Thanks, George. I was tapping away on a tablet, so my message perforce had to be short.
Yes, don't keep around rags used to mop up spilled linseed oil, especially if they're cotton, unless you have facilities for keeping them under water, which is what oil painters do. They can combust spontaneously in the presence of oxygen, that is, air.
However, sprayed- or painted-on linseed oil is safe enough not to be considered a hazard even by the nanny-state EU, and I don't know of a single fire starting on a painter's canvas in half a millennium. After the linseed oil has dried the residual layer is perfectly safe. Drying is a slow process that goes on for years but the residual layer after the volatile components have evaporated generally feels touch dry within a handful of days and oil painter practice is to varnish over linseed-mixed pigments from six months upward, when an oil painting is "presumed dry".
