Sand made of crocum [ferri]
You can use the mixture made of crocum ferri to cast silver or gold, this mixture will make these casts firm, and prevent these from cracking and making burrs. And I believe that for flat things, it would withstand severa casts. Which however is not commonly done for gold and silver.
at left bottom margin of folio 134r
Try crocum ferri for lead and tin.