Linseed oil is a colorless, yellowish oil obtained from the dried and ripened seeds of the flax plant (Linum usitatissimum). The oil is obtained by squeezing, sometimes followed by the extraction of the solvent. Linseed oil is a drying oil, which means it can polymerize into a solid form. Due to its polymer-forming properties, linseed oil can be used alone or mixed with combinations of other oils, resins or solvents as a primer, drying oil finish or varnish in wood finish, as a binder for pigments in wood oil paints, as a plasticizer and hardener in mastic and in the manufacture of linoleum. The use of linseed oil has declined in recent decades with an increased availability of synthetic alkyd resins, which work the same but resist yellowing
Linseed oil is an edible oil required as a food supplement, as a source of α-linolenic acid (an omega-3 fatty acid). In some parts of Europe, it is traditionally eaten with potatoes and quark. It is considered a delicacy due to its substantial taste, which enhances the flavor of the quark, which is otherwise bland.
Linseed oil is a triglyceride, like other fats. Linseed oil is characteristic for its unusually large amount of α-linolenic acid, which has a distinctive reaction with oxygen in the air. Specifically, the fatty acids in a typical linseed oil are of the following types:
Triple unsaturated α-linolenic acid (51.9-55.2%),
saturated palmitic acid (about 7%)
stearic acid (3.4-4.6%),
monounsaturated oleic acid (18.5-22.6%),
doubly unsaturated linoleic acid (14.2-17%).
With a high content of di- and tri-unsaturated esters, linseed oil is particularly sensitive to polymerization reactions following exposure to oxygen in the air. This polymerization, which is called "drying", causes the stiffness of the material. The drying process can be so exothermic that it poses a fire hazard under certain circumstances. To avoid premature drying, linseed oil products (oil paints, putty) should be stored in airtight containers.
Like some other drying oils, linseed oil fluoresces under UV light after degradation.