I'd heard about spontaneous combustion of linseed oil soaked rags, but was a bit skeptical of some of the stories I'd read. When oiling my frame this weekend, I decided it was a good opportunity to test it for myself. I oiled the frame on Saturday (50:50 oil/turps), then wiped it down with a rag (~ 1 sq/ft in size) and hung the rag out to dry. As expected, there were no problems. The next day, I used the same rag to wipe down the second coat (75:25 oil/turps), at which point it was thoroughly saturated. I wadded it up and set it on a concrete step with no combustible materials nearby. It was sunny and ~80 degrees.

Half an hour later, while steaming the coaming rim, we noticed a strong linseed oil smell. We assumed it came from the steamer, but couldn't figure out why, since the wood in it wasn't oiled. Half an hour or so after that, I went to check on the rag. It looked the same as when I set it down, but it was beginning to smolder. As we watched, it began to turn a nice golden brown on the outside. I prodded it open with a stick and it was black and charred inside, with parts of it already burned away. That was proof enough for me, and I doused it with a hose, then hung it out to dry. There were never any flames, but I have no doubt that there would likely have been if I hadn't doused it.

This experiment basically confirmed what I had previously thought:

- Hanging an oil soaked rag out to dry is a safe way to handle it, as it cannot build up any heat. Once they're dry, they're supposedly safe, but I still haven't had the nerve to throw this charred rag in the trash, since the can is full of wood shavings (can you say "tinder"?).

- Leaving it in a ball is the worst thing you can do.

- Even under "ideal" conditions, it takes some time for combustion to take place; in this case, over an hour. It isn't likely to happen in just a few minutes, though that doesn't mean one should leave wadded up rags lying around.

The strong oil smell gave ample warning that something was amiss. Despite having a fan blowing air out of the garage, we smelled the oiled rag outside so strongly that we thought the odor was coming from the steamer right beside us (the steam was blowing toward us). Now that I know what it smells like, I feel that confident that I'll recognize it if I ever smell it again. However, I don't plan to let that happen unintentionally! I'll continue to use oil finishes and carefully hang out the rags as I have in the past.

I hope this puts the risks of spontaneous combustion into perspective somewhat. Those of you who use oil finishes may want to repeat this test under controlled conditions. I'd be interested in your results.