Drying oil

The oil hardens through a chemical reaction in which the components crosslink (and hence polymerize) by the action of oxygen (not through the evaporation of water or other solvents).

The use of natural drying oils has declined over the past several decades, as they have been replaced by alkyd resins and other binders.

[1] Rags, cloth, and paper saturated with drying oils may spontaneously combust (ignite) after a few hours as heat is released during the oxidation process.

The "drying", hardening, or, more properly, curing of oils is the result of autoxidation, the addition of oxygen to an organic compound, and the subsequent crosslinking.

Bonds form between neighboring fatty acid chains, resulting in a polymer network, often visible by the formation of a skin-like film on samples.

A large number of the original ester bonds in the oil molecules undergo hydrolysis, releasing individual fatty acids.

Carboxyl groups in the polymers of the stationary phase ionize, becoming negatively charged and form complexes with metal cations present in the pigment.

[citation needed] In technical terms, these oil drying agents are coordination complexes that function as homogeneous catalysts.

Rags, cloth, and paper saturated with drying oils may combust spontaneously (ignite) due to heat released during the curing process.

This hazard is greater when oil-soaked materials are folded, bunched, or piled together, which allows heat to accumulate and accelerate the reaction.