I have made aluminum powder in a ball mill a few times, but my last batch began to smoke and caught fire soon after I poured it from the tumbler. Nothing unusual was afoot. It was poured into a paper plate. The ball mill aggregate was steel bearings and a few rounded rocks, same as I had used before. The only thing different was I didn't place any charcoal along with the aluminum during the milling? So why the spontaneous combustion?