The most common action is a physical one, in which the opacity of a pigment is gradually lessened in course of time by the more complete interpenetration of the oil between the particles. Thus yellow ochre and raw sienna, for example, darken in colour because they become more translucent, just as a piece of oiled cream-laid paper is darker and yellower than the same paper when dry. The light which falls upon it plunges into it more deeply, and on reflection is more highly coloured. In the case of such pigments as we have named, and several others, another cause is at work darkening and modifying the colour: this is the yellowing of the oil itself. And it is the pigments which require the largest proportion of oil for grinding which exhibit in a marked degree the phenomena in question.

A second action between a pigment and the oil with which it has been ground is the peculiar gelatinous or 'livery' condition quickly assumed by some oil-paints.

This change is particularly noticeable with the cochineal and madder lakes. I have succeeded in obviating it, by carefully drying the pigments at a temperature just under 100° C, before grinding them with oil, and by substituting for raw linseed oil a mixture of the 'manganese oil,' described in the present chapter, with some poppy oil. Those pigments which dry easily should be ground with more of the latter oil, those which dry with difficulty with more of the former. Sometimes pigments harden quickly in the tube itself; this change is due either to the siccative character of the pigments, or to the introduction of an actual 'dryer,' or to the too copious use of a strongly siccative oil with those pigments which are naturally slow in drying.

The third action between a pigment and the oil with which it has been ground appears to be of a distinctively chemical nature. The most striking example of it known occurs with flake-white. When normal flake-white, that is hydrato-carbonate of lead, is ground in oil and afterwards exposed to the air, it gradually becomes very hard - much harder than the great majority of other oil-paints under the same conditions, zinc-white for example. It is not the hardness of the lead-compound plus the hardness of the oxidized oil, but a hardness combined with toughness of a higher order. As those varieties of white-lead which consist wholly of the carbonate do not possess this quality in anything like the same degree, so it must depend upon the lead hydrate which is intimately associated with the carbonate in typical white-lead. It has been usual to conclude that the phenomenon is due to the formation of a lead soap, a linoleate of lead, by the interaction of the free acids in the oil with some of the lead hydrate. This view is supported by some, yet hardly decisive, experimental evidence. Thus the longer the oil and the lead-white remain in contact the more marked is the change, especially if the temperature be raised somewhat above the normal.

If a collapsible tube, filled with ordinary flake-white ground in oil, be tightly closed so as to exclude the air and then be submersed, in water kept hot, for a few days, the change in question is hastened. Consequently it becomes impossible to extract from the paint so treated, by means of ether or other suitable solvent, quite the amount of oil originally present. One could understand this result had the paint been allowed to absorb oxygen from the air, when some linoxine, insoluble in most solvents of oil, would have been produced. Whatever the action may be, and whatever the true explanation, the phenomenon is beyond question Further reference to this subject will be found in Chapters XIII. and XXIII.

A fourth action between a pigment and the oil with which it has been ground is occasionally observed with certain colours of organic origin, which actually dissolve in and stain the oil. Bitumen, gamboge, and several alizarin preparations exhibit this phenomenon.

2. The different amounts of oil required by different pigments may now be considered. As a rule, the densest or heaviest pigments require the least oil. A few pigments require an excess of oil in order to protect them from moisture or other injurious agents. Different authorities do not agree at all closely as to the amount of oil needed to make a workable oil-paint from the same pigment. The following list gives the weight required by 100 parts in weight of 22 pigments:

The discrepancies between the corresponding figures in the vertical columns are due, amongst other causes, to differences in the modes of preparation of the dry pigments; to natural variations in the native earths employed; to the dissimilar standards of solidity or fluidity aimed at in the finished paint; and to several other causes which it is needless to particularize, but amongst which may be named different modes of grinding and the employment of different kinds of oil.

The great differences in the above amounts of oil do not cause such serious results in the conduct of the process of oil-painting as might have been expected at first, for they correspond in a measure to the relative bulks of the several pigments. We can use more copal or amber varnish to balance the excess of oil in some pigments, and so secure a uniformity of structure, texture, and rate of drying in the different parts of the work. It is, however, often convenient to remove some of the excess of oil from a pigment before using it, especially with the colours prepared by some makers.* This can be done by leaving the oil-paint on a pad of blotting-paper; but 3-inch cubes of plaster-of-Paris afford a far cleaner and surer method for the absorption of oil. It may be further remarked that the quantities of oil required by some of the pigments in the above table may be reduced by grinding them under greater pressure. Raw sienna, burnt sienna, and ivory black should be dried at 100° C. just before grinding, and then yield workable paints with less oil.

The subsidence of vermilion from the oil in which it has been ground may in some measure be prevented by using 'manganese oil' instead of raw linseed oil, and adding to it a small quantity of linoleate or oleate of alumina, or of beeswax, or of hard paraffin wax or ceresin, having a melting point not under 65° C. Some artists find it a good plan to keep their tubes of vermilion and of other heavy pigments in an inverted position - that is, with the cap downwards.

* Figure for rose madder.

* Dr. H. Stockmeier, of Nurnberg, found the following percentages of oil in certain oil-paints from different sources which he analysed: