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Oil obtained from the dried, ripened seeds of the flax plant

Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colourless to yellowish oil obtained from the dried, ripened seeds of the flax plant (Linum usitatissimum). The oil is obtained by pressing, sometimes followed by solvent extraction. Owing to its polymer forming properties, linseed oil is often blended with combinations of other oils, resins or solvents as an impregnator, drying oil finish or varnish in wood finishing, as a pigment binder in oil paints, as a plasticizer and hardener in putty, and in the manufacture of linoleum. Linseed oil use has declined over the past several decades with increased availability of synthetic alkyd resins—which function similarly but resist yellowing. Linseed oil is an edible oil in demand as a dietary supplement, as a source of α linolenic acid, an omega 3 fatty acid. In parts of Europe, it is traditionally eaten with potatoes and quark.

Linseed oil is a triglyceride, like other fats. Linseed oil is distinctive for its unusually large amount of α linolenic acid, which oxidises in air. The fatty acids in a typical linseed oil are of the following types:
The triply unsaturated α linolenic acid (51.9–55.2%),
The saturated acids palmitic acid (about 7%) and stearic acid (3.4–4.6%),
The monounsaturated oleic acid (18.5–22.6%),
The doubly unsaturated linoleic acid (14.2–17%).

Having a high content of di and tri unsaturated esters, linseed oil is susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization, which is called autoxidation, results in the rigidification of the material. To prevent premature drying, linseed oil based products (oil paints, putty) are stored in airtight containers. Rags soaked with linseed oil pose fire hazard because they provide a large surface area for rapid oxidation. The oxidation of linseed oil is exothermic, which may lead to spontaneous combustion. In 1991, One Meridian Plaza, in Philadelphia, was severely damaged in a fire, in which three firefighters perished, thought to be caused by rags soaked with linseed oil.

Most applications of linseed oil exploit its drying properties, i. e., the initial material is liquid or at least pliable and the aged material is rigid but not brittle. The water repelling (hydrophobic) nature of the resulting hydrocarbon based material is advantageous. Garden furniture treated with linseed oil may develop mildew. Oiled wood may be yellowish and is likely to darken with age. Even though the oil feels dry to the touch, studies show linseed oil does not fully cure. Linseed oil is a common finish for wooden items, though very fine finish may require months to obtain. Studies show the fatty acid structure of linseed oil has problems cross linking and oxidizing, frequently turning black.

Boiled linseed oil is used as sizing in traditional oil gilding to adhere sheets of gold leaf to a substrate (parchment, canvas, Armenian bole, etc.). It has a much longer working time than water based size and gives a firm smooth surface that is adhesive enough in the first 12–24 hours after application to cause the gold to attach firmly to the intended surface.

Linseed oil is used to bind wood dust, cork particles, and related materials in the manufacture of the floor covering linoleum. After its invention in 1860 by Frederick Walton, linoleum, or "lino" for short, was a common form of domestic and industrial floor covering from the 1870s until the 1970s, when it was largely replaced by PVC ("vinyl") floor coverings. However, since the 1990s, linoleum is returning to favor, being considered more environmentally sound than PVC. Linoleum has given its name to the printmaking technique linocut, in which a relief design is cut into the smooth surface and then inked and used to print an image. The results are similar to those obtained by woodcut printing.

Raw cold pressed linseed oil – commonly known as flax seed oil in nutritional contexts – is easily oxidized, and rapidly becomes rancid, with an unpleasant odour, unless refrigerated. Linseed oil is not generally recommended for use in cooking. In one study, the content of Alpha linolenic acid (ALA) in whole flaxseeds did not decrease after heating the seeds to temperatures of up to 178 °C (352.4 °F) for one and a half hours. Food grade flaxseed oil is cold pressed, obtained without solvent extraction, in the absence of oxygen, and marketed as edible flaxseed oil. Fresh, refrigerated and unprocessed, linseed oil is used as a nutritional supplement and is a traditional European ethnic food, highly regarded for its nutty flavor. Regular flaxseed oil contains between 57% and 71% polyunsaturated fats (alpha linolenic acid, linoleic acid). Plant breeders have developed flaxseed with both higher ALA (70%) The USFDA granted generally recognized as safe (GRAS) status for high alpha linolenic flaxseed oil.

Stand oil is generated by heating linseed oil near 300 °C for a few days in the complete absence of air. Under these conditions, the polyunsaturated fatty esters convert to conjugated dienes, which then undergo Diels Alder reactions, leading to crosslinking. The product, which is highly viscous, gives highly uniform coatings that "dry" to more elastic coatings than linseed oil itself. Soybean oil can be treated similarly, but converts more slowly. On the other hand, tung oil converts very quickly, being complete in minutes at 260 °C. Coatings prepared from stand oils are less prone to yellowing than are coatings derived from the parent oils.

Boiled linseed oil is a combination of raw linseed oil, stand oil (see above), and metallic oil drying agents (catalysts to accelerate drying). The lead oxide forms lead "soaps" (lead oxide is alkaline) that promote hardening (polymerisation) of linseed oil by reaction with atmospheric oxygen. Heating shortens its drying time.

Raw linseed oil is the base oil, unprocessed and without driers or thinners. It is mostly used as a feedstock for making a boiled oil. It does not cure sufficiently well or quickly to be regarded as a drying oil. Raw linseed is sometimes used for oiling cricket bats to increase surface friction for better ball control. It was also used to treat leather flat belt drives to reduce slipping.