Amounts of Oil Required

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.² 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.

1	Figure for rose madder.
2	Dr. H. Stockmeier, of Nurnberg, found the following percentages of oil in certain oil-paints from different sources which he analysed: Flake-White (Roberson and Co.) 16.2 Light Red (Winsor and Newton) 41.9 Burnt Sienna (Dr. Schoenfeld) 59.2 Chinese Ochre (G. B. Moeves) 45