When I worked at an aluminum plant, all types of metal came in as alloys. While tin would alloy w/ aluminum, lead wouldn't. It would hang, to a certain extent, in between the other metals, but if there was very much, it would run right on through the ingots. SO, if you can melt aluminum and wheelweights, for instance together, the aluminum alloy, whatever it happens to end up being, will solidify first while the lead won't alloy with aluminum, hopefully the tin would, and you'd have lead running out. Efficiency is the big question, but it might be fun to try with a forced draft charcoal fire.