LINSEED is the seed of the common flax or lint, Linum usitatissimum, from which also the well-known fibre flax is obtained. The plant itself is figured and described under FLAX, vol. ix. p. 293. The fruit of the flax plant consists of a globose capsule which splits into five cocci, each containing two seeds. These seeds, the linseed of commerce, are of a lustrous brown colour externally, and a compressed and elongated oval. form, with a slight beak or projection at one extremity. The brown testa contains, in the outer of the four coats into which it is microscopically distinguishable, an abundant secretion of mucilaginous matter ; and it has within it a thin layer of albumen, enclosing a pair of large oily cotyledons. The seeds when placed in water for some time become coated with glutinous matter from the exudation of the mucilage in the external layer of the epidermis ; and by boiling in sixteen parts of water they exude sufficient mucilage to form with the water a thick pasty decoction. The cotyledons contain the valuable linseed oil referred to below. Linseed grown in tropical countries is much larger and more plump than that obtained in temperate climes, but the seed from the colder countries, on the other hand, yields a finer quality of oil. Fliickiger and Hanbury found that six seeds of Sicilian linseed, thirteen of Black Sea, and seventeen of Archangel linseed weighed respectively one grain. The average composition of linseed may be fairly represented by the following analysis by Dr Thomas Anderson : - albuminous substances, 24.44 ; oil, 34.00 ; gum sugar and cellulose, 30.73 ; ash, 3.33 ; water, 7.50. Linseed is cultivated and secured as a crop in all European flax-growing countries, where the seed is probably not less valuable than the fibre. It is also obtained from Egypt and India, being cultivated in the latter country solely on account of the seed.
Apart from its value as a source of oil, and for sowing, linseed is not a product of much economic importance. It formed an article of food among the Greeks and Romans, and it is said that the Abyssinians at the present day eat it roasted. The oil is to some extent used as food in Russia, and in parts of Poland and Hungary. Linseed meal, partly on account of its bland oily constitution, is a valuable material for poultices. At one time the crushed seeds were the officinally recognized cataplasmic material, but the readiness with which that preparation became rancid through the oxidation of its abundant oil frequently rendered it a dangerous application for open sores. The lini farina of the pharmacopoeia is now the powdered meal of the cake left after expression of the oil, with a proportion of olive oil added when about to be used. An infusion of linseed under the name of "linseed tea" is a popular diluent in bronchial and other inflammatory affections. The abundant mucilage in linseed meal makes it a most useful material for luting stoppers in chemical jars, and other such joints in glass-work. Linseed cake, the mare left after the expression of the oil, is a most valuable feeding substance for cattle. According to a recent analysis by Dr Voelcker (Journ. Roy. Agric. Soc., 2d ser., vol. xvii. p. 659) it contains in 100 parts - oil, 10.90 ; albuminous compounds, 24.56 ; mucilage, sugar, and digestible fibre, 31.97; woody fibre, 11.47; ash, Go20; moisture, 1490.
Linseed is subject to extensive and detrimental adulterations, resulting not only from careless harvesting and cleaning, whereby seeds of the flax dodder and other weeds and grasses are mixed with it, but also from the direct admixture of cheaper and inferior oil seeds, such as wild rape, mustard, sesame, poppy, kc., the latter adulterations being known in trade under the generic name of " buffum." In 1864, owing to the serious aspect of the prevalent adulteration, a union of traders was formed under the name of the "Linseed Association," the members of which bind themselves to give compensation for all adulterations in excess of 4 per cent. of foreign matter. Highly adulterated linseed is, however, still very common outside the field of operations of the Linseed Association.
The quantity of linseed imported into the United Kingdom during the year 1881 was 1,829,838 quarters, of a value of £4,395,061. About one-half of this amount, 937,059 quarters, valued at £2,299,877, came from the East Indies, and the imports from Russia, amounting to 728,338 quarters, valued at £1,694,720, account for the greater part of the remainder.
