Auto-ignition is not spontaneous!    • AVE Almost Burned My Shop Down! - Oil...  
More rigorous test of oily rags catching on fire:
-Toxic amounts of phenol at low temperature 230F - 110C
-Can take a long time, depends on ambient temp. 24-36 hrs at 60F - 15C.
-Carbon Monoxide may aid ignition
-Cotton faster than paper.
-Prodigious amounts of smoke prior to ignition
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