Action of Pigments on Oils

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.