[Avodah] The Burning of the Golden Calf

[Joshua Meisner]

On Jan 7, 2008 3:52 PM, Moshe Y. Gluck <mgluck at gmail.com> wrote:
> 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[1]  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.

[1] http://www.gold.org/jewellery/technology/caratage/tarnishing.html

Joshua Meisner