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.