: Does anyone know?

: Solvents, as they evaporate rapidly, take heat out of the environment as they
: change from liquids to vapor. They seem an unlikely candidate for
: combustion without an outside flame source.

: What process is going on that leads to spontaneous combustion?

Oxidation.

It is the chemical name for things we also call "rusting", "burning", and in the case of paints and some oils, "drying".

It is not the evaporation of the solvents, but the polymerization of the oils in the presence of air which creates heat. If that heat is trapped in an area where it can build up then a fire will occur when the temperature exceeds the flashpoint of the oils, or any associated solvents that might be nearby. A stack of oily rags might build up a temperature that is high enough to ignite some other solvent in the area, even if the temperature in the pile of rags in not high enough to ignite the rags.

For example: Those who have followed the discussions on finishing have noted the differences between two types of linseed oil. Plain linseed oil, and boiled linseed oil. These oils, and many others, combine with oxygen from the air to form linked chains of molecules that are the base for the flexible film that binds pigments in oil-based paints.

Plain linseed oil it will take forever to harden, however, boiled linseed oil will "dry" in a few days as the process of boiling it actually cooks it a bit and starts the polymerization process -- which continues in the presence of oxygen.

Think of cooking an egg and you'll get a vivid mental picture of how low heat can make a big change. You go from a runny raw egg to an omelet. Or, when epoxy resin is mixed with hardener you get polymerization of the plastic resin, with the release of heat, which also speeds up the process.

When you get a pile of oily rags the inner ones are well insulated by those on top of them. There is little air flow to cool things down, but enough airflow to bring some oxygen in contact with the oils. These react by slowly changing their molecular shape, and by releasing some heat -- which is retained in the stack of rags. If allowed to continue the heat can raise the temperature of the pile above 400 or 500 degrees fahrenheit, which will do it.

Similar problems happens in compost heaps and haystacks.

Hope this helps.

PGJ