Apologies if I display my archaeological ignorance here but I think it may be of value to have a geoscientist's comments. I concur with the fact that the gold-silver solid solution series is complete, but the addition of a little copper (or other impurities) to the solid solution can change this. Copper is relatively cheap, it will reduce the melting temperature (by up to 250 degrees) and increase the strength and hardness of the gold and thus give many advantages for working if the gold is melted. But it will not be homogeneously included in the solid metal, in which case you may see structures developing that relate to decomposition. Also, in my (limited) experience, gold is usually very efficiently recycled throughout archaeological time, and because of the high melting temperatures and malleability, the gold is commonly mechanically worked rather than melted. I don't know if this will be relevant for coins, but if it is, then you will see a variety of textures relating to the mixing of gold from different sources (with different original silver contents) as preexisting objects have been worked into shape. In that case, I would expect etching to be a very effective way of enhancing structures.
Jens
-----Original Message-----
From: Arch-Metals Group [mailto:[log in to unmask]] On Behalf Of DAVID SCOTT
Sent: 07 December 2009 23:05
To: [log in to unmask]
Subject: Re: microstructure of electrum
Electrum, since the gold-silver alloy series is a solid solution, should be homogeneous, but it is possible that cast materials may show some segregation, but I have only really observed that as far as I recall in materials with a little copper in them, then with more copper, they can be wildly out of equilibrium, otherwise we expect a solid solution. Useful information can be obtained from the electrums by etching in potassium cyanide/ammonium persulphate mixtures as grain boundaires, grain size, strain lines, etc , twin lines, distorted twins all show up....occasionally I use aqua regia at various dilutions for etching these, so the answer is yes! Useful information is obtained by etching electrum, most of my examples are unpublished or obscure, will tackle some publication of them in a year or so. Let me know how it goes. Don't forget that the gold alloys are very soft and easily distorted by cutting or polishing. Electropolishing is used sometimes to get over this, or etch/polish techniques. Sometimes etch-pits on the surface of the etched section have to be accepted as the best result if the grain boundaries and other features can be seen.
On Today 2:07 PM, [log in to unmask] wrote:
> Hi David,
>
> Is it possible to etch electrum and get any useful information? Would
> it almost always be solid solution... would that depend on the ratio
> of gold to silver?
>
> Do you know of any articles/results or photomicrographs or anyone who
> specializes in studying electrum? I'm doing some research on some Lydian
> electrum coins in the Harvard collection.
>
> Thanks,
> Jill
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