Recently, I've taken an interest in composition analysis of silver coins. It's intriguing to see what's really in a coin, and how this might reflect on mint practices and where they sourced their metals. One thing that immediately strikes me is that Sasanian silver has a detectable amount of gold--something which you don't always see in modern silver or even in the Roman imperial silver I discussed earlier. I'm unsure how much Sasanian coins are faked, but perhaps the gold content could be a way to tell genuine coins from modern fakes? XRF will undoubtedly play a larger role in forgery detection in the future. Below is a brief table of some results of the analysis--the whole paper can be found here (pdf).
What I see from this chart is there are many rulers and many mints but I see no obvious pattern. There are two kinds of Sasanian fakes. Tourist grade fakes might be of any ruler and might be detected by the gold question. Fakers of Sasanian rarities might see that the best way to replicate the appropriate metal would be to melt or overstrike poor examples of common rulers. A fake Buran on a flan provided by an authentic but junk Xusro II would not be exposed by this test.
Where did the Sassanians source their metal? If found in nature, its common enough to find a silver-gold alloy known as electrum. Heck, the first coins were made from electrum. I would have thought metallurgy would have progressed to the point where they could remove most of the gold, but its not unbelievable that there would be a bit left. Do you have any fakes that you could test, to see what they are made of?
I read the paper, although not with complete understanding of some passages due to not having read the cited reference papers and not having knowledge of mining and extraction methods. I wonder why their sample size was so tiny? Those Sassanid drachms are available in massive quantities. Maybe they were limited to coins with documented find locations? Still, I'd think there would be considerably more available for nondestructive study. The resulting graph looks pretty but it seems premature to draw conclusions when only one or a few coins per emperor or mint were studied. As for the data's utility in identifying fakes, Doug already mentioned the biggest problem. Kurt-- can you tell me more about the detection depth of XRF? I wonder how accurately it detects a coin's true composition if the coin has been cleaned. For instance, if you clean an Ag-Cu alloy with acid, copper will be removed from the surface, causing the surface to have a higher apparent silver content than the interior ("depletion gilding"). The coins in this study were all soaked in formic acid. I do not know the effect it has on other elements, but it would also lead to a higher apparent percentage of gold at the surface.
I recall once owning a Greek silver coin that under a microscope showed a tiny speck of gold. I was told by person I trusted that the issue was known to have high gold content due to impurities they lacked the technology to remove. I'd have to see a lot of tests before I'd have an opinion on whether 1% is a lot or just normal.
i wondered why they only from arch. sites, i assume they wanted coins only from digs so they would avoid a situation like DS suggested, or coins that have been subjected to a wide variety of cleaning techniques. it would be cool to see a sample of coins from each ruler, mint that allowed you to say something statically about coins in general vs. just these coins studied. bit still..very interesting.
Good point, chrsmat. If the coins were taken from the site and all cleaned in a manner that didn't affect the surface composition, the XRF data might be more meaningful.
A few years back I was offered the opportunity to have some of my coins tested for metal by having a tiny hole drilled in the edge. I declined. Proper XRF will require having a spot polished on the edge so the reading would be coin not patina. I'd decline. I do not accept any surface sampling method as valid because we know many coins were struck on flans prepared with acids (on purpose or just as a factor of handling). Surface enrichment and leeching of metals from burial conspire to make the surface different from the core. How significant is this? That would require a study comparing the surface results with fully destructive results on the same specimen. Not on my coins! When someone perfects a method of sampling subsurface composition remotely AND has demonstrated that the results do agree with destructive testing, we might consider it. As it is, I see numbers like 96.1 plus or minus 4.8 troublesome. Warren: As a statistician, can you explain the math here? Disclosure: I did not read the paper.
While I understand your point, surface contaminants, patina, and toning would definitely not add gold, iron, silver, or lead. Any surface contaminants would be things like carbon, silicon, or sulfur.
Another curious tidbit from the paper... None of the coins they examined had any sulfur. This lack of sulfur led to their conclusion about which ores were refined and used in these coins. No sulfur? Maybe it was depletion-gilded away when they cleaned the coins with formic acid.
From what I've read, many XRF units only detect the heavier elements, or perhaps in this case it was programmed to provide data for a specific range of metals? I would have to look up the specific unit they use and see what elements it can detect. Regarding detection depth and patina issues: I and a fellow collector plan to analyze a bunch of US silver coins with XRF. To eliminate the patina issue, I plan to use junk silver with a large selection of dates, and prepare the test area by polishing off all the surface patina. I believe the paper I posted discussing Roman imperial silver discusses the issue with surface leaching/patina.
But At least this can be used to detect the cheaper sasanian coins . I dont think a faker would create a special metal based on these percentages just to fake a coin that worth few hundred bucks or less.
Cool thread and article. A friend has a book this that I would use to reference the effective depth XRF testing. Although not a ancient coin book here is great reference for XRF: https://www.amazon.com/Counterfeit-Portrait-Eight-Reales-Real-Reales/dp/0990802906
