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. [Illustration: FIG. 41.]