Why Does Oil Paint Dry When Cooking Oil Does Not? It Is Not Drying at All

The word drying is misleading, since nothing leaves the film. These oils contain molecules with several double bonds along their chains, and those bonds react with oxygen from the air, which produces reactive fragments that then link neighbouring molecules together. Over days and then months and years, an increasing proportion of the molecules become joined into a single continuous network, and a liquid made of separate molecules becomes a solid made of one enormous one. The film therefore gains weight as it hardens rather than losing it, which is the opposite of what happens when a solvent evaporates.

The behaviour depends directly on how many double bonds the molecules carry:

• •Linseed oil has many and hardens fastest, which is why it dominates
• •Tung and poppy oils also harden, with different colour and speed
• •Walnut oil hardens more slowly and yellows less
• •Olive and other cooking oils have too few and never harden
• •Semi-drying oils fall between and stay tacky
• •Metal compounds are added as driers to speed the reaction

The reaction never entirely stops, and that has consequences a conservator has to manage. A film continues to cross-link for decades, becoming harder, more brittle and less able to accommodate movement of the support beneath it, which is why old paintings crack in characteristic patterns. Linseed films yellow as they age, noticeably in whites and pale blues, and yellow faster in darkness, which is why a painting kept in a dark store can be improved by light exposure. Different pigments alter the speed enormously, so layers dry at different rates, and painting a fast-drying layer over a slow one produces cracking that is entirely predictable and is why the rule of working fat over lean exists.

Raw pressed oil is slow and is almost always treated before use, and the treatments change the working properties considerably. Boiled oil in the modern sense is generally not boiled at all but has metal driers added, which speed the reaction and shorten the drying time from weeks to a day. Genuinely heat-treated oil, thickened by prolonged heating without oxygen, becomes viscous and dries to a tougher glossier film that levels itself, which suits fine painting. Blowing air through the oil thickens it differently. Refining removes the mucilage that would otherwise cloud the film and slow the reaction, and cold-pressed oil is paler and yellows less than hot-pressed.

The reaction with oxygen releases heat, which is a genuine and frequently underestimated hazard. A rag soaked in linseed oil has an enormous surface area exposed to air and very poor heat conduction if crumpled in a bin, so the heat generated cannot escape, the temperature rises, the reaction accelerates because it goes faster when warm, and the cycle can reach the ignition point without any flame or spark being involved. Spontaneous fires started this way destroy workshops and houses every year. The remedy is to spread rags flat to cool, or immerse them in water in a sealed metal container, and never to leave them crumpled.

Oxygen reacts with double bonds along the oil molecules and links them into one continuous network, so the film gains weight as it hardens rather than losing anything, which is why the name is wrong. Linseed has the most double bonds and hardens fastest. Cross-linking continues for decades, which is why old films crack and yellow. The heat released can ignite crumpled rags unaided.