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Most everyone's heard about piles of oily/greasy rags igniting. Linseed oil seems to be a popular culprit for it, but I've also recently heard claims of common automotive oil being involved. I've run across a couple recommendations, but I have no idea if there are any better ways to handle them. Also wondering if there's a certain threshold as to quantity of material required for them to ignite. Is it a problem just with natural oils, or are synthetics also involved?

Recommendations I'm aware of include:

  • Seal rags in side a metal bucket (with lid).
  • Allow rags to dry (spread out), then throw out normally.

The metal bucket sounds good, but what happens when you have to transfer them into a trash bag? Spreading them out to dry seems unproductive, especially with synthetic auto oil which never seems to dry.

I've experimented with putting a pile of synthetic auto oil soaked rags in a plastic bag (worst case scenario based on my reading) and not only did they NOT catch on fire, but there was no detectable increase in temperature. I've seen so much conjecture on this important topic, hopefully there are people on hear that can bring some science to bear on the problem! :-)

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I've read this only occurs with animal and vegetable (Linseed oil) oils, not mineral oils (motor oil). –  Tester101 Feb 28 '12 at 15:44
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If you're using a metal bucket it needs a hinged, heavy lid. Then if you do get combustion, you drop the lid and smother the fire. But I wouldn't recommend it, unless you're simply batching the rags for proper disposal later in the day. –  Alex Feinman Feb 28 '12 at 16:22
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What ever you do, do not attempt to clean them in a washing machine. The oily towels are thrown in, the water comes on, the water fill the tub with water. The fumes are pushed out of the wash tub. Then the motor fires up. KABOOM!!! –  lqlarry Feb 29 '12 at 1:46
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@lqlarry - is combustion in a washing machine really a problem? Motor oil vapor doesn't seem particularly flammable. When I worked at an auto repair shop, we tossed soiled rags into a metal trashcan, which got dumped into the normal dumpster and never had a problem with self-combustion. This probably wouldn't be allowed today - the oily rags probably need to be treated as hazardous waste. We didn't launder our rags, but I did end up with oil soaked clothing at times, that I'd just launder in the normal machine with extra detergent. –  Johnny Mar 6 at 1:15
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@Johnny - I know that flammable rags have caused problems in laundromats. Maybe lightly saturated might not cause a problem, but explosions have happened due to flammable rags and the spark of a motor starting. –  lqlarry Apr 13 at 8:22

3 Answers 3

up vote 8 down vote accepted

The trouble with linseed oil, especially boiled linseed oil, is that it generates heat as it dries. If you leave BLO-soaked rags in a pile, the drying process proceeds but heat is trapped. The drying process accelerates as the rags warm up, and the temperature rises to the point of ignition. Spreading the rags out lets the heat escape so that the drying process can proceed safely. I've heard of people just spreading the rags out on their shop floor without problem, but I'd suggest putting them outside just to be safe.

I don't know if the risk of spontaneous combustion is as high with motor oil, but whether it is or not the fact remains that oily rags are highly combustible and should be handled with care. It's worth putting them in an appropriate container to prevent sources of ignition from reaching the rags, to limit the oxygen that reaches the rags, and to contain a fire should one occur.

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to avoid fire you can do a few things:

  1. remove fuel: not as easy, but washing the rags and disposing of the waste water properly should reduce the risk here

  2. remove air: storing the rags in a sealed can or under sand or submerged in water (you can add detergent to wash them at the same time) will take care of that

  3. remove heat: if you keep the rags under the combustion temp there will be no combustion (a fridge or freezer will do).
    You can avoid heat buildup from curing (as mentioned by Tester101) by spreading them out or hanging them on a clothesline (remember to properly ventilate)

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The few times I've worked with substantial amounts of drying oils, I used the "bucket of soapy water" method. A water-dampened rag is definitely not going to catch on fire. –  Mark Bessey Feb 28 '12 at 18:44

From the Material Safety Data Sheet (MSDS) of Klean-Strip® Boiled Linseed Oil.

Flammable Properties and Hazard

RISK OF FIRE FROM SPONTANEOUS COMBUSTION EXISTS WITH THIS PRODUCT. Oily rags, waste, and other oily materials can cause spontaneous combustion fires if not handled properly. Immediately after use, and before disposal or storage, you MUST (1) Spread out all oily materials outside to dry by flattening them out to their full size in an airy spot for 24 hours at temperatures above 40 degrees F, or (2) Wash them thoroughly with water and detergent and rinse. Repeat until you have removed all oil from all clothes, tools, rags, paper, clothing, mops, and any other materials contacted during use or as a result of an accidental spill. Make certain all wash and rinse water is disposed of properly.

It also says the Flash Point is 250°F, but has "No data" on the Autoignition Temperature.

The principle at work here is that Linseed Oil cures due to Reduction Oxidation, a chemical reaction which creates heat (Exothermic Reaction). If the heat cannot dissipate quickly enough (insulated by a pile of rags, for example), the process can accelerate creating more heat and eventually resulting in combustion.

It's always a good idea to check the MSDS for any chemicals you use.

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MSDS on motor oil "Flammable Properties: Flammable.". No mention of spontaneous combustion. –  Tester101 Feb 28 '12 at 16:33

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