Hardcase, good advice, no argument from me on it at all.
However, my technique involves a French Fry spoon that is impossible to take away any tin or "good stuff". So I'm not wasting flux on the trash.
Here's some of my Ingot casting tools:
Starting with Wheel Weights my turkey fryer and cast iron pot:
French Fry spoon to get the trash out:
Fluxing with Gulf Wax. Yes it ignites, that's ok.
Stir and scrape the bottom and sides agressively then remove using a tea strainer. The lead goes right through it leaving the "bad stuff" in the strainer: