Oily Rags and Spontaneous Combustion

It really happens

Spontaneous combustion of oily rags is not folklore. The National Park Service describes a case where a crew’s rag soaked with linseed oil ignited despite precautions ¹, and the Wood Handbook carries an explicit caution: sponges, cloths, and paper towels wet with oil-based stain, any other oil or oil-alkyd, or urethane finish are “particularly susceptible to spontaneous combustion” ². Fires from oily rags burn down shops, garages, and houses every year, often hours after the work was finished and everyone has gone to bed.

Why it happens

Drying oils cure by reacting with oxygen, and that reaction gives off heat ³. On a tabletop the heat escapes harmlessly. But a rag holds a thin film of oil spread over an enormous surface area, with plenty of air in its fibers, so the reaction runs fast. Crumple that rag into a ball, or pile several in a bucket or trash can, and the heat has nowhere to go. As the temperature rises, the reaction speeds up, which makes more heat. Eventually the rags reach their ignition point and burn.

Linseed oil (raw or boiled), tung oil, oil-based stains, Danish and “tung oil” finishes, oil-based varnishes and polyurethanes, and linseed oil paint all carry this risk. Even cooking oils can do it in large quantities. Water-based finishes don’t.

What to do, every time

Pick one method and make it automatic:

1. Under water, in metal. Put rags in a metal can, fill it with water, and close the lid. Water cuts off the oxygen and absorbs heat. The Wood Handbook’s instruction is to immerse them in water and seal them in a water-filled, airtight metal container immediately after use ². Dispose of the can through your local hazardous waste program or once the oil has hardened, as your municipality allows.
2. Spread flat to cure. For home projects, the National Park Service recommends spreading the rags in a single layer on concrete, out of direct sunlight, with the corners weighted against wind, until they’re stiff and brittle ¹. Once fully hardened, they’re inert. Hanging rags individually over a line outdoors works the same way.
3. Approved containers. Shops using a lot of rags should use a listed oily-waste can, a self-closing metal can designed for this, and have it emptied regularly.

Never:

• toss oily rags in a trash can, bucket, or cardboard box;
• stuff them in a plastic bag;
• leave them in a pile on the bench “until later”;
• throw them in a dryer, or wash and dry oil-soaked shop towels in a home dryer.

The same goes for

• Paper towels and sanding dust soaked with oil finish.
• Steel wool used with oil finishes; steel wool can also self-heat.
• Wood shavings and sawdust that have absorbed oil.
• Brushes are less risky, but store them in oil or clean them; don’t wrap oily brushes in rags.

What to take from this

Curing linseed and other drying oils give off heat; a crumpled pile of oily rags traps it until they ignite, sometimes hours later. Every time, either seal the rags under water in a metal can or spread them flat, one layer, outdoors on concrete until stiff. Never put them in the trash, a bag, or a heap.