Cate, just to expand on what Codeman said about storage of oil-soaked rags, spontaneous combustion can occur when a bunch of such material isn't stored in an air-tight container. There always is some low level of oxidation of the solvents going on, with release of heat. When the rags are spread out, the heat easily dissipates, with no increase in temperature and all is well (provided there is no ignition source). When the rags are bunched together and not sealed off from air, then oxygen in the air diffuses through the mass to sustain further oxidation, and also the bunched up rags are somewhat self-insulating. This results in an increase in temperature, which in turn raises the rate of oxidation and temperature increase. For every 10 degrees (centigrade, I think), the reaction rate roughly doubles. Of course, as the temperature goes up, so does the rate of heat dissipation. But heat loss goes linear with temperature, while the rate of oxidation is exponential. Given sufficient volume of solvent-soaked rags, the temperatures scoots up to the point where the mass ignites with visible flame. Burning is just a fast rate of oxidation.
Keep the rags spread out in a ventilated area or in a very tightly closed and preferably metal container until you can get them to the dump.
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