An ordinary oily rag left behind after a repair job or after finishing wood can start a fire. Fire safety experts warn that cloth soaked with vegetable or other organic oils can ignite by itself within just a few hours.
Houses, sheds and garages, as well as woodworking shops and furniture shops where wood is treated with oil, can burn down without any open flame and without a shorted wire. The cause is oily rags that are not disposed of properly and go into the trash can with the rest of the garbage. Let's look at why oil makes materials ignite and what to do with used rags.
Can Oily Rags Really Catch Fire?
Yes. A typical case: someone oils a wooden staircase or stains a deck, balls up the rags and drops them on a pile of wood scraps or into a trash can, then leaves for lunch. The pile heats up from the inside and starts to smoke within hours, with no flame or spark anywhere near it.
Do not throw oily rags into the trash can, and never leave them in a pile. Oil-soaked rags left crumpled or heaped together can heat up on their own until they burst into flame.
Why Oily Rags Spontaneously Combust
Spontaneous combustion, the self-ignition of materials at ordinary temperatures without any outside source of fire, is a significant, though not always obvious, danger. Despite the seeming "lack of cause," the phenomenon rests on specific chemical processes of self-heating.
The chemistry of self-heating
Exothermic oxidation is a reaction of a substance with oxygen in the air that gives off heat. Many organic materials oxidize slowly, but the key factor for spontaneous combustion is the balance between how fast heat is produced and how fast it escapes into the surroundings. If heat is generated faster than it dissipates, the temperature of the material gradually rises and eventually reaches the ignition point.
Oily rags are the classic and most often cited example. But the range of materials that can combust on their own is much wider. It includes hay, compost, sawdust and some kinds of coal. What they have in common is a porous structure or a large surface area, which lets oxygen in and at the same time makes it hard for heat to get out quickly.
According to the NFPA's oily rags safety tip sheet, the oils in oil-based paints, stains and varnishes release heat as they dry, and a pile of oily rags traps that heat until it starts a fire. This does not happen with water-based finishes.
Oil on cloth versus oil in a can
When oil is in contact with air (oxygen), it oxidizes and gives off heat energy. But oil standing in an open can or bucket will not catch fire. Its contact surface with the air is small. Porous and fibrous materials soaked in oil, fabric and rags, are the ones prone to spontaneous combustion. A number of conditions have to come together: a certain fabric and how tightly it is crumpled, a certain oil, a certain amount of oxygen.
The conditions for ignition
A material that oxidizes easily is not enough on its own for a fire. Several critical factors have to coincide:
- A large contact surface with the air: a thin layer of oil on a developed surface (cloth fibers, sawdust, porous material) gives the oxidation reaction the largest possible area.
- Enough oxygen: air has to keep flowing to the reaction zone.
- Heat insulation: the material has to be laid out in a way (a crumpled rag, a heap of sawdust) that lets the heat build up instead of escaping into the surroundings. It is the blocked heat loss that lets the temperature keep climbing. Such conditions appear when oily materials are stacked in piles, stacks or bags.
- Air temperature: the process can start at fairly low temperatures, even in cool weather, but warmer surroundings speed up the chemical reactions and shorten the time to a possible fire. The higher the starting temperature, the faster the reaction goes.
- Time for the reaction: the period before self-ignition can run from a few hours to several days or longer.
The ideal combination is gauze or cotton cloth lightly moistened with oil and crumpled up. That gives a large oxidation surface and little heat loss. Enough air reaches the material for oxidation, and because it is crumpled, the heat released builds up and gradually reaches the ignition temperature of the cloth.
Heating an oily material up to its ignition temperature can take anywhere from a few hours to several weeks and even months, which makes it especially treacherous. The material can smolder unnoticed inside a pile or stack until it reaches the temperature of open burning. It all depends on the volume of oily material, the kind of oil, the amount, the air temperature, how heat is exchanged and so on. These are the conditions that often come together on construction sites, in carpentry and paint shops, and wherever oily waste is stored the wrong way.
Which Products Put Rags at Risk
Oils and fats, especially of plant and animal origin, deserve special attention. Their tendency to self-heat is tied directly to their chemical structure, to the unsaturated fatty acids in them. These acids contain double bonds between carbon atoms, and these bonds react easily with oxygen. This process, called autoxidation or oxidative polymerization, gives off heat.
