All That Glitters, Phase 4

The times continue strange in UK higher education, as you may have seen. Many of us are on strike for what is now the third week, more of us than ever now, and the employers’ representatives appear to be refusing to negotiate in person and then changing their mind by Twitter overnight. I don’t know what may happen in the next 48 hours and of course in case classes happen, they all have got to be got ready on the few days when we’re not on strike, in case something is resolved that means we go back to work. But, what this does mean is that my conscience is pretty clear about blogging. Having taken my first steps down a new road in the previous post, it thus behoves me to look around myself and say, ‘What was I doing in July to September 2015 that I haven’t already told you about?’, and the answer to that is not limited to but certainly includes, ‘zapping gold coins with X-rays some more’. So this is about our fourth set of tests.

Jonathan Jarrett and a gold solidus with XRF machinery in the Department of Chemistry, University of Birmingham

Posed, obviously; I may look intrepid, but you have no idea how tightly I was holding that coin. It rolling under the machine would have spoiled several people’s day quite badly…

If you remember, where we were with this is that having got money to evaluate techniques by which we might be able to use X-ray fluorescence spectroscopy to measure the metal content of Byantine coins, with an especial eye on trace elements that might betray metal sources, we had fairly quickly established that the kind of portable machinery which we could bring to the coins in their museum wouldn’t tell us what we needed to know. So the working set-up for these experiments was now that, after having checked our insurance quite carefully, as soon as I could get into the Barber’s coin room of a morning I would remove from it about 100 g of high-purity gold in the form of 20-odd Byzantine and other coins, then University security would turn up (in theory) and transport me to the School of Chemistry (in theory). We would then do as much zapping as could be done, with at least two people present where the coins were at all times, before Security turned up again (in theory) in time to get me and the coins back into the Barber before it closed. And this time we did this for four days running. I won’t tell you how many ways this process could go wrong, but I haven’t flagged them all. But Chemistry were lovely and very generous both with expertise and with biscuits, and though we never had quite the same team there two days together it was all quite a good group exercise anyway. So, what were we doing this time and how did it go? The answer is a long one, so I’ll put it behind a cut, but do read on!

Jonathan Jarrett processing experimental data in the Department of Chemistry, University of Birmingham

I broke my camera taking this photo; it was sat on the analyser and fell off just after this. It was fairly easily fixed, but still… I can’t blame anyone else for that one!

Well, we came into these sessions with some questions that we had planned to have and some we had been faced with by our previous experiments. The first of the latter was about the big S8 Tiger on which we were doing our analyses. It could subject our acetone-cleaned coins to beams and measure their output for any of 2, 8 or 18 minutes, and we needed to know if we had to use the 8- or the 18-minute cycle. So the first test we did, which ran over most of the first two days, was to test on the longest cycle the coins we had already tested on the 8-minute one. Since our operating window was between as soon after 10:10 as Security could deliver me and about 16:40 in the afternoon (long enough to get back before closing), we functionally had just over 6 hours to work each day, which is only about 16 coins a day on the 18-minute cycle counting changeover time. We were very much hoping to do more than that, so in this sense the first blow was that our analyst team member, working over the spreadsheets in his London lair, ruefully had to conclude that there was somewhat less of the worrying variation we’d already noticed in the long cycle than in the short one, so that if we wanted any kind of useful figure it was the 18-minute cycle we had to use. But so it was.

Bruker S8 TIGER XRF analyser open for business

The maw of the S8 TIGER… I have used this photo before, but to be fair, if I’d taken more of the machine they’d have looked just the same…

The next blow, or perhaps I should say, negative experimental outcome, came with one of the tests we had planned from the beginning. An obvious potential problem with evaluating objects with an energy beam is: what if they’re not uniformly composed? If you point your beam at a crusty spot of iron and then at some nice smooth gold, obviously different readings will result. Now, the S8 Tiger works on areas, not points, so that was less of a problem than I make it sound there, but we still wanted to know if non-homogeneity was something we were dealing with. The way we hit upon was to use a 5 mm mask on the sample cup, rather than the normal 8 mm one, which let us expose different areas of the same coin.

