Oils

The common usage of the term 'oil' is wider and less definite than that sanctioned by chemists. We must exclude from the category of true oils petroleum and the liquid paraffins, spirit of turpentine and the volatile essences of plants, the hydrocarbons of coal naphtha, as well as a number of other liquids which present certain superficial resemblances to the oils proper. Fats, however, belong to the same group, their solidity at ordinary temperatures being, so to speak, an accidental rather than an essential difference.

The true oils are often called fixed oils, for they cannot be boiled and distilled without change, thus differing from really volatile liquids. They are glycerides - that is, compounds from which glycerin, on the one hand, and fatty acids, on the other, are obtainable. These glycerides are named after the fatty acids which they yield. Thus olein is the glyceride of oleic acid, linolein the glyceride of linoleic acid. In reality three kinds or varieties of glycerides of each fatty acid are possible, but the oils used by painters consist almost entirely of one of these kinds. The formation of one of these glycerides may be expressed in words thus: One molecule of glycerin, reacting with three molecules of a fatty acid, yields one molecule of the glyceride in question and three molecules of water. Conversely, under other conditions, one molecule of a glyceride, reacting with three molecules of water, produces one molecule of glycerin and three molecules of fatty acid. If, in this last reaction, we substitute for the water three molecules of an alkali, such as potash, we obtain glycerin as before; but, in lieu of the free fatty acid, we find that an alkaline salt of the fatty acid has been formed - such salt is a soap.

Alkaline soaps, namely, those of potash, soda, ammonia, are soluble in water, which fatty acids - at any rate, those with which we are here concerned - are not. There are, however, other soaps which are insoluble in water, namely, the lime, lead, copper, and many similar metallic salts of fatty acids.

Oils, though insoluble in water, are easily soluble in spirit of turpentine and other volatile plant essences; in benzene, chloroform, and liquid paraffins; they are, in fact, miscible in all proportions with these liquids. There are other liquids in which the oils are less soluble, such as alcohol, acetone, and glacial acetic acid.

Oils are divisible into two classes, one of which includes those which dry up and harden, forming a kind of elastic varnish, by exposure to the air. The oils of the other class do not harden, but become sticky, and rancid in smell; these oils, however, if submitted to the temperature of boiling water for some time, do in some instances become dry and hard, but the varnish they yield under these circumstances is dark in colour and brittle; it has been suggested that some of these should be grouped together in a third class as 'semi-drying' oils. The painter's concern is almost exclusively confined to the oils of the first group, generally known as drying oils. To the most important of these attention will be directed presently, but the general methods of extracting them first demand a few words of explanation. There are two different processes in use. In one of these, which has been practised widely from very early times, the oil is obtained by pressure; in the other process, invented some seventy years ago, the oil is extracted by means of an appropriate solvent. We may dismiss this latter process almost summarily, for the product which it yields, though much greater in quantity, is decidedly inferior to that obtained by pressure.

It is less fluid, and contains a larger proportion of solid fats. The solvent commonly employed to dissolve out the oil from oil-yielding materials is carbon bisulphide (CS₂), a compound of carbon and sulphur, which may be prepared cheaply by passing the vapour of sulphur through red-hot charcoal. Of the pressure-process for obtaining fixed oils there are two modifications. In the more usually adopted of these, the oily seed or other material is first heated, and then pressed while still hot; in the other modification the pressure is applied to the cold seed, etc. Heat and pressure give a more abundant yield of oil, but the product is less pure and less well fitted for use in painting. The bulk of the oils of commerce are thus obtained. Cold-pressed oils remain clear in cold weather, are more fluid than hot-pressed oils, and contain a smaller proportion of solid fats and of free fatty acids.

The most important drying oils are those of linseed, poppy-seed, and walnut kernels; others are obtained from niger-seed, sunflower-seed, and hemp-seed. The first place is due to linseed oil.