Ki Tisa - The Burning of the Golden Calf

A poster on Avodah asked the following question:

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?

IANAMetallurgist, but it sounds like it could be sulfur (or some natural compound containing such). Although gold is inert to the majority of chemical attacks, most gold jewelry contains a significant fraction of other metals such as silver and copper, as pure gold is too soft and malleable to be very useful. These alloys were also known in the ancient world. Metal sulfides have inferior mechanical properties compared to the metals themselves, so that once the surface of a piece of metal is sulfidized, it would crack and flake off, especially at high temperatures (and if agitation is used), revealing a new clean layer of metal to be attacked, until eventually one would get a blackish powder. I'm not sure how long this "eventually" would take, but if the metal was concurrently mechanically ground to maximize its surface area (as per "vayitchan ad asher dak"), the rate of the reaction would be accelerated.

I would not have thought that this process could cause a color change in gold of a relatively high purity, but I found a report by the World Gold Council which reported a phenomenon of the blackening of gold in the Middle East and India at purities considerably higher (up to 22K) than that which tarnishes elsewhere in the world, and suggested that the cause may be significant exposure to foods and spices that are high in sulfur compounds. At high temperatures and using large amounts of the sulfidizing reagent (i.e., an amount of mass equal to that of a significant fraction of the non-gold components), the
reaction may well proceed at a much faster rate.