I attended a call out recently to a fire in an apple orchard workers cottage. We recently read the report of the cause and I was astonished at the findings.
I questioned it and went on to do my own research and the phenomenon of spontaneous combustion is a real and present danger few people appear to know about.
Spontaneous combustion occurs when an object suddenly bursts into flame without obvious cause. No match, no sparks, no lightning, no electrical short or smoking cigarette... nothing!
What causes it? With linseed oil and other oils used to finish wood, including some exterior deck sealers and wood stains, heat is generated during the drying process. This is because these oils do not dry like paint (through the evaporation of a solvent or water). Instead, they dry through the same process that generates fire... oxidation.
Despite the stories about spontaneous combustion it's a difficult phenomenon to reproduce ... however, spontaneous combustion is real!
And that's the key! Burning is rapid oxidation. For spontaneous combustion to occur, enough heat must accumulate so fire can start. You would never see a piece of furniture spontaneously combust because the oil oxidizes in open air so the surface never even gets warm to the touch! But a pile of oil-soaked rags can...
In every case of spontaneous combustion of drying oils that I have found, the cause has been a bunch of oil soaked rags. It seems that, as the oil oxidizes, the rags act as an insulator, allowing the oxidizing oil to become hot enough to cause the cloth to smoke and eventually ignite. The bigger the pile, the greater the possible heat and the greater the risk.
Room temperature is also a factor. The warmer it is, the quicker the rags can reach ignition temperature.
Well, that's it. If you ever had any doubts of the danger of spontaneous combustion with linseed oil, consider them vanquished. Like me, none of these readers had a clue, and fortunately none of them lost their lives.