According to the purpose for which they are to be used pigments require a different method of preparation. The preparation of pigments for certain special purposes, such as sap colours, cake colours and water colours, has been already described. The present chapter deals with the preparation of those pigments which are used in large quantities by artists and for ordinary painting.

Colours for artistic purposes require different treatment to those used for ordinary painting. It is important that both should be ground to a completely homogeneous mixture with the binding medium. At first sight this appears to be a simple operation, but in practice there are difficulties which are not too easily overcome. Artists’ colours are generally ground with a drying oil. The drying oils are vegetable oils which, when exposed to the air in a thin layer, in a short time become very viscous, and finally completely resinify; linseed, poppy, and nut oils possess this property. As a rule, artists’ oil paints are ground with poppy oil. To prevent the paint from drying to a solid mass by exposure to air it is enclosed in protecting vessels. Formerly the ground paint was sold in small bags of bladder. These are now no longer used; instead, collapsible tubes made of a soft tin alloy are employed. They are closed at one end; the other has a neck upon which a metal cap screws down. Pigments ground with just sufficient oil to form a thick paste may be preserved in these air-tight tubes without altering in consistency.

Cheaper pigments, such as white lead, white zinc, chrome yellow, etc., which are principally used in ordinary painting, are ground with raw or boiled linseed oil. Ordinary boiled oil is made by boiling linseed oil with litharge; it contains a certain quantity of lead in solution. It has been repeatedly stated in this work that lead compounds are very sensitive to sulphuretted hydrogen. Pigments which are not themselves altered by sulphuretted hydrogen acquire a dark shade when they are ground with ordinary lead-boiled oil, because the lead is slowly but certainly converted into black lead sulphide on exposure to air. Thus, in order to retain the original beauty of colour, lead-boiled oil should be replaced by oil boiled with manganese borate, which is at least as cheap and has the advantage that the oil does not darken in air. Now that zinc oxide is so cheap it is more and more used in place of white lead, it is not sensitive to sulphuretted hydrogen, and does not even change its colour in an atmosphere of the pure gas. It seems to be quite illogical to use this pigment with a lead-boiled oil. A coating would be quite discoloured in the course of time, whilst, if used with manganese-boiled oil, it would retain its white shade unaltered.

The dry pigment was formerly mixed with the oil by manual labour. The pigment was spread out upon a smooth stone slab, the oil poured over, and the two substances rubbed to a uniform paste by means of a glass or stone rubber known as a muller.

Paint Mills are constructed in various ways. The mixture of colour and oil is ground either between two metal plates or between two rollers pressed close together.

Fig. 41 represents the construction of a paint mill in which the grinding is accomplished by a rotating disc. The mixture of pigment and oil stirred together is brought into the hopper, T, from which it must pass between the rotating grinding disc, M, and the lower surface of the hopper. The two substances are thus mixed together. The ground paint flows out of the ring-shaped vessel surrounding M into a receiver below. The grinding disc is driven by means of horizontal and vertical toothed wheels, the latter of which is connected with a pulley driven by power. By means of a screw on the lower part of the plate in which is the bearing of the axle of the grinding disc, the distance of the latter from the lower surface of the hopper can be adjusted. The process begins by adjusting the disc at a considerable distance from the hopper. When the paint has once gone through the apparatus the disc is raised so that the colour is more finely ground. The ground paint is returned to the hopper until it is quite uniform.

This form of mill can be made in different sizes. Fig. 42 shows the construction of a form for grinding by hand. The actual grinding arrangements are exactly the same as those previously described; the difference lies simply in the use of a fly-wheel turned by a handle. The illustrations are due to the kindness of W. Sattler of Schweinfurt, who makes paint mills as a speciality in different sizes and of excellent quality.

In the second form of paint mill the paint is passed between smooth rollers moving in opposite directions with different speeds, which thus exert a grinding action in addition to the crushing effect. The two rollers are provided with cog-wheels with a different number of cogs in order to give the different speeds.

In order that the paint may be sufficiently ground in one operation, roller mills are made with several pairs of rollers, one above the other, the lower rollers being somewhat nearer together than the upper. The paint is fed on to the top pair of rollers, and, after going through these, passes to the next, and finally, after going between all the pairs of rollers, collects in a receiver below in a finished condition. By the use of such mills the paints can be ground sufficiently fine in one operation, if a proper number of rollers is used. When only two or three pairs are used and the paint is to be very finely ground, it must be passed through the whole mill two or three times.