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Linseed oil, also known as flax seed oil or simply flax oil, is a clear to yellowish drying oil derived from the dried ripe seeds of the flax plant (Linum usitatissimum, Linaceae). It is obtained by pressing, followed by an optional stage of solvent extraction. Cold pressed oil obtained without solvent extraction is marketed as flaxseed oil.

Uses

Paint binder

Linseed oil is the most commonly used carrier in oil paint. It can also be used as a painting medium, making oil paints more fluid, transparent and glossy. It is available in varieties such as Cold Pressed, alkali refined, sun Bleached, sun thickened, and polymerised (stand oil).

Putty

Glazing putty, consisting of a paste of chalk powder and linseed oil, is a traditional sealant for glass windows that dries hard within a few weeks and can then be painted over.

Wood finish

When used as a wood finish, linseed oil dries slowly and shrinks little upon hardening. Linseed oil does not cover the surface as varnish does, but soaks into the (visible and microscopic) pores, leaving a shiny but not glossy surface that shows off the grain. Wood treated with linseed oil is resistant to denting and scratches and easily repaired, but the surface is not as hard as a modern varnish, and the wood will slowly absorb moisture if allowed to stay wet. Soft wood is protected from denting, but requires numerous applications and even more drying time than harder wood. Garden furniture treated with linseed oil may develop mildew. Oiled wood is yellowish and darkens with age.

Linseed oil is a traditional finish for gun stocks, but a very fine finish may require months to obtain. Several coats of linseed oil is the traditional protective coating for the raw willow wood of cricket bats, and thus has a special cultural place in cricket-playing countries.^{citation needed} Fire departments treat the wood handles of hand tools that have metal implements (e.g. axes, plaster hooks) with Linseed oil to prevent buildup of static electricity.^{citation needed} Linseed oil is also often used by billiards/pool cue-makers cue shafts, as a lubricant/protectant for wooden recorders, and used in place of epoxy to seal modern wooden surfboards.

Nutritional supplement

Food-grade flaxseed oil is cold-pressed, obtained without solvent extraction, and marketed as edible flaxseed oil. Fresh, refrigerated and unprocessed, linseed oil is used as a nutritional supplement. It contains high levels of omega-3 fatty acids, especially alpha-linolenic acid, which has been suggested to be beneficial for reducing inflammation leading to atherosclerosis,¹ preventing heart disease and arrhythmia,² and is required for normal infant development.³ However recent well-controlled placebo studies suggest that regular consumption of flax seed oil may not reduce the risk of stroke, heart disease, or cancer any greater than placebo.⁴ Regular flaxseed oil contains between 52 and 63 % alpha linolenic acid (C18:3 n-3). Plant breeders have developed flaxseed with high alpha linoleic acid content (70 %) and very low alpha linolenic acid content (< 3%).⁵

Although flax seeds themselves contain lignans, a class of phytoestrogens considered to have antioxidant and cancer preventing properties,⁶⁷⁸ the extracted linseed oil does not contain the lignans found in flax seed,⁶ and therefore does not have the same antioxidant properties. In fact, flax seed oil is easily oxidized, and rapidly becomes rancid with an unpleasant odor unless refrigerated. Even when kept under cool conditions it has a shelf life of only a few weeks.⁹¹⁰ Oil with an unpleasant or rancid odour should be discarded. Rancid oils contribute to the formation of free radicals and may be carcinogenic.¹¹¹²¹³ Oxidation of flax seed oil is major commercial concern, and antioxidants may be added to prevent rancidification.¹⁴

Nutrient content

Flax seed oil contains no significant amounts of protein, carbohydrates, or fibre.

Additional uses

Boiled linseed oil

Boiled linseed oil is used as a paint binder or as a wood finish on its own. Heating the oil causes it to polymerize and oxidize, effectively making it thicker and shortening its drying time. Today most products labeled as "boiled linseed oil" are a combination of raw linseed oil, petroleum-based solvent and metallic dryers. The use of metallic dryers makes boiled linseed oil inedible. There are some products available that contain only heat-treated linseed oil, without exposure to oxygen. Heat treated linseed oil is thicker and dries very slowly. This grade of linseed oil is usually labeled as "polymerized" or "stand" oil, though some types may still be labeled as "boiled".

Spontaneous combustion

Rags dampened with boiled linseed oil are considered a fire hazard because they provide a large surface area for oxidation of the oil. The oxidation of linseed oil is an exothermic reaction which accelerates as the temperature of the rags increases. If rate of heat accumulation exceeds the rate of dissipation, this reaction may eventually become sufficiently hot to cause spontaneous combustion of the rags. Linseed oil soaked rags should never be stored, least of all in an enclosed container. Instead, rags should be washed, soaked with water, or incinerated. A fire that destroyed the El Rey Theater-Golden West Saloon in February 2008 was ignited by the spontaneous combustion of linseed oil soaked rags left in a plastic container. ¹⁶

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