The term “spontaneous combustion” may seem like an idea from a fantasy movie or tabloid newspaper, but in fact, spontaneous combustion is a serious source of fires in home and garage workshops, as well as on farms. According to the U.S. Fire Administration, spontaneous fires are one of the leading causes of fires in agricultural storage facilities (i.e., barns, silos, stables, etc.).

How Spontaneous Combustion Occurs

The name is a little misleading, though. Spontaneous combustion doesn’t occur without cause. All fires, including those ignited “spontaneously,” require three elements: fuel, oxygen and a source of heat. Normally, we think of a source of heat as something with an open flame, but, in spontaneous combustion, there is no flame causing the heat.

Spontaneous Combustion and Rags

Spontaneous combustion becomes a possibility when flammable finishes like linseed or tung oil combine with air and oxygen in a natural chemical reaction that creates heat. In farm situations, the chemical reaction involves organic materials like hay, straw or grains beginning to ferment or break down—a process that creates natural heat. If you have ever noticed the heat generated in a gardener's compost pile, the same principle applies to hay or straw stored in a barn.

In open-air environments, the heat generated from these natural chemical reactions is usually not a problem, and may not even be noticed, since the heat is easily dissipated and never builds up to a temperature that can ignite the materials. But when the oxidizing chemical reaction is confined in a way that prevents the heat from dissipating—such as when oily rags are bunched up in a closed area—it is possible for the heat to climb to a level that will ignite the substances. If other combustible materials are nearby, this little act of magic can quickly develop into a full raging fire. The reason why there are so many instances of farm fires caused by spontaneous combustion is that substances like hay and straw have a relatively low ignition point, to begin with.

Preventing Spontaneous Combustion

Preventing spontaneous combustion from occurring is as simple as practicing a little routine housekeeping. Anytime you have an oily rag left over after some wood-finishing or another project, hang it up to dry, preferably outdoors. You can use a clothesline or a fence, but just be sure to isolate each rag individually. Don’t pile them on top of each other. And if you need to hang them indoors, keep them away from heat sources, such as water heaters or furnaces.