I use only 100% pure bees wax, I use about the size of a Pea for every #3 of lead I have melting.
It'll smoke so keep away while it cleans, use a shallow paddle to skim off any floating stuff. Stir up from the bottom and repeat and
the lead will be clean as new.
The hotter the mold the cleaner the casts will be.
And make use there is plenty of Ventilation, work outside if possible.
Aloha Nui Loa