LINSEED OIL. A most valuable drying oil obtained by ex pression from linseed, with or without the aid of heat. Prelimi nary to the operation of pressing, the seeds are crushed and ground to a fine meal. Cold pressing of the seeds yields a golden-yellow oil, which is often used as an edible oil. Larger quantities are ob tained by heating the crushed seeds to 16o° (71°C.), and then expressing the oil. So obtained, it is somewhat turbid and yellow ish-brown in colour. On storing, moisture and mucilaginous matter gradually settle out. After storing several years it is known com mercially as "tanked oil," and has a high value in varnish-making. The delay attendant on this method of purification is avoided by treating the crude oil with 1 to 2% of a somewhat strong sulphuric acid, which chars and carries down the bulk of the im purities. For the preparation of "artist's oil," the finest form of linseed oil, the refined oil is placed in shallow trays covered with glass, and exposed to the action of the sun's rays. Numerous other methods of purification, some based on the oxidizing action of ozone, have been suggested, but have not yet superseded the older methods. The yield of oil from different classes of seed varies, but from 23 to 3o% of the weight of the seed operated on should be obtained.
Commercial linseed oil has a peculiar, rather disagreeable sharp taste and smell; its specific gravity is given as varying from 0.928 to 0.953, and it solidifies at about —27°. It deposits stearine at 25° and melts at By saponification it yields a num ber of fatty acids—palmitic, myristic, oleic, linolic, linolenic and isolinolenic. Appreciable quantities of stearic acid and traces of arachidic acid are also said to be present. Exposed to the air in thin films, linseed oil absorbs oxygen and forms "linoxyn," a resinous semi-elastic, caoutchouc-like mass, of uncertain composi tion. The oil, when boiled with small proportions of litharge and minium, undergoes the process of resinification in the air with greatly increased rapidity.
Its most important use is in the preparation of oil paints and varnishes. By painters both raw and boiled oil are used, the lat ter forming the principal medium in oil painting, and also serving separately as the basis of all oil varnishes. Boiled oil is prepared
in a variety of ways—that most common being by heating the raw oil in an iron or copper boiler, which, to allow for frothing, must only be about three-fourths filled. The boiler is heated by a fur nace, and the oil is brought gradually to the point of ebullition, at which it is maintained for two hours, during which time mois ture is driven off, and the scum and froth which accumulate on the surface are ladled out. Then by slow degrees a proportion of "dryers" is added—usually equal weights of litharge and minium being used to the extent of 3% of the charge of oil; and with these a small proportion of umber is generally thrown in. After the addition of the dryers the boiling is continued two or three hours; the fire is then suddenly withdrawn, and the oil is left covered up in the boiler for ten hours or more. Before send ing out, it is usually stored in settling tanks for a few weeks, during which time the uncombined dryers settle at the bottom as "foots." Besides the dryers already mentioned, lead acetate, manganese borate, manganese dioxide, zinc sulphate and other bodies are used.
Linseed oil is also the principal ingredient in printing and litho graphic inks. The oil for ink-making is prepared by heating it in an iron pot up to the point where it either takes fire spontaneously or can be ignited with any flaming substance. After the oil has been allowed to burn for some time according to the consistence of the varnish desired, the pot is covered over, and the product when cooled forms a viscid tenacious substance which in its most concentrated form may be drawn into threads. By boiling this varnish with dilute nitric acid vapours of acrolein are given off, and the substance gradually becomes a solid non-adhesive mass the same as the ultimate oxidation product of both raw and boiled oil.
Linseed oil is subject to various falsifications, chiefly through the addition of cotton-seed, niger-seed and hemp-seed oils ; and rosin oil and mineral oils also are not infrequently added. Except by smell, by change of specific gravity, and by deterioration of drying properties, these adulterations are difficult to detect. (See OILS, FATS AND WAXES.)