With linseed oil in a rag, for example, if you hang the rag off a stick, or spread it out, it wont ignite because the heat generated is dissipated by the surface.

But if you wad it up in a ball, then the internal heat cant dissipate as fast as it's generated, and it will ignite.

: 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?