Lead's affinity for oxygen in the cupellation refining process My inorganic chemistry textbook1 says:

Cupellation is a process, where crude gold is taken in a small bowl called cupell and melted in the presence of air. Due to high oxygen affinity of $\ce{Pb}$, it is converted to $\ce{PbO}$ (volatile) and escapes from the system.

Now, the affinity for sulphur and lead is explained by HSAB (soft acid-soft base). But how to account for the affinity of oxygen (hard base) and lead (soft acid)?
References

Guha, Sudarsan. Concise Inorganic Chemistry for JEE (Main and Advanced). 4th ed., Wiley India Pvt. Ltd., 2017.
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.