Strangely, I was recalling the dangers of using wood and aluminium on the same belt sander, when the letter from Canada appeared.
Even using for Steel and Aluminium can be dangerous.
Railways use Thermite, a mixture of finely divided Aluminiium and steel, as a means of welding rails together, because of the tremendous heat generated.
Steel wool will burn if a match is applied. The steel is sufficiently finely divided and there is pleanty of air to provide oxygen for combustion.
It would not be surprising to learn that Alumium swarf will burn quite readily.
Howard