Can oily rags really catch fire?

It sounds like an old workshop myth and it is not. Linseed, tung, danish and hardwax oils cure by reacting with oxygen, and that reaction is exothermic — it gives off heat. A rag spread out loses that heat to the air as fast as it is produced. A rag screwed into a ball, or a pile of them in a bin, insulates itself: the heat accumulates, the higher temperature makes the reaction faster, and the cycle runs away until the cloth reaches its ignition temperature and starts to burn.

Safety

Step by step

1. Know which products do this.Any drying oil: raw and boiled linseed, tung, danish oil, teak oil, hardwax oil, oil-based wood stains and oil-modified varnishes. It also applies to the wax-and-oil blends used on furniture. It does not apply to water-based finishes, emulsion, or non-drying oils such as mineral oil.
2. Lay rags out flat and singly to dry.The simplest and best method. Spread each cloth out fully on a non-combustible surface — a paving slab, a metal sheet, a wire line — outdoors and away from the building, weighted if it is windy. Leave them until they are completely stiff and dry, at which point the reaction is finished and they are safe to bin.
3. Or drown them.Drop used rags straight into a metal container of water with a lid, and keep it outside. Water excludes oxygen and takes the heat away. Empty it with your household waste in line with your council's rules once you are finished with the job.
4. Never leave them bundled, even briefly.A ball of rag on a bench at the end of the day, a wad in a bin liner, a cloth stuffed in a pocket or a tool bag, or a heap in a bucket. These are the actual circumstances in which this happens, and the fire typically starts several hours later when nobody is there.
5. Treat sanding dust from oiled surfaces the same way.Dust from sanding an oiled floor, or the contents of a sander's dust bag after working on freshly oiled timber, contains the same curing oil in a fine, well-aerated, insulating form. Empty the bag outside into a metal container, and do not leave it in the machine overnight.
6. Include applicator pads and buffing bonnets.Foam pads, cloth applicators and floor buffing bonnets soaked in hardwax oil are exactly the same hazard as a rag and are more often forgotten because they are equipment rather than rubbish.
7. Tell whoever else is in the house or on the job.Most incidents happen because someone tidying up put the rags in the bin. It takes one sentence to prevent.

Tips

• If a pile of rags feels warm, that is the reaction underway. Separate them immediately, outdoors, and do not put them in a bin.
• Keep a lidded metal container with water in it in the workshop or garage specifically for this. It removes the decision entirely.
• Once a rag has dried out flat and gone stiff, the oil has fully cured and it is no longer a hazard.

Common mistakes

• Putting oily rags in a household bin at the end of the day — A bin is an insulated container full of other combustible material. This is the exact circumstance in which these fires start, usually several hours later.
• Assuming a garage or a shed is safe because it is outside the house — It is still an enclosed space, usually full of flammable material, and often attached to the house or close to a car.
• Leaving a sander's dust bag full of oiled dust in the machine overnight — Fine dust from an oiled surface is well-aerated and insulated at once — the ideal conditions for self-heating.

Questions people ask