A gold solidus of Emperor Constantine VI and Empress Eirini struck at Constantinople in 785-797, Barber Institute of Fine Arts B4598, in a sample cup fitted with 8 mm mask

A gold solidus of Emperor Constantine VI and Empress Eirini struck at Constantinople in 785-797, Barber Institute of Fine Arts B4597, in a sample cup fitted with 8 mm mask


A gold solidus of Emperor Justinian II struck at Carthage in 685-695, Barber Institute of Fine Arts B4399, in a cup with a 5 mm mask

A gold solidus of Emperor Justinian II struck at Carthage in 685-695, Barber Institute of Fine Arts B4399, in a cup with a 5 mm mask, mainly showing Justinian’s face


A gold solidus of Emperor Justinian II struck at Carthage in 685-695, Barber Institute of Fine Arts B4399, in a cup with a 5 mm mask

The same coin again, in the same cup, but now mainly showing Justinian II’s neck

So we did that with a few coins, and yes, they did vary more in what we saw. Unfortunately, as before, they didn’t vary very consistently, and the other problem was that because we were exposing less of the coin, we were also getting less actual data back, which might have accounted for the greater variation just by itself. It would have been less of an issue had the 8 mm scans not been quite so variable to start with, but as it was, we decided fairly quickly that we would not believe in any statistical processing that might even try to cope with all of this, and so we abandoned this part of the plan. The one thing I will claim we detected: there was one coin with a visible corrosion spot on its reverse, and that tested with way way more iron than anything else we ran, and only on that side. I’m pretty sure that was a genuine result. But when you’re checking your X-ray machine by eyeball, it’s fair to ask why you’re bothering with the X-rays…

A gold tremissis of Emperor Constans II struck at Ravenna in 641-668, Barber Institute of Fine Arts B4235

A gold tremissis of Emperor Constans II struck at Ravenna in 641-668, Barber Institute of Fine Arts B4235, and that black blotch on the reverse left of the cross? is mainly iron. Pretty sure about that.

All of this was making us quite nervous about how much we could in fact believe what the machine was telling us. This wasn’t helpful, really, but we gave into two impulses to try and catch it out before one of our team quite rightly stopped us wasting any more time on sidetracks. Firstly, we re-ran one of the first coins again, to check for drift; but as said, the variation was so high anyway that we couldn’t tell putative drift from normality. Secondly, we were pretty sure that there should have been tin in some of these coins, and we were seeing none; one of our team had a suspicion that tin might be being mistaken for something else with an energy value close to it on the spectrum. So on the second day I included in our sample a stalwart, noble, British penny of 1967, when they actually did have tin in, and we tested that on the quick cycle as a last-minute thing at day end. And tin duly showed up in substantial proportion, and so again we had to face that there probably was no tin and the machine was probably not screwing up especially; what was wrong was what we were asking it…

A copper penny of Queen Elizabeth II struck at London in 1967, Barber Institute of Fine Arts BR028, in an 8 mm sample cup for analysis

See that dull sheen? Good British tin-bearing copper, no less! In the form of a copper penny of Queen Elizabeth II struck at London in 1967, Barber Institute of Fine Arts BR028

Nonetheless, we had two more days’ machine time booked and a plan, so in case anything could be got from the whole mess, we carried on. Up till this point, everything we’d tested fully had been a full-size solidus of Constantinople; now, we added to the bag both coins from other mints (namely Carthage, Alexandria, Syracuse, Ravenna, and that old favourite, ‘unknown’) and also half- and third-solidi (more usually known as semisses and tremisses). We were wondering, firstly, if the provincial mints were working to the same standard as the centre, already knowing that their graphical quality control could be somewhat shaky as you can see below, and secondly whether the fractions were being struck with the same metal as the full-size coins.

