Everything you need to know about linseed oil

Linseed oil is extracted from flax seeds. The climate in the area in which the flax is grown has a significant impact on the composition of the oil and the ratios of the unsaturated fatty acids it contains.

Linseed oil is one of the fast-drying oils, as it polymerizes easily when exposed to the oxygen in the air (in other words, it ‘dries’), forming a strong transparent film. This characteristic is due to the high content of glycerides of unsaturated fatty acids – linoleic, linolenic and oleic acids. Sunlight makes the oil dry more quickly, but after 10-14 days it softens and can even become sticky again. The purer the oil, the faster the drying process. Slower drying results in a better coating. The oil will not dry if left in a dark place or if there is no oxygen around it.

Linseed oil can be either refined or unrefined. The refined form is obtained through pressing and extraction, whilst the unrefined form is obtained through pressing alone. Only the refined and bleached form of the oil is used in painting.

Linseed oil is further subdivided into cold-pressed and hot-pressed varieties, depending on the production method used. The color of cold-pressed linseed oil ranges from lemon to dark lemon, while hot-pressed linseed oil ranges from golden yellow to brown.

Linseed oil contains pigments that determine its color. These pigments are not resistant to sunlight – they fade in the sun, so one of the ways to whiten (or lighten) linseed oil is by exposing it to light.

Special highly dispersed adsorbents are used to remove coloring compounds from the oils during the refining process.

Linseed oil stays in liquid form when cooled, until the temperature reaches -16°C, at which point coagulation (the formation of flakes) occurs; when heated to 40°C, the oil becomes transparent again.

 

These days, linseed oil – particularly the pure, bleached variety – is used in the production of varnishes for paintings. The extraneous byproducts are removed by heating the oil to 85°C and adding bleaching earth (clay with high adsorption properties). This improves oxygen uptake, thereby speeding up the drying process. It is used in the production of white varnish and artists’ paints.

Compacted linseed oil, for painting, comes from a special type of processing of linseed oil, whereby the oil has first been aged and freed from protein mucilage and other harmful impurities through refining and double bleaching. This oil is used in oil painting as a paint thinner. Its properties differ from those of ordinary bleached linseed oil in that the film it creates is less likely to wither and result in ‘settling’ (shriveling), less prone to yellowing, and more resistant to moisture. That being said, this variety of oil does not get absorbed so well into the base material. Compacted oil enhances the optical properties of colors and prevents some pigments from turning lighter.

Oxidized linseed oil for painting is a refined, bleached and aged oil that has then been thickened through oxidation with oxygen, in the sunlight. This oil is the classic medium described in ancient treatises. Nowadays, this oil is oxidized by blowing air through a layer of oil heated to 110-150°C. It differs from other types of linseed oil in that it dries more quickly and results in the lowest amount of yellowing seen in any of the films of varnish. Hydroxyl groups appear as a result of oxidation, and consequently the oil has strong emulsifying properties.