This has prompted me to review everything I have in my files about the subject.
"Spontaneous combustion" of linseed oil requires a few things (this is not
exhaustive)

The fuel-- linseed oil

oxygen-- not for burning, but to feed the polymerization of the oil

conditions in which heat can build up-- if the heat from the reaction can get
out as fast as it is produced, the temperature will stop rising. This is
strongly influenced by the acceleration of the reaction at higher temperature
(thermal runaway is one term)


Soaked rags tend hold heat, due to the trapped air, and oxygen. Fresh oxygen may
or may not be able to get to the core of a pile or wad or bucket, but not a lot
is needed to support the reaction, so in many  cases it may not matter. Soaked
rags have the fuel and may provide conditions for heat to build up before the
fuel is exhausted. I could find no precise numbers in the pile of (electronic
and paper) safety documentation I have, but there is indication that there is a
pretty strong influence due to the particular formulation of the product and the
particular analysis of the oil itself.

It appears that risk is increased with catalysts ('driers') and the presence of
lignocellulose (like a cotton rag or wood shavings....). Treatment of the oil
(thermal or other that destroys peroxy radicals and ties up some of the O2
sensitizers) tends to reduce the risk.

I would *guess* that thinned oils, especially the fast thinned ones that are
already partially oxidized, are  lower risk (less reactive material, evaporation
of the solvent will take heat with it) that pure oils, but it is only a guess.
The literature I have does not differentiate (the focus is primarily on
catalyzed coatings-- urethanes, epoxies, polyesters, and so on... One hat I
periodically were at work is health/safety/compliance)


An accessible paper that gets deep into the subject is: https://firesci
encereviews.springeropen.com/articles/10.1186/2193-0414-1-3">https://firescience
reviews.springeropen.com/articles/10.1186/2193-0414-1-3

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