What, exactly, spontaneously combusted? Rags soaked with stain? I don't quite get the cause here. I know that enclosed, fresh grass clippings can spontaneously combust, especially on a hot day and in a garage. Since the FD says the fire started on the OUTSIDE of the house, it would really be helpful to know what combusted. Thanks!


Editor's note: An April 27 fire that significantly damaged a two-story house under construction in Portola Valley's Westridge neighborhood was likely caused by spontaneous combustion associated with materials used to stain exterior cedar siding, firefighters said.

Linseed oil is a common component of stains and varnishes. Put some rags soaked with this in a container exposed to air and moisture and the oil will oxidize. If the heat is trapped the oxidation will raise the temperature of the rags to the point of combustion. So...if the container is next to the house....you get the result here. One can only hope the painter has a really good liability insurance policy