Linseed Oil, Or Flaxseed Oil, an oil expressed from the seeds of flax, and very extensively employed in the arts, and particularly in the preparation of paints for woodwork and other surfaces. It is the type of a class called drying oils, on account of possessing the property of forming a hard resinous mass from oxidation and exposure to the air. Thus employed it makes with the powdered substance called the body a paste, and on drying acts both as a cement and a varnish. The seeds, either in their raw state or roasted, are ground in mills, and the powder is then subjected to powerful hydraulic pressure. By roasting, the gummy matter in the interior coating of the seeds is destroyed, and the oil is obtained more free from mucilage, but it is of higher color and more acrid than that expressed from the raw seeds. Linseed oil freshly pressed is of a golden yellow color, which turns to dark brown with age. When fresh and cold pressed it is without disagreeable taste, but the commercial oil has a peculiar smell and taste. Its specific gravity is 0.940. At 000° F. it boils, and at about zero, or 4° below, it solidifies. Exposed a short time to the air, it becomes rancid; but on being agitated with warm water and allowed to stand till the two fluids separate, the oil may be decanted sweet.
Its chemical composition, according to Sacc, is that of an oleate and margarate of glycerine, containing no stearate. Sulphur dissolves in hot linseed oil with a red color, partially crystallizing on cooling; on longer heating, the oil takes up with solution of sulphydric acid one fourth its weight of sulphur, forming a brown viscid mass (fatty balsam of sulphur). Boiled with phosphorus, it acquires a scarlet color. On mixing from 15 to 25 parts of chloride of sulphur with 100 parts of linseed oil, products resembling caoutchouc are formed, hard in proportion to the amount of sulphur. Linseed oil takes fire with fuming nitric acid; and shaken with very dilute nitric acid, it is decolorized and finally converted into a varnish. It dissolves oxide of lead when heated, by which it is decolorized and rendered more drying, forming what is called boiled oil. According to Bueholz, it dissolves in 5 parts of boiling and 40 parts of cold alcohol. Besides the uses already named for this oil, it is largely applied in the manufacture of varnishes as a vehicle for the harder resins, to which it imparts softness and toughness.
But for the best varnishes it is necessary to clarify the oil by repeated skimming while it is allowed to simmer at nearly the boiling point, afterward boiling it with calcined magnesia, and letting it stand at least three months for the impurities to subside with the magnesia. - Linseed oil is an important commercial product, and is largely imported into the United States from Europe. The English import the seed from the East Indies, Russia, Germany, Holland, and America. From this they obtain the oil, and make of the residue, called oil cake, an important article of food for cattle. This incidental product of the manufacture of linseed oil is also largely used for the same purpose in the United States. - As the chief use of linseed oil is in decorative painting for the sake of its drying quality, it is essential that it should be free from mixture with other oils of a different nature, and from all other foreign ingredients possessing properties incompatible with this application. It is unfortunately the case, particularly with seed obtained from tropical regions, that other oleaginous seeds of plants that have grown up with the flax are intermixed with those of the latter; and the oil they furnish not possessing the same drying character, the product is thereby seriously impaired.
Linseed oil is moreover intentionally adulterated, as with common rosin dissolved in it, also with rosin oil, and with various fats and non-drying oils. The effect of rosin is to render the paint when apparently dry easily affected by warmth, even that of the hand, so as to be softened and made sticky. To detect the rosin or rosin oil, it is sufficient to heat a small portion in a porcelain cup, when the peculiar odor of the substance will be noticed if only 1/1000 part be present. To detect the presence of fats or non-drying oils, the practice is to touch the centre of a drop or two of the oil placed upon a white plate with a drop of sulphuric acid conveyed at the end of a glass rod; changes of color are chereby induced, and the formation of concentric rings of various shades, which suggest to an experienced eye the nature of the oil and of its adulterations. More perfect methods of testing linseed oil are very much to be desired. - In medicine linseed oil has sometimes been used internally as a laxative in the dose of a fluid ounce. It may be used externally as an ingredient of resin cerate, or of the lini-mentum calcis, sometimes called Carron oil.
It is occasionally employed as a vehicle for the external application of carbolic acid or in the carbolic acid putty.