When you drop zinc on a hard surface (like concrete) it goes 'tink'. When you drop lead it goes 'thunk'. If you think you won't tell the difference between a tink and a thunk, try it sometime. You won't mistake the two.
Zinc melts at a higher temperature and is lower in density. Melt the lead with moderate heat and the zinc will float unmelted.
I tend to set aside any that I have doubts about and throw them in when I've got an inch or two of molten lead ..if they float then I skim them back out.
Zinc weights usually have rivets on the back, rather than having the clip cast into the weight too. I pick out the ones I notice and just scoop out the ones I miss with the rest of the clips.
Lead melts at 725 degreees F. If you'll run your smelting pot at 725-750, you'll be well under the zinc melting point and you can simply scoop them out with the steel clips.
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