Oleum Lini (U. S. P.)—Linseed Oil.
Related entry: Linum (U. S. P.)—Linseed
"A fixed oil expressed from linseed without the use of heat. It should be kept in well-stoppered bottles"—(U. S. P.).
SYNONYM: Oil of flaxseed.
Preparation.—When prepared by cold expression the yield varies from 15 to 20 per cent. Prepared by the aid of heat, the gummy substance in the tegument of the seeds is often removed by roasting or steaming them previous to expression, and after expression, the oil in the expressed fluid separates and floats above the mucilaginous water. The yield by hot expression varies from 24 to 28 per cent. For medicinal purpose only that prepared without heating is admissible, because otherwise it has a darker color and an acrid taste. The press-cake remaining in the expression of the oil is known as oil cake (see Linum). The oil as first obtained is called raw oil, and is mostly purified by agitation with about 1 per cent of strong sulphuric acid, the latter being removed by boiling water. The oil thus obtained is called refined oil.
Description.—The U. S. P. describes linseed oil as a "yellowish, or yellow, oily liquid, having a slight, peculiar odor, and a bland taste. When exposed to the air, it gradually thickens, and acquires a strong odor and taste; and if spread, in a thin layer, on a glass plate, and allowed to stand in a warm place, it is gradually converted into a hard, transparent, resin-like mass (absence of non-drying oils). Specific gravity 0.930 to 0.940 at 15° C. (59° F.). It does not congeal above -20° C. (-4° F.). Soluble in about 10 parts of absolute alcohol, and, in all proportions, in ether, chloroform, benzin, carbon disulphide, or oil of turpentine"—(U.S.P.). When cooled to -27° C. (-16.6° F.) linseed oil congeals to a yellowish mass. Upon exposure to the air, old oil is liable to become rancid. On account of its drying properties, facilitated by warmth, linseed oil is a most important article, being used in the making of paints and varnishes, of printer's ink, oil cloth, etc. Its affinity for the oxygen of the air is so great that it is liable to inflame cotton waste and other fibrous material soaked with it.
BOILED LINSEED OIL.—For technical purposes, the raw oil absorbs oxygen and hardens too slowly. This process is promoted by heating the oil to a temperature of 130° C. (266° F.) while a current of air is made to pass through it; its temperature is then raised until bubbles arise, due to decomposition of the oil. Then it is called boiled oil, as contrasted with the raw or unboiled oil. Boiled oil is thicker, darker, has a somewhat higher specific gravity (0.939 to 0.950) and dries more rapidly than raw oil, hence is preferred by painters for outside work when it is desired that the paint dry rapidly. The drying properties are materially increased by incorporating during the process certain metallic oxides, as litharge, ferric oxide, red lead, manganese dioxide, especially lead acetate, manganous berate, etc., whose function seems to be partly to facilitate the transmission of oxygen, and partly to form more readily oxidizable metallic salts of the fatty acids. The nature of these substances, as well as the mode of manipulation, is usually kept secret. Of late, oil-soluble "driers" or "siccatives" have come into use—namely resinates of certain metals, as lead and manganese. (Concerning these, see A. H. Allen, Commercial Organic Analysis, Vol. II, Part I, 3d ed., Philadelphia, 1899, p. 150.)
Chemical Composition and Tests.—Linseed oil consists of 10 to 15 per cent of stearin, palmitin, and myristin; the remainder is chiefly isolinolenin (the glyceride of isolinolenic acid, C18H30O2, of the type CnH2n-6O2), and smaller amounts of linolein (the glyceride of linoleic acid, C18H32O2, type CnH2n-4O2), and olein (the glyceride of oleic acid, C18H34O2, type CnH2n-2O2). Unsaponifiable constituents amount to about 1 per cent (also see Linum). The drying qualities of linseed oil depend on the presence of the highly unsaturated linolein, linolenin, and isolinolenin. Upon drying, linseed oil becomes gradually converted into a hardened varnish, which is insoluble in ether. Chemically, it is an ester, called hydroxy-linolin (Mulder's linoxyn). Linseed oil is subject to many sorts of adulteration. Flaxseed itself is often found mixed with oil-bearing weed seeds, adulterated with hemp-seeds, and the oil may be adulterated with cotton-seed, niger-seed, and fish oils, mineral oils, and turpentine. These additions influence the specific gravity, congealing point, iodine absorption, and other physical and chemical constants of pure linseed oil. (For details regarding the analysis of linseed oil, see A. H. Allen, loc. cit., pp. 152-155.)
The U. S. P. gives the following tests for linseed oil: "It should not more than slightly redden blue litmus paper previously moistened with alcohol (limit of free acid). If 2 Cc. of the oil be shaken with 1 Cc. of fuming nitric acid and 1 Cc. of water, it should neither completely nor partially solidify, even after standing for 1 or 2 days (absence of non-drying oils). If 10 Cc. of the oil, contained in a small flask, be mixed with a solution of 3 Gm. of potassium hydrate in 5 Cc. of water, then 5 Cc. of alcohol added, and the mixture heated for about 5 minutes on a water-bath, with occasional agitation, a dark-colored, but clear and complete solution should be obtained. If this liquid be diluted with water to the measure of 50 Cc., then cooled, and shaken with 50 Cc. of ether, the clear, ethereal layer, after having separated, should not show a bluish fluorescence, and, when carefully decanted, and allowed to evaporate spontaneously, should leave not more than a slight, and not oily, residue (absence of paraffin oils)"—(U. S. P.).
Rancid linseed oil may be again made sweet by shaking it with warm water, allowing it to stand a while, and finally decanting.
Action, Medical Uses, and Dosage.—(See Linum.)
Related Oils.—The following are drying oils:
CANDLE-NUT OIL.—This oil is obtained by boiling in water the crushed seeds of Aleurites triloba, Forster. (Aleurites moluccana, Willdenow), found in tropical regions, in the isles of the Indian and Pacific Oceans, India, and the West Indies. The oil is known by several names, as Kekune oil, in Ceylon; Bankul oil, in India; Spanish-walnut oil, in Jamaica; Kakui oil, in the Sandwich Isles, where it is used as a mordant for vegetable colors. The tree yielding the fruit is known as the Candleberry tree, from the fact that the seeds, strung upon palm-fibers, are used for lighting purposes by the Polynesians. The nuts taste like walnuts, and are used as food by the natives of New Georgia. The tree is called Indian akrot (walnut) in India. On account of the oil being much used by painters, it is sometimes known as country-walnut oil or artist's oil (see Treasury of Botany). The oil is limpid, transparent, syrupy, amber-yellow, odorless, and rapidly drying. The yield is about 60 per cent. It consists of palmitin, stearin, myristin, and olein, the latter being somewhat like linolein from linseed oil. The oil is laxative (see Wood Oil of China).
GRAPE-SEED OIL.—From 10 to 20 per cent of a slowly drying, odorless, pale-yellow or brownish oil is obtained from the seeds of grapes (Vitis vinifera, Linné.. It has a feebly bitter taste. At near -16° C. (3.2° F.) it congeals. Its chief constituent is the glyceride of erucic acid (C22H42O2). At 34° C. (93.2° F.) this acid fuses, and, if melted with caustic potash, yields arachic and acetic acids. The lesser constituents are stearin and palmitin.
NIGER-SEED OIL.—A yellow oil, intermediate between the drying and non-drying oils, obtained to the extent of about 40 per cent from the black akenes of Guizotia oleifera, De Candolle (Verbesina sativa, Roxburgh), a composite plant of India and East Africa. It has a nut-like flavor, and congeals at -10° C. (14° F.). It is composed of two kinds of olein, one closely analogous to linolein, and myristin and palmitin.
MADIA OIL.—The akenes of Madia sativa, Molina, a composite annual found native in Chili and cultivated in the Old World, yields a bland, yellow, fixed oil (about 40 per cent), having a distinctive odor, a specific gravity of 0.930, and congeals at about -20° C. (-4° F.). It readily turns rancid on exposure; and gradually assumes a semisolid state.
WALNUT OIL.—(See Juglans.)