NEI Handtools Casting Flux (10 gram)

Carnuba flake casting flux used to remove most zinc, copper, brass and foreign materials from molten metal. Melted lead temperature should be as low as practical, about 620 – 655 degrees. Use only a few flakes per fluxing. Stir lead and allow dross to rise to surface for skimming.

This is just the best stuff for getting perfectly clean lead.