Lead's affinity for oxygen in the cupellation refining process
Indeed if you do the cupelation, the lead will form litharge and some parts of the litharge will volatilize.

But usually the cupel is made out of bone matter - this is very porous.

While the molten lead and the gold will stay in the cupel, the litharge will be absorbed by the cupel like a sponge.

If all lead is converted to litharge and all litharge is absorbed by the cupel only the gold shall be left - so far to the theory.