Jyrki I searched for your paper but was only able to find your brochure on PMI (pretty slick I gotta say!). If you could please email it to me this would be very appreciated.
Shawn, the devil's advocate in me was skeptical from the get go but I
had to give the analyzer a tip of the hat when it successfully identified some exotic "stumpers" I threw its way (zirconium, cobalt and tungsten). I did stump it with a pure sample of beryllium and another with the
rare earth metal erbium but, to be
fair, in both cases I knew these were out of spec beforehand. Also, if the machine was unreliable I would expect
poor scores on repeatability which wasn't the case. All the same, I grilled the tech on handheld versus benchtop analyzers and it seems that the key
part of the hardware, the vacuum
tube and X-ray source, as well as the
software, are the same and that only the form factor changes. He could've been misinformed or outright lying, of course, but it does make some sense that the manufacturers would streamline their parts bin to cut down on development and manufacturing costs. He explained that what you get as you go up in
price, up into the six figures territory, is more resolution. More resolution is nice - that being the ability to discern even the smallest amounts of trace elements - but is not critical to the greater
part of answering our most pressing questions.
And just to be thorough, in case anyone
still reading this
thread isn't bored to tears yet, there are actually three processes at play that prevent an easy understanding of the composition of ancient silver coins. First, as already discussed, the flans
had their silver content artificially elevated on their surfaces via annealing (and possibly acid washing). Secondly, a heated silver-copper
alloy when struck will have a higher amount of silver concentrate at the point of impact in a process I don't understand well at all but which is due to the limited solubility of copper and silver with each other while in a molten state. Whatever the chemistry at play, the bottom line is that it results in precipitation of microscopic blobs richer in silver and counterpart
grains that are richer in copper to essentially form two
alloy sub-groups. In the blink of an eye somehow these two stratify. The third and final process finds the surface copper slowly eroding through oxidation, molecular migration and mechanical stress. With the steady deterioration of these copper-rich
grains comes a weakening of the overall
matrix through internal fracturing. You see this popularly defined as "
crystallization" and is the phenomenon of the brittle ancient silver coin that can shatter if mishandled. What is as yet undetermined, and this is the key point, is what a hypothetical section from surface to core would look like. Is it Ag-rich only in the first few microns at the surface with the rest fairly evenly distributed or is the core, like the surface, especially rich in copper and other base metals and only the middle layer relatively homogeneous?
Given the above, I remain very skeptical of Woytek's figures. The data sampling of 71 coins is great so long as you trust the methodology is sound... except this is precisely what is controversial! Just as it is inaccurate to measure the surface and extrapolate from there it is, or at least I believe it to be, equally inaccurate to measure the hyper-debased core and make the assumption that it's representative of the
bulk of the material. I really have no personal investment in saying
his results or mine are ultimately correct, I just want to know the facts. When I can again get the opportunity to use an analyzer I intend on running tests that I think will better determine the most authoritative measurements by taking a sample coin, testing
as is, wearing it down and testing it again. I will repeat this a few times over and from that data generate a plot of the silver-copper gradient. Then, to be truly thorough, melt the coin (in an inert atmosphere!) and test it one final time. I suspect, but do not yet have proof, that the "sweet spot"; that is, the point where the two lines of each of these elements bisect on a graph, is below the surface but above the core. In other words, simply surface testing a worn coin should give a reasonably reliable analogue to the ideal scenario where you melt the coin and test it in its most homogeneous state.
Even in the case where surface analysis is completely unreliable in determining overall composition (for silver coinage) it
still provides very valuable data. Knowing whole-coin composition is important from a historical perspective but surface analysis potentially gives us a tool for determining age, modern and ancient counterfeiting and perhaps even a glimpse into manufacturing processes.
Ras
ps. Melting denarii in an argon-filled chamber, come to think of it, yeah, that's.... that's not going to be an easy task to pull off :-(