Since the output from the stamp mill will require further purification.
A collector metal such as lead has a much lower melting temperature than gold and gold willingly alloys with the lead.
The button removed from the cooled slag then cupelled, as the lead melts the oxides shed off being absorbed into the bone ash leaving behind a button of gold with perhaps a few other metals of value ie: silver and platinum.
Gold and silver are relatively easy to refine, it's the platinum sisters that can cause you health issues if handled improperly.
