[sidewindr]

I meant to say sludge not slugs, as stated above it would turn into an "oatmeal" you'll know it, there's no mistaking it. Unless the ww were melted at a high temperature the zinc just floats and can be scooped out. Most know this and the risk of it being there is low, but some don't.They usually are those that don't cast themselves.
I have some good scrapyards nearby and I smelt my own now also, I use my thermometer and never get any hotter than 650F, even down to 575F. I can seperate certain brands of ww that have higher pure lead %
because they will not melt until around 630F , so they'll float also,they are not zinc. I now have a couple piles of ingots that are sperated by ww brand( most ww have a stamp or pattern on them so it's easy to sort them out) ,some are older and have higher antimony and tin. Someday I plan to do some testing to see just what the difference really is.
If zinc is mixed in, skimming the oatmeal can get some of it out, how much I don't know( a small % of zinc is usable as bullets) but to get good clean lead requires powdered sulfur mixed in outdoors and downwind. It is doable, but not something I'm interested in. I would just find something else .to cast out of the mixture(heating it high enough to fully melt the zinc).