A gold solidus of Emperor Justinian II struck at Constantinople 685-695, Barber Institute of Fine Arts B4384

A gold solidus of Emperor Justinian II struck at Constantinople 685-695, Barber Institute of Fine Arts B4384

A gold solidus of Emperor Justinian II struck at Carthage 685-695, Barber Institute of Fine Arts B4399

A gold solidus of Emperor Justinian II struck at Carthage, the one in the 5 mm mask above in fact, B4399

A gold solidus of Emperor Justinian II struck at Syracuse in 685-695, Barber Institute of Fine Arts B4407

And a gold solidus of Emperor Justinian II struck at Syracuse in 685-695, Barber Institute of Fine Arts B4407. You will notice that these mints did not all use the same design…

Now, I or someone still needs, even now, to do the number crunching that would genuinely prove this was what our results showed, but the answer to this seemed to be: all denominations were always similar in metal at Constantinople, and Alexandria, Carthage and Ravenna had only minor differences at the full size—and we had too few small coins of those mints to be sure if they were consistently different—but Syracuse’s metal content went steadily downhill after the mid-seventh century (and is visibly worse even by modern naked eye by the late-eighth), and its fractions could be quite surprisingly different in metal (by which I mean, even less pure).

Graph of fineness of several Constantinople and Syracuse gold coins, average figures over 50-year intervals

Graph of Constantinople and Syracuse gold fineness, average figures over 50-year intervals; it’s patchy data with a shaky basis but I still think it’s indicative

We had allocated random running numbers to the coins to stop us trying to guess what we should see, but it was remarkably easy and accurate to guess a coin of Syracuse, simply because they were the grubbiest most coppery things we were testing. The graphs separate pretty clearly, as you can see. And, excitingly, it was clear that the ‘unknown’ coin belonged with the lower line, not the Constantinoplitan, so we were actually able to assign it to the West, which we could not have done without analysis. So hurray! The machinery could in fact detect meaningful difference! But, of course, this we probably could have done with the cheap portable machinery too…

Gold solidi of Leo III and Constantine V from different mints compared

Three solidi of Emperors Leo III and Constantine V, one nice-looking and ‘therefore’ from Constantinople, one kind of cartoonish and ‘therefore’ from Syracuse, and one somewhere between… but now much more likely Italian than capitoline! This is a slide I’ve grabbed from a presentation I’ll tell you about at a later stage.

The last thing we did in this cycle was throw a few non-Byzantine things into the machine as well. These were a dinar of Shah Vahran IV of Persia, an Ostrogothic solidus of Ravenna, a one-third-dinar of Caliph ‘Umar, and the three-figure Arab-Byzantine solidus that’s probably from Syria which I’ve shown you here before, long ago. I don’t have time to explain why we picked these things, but they were probably all ultimately made from Roman or Byzantine coins. The Persians rarely issued gold but seem to have done so when they had received a major Roman tribute; the two Islamic conquest-era coins were trying to replace the Byzantine tax system and its gold-coinage liquidity; and there are political arguments by which the Ostrogothic coin actually was Byzantine, by the terms of the time. So what were the results like?

A gold solidus struck in the name of Emperor Justinian I at Rome in 527-536, Barber Institute of Fine Arts VV0018

A gold solidus struck in the name of Emperor Justinian I at Rome in 527-536, Barber Institute of Fine Arts VV0018. There are certainly some things slightly awry about this coin but it turns out the metal content isn’t one of them!

