During recent years many improved methods of treating raw linseed oil have been devised. The 'boiling' has been carried out in aluminium vessels in lieu of those of iron, which become much corroded through the action of the free acids of the oil upon this metal. The iron compounds thus formed are undesirable ingredients of the boiled oil. Then, again, superheated steam has taken the place of direct fire-heat in treating the oil, which is mechanically stirred while a current of oxygen or of air is at the same time sent through the liquid. In yet an other process, the raw oil in the form of spray and heated is brought into intimate contact with a stream of heated air. There exists also an improved method of preparing a drying oil by the agency of light. As it has been found that sunlight acts mainly in virtue of its ultra-violet or actinic rays, an artificial light, singularly rich in such rays, has been substituted for the light of the sun. This is found in the mercury lamp, in which an electric spark, passing between two mercury poles enclosed in a quartz-glass tube, originates a peculiar light of high activity.

A battery of such lamps is so arranged that their radiation impinges upon the oil, which is at the same time kept at a temperature of 80° C, while a finely divided stream of oxygen gas is forced through it. The pale, quickly drying oil thus produced is said to possess the property of drying uniformly throughout without the production of a skin on the surface.

The changes which occur during the oxidation of linseed oil, as described in the preceding paragraphs, are, as will have been seen, complex; but there is some formic acid formed, so that the product is sour carbonic acid gas and water are also produced. It has been shown that there are many ways of bringing about this oxidation. A very common one is to heat the oil to a temperature of at least 100° C., and to blow air through it, or air containing ozone. Many substances favour the absorption of oxygen by linseed oil under the above conditions. Amongst these may be named manganese dioxide, borate, oxalate, resinate, or linoleate; cobalt resinate; red-lead, litharge, or lead acetate; green vitriol, iron in the presence of water, etc. It is better to use one of the manganese compounds, and an excellent result is obtained with the borate of this metal. On the small scale, the operation may be thus carried out: Tie up in a small piece of muslin 20 grains of dry and powdered manganese borate. Suspend the bag in a glass quart flask, into which a pint of linseed oil has been placed, so that the bag is just covered by the oil; lightly plug the mouth of the flask with some carded cotton.

Stand the flask in a warm place, where the temperature does not fall below 40° C, nor rise above 100° C., In a fortnight's time, the oil will have become strongly siccative, so that when it is spread in a thin layer on glass, or paper, it will dry up to a tough varnish within twenty-four hours. If the oil and manganese borate be maintained, by means of a water-bath, at a temperature of 100° C., the change will occupy less time, and the product will be just as good; but it is not advisable to boil the oil with the borate, although the change may be thus effected in less than an hour. The oxidation may be further hastened by occasionally blowing a little air into the oil through a glass tube kept permanently in the flask. When the rapid-drying quality of the oil has been proved, by experiments made with a drop or two withdrawn for that purpose, the flask is allowed to get cold, and the oil poured into a corked glass bottle, so as to fill it. In the course of the next few weeks, a slight deposit will be formed in the bottle; when this has occurred, the clear oil should be poured off into other bottles, and preserved for use.

According to the purpose for which the prepared oil is to be afterwards used, the treatment with the borate must be more or less prolonged; but care should be taken not to carry it so far that the oil becomes ropy or viscous, unless it is intended to make linseed oil varnish. In the subsequent chapters of this book we shall often refer to this siccative linseed oil as 'manganese oil.' To the above directions for preparing this oil may be added the remark that if the operations be conducted in a strong light, the oil will be bleached, as well as rendered highly siccative. No satisfactory explanation of the action of the manganese borate (and of many other substances used for the same purpose) has been offered. But it seems probable that the absorption of oxygen by the oil is favoured by the removal of certain impurities, and this the borate of manganese may effect: it has been suggested that the action is in part catalytic.

The increasing specific gravity of the 'manganese oil,' as the process is prolonged, may be used as an indication of the point at which the heating may be discontinued. When the oil has acquired a specific gravity of .945, it is generally sufficiently siccative for grinding with non-drying pigments, and as an addition to certain varnishes. For these purposes it may even attain a specific gravity of .96; but when it shows .99, or .995, it constitutes a thick varnish, which needs dilution with a suitable solvent. It may be well to remark here that the various processes for rendering linseed oil more rapidly-drying may be regarded as resulting in two actions, partly consecutive, partly simultaneous. The first action, if it could, or did, occur alone, would yield a purified oil apt to dry quickly, but very slightly altered in composition; the second action is more profound, and gives rise to a thickened, denser product, in which the drying process has already commenced. In practice, the first action occurs almost, but not quite, uncomplicated with the second, when linseed oil is warmed with borate of manganese in a vessel to which atmospheric air has very limited access; the second action, which is of necessity associated with the first, takes place when a stream of air is blown through warm linseed oil, even in the absence of manganese borate, but far more quickly in its presence.