Linseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant (Linum usitatissimum, Linaceae). The oil is obtained by cold pressing, sometimes followed by solvent extraction. Due to its high levels of α-Linolenic acid (a particular form of Omega-3 fatty acid), it is used as a nutritional supplement.
Linseed oil is a "drying oil", as it can polymerize into a solid form. Due to its polymer-forming properties, linseed oil is used on its own or blended with other oils, resins, and solvents as an impregnator and varnish in wood finishing, as a pigment binder in oil paints, as a plasticizer and hardener in putty and in the manufacture of linoleum. The use of linseed oil has declined over the past several decades with the increased use of synthetic alkyd resins, which are functionally similar but resist yellowing.[1] It is an edible oil but, because of its strong flavor and odor, is only a minor constituent of human nutrition in the U.S., although it is marketed as a nutritional supplement. In parts of Europe, it is traditionally eaten with potatoes and quark (cheese). It is regarded as a delicacy due to its hearty taste, which spices the bland quark.[2]
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Linseed oil is a triglyceride, like other fats. Linseed oil is distinctive in terms of fatty acid constituents of the triglyceride, which contain an unusually large amount of α-linolenic acid, which has a distinctive reaction toward oxygen in air. Specifically, the constituent fatty acids in a typical linseed oil are of the following types:[3]
Having a high content of di- and triunsaturated esters, linseed oil is particularly susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization, which is called "drying," results in the rigidification of the material. The drying process can be so exothermic as to pose a fire hazard under certain circumstances. To prevent premature drying, linseed oil-based products (oil paints, putty) should be stored in air-tight containers.
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.
Linseed oil is a common carrier used 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). The introduction of linseed oil was a significant advance in the technology of oil painting.
Traditional glazing putty, consisting of a paste of chalk powder and linseed oil, is a sealant for glass windows that hardens within a few weeks of application and can then be painted over. The utility of putty is owed to the drying properties of linseed oil.
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 of the wood. A linseed oil finish is easily repaired, but it provides no significant barrier against scratching. Only wax finishes are less protective. Liquid water will penetrate a linseed oil finish in mere minutes and water vapour bypasses it almost completely.[4] Garden furniture treated with linseed oil may develop mildew. Oiled wood may be yellowish and is likely to darken with age. Because it fills the pores, linseed oil protects wood from denting by compression.
Linseed oil is a traditional finish for gun stocks, although 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. Linseed oil is also often used by billiards or pool cue-makers for cue shafts, as a lubricant/protectant for wooden recorders, and used in place of epoxy to seal modern wooden surfboards. It is used to coat cricket bats so that the wood will retain some moisture. New cricket bats are coated with linseed oil and knocked to perfection so they last longer.[5]
Additionally, a luthier may use linseed oil when reconditioning a guitar, mandolin, or other stringed instrument's fret board; lemon-scented mineral oil is commonly used for cleaning, then a light amount of linseed oil (or other drying oil) is applied to protect it from grime that might otherwise result in accelerated deterioration of the wood.
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 its virtual replacement by PVC floor coverings. 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.
Although flax seeds contain lignans, a class of phytoestrogens considered to have antioxidant and cancer-preventing properties,[6][7][8] the extracted linseed oil does not contain the lignans found in flax seed,[6] and therefore does not have the same antioxidant properties. Some brands of supplement have lignans added during production. 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.[9][10][11] Oil with an unpleasant or rancid odor should be discarded. Oxidation of flax seed oil is a major commercial concern, and antioxidants may be added to prevent rancidification.[12] The alpha linolenic acid (ALA) in flaxseed oil is suitable for cooking, as it (and the lignans in flax seeds themselves) can withstand temperatures up to 350 degrees F for two hours.[13]
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 and is a traditional European ethnic food, highly regarded for its hearty taste. It contains the highest level of the omega-3 fatty acid ALA among vegetable oils.[14] Regular flaxseed oil contains between 52% and 63% ALA (C18:3 n-3). Plant breeders have developed flaxseed with both higher ALA (70%) and very low ALA content (< 3%).[15] The USFDA granted generally recognized as safe (GRAS) status for high alpha linolenic flaxseed oil.[16]
According to the Flax Council of Canada, ALA is required for normal infant development[17] and may be beneficial for reducing inflammation leading to atherosclerosis,[18] and for preventing heart disease and arrhythmia.[19] However, recent well-controlled placebo studies suggest the regular consumption of flaxseed oil may not reduce the risk of stroke, heart disease, or cancer.[20]
Studies[21][22] have shown a relationship between ALA and an increased risk of prostate cancer. This risk was found to be irrespective of source (e.g., meat, vegetable oil).[23] Alternatively, at least one meta analysis has found a weak protective association between dietary ALA intake and prostate cancer risk.[24]
| Typical fatty acid content | % [25] | % European[26] |
|---|---|---|
| Palmitic acid | 6.0 | 4.0–6.0 |
| Stearic acid | 2.5 | 2.0–3.0 |
| Arachidic acid | 0.5 | 0–0.5 |
| Palmitoleic acid | - | 0–0.5 |
| Oleic acid | 19.0 | 10.0–22.0 |
| Eicosenoic acid | - | 0–0.6 |
| Linoleic acid | 24.1 | 12.0–18.0 |
| Alpha-linolenic acid | 47.4 | 56.0–71.0 |
| Other | 0.5 | - |
Nutrition information from the Flax Council of Canada.[6]
Per 1 Tbsp (14 g)
Flax seed oil contains no significant amounts of protein, carbohydrates, or fiber.
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.[28]
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, 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 (catalysts to accelerate drying). The use of metallic dryers makes boiled linseed oil inedible. Some products 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".
Rags soaked with linseed oil stored in a pile are considered a fire hazard because they provide a large surface area for oxidation of the oil, and the oil oxidizes quickly. The oxidation of linseed oil is an exothermic reaction, which accelerates as the temperature of the rags increases. When heat accumulation exceeds the rate of heat dissipation into the environment, the temperature increases and may eventually become hot enough to make the rags spontaneously combust.[29]
In 1991, One Meridian Plaza, a high rise in Philadelphia was severely damaged and three firefighters perished in a fire caused by linseed oil-soaked rags.[30] An apartment in Sacramento also caught fire due to the spontaneous combustion of linseed oil-soaked rags.[31]
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