Well, firstly, the Persians pretty clearly didn’t bother with maintaining Roman standards; that coin came out markedly low. And why would they have, after all? Gold coin didn’t fit into their normal value system, and was fairly clearly being made for publicity purposes, so as long as it looked OK, why not stretch that gold as far as it would go? But the other three were pretty much on the mark; indeed, the Arab-Byzantine one, perhaps just because it’s quite smooth or by fluke, but still, came back pretty much the purest coin we’d tested. And I suppose that would be a necessary means of ensuring trustworthiness as you changed systems around people. But that’s not the usual historical assumption, even so; we expect the strong state to maintain standards and its chancy supplanters (not that one can really still say that of political Islam by the time of ‘Umar, but you know what I mean) to let them slide. But actually, if you just had this data to go on, you’d probably theorise differently: being sure of your grip on the system means you have latitude to mess with it in a way that new operators in uncertain times just couldn’t afford to if they wanted to survive long enough to get that grip…

A gold dinar of Shahanshah Vahran IV struck at an uncertain mint between 388 and 399, Barber Institute of Fine Arts S0113

A gold dinar of Shahanshah Vahran IV struck at an uncertain mint between 388 and 399, Barber Institute of Fine Arts S0113, and as you can probably see yourself not as careful about the gold content as their neighbours, because why would a victorious Persia need to care?

The other thing that we now had that was beginning to look like a real pattern was the impossibility of making any distinctions on the basis of trace elements. As I disgustedly said at one point in the testing, “everything’s in everything!” That wasn’t quite true: we saw tin and tungsten in one single test each, lead only twice, and numerous elements not at all, and many only infrequently (like platinum or bismuth). On the other hand some elements could be more or less guaranteed: copper, and almost always iron, aluminium, calcium and potassium; and actually always silicon, irrespective of the preservation context of the coins. Sodium, on the other hand, was a comparative rarity, even on two coins which had actually been found in the sea. Lots of these signals were at levels too small to be very confident in, but not all. This applied to the outsider coins as much as the Byzantine ones, too. But then, that shouldn’t have surprised us given that, as I said above, they were probably all made from other gold coins in the first place…

And somewhere in thinking about this, it dawned on me that of course that is probably what we should see, because of course Byzantine coins were also mostly made from other coins; almost all of them would have been struck from gold that had been gathered in from anywhere possible to one of a few, sometimes just one, central collection point, melted down into ingots, shipped to a mint and melted down again to make coins with, a yearly slow stirring of a pot of gold the size of the whole Mediterranean. Of course everything’s in everything; how could it not be? And we were seeing that, in so far as we were seeing anything measurable at all. But again: given this somewhat obvious conclusion, why were we bothering to look in the first place? And the answer was, because other have and have found stuff. Or at least they thought they had. But by now, we were beginning to wonder if what we did have out of all this experiment was reason to doubt them…


If you want to know more about how the late Roman and early Byzantine tax system worked, your best recourse is Michael Hendy, Studies in the Byzantine Monetary Economy c. 300-1450 (Cambridge 1985), pp. 284-303. For the rest of this post I’m going to just cruelly assume you can cope, or I’ll have to footnote every second sentence…

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5 responses to “All That Glitters, Phase 4

  1. Just nice to have this blog back.

  2. I can’t explain why I find your reports of these experiments so fascinating, considering how far they are from my own work; maybe it’s due to my youthful dream of being an archaeologist, but I have to think a lot is due to your skill as a story teller. Even if experiments confirm what you expect, or show that you need to rely on other methods, that’s useful information.

    Methodologically, it’s helpful to see that even scientific experiments can yield ambiguous or less-than-helpful data; it’s not just being in “fuzzy studies” that makes interpretation the key element.

    • Your praise is too kind; I’m just relieved to know it’s all more or less comprehensible! I was looking at it in draft and thought, ‘man this gets tangled. I’ll just put it behind a cut and hope…’

  3. Has anyone ever attempted recovery of ancient DNA from coins and suchlike artefacts? Come to think of it, painters often use their own spit. Has anyone tried to recover ancient DNA from – for example – the wonderful cave paintings in Spain?

    • Not as far as I know, to either question, though my knowledge would certainly not be complete. Still, with anything that has been touched by so very many people, contamination would be very hard to exclude. With the coin, I’m not even sure whose DNA you’d be after; many many historic hands to choose from!

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