Linseed oil rags and the iodine number
The practical measure of how unsaturated an oil is, and so of its potential to combust on its own, is the iodine number. It shows how much iodine can attach at the double bonds in a set amount of oil. The higher the iodine number, the more unsaturated bonds there are and the more actively the oil oxidizes and releases heat.
Linseed oil, traditionally used to treat wood and as a base for boiled linseed oil, has one of the highest iodine numbers (about 175-192). This makes linseed oil rags especially dangerous. Linseed oil applied to gauze or cotton cloth will catch fire fastest of all. Soybean oil (an iodine number of about 120-140) is less active, but under certain conditions the materials soaked with it can also self-heat. Mineral oils, which contain no unsaturated fatty acids, pose practically no such danger.
The oils that carry the greatest risk:
- Linseed oil, raw or boiled, and finishes made from it.
- Tung oil.
- Danish oil and teak oil, which are usually linseed or tung oil blends.
- Walnut oil and other drying vegetable oils.
In theory, a cloth or sponge soaked with cooking oil can catch fire under certain conditions. So if you spill cooking oil or any other vegetable oil, be careful: the material that soaked it up can easily flare up in the trash can. That is a fire out of nowhere.
Stain rags, varnish and paint
It is also important to understand that driers, catalysts that speed up the drying (oxidative polymerization) of oil paints, varnishes and penetrating finishes, greatly increase the risk that materials soiled with these products will combust on their own. Drying vegetable oils used to make varnishes and paints always have driers added to them, so stain rags and rags used with oil-based varnish need the same care as rags soaked with straight oil.
How to Dry Oily Rags Safely
Understanding how spontaneous combustion works makes it possible to prevent it. The key rule for working with materials soaked with vegetable oils (especially drying oils), boiled linseed oil or products that contain them: never leave them crumpled or piled up.
- Do not leave them in a pile. Do not heap the rags together and do not crumple them; open them up and lay them out one by one so the heat can escape.
- Dry them outside. Hang the rags outdoors one at a time, or spread them out flat on the ground and weigh them down, away from buildings.
- Wait until the oil has fully dried. Even dry, they still go into a closed metal container with water, as described below.
- Keep them out of the dryer. Washing does not get all the oil out, and the heat of a clothes dryer speeds up the same reaction, so rags with oil or finish on them never go into one.
The same goes for foam brushes and sponges used with oil: do not leave them in a heap.
How to Store Used Rags
- Soak them right away. If you can't spread them out to dry, drop them into a bucket of water so they stay wet. Water cuts off oxygen and carries heat away.
- Use a metal container. Put dried rags in a metal container with a tight cover, filled with water and detergent.
- Keep it cool and shaded. Keep the container in a cool place out of direct sunlight and away from anything that burns easily.
- Store the products properly too. Clean wiping materials and the oils and finishes themselves should be stored according to the manufacturer's instructions, without soiling them or stacking them in conditions that encourage self-heating.
Disposable paper towels can be an alternative to oily cloth rags, but they need the same handling.
How to Dispose of Oily Rags
- Keep them out of the regular trash. Do not throw rags into the household trash can, especially when they are balled up.
- Treat them as hazardous waste. Take them to a household hazardous waste drop-off or haul them separately from household garbage.
- Check local rules. Check with your city about disposal. Rules in El Cajon and other cities can differ.
Garage Fires and Smoke Damage
Garages, workshops and job sites are exactly where oily rags tend to be left: after staining a deck, oiling furniture or refinishing doors. Even a small fire that is put out quickly can leave soot and smoke odor on walls, stored items and the house next to it, and water used to put out the fire adds its own damage.
If a fire has already happened, fire and smoke damage restoration services cover soot cleanup and odor removal, and water damage restoration after firefighting deals with the water left behind. Belongings from a smoky garage can be moved out and cleaned through pack-out and contents cleaning. For a different kind of household fire, see our guide on cleaning up after a kitchen fire.
You can read more about our restoration team, see what El Cajon homeowners say, browse more home safety articles or go back to the El Cajon restoration homepage.
Spontaneous combustion is not a myth. It is a real danger that comes from basic chemistry. Vigilance and strict handling rules for oily materials are the best protection against "fire out of nothing."