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 fast drying. Similar thickened linseed oil from the art store is alkali-refined, and though thickened (therefore supposedly polymerized) still dries slowly. Why? (It is boiled in the absence of O2, so can polymerization happen without oxygen?) So does polymerization not affect rate of curing? I can 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.