Tag Archives: XRF

All That Glitters, Experiment 3

A problem with finally picking up this blog a bit is that some of the old stories in it have been sitting idle for really quite some time. Does anyone remember, for example, that while I was working at the Barber Institute we got a small amount of money to zap gold coins with x-rays? If you do, you may remember that I’d already reported on the first two experiments we did in that project when everything here ground to a halt in 2016. Well, now I resume, with a brief account of the third experiment we did, and before long who knows, we may be through the rest!

Bruker S8 TIGER XRF analyser open for business

The S8TIGER WD-XRF spectrometer in the Department of Chemistry, University of Birmingham, with five sample cups waiting and one under analysis

A brief recap may be in order first of all, though. The very short version of the project’s purpose was: we had all seen an increasing amount of work using X-ray fluorescence spectrometry to analyse the fineness of precious-metal coins and sometimes even assign origins to its metal based on the trace elements therein. We had our doubts about this, but the manufacturers of some of the best machinery involved wanted to quell our doubts about this and offered us quite a lot of help with it, so we came up with a plan and, somewhat to our surprise, got both permission from the Henry Barber Trust to muck around with their coins and funding from the Royal Numismatic Society to do it, and so set about it. We began this in December 2014, with two experiments. The first was designed to test the viability of hand-held, portable, energy-dispersive machinery that could be brought to the coins; the second did comparator analyses using a fixed-location wavelength-dispersive spectrometer to which we had to bring the coins. This showed us that to detect anything at all of trace elements we would need the big machine, but also that it was picking up a whole load of stuff that seemed very likely to be extraneous material on the surface of the coins, probably from the soil in which they had long ago been found, rather than the stuff of which they were actually made. And that is about where we come in!

Gold solidus of Emperor Constantine VI and Empress Eirini struck at Constantinople 785-797, Barber Institute of Fine Arts B4599

The coin whose results (and details) are given in the first table below, set up for the test in which it did so

So, the third experiment was very simple. Using the same set of coins as we had previously run, I got hold of some conservation-quality acetone and a toothbrush, took them and the coins down to a workroom in the depths of the Barber and gave them all a careful but thorough scrubbing and let them dry, all except one that we left uncleaned as a check. Then I packed them up again and we bore them off to the Department of Chemistry on 22nd July 2015. There we ran them again, on the shorter of the big machine’s two cycles, and this gave us some results both before and after cleaning.

Unfortunately, if anything, the post-cleaning results showed more extraneous grot. That ‘if anything’ will tell you, though, that consistency was hard to observe. The variation wasn’t vast amounts, you understand, between half and one-and-a-half per cent, but cumulatively certainly enough to push our figures for the actual gold content up or down by two or three per cent, which would for some people be enough on which to found a theory about debasement… Here are the top ten elemental results from just one side of one coin by way of example:

Coin 170 (obverse)1 Percentage observed first time Percentage observed second time
Gold 86.51% 84.49%
Silicon 3.44% 4.63%
Silver 1.61% 1.43%
Aluminium 1.27% 1.19%
Potassium 0.80% 0.63%
Sodium 0.74%
Copper 0.44% 0.34%
Calcium 0.32% 1.96%
Sulphur 0.28% 0.48%
Iron 0.27% 0.50%
Total percentage of coin observed 96.38% 96.70%

Down by two per cent gold, and that mostly gone to silicon and calcium, even though some other small elements had dropped in the readings as well. Also, what this arrangement doesn’t show, the second time small but possible readings showed up for chlorine, magnesium and, way down the scale, platinum, all absent the first time through. Surely cleaning with acetone couldn’t have added calcium to the coin? But each coin we ran had its own little tale of additional mess to tell, even (and this should have told us something) the uncleaned check. Here are its top-ten figures for the obverse:

Coin 21 (obverse)2 Percentage observed first time Percentage observed second time
Gold 82.57% 87.43%
Silicon 1.82% 1.61%
Aluminium 1.01% 0.81%
Silver 0.74% 1.10%
Sodium 0.65%
Potassium 0.47%
Chlorine 0.45%
Calcium 0.44% 1.78%
Iron 0.31% 0.31%
Copper 0.24% 0.26%
Total percentage of coin observed 89.12% 93.80%

Some things are beguilingly similar, yes—iron and copper levels stay about the same—but others are not. The first test picked up sodium, potassium and chlorine but none of those turned up second time even though we’d actually got better readings with more of the sample observed (possibly because we gave it a flatter bit to look at). Were we just looking at a different bit? If so, that bit apparently had less silicon in it, but a whole whack more calcium in it. What can you do with these sorts of data?

Gold solidus of Emperor Anastasius I struck at Constantinople 491-518, Barber Institute of Fine Arts B0006, in an WD-XRF sample cup

The uncleaned check likewise set up for the results above, though I have to say it’s not like it looks dirty

So this was somewhat daunting, especially as we had four solid days of testing booked in for the following week and now somewhat less certainty that it would produce anything. At the time, my best guess was that the acetone had removed only handling patina, thus exposing the actual surface of the coin and everything that had adhered to or reacted into it while it was in the ground. Actually, looking back, I think we were already seeing here the conclusion that the final experiment would necessarily lead us to. But that would be getting ahead of the story, and even though this story is so very far behind, I have hopes of telling it as I wanted to anyway. So, till next post but, er, two, if I have my plans right (and no-one else dies—did you see Mark E. Smith has left us for the bar of the great and final WMC since the last post?), I shall leave the question hanging…

1. We used randomly-allocated running numbers throughout these experiments, so that we couldn’t try and guess what the results should be according to where the coins fitted in Byzantine history (or the previous curves laid down by work like Cécile Morrisson, Jean-Nöel Barrandon and Jean Poirier, “La monnaie d’or byzantine à Constantinople : purification et modes d’altérations (491-1354)” in Morrisson, Claude Brenot, Barrandon, Poirier and Robert Halleux (edd.), L’or monnayé I : purification et altérations de Rome à Byzance, Cahiers Ernest Babelon 2 (Paris: C. N. R. S. 1985), pp. 113–187. This one was actually Birmingham, Barber Institute of Fine Arts B4599, a solidus of Emperor Constantine VI and Empress Eirini struck at Constantinople between 785 and 797, online here. I’m actually almost loath to identify the coins here, though, lest it be thought I’m actually publishing figures for their metal content. As is probably clear, though, that’s something I’m sure we weren’t reliably getting.

2. And this one was Barber Institute of Fine Arts B0006, a solidus of Emperor Anastasius I struck at Constantinople between 491 and 518, online here, with the same reservations as in the previous note.


All That Glitters, Experiment 2

Somehow my posts about me and my work—and that may not be what you’re here for but, you know, I like it—have got behind my seminar reports in such a way that they’re into February 2015 and I’m still in December 2014. Let me resolve some of that disparity by giving you a short report on the second day of experiments in the collaborative project I’m in for analysing Byzantine gold coinage by X-ray fluorescence, which was 14th December. (If you need background I announced this project ages ago here and dealt with some of our starting questions and the first day’s experiment here.)

Cover of J. O. Jeppson, The Second Experiment

Our results have so far not been this dramatic, but then, I’m guessing that our first experiment wasn’t quite as adventurous as this must have been

To recap, we had established that if our experiments were to tell us anything much about elements other than gold, silver, copper and maybe one or two other pre-determined elements, we were going to need not the energy-dispersive machinery we’d been using on the first day but the bigger, more expensive and, most importantly, immobile wavelength-dispersive machinery in the Department of Chemistry in the University of