Is there such a thing as spontaneous combustion?

Dear Cecil:

Is there such a thing as spontaneous combustion? I'd heard it was a myth.

Cecil replies:

Spontaneous human combustion is a myth. (See my column on the subject.) Spontaneous combustion of other things is all too real. Often it involves vegetable oils such as linseed oil or tung oil, but sometimes just damp hay is enough. In the typical case, somebody refinishing furniture or a wood floor piles a bunch of oil-soaked rags in a bucket, corner, or other confined place overnight. The oil oxidizes, a process that generates heat. (In the case of damp hay, bacteria start digesting it, which also generates heat.) If the heat can’t dissipate, it builds up, dramatically speeding up the oxidation reaction and generating still more heat. Eventually the oil, rags, or whatever other potential fuel is on hand reaches “autoignition” temperature — that is, the point at which the stuff bursts into flame without the need of an external ignition source. This can occur at surprisingly low temperatures, only a few hundred degrees above room temperature. So spread your rags out to dry in a well-ventilated place, lest there be a hot time in the old house tonight.

Send questions to Cecil via cecil@straightdope.com.

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