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The autoignition temperature of a substance is the temperature at or above which a material will spontaneously ignite (catch fire) without an external spark or flame.
Rags soaked with linseed oil are notorious for spontaneously (auto)igniting. The rags provide a high surface area for atmospheric oxygen to oxidize the hydrocarbon components, an exothermic (heat-releasing) chemical reaction. As the pile heats up, the rate of oxidation increases further until the temperature within the rag pile reaches the autoignition point. Such fires are unfortunately commonplace. For example, a 1991 fire started by the autoignition of 25 lbs of linseed oil-soaked rags gutted an office building and killed three firefighters.
Fire codes, therefore, require spontaneously combustible rags and other autoignitable materials to be stored in covered receptacles that limit oxidation and self-heating by excluding oxygen.
Other hydrocarbons such as turpentine, fish oil, lard etc. are also capable of autoignition. Always treat oily rags of any sort as a potential autoignition hazard.
Autoignition temperatures are determined specialized equipment that is very similar to that used for flash point determinations.
Storing a substance anywhere near its autoignition temperature is a severe safety hazard. Be careful storing substances in hot areas such as 1) sheds or cabinets exposed to direct sunlight, 2) adjacent to furnaces, hot water heaters or boilers or 3) places where flames or heat are often used.
Knowing a substance's autoignition temperature is also very useful in the event of a fire.
See also: combustible, flash point
Additional definitions from Google and OneLook.
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