I've seen a batch ruined by zinc wheel weights. The alloy won't fill out properly in a mold afterwards and is useless, but it sure does make a pretty rainbow on top of the liquid alloy. Some say it can be fluxed and skimmed off, but I don't think it can.
The key to avoid this from happening is the barbeque method, you cook down the wheel weights low and slow just like making good ribs. There is enough variation between the melting point of lead and zinc to where you do not have to pre-sort the weights prior to making ingots, but a good thermometer is a necessity. The zinc and steel weights, the valve stems and caps, wheel weight clips, they all float to the top and can be skimmed off easily.
I've not cast ingots in over ten or twelve years, and I still have a huge stash left. In this photo, I had cast just over 500 ingots from scrap wheel weights. Those little ingots have some heft, so there is a massive number of bullets just from this one session.
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