when wetted with water they again dissolve, which is not the case with pigments ground in oil. Yet for certain purposes they are commonly used, as in the manufacture of playing cards.
In the manufacture of these pigments only colouring matters soluble in water can be used, the number of which is restricted. According to a particular method a tin lake is first made, which is decomposed by a strong base, such as ammonia, so that the colouring matter again goes into solution. The very deep coloured liquid is then mixed with a thickener and an indifferent white substance such as flour or starch, formed into thin sticks, and sent into the market.
A decoction of buckthorn berries is used for the yellow sap colours. It is considerably evaporated, and then mixed with 2 to 3 per cent. of alum, after which starch paste or gum Arabic and sugar are added in proper quantity, and the mixture evaporated at a low temperature so that it is not browned by over-heating.