[Avodah] The Burning of the Golden Calf

[Moshe Y. Gluck]

[I'm not sure if this is an Areivim or Avodah question, so I'm sending to
both and the respective moderators will decide.]

The Pasuk (Shemos 32:20) recounts that Moshe burned, then ground the Eigel.
Ibn Ezra explains that 'burned' doesn't mean 'melted' as one might have
thought (given that the Eigel was Zahav), but that there is a substance that
when inserted into a fire with gold makes it irreversibly black. Does anyone
know what this substance is?

KT,
MYG