What are drying oils?

Drying Oils

Drying oils are natural oils that dry to a solid film after a period of exposure to air. They react with oxygen in the air to crosslink chemical bonds in the oil. Drying oils are permanently flexible, and never dry completely ‘hard’, unlike some varnishes for instance. Drying Oils are therefore able to cope with natural movement in the wood due to temperature, humidity and age.

Different drying oils dry at different rates. The fastest entirely natural drying oil is Pure Tung Oil, a thin film of which will dry to a low sheen film on wood in 24 to 72 hours at room temperature.

Raw Linseed Oil, which hasn’t been treated in any way is the next fastest natural drying oil, but it can take many days to dry.

Boiled Linseed Oil is natural Linseed Oil which has been processed to improve its drying time. The processing usually involves some heat treatment, ‘boiling’ air through the oil to partially oxidise it, and/or adding a small amount of driers. Boiled Linseed Oil usually has a glossy finish when dry.

Semi-drying Oils

Some natural oils, for example sunflower oil and soyabean oil are classed as ‘semi-drying’ oils. They only partially dry on exposure to air, slowly leaving a tacky surface. They need chemical treatment or the addition of relatively large quantities of chemical driers to encourage the drying process.

Non-drying Oils

Some vegetable oils will not dry naturally at all. These include coconut oil and olive oil. Care should be taken if considering using ‘preparations’ containing these oils, as, although they may be absorbed into timber and provide temporary treatment, at best they will leach out over time, and in the worst case they can go rancid, causing stains and encouraging rot and mildew.