If you are convinced the junk you are scraping off the top is actually lead, then save it in a tin can and drop it into your next smelt.
I cast with clean ingots in my basement with a vent fan. I don't use wax indoors, I switched to a dry 3/8" dowel about 12" long. I stir the pot with this and make sure to scrape the sides of the pot while doing so. Then I use a large kitchen table spoon to scrape the top of the melt. I put this slag into a tuna can for months and years. Eventually I convince myself that this is not lead but actually contamination and not wortth trying to incorporate into a smelt.....then I throw it into the trash and forget about it.
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