I was saddened to hear that a member of my own guild lost his shop and tools last month to a fire started by a bunch of oily finish rags. It is a common cause of shop and worksite fires, but it is easy to prevent.
Oily rags are spontaneously combustible. That means that even in a very small pile, they will generate enough heat to ignite themselves in about two hours or less. Oil cures by oxygen polymerization, which is a fancy way of saying 
oxidation. Remember high school chemistry? Fire was described as “rapid oxidation.” This is the slower version of the same thing, and it too generates heat. In fact, it generates enough heat to catch the rags on fire. It can happen with any drying oil or finishing oil: linseed or tung oil, oil stains, and even oil varnish, polyurethane, oil based filler, or danish oil mixtures.
To prevent the heat buildup, lay out oily rags ONE LAYER THICK. Drape them over the edge of your workbench, trashcan, or even a clothesline. That will allow enough air flow to dry them quickly AND to dissipate the heat as it is generated. When the rags are stiff and crusty, usually in 24 hours, no liquid oil will be left uncured. At that point they are landfill safe and can go into the household trash.
Please, take extra care with oily rags. Lay them out to dry and don’t let them sit in piles. I don’t want to hear that any of my readers has had a spontaneous combustion fire in his or her shop.
Gentle reminder: A modest donation to this finishing blog can keep it going to help others. Thanks!
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