Mike
What causes self ignition or self combustion as some call it in the rags is when you use linseed
oil on a rag and wad it up it gains heat within the rag while the linseed
oil is still wet. It will reach unreal temperatures before the rag itself catches on fire.
I had a fire Marshall do this very test in the summer of 2001. I had a 5,400 square foot home get burned to the ground and they started asking me about self combustion. I was curious and as we talked he went out to one of the firetrucks and got a red mechanics type rag, poured linseed
oil on it and threw it in the driveway. He then got a temperature gun (?) he pointed it at the rag and the temperature just kept getting higher and higher. After about a half hour or so it started smoking and then caught itself on fire.
The reason the decks do not catch fire is they have a chance to air out so the temperature does not continue to rise. have you ever noticed when you use paint thinner or even gasoline to wash your hands how your hands fell hot? Thst is the chemical reaction doing that.
Next time your at Home Depot get a can of it and read the back label.