The cause of the fire was determined to be from the spontaneous combustion of some paper towels, that were soaked with old paint remover. The owner had been out in the barn stripping a piece of furniture on a cool summer morning. As he was wiping off the remover with the paper towels, he simply tossed them on the floor, so he could throw them away later, all at once. Stripping furniture was not new to him. He had had done this hundreds of times. He had a system and it worked fine for him until that one particular day. When he was almost done with the stripping he became sidetracked and left the paper towels where he had thrown them. Later on in the day the weather warmed up. By the afternoon it was hot outside. The direction of the the light from the sun changed and it started to shine inside the barn window. It is believed that at some point the sun managed to shine through the barn window and into a large mirror not far from the pile of paper towels. That is all it took. It was over in a single afternoon.
So here is my simple tip of the week, to keep this from happening in your workshop. Do like I do. Keep a metal bucket handy, about half full of water. As you discard old paper towels and rags, throw them in the bucket of water, and let them soak until they are ready for your trash pickup. Also, be especially careful with old steel wool. Steel wool is what they sometimes teach boyscouts to make campfires with. When steel wool is contaminated with something like mineral spirits or paint thinner it makes an easily ignitible combination. So, also throw your old used steel wool in the bucket when you are done with it too.