Linseed Oil, the most valuable and characteristic of the series of drying oils, is obtained by expression from the seeds, with or without the aid of heat. Preliminary to the operation of pressing, the seeds are crushed between a pair of revolving rollers, and ground to a fine meal under heavy edge stones on a stone bed. For the extraction of the fine quality of oil known as cold-drawn the meal is, without further preparation, filled into woollen or canvas bags and enclosed in horse-hair envelopes for pressure, either in a Dutch mill worked by means of wedges and falling stampers or in a screw press, or, what is now more prevalent, in a special form of hydraulic press. The oil so obtained is of a clear yellow colour, and is comparatively devoid of odour and taste. The cake left by cold pressure is again ground up, heated in a steam kettle to about 212° Fakir., and while hot submitted to further pressure, which results in the exudation of a less pure oil of a brownish-yellow colour. In general practice, cold-drawn oil is little prepared ; the linseed after grinding is submitted to a high heat, whereby the mucilage in the epidermis is destroyed, and the oil flows more freely ; and in many crushing establishments the oil is obtained by a single operation under the press. The yield of oil from different classes of seed varies, but from 23 to 28 per cent. of the weight of the seed operated on should be obtained. A good average quality of seed weighing about 392 lb per quarter has been found in practice to give out 109 lb of oil.
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 does not solidify under the influence of very low temperature. It is soluble in 32 parts of alcohol, sp. gr. 0.82, in 6 parts of boiling alcohol, and in 1.6 of ether. By saponification it yields about 95 per cent. of fatty acids, principally linoleic acid (C16H26O2), a body peculiar to the drying oils, and by treatment with oxide of lead about nine-tenths of the resulting lead salt is found to be linoleate of lead. The oil may be perfectly bleached by treatment with a solution of green sulphate of iron, with repeated shaking and exposure to the light for a period of four to six weeks. Exposed to the air in thin films, linseed oil absorbs oxygen and forms a resinous semi-elastic caoutchouc-like mass, oxylinoleic acid, C16H26O5. The oil, when boiled with small proportions of litharge and minium, undergoes the process of resinification in the air with greatly increased rapidity. Saec found by boiling 2500 grains of raw oil for ten minutes with 30 grains each of litharge and minium, and weighing after twenty-four hours exposure to the atmosphere, that the oil had lost only GO grains. A second sample he boiled till there was a loss of 5 per cent. in weight, when the product assumed the consistency of molasses ; and a third portion boiled to a loss of 12 per cent. became a caoutchouc-like mass. The first of these products he found dried, on exposure, to a fine transparent varnish ; the second did not resinify after fifteen days' exposure ; and the atmosphere had no effect on the third portion. The weight of the film of the first after complete resinification was increased 50 per cent. through absorption of oxygen, and the rate at which absorption took place was much influenced by heat.
To these physical properties the varied industrial applications of linseed oil are principally due. Its most iinportant use is certainly found in the preparation of oil paints and varnishes. By painters both raw and boiled oil are used, the latter not only forming the principal medium in oil painting, but 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 furnace, and the oil is brought gradually to the point of ebullition, at which it is maintained for two hours, during which time moisture is driven off, and the scum and froth which accumulate on the surface are ladled cut. Then by slow degrees a proportion of " dryers " is added - usually equal weights of litharge and minium being used to the extent of 3 per cent. 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 ill the boiler for ten hours or more. Before sending out, it is usually stored in settling tanks for a few weeks, during which time the ivncombiiied dryers settle at the bottom as "foots." Besides the dryers already mentioned, acetate of lead, borate of manganese, binoxide of manganese, sulphate of zinc, and other bodies are used. The theory of the influence of boiling and of the addition of these bodies on linseed oil is not well understood. By Liebig it was suggested that they simply removed the mucilaginous and other foreign constituents of the oil which by their presence intercepted the action of oxygen ; but by Chevreul and others the opinion was held that the chemicals used, by giving up oxygen to the oil, thereby induce a more rapid and energetic absorption from the air. However this may be, it does not appear at least that boiling is essential for the production of that active condition of the oil, as it may also be induced by treatment of cold raw oil with lead acetate and other agencies. Boiled oil is now very largely used in the manufacture of linoleum floor-cloth. See LINOLEUM.
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 pi oduct wdien cooled forms a viscid tenacious substance which in its most concen-trated 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. (J. PA.)
The above article was written by: James Paton.