It should be observed that it is of great importance for the colour maker to have a practised eye, sensitive to fine differences of colour. He may then produce colours faultless in shade, for if by chance a colour does not turn out well it may be improved by judicious admixtures. It is quite impossible to give definite rules for the mixing of colours; the estimation of a shade cannot be taught in words, it demands long practice. By the use of a system, provided the user has normal eyes, very slight differences of colour can be readily estimated. A scale of colours should be made from pigments of pure and definite shades, in which scale successful mixtures are gradually inserted, so that a series is produced in which the separate pure colours blend regularly into one another. Then, when it is required to produce a pigment corresponding to a certain shade on the scale, with a little practice the shade can be readily produced, for the scale indicates which colour predominates. The so-called sap colours are generally characterised by very pure shades. They are produced by decomposing a lake by an acid or a strong base, evaporating the solution of the colouring matter so obtained at a gentle heat and mixing with gum, starch, or some other thickening material to produce a mass of such consistency that it can be formed into balls or sticks. The decoctions of the dye-woods always contain other substances in addition to the colouring matters, which are precipitated with the colouring matter, and somewhat injure its shade. When such a lake is decomposed the colouring matter is obtained in a purified form, thus showing its full beauty. This solution of the purified colouring matter might be again precipitated with a salt solution and lakes be produced which would be much brighter than the original, but the losses incidental to this double precipitation would be so great that the lake would be made much too expensive. CHAPTER XLIX. YELLOW LAKES. Yellow colouring matters are widespread in nature; from them are obtained many yellow pigments used in dyeing and in the manufacture of colours. The colours produced by the yellow vegetable colouring matters are not particularly bright; thus the yellow lakes are used chiefly to produce cheap pigments, whilst the inorganic pigments are used for bright yellows. =Dutch Pink.=—Several species of buckthorn (_Rhamnus_) contain a yellow colouring matter—xanthorhamnin—which is obtained pure by extracting the yellow berries with hot alcohol. On cooling, the impure colouring matter separates; by repeated recrystallisation from alcohol it is obtained in the form of crystalline needles, which are soluble in water and alcohol.