A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air. The oil hardens through a chemical reaction in which the components crosslink by the action of oxygen (not through the evaporation of water or other solvents). Drying oils are a key component of oil paint and some varnishes. Some commonly used drying oils include linseed (flax seed) oil, tung oil, poppy seed oil, perilla oil, and walnut oil. Their use has declined over the past several decades, as they have been replaced by alkyd resins and other binders.
Since oxidation is the key to curing in these oils, those that are susceptible to chemical drying are often unsuitable for cooking, and are also highly susceptible to becoming rancid due to autoxidation, the process by which fatty foods develop off-flavors. Rags, cloth, and paper saturated with drying oils may combust spontaneously (ignite) in a few hours, due to accumulation of heat released during the oxidation process.
Read more about Drying Oil: Chemistry of The Drying Process, Constitution of Drying Oils, Comparison To Waxes and Resins, Safety
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