Linseed oil is used in painting to increase the fixing strength of oil paints, adjust their thickness and viscosity, and to enhance luster. It is also blended into paints as the fixing ingredient (medium) to affix the pigment to the surface of the painting.
The oils used in oil painting are primarily distinguished based on two characteristics: their ability to function as solvents and their ability to function as drying oils. Linseed oil is considered a drying oil, as it possesses the ability to harden through oxidative polymerization when the unsaturated fatty acids it contains are exposed to oxygen. It is a drying oil extracted and refined from linseed (flaxseed), which contains a large amount of linolenic acid. Compared with poppy oil (from poppy seeds), linseed oil dries more quickly and forms a stronger coating. However, because it has a tendency to yellow when stored in dark places, it is important to be careful when mixing it with white or light-colored paints. The yellowing may be reversed by exposing it to sunlight. Linseed oil is also available in processed varieties that resist yellowing, such as “stand oil,” which has been polymerized through heating, and “sun-bleached oil,” which has been exposed to sunlight for long periods.
The standard method of using linseed oil involves mixing it with an appropriate quantity of solvent, diluted using an essential oil such as turpentine, in some sort of vessel, such as an oil can; as needed, the solvent is then added to the tip of the paintbrush, by which it is then mixed into the paints to produce the desired degree of viscosity and thickness. The paint is then applied to the surface. The solvent should have larger amounts of essential oils and smaller amounts of drying oils at the beginning of the painting process, with the quantity of drying oil being gradually increased as one approaches completion of the painting.
When using linseed oil, one should work in a well ventilated space away from high temperatures and exposed flame.
Linseed oil is available for purchase at most painting supply stores.