Colours and Their Preparation.—Colours, in the state in which they reached the painter when he prepared them himself, and in the state in which they now reach the manufacturer, are in the form of blocks, either crystalline or powdery, but generally of the consistency of soft chalk. Their quality undoubtedly de pends above all on the purity of the raw materials used. The chemist isolates the colouring matter from the crude materials which he receives. In some cases the material is earth, as in the case of ochres; the earth is in its natural state in the case of yellow and red ochre, and it can be calcined so as to obtain other colours such as burnt sienna. Other colours are obtained from elementary materials; thus different kinds of black are produced from lamp black, bone charcoal and charcoal obtained from vine stocks. Ultramarine is also a natural stone—lapis lazuli. But unlike the coloured earths, which are abundant, natural ultra marine is so rare that it is scarcely used. By means of laboratory research a fairly stable blue resembling natural ultramarine has been obtained from sodium sulphide and silicate of aluminium.
It would be desirable if painters could use only colours ex tracted from simple sources such as earths. A colourist can do a great deal merely with yellow ochre, red ochre and black. These materials are little subject to alteration. Even the most skilful artist, however, cannot go far without the use of white; and this is the first stumbling block. The white which is known as flake white (in French, blanc d'argent) is a carbonate of lead, or in other words a purified form of white lead. It turns black in the course of time. The least that can happen is that it will turn yellow. Zinc white has not this disadvantage, but it cracks, does not spread easily and dries slowly. Moreover, even if the problem of the stability of white were solved, there would remain that of other colours which are absolutely necessary to the painter, such as bright yellow, whether strontian or cadmium, red, such as vermilion or even madder lake, blue, green and brown. These colours are the product of chemical combination and are only relatively stable. Some turn black, others absorb the colours mixed with them, while others become opaque.
However, not all of the hundreds of colours at the painter's disposal are equally bad. In order to choose the lesser of two evils, the painter should only add to the tones obtained from earths the indispensable minimum of other less durable colours. Prussian blue, which was discovered in 1710 and was used to excess in the early 19th century, is today almost abandoned.
Unless it is laid on as a light glaze, or supported by a considerable admixture of white, it tends to turn black and to absorb the colours mixed with it. A green composed of Prussian blue and a yellow (especially if this is not one of the earth yellows) gradually turns blue; similarly a violet into which Prussian blue enters gradually loses its red elements. Cobalt blue has the serious defect of appearing violet by artificial light.
It is possible to make a verbal distinction and to say that Rubens and Rembrandt did not use bitumen ; they used a bituminous colour, and so did all the Dutch and Flemish painters; indeed all painters of whatever school have used a transparent brown resembling bitumen and fulfilling the same function ; but it was recognised that the use of this colour was subject to cer tain strict technical conditions. At the end of the i8th century an attempt was made to recover the exact tone of bitumen which was believed to be that used by Rembrandt ; it was believed to be very durable, and most of the painters of the day made abundant use of it. The terrible consequences are well known at the present time. The worst of it is that the damage due to bitumen, which never dries, combined with varnish which dries rapidly, may not become apparent until after many years.
The industrialisation of the manufacture of colours thus gives rise to a number of problems and technical points with which modern painters are unacquainted. They ought themselves to try experiments on the power of resistance of colours by exposing them to sunlight, and to fumes containing sulphur, sulphuretted hydrogen and ammonia ; and in addition it would be in their interest to simplify their palette. Experience has shown, for example, that Naples yellow becomes dull and greenish; that umber darkens and rapidly alters the colours mixed with it ; that cadmiums lose their brightness in time; that ultramarine be comes opaque ; that bitumen should only be used as a light glaze: and that colours which may be highly recommended must pass the test of time, and cannot be used with certainty until they have done so. It is only possible to calculate their palettes by mak ing a close study of the old masters' pictures in a good light. It is possible to train oneself in a study of this kind by beginning with modern pictures. If the painting of a picture is closely examined, it is almost always possible to discern, in a single brush stroke, particles of the pure colours of which it is composed. If a study of several pictures by the same painter gives the same results, they may be taken as accurate.