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I am having trouble understanding the relationship between polymerization and oxidation of drying oils, specifically linseed oil used for fine art. In refining my own oil by filtering it then leaving it in the summer sun in a pan, the oil thickens and becomes strong and very fast drying. Similar thickened linseed oil from the art store is alkali-refined, and though thickened (therefore supposedly polymerized) still dries slowly. Why? So does polymerization not affect rate of curing? (Art store thickened oil is boiled in the absence of O2, so can polymerization happen without oxygen?) I can use an aquarium pump to blow air through some home-filtered oil for a number of days and rapidly thicken the oil. Is the thickening because it is starting to cure/oxidize, or is it because of polymerization, or both? I am trying to learn the relationship between these concepts, and how they affect cure rate and strength of the dried film.

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    $\begingroup$ Could you give us some ideas of drying times? How are you applying these oils? To what? $\endgroup$ Dec 17, 2022 at 10:38

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can polymerization happen without oxygen

Yes, under heat the Diels-Alder reaction can occur between double bonds of unsaturated oils without oxygen. "Boiled oil" is partially polymerized. Solvent (mineral spirits) is used to thin it and "dryer" chemicals are added into the formula for faster drying.

Oxygen can also "cross-link" untreated oils, such as pure linseed, tung, or soya oil, as well as processed oils, but this usually takes longer.

However, drying rates of solvent based product can take markedly longer at lower temperatures simply because mineral spirits does not evaporate as well. The solvent must evaporate before even "boiled" oil can dry. "Store bought" product also may not be pure linseed oil$^1$.

In coatings jargon, there are also two degrees of dryness: set to touch and tack free. The dryers generally help form a film on a surface faster. So lightly dragging fingertips without sticking should happen sooner. The whole coating being firm with finger pressure (tack free) can vary, especially if the solvent is slow to dry.

$^1$ "tung oil finish" is an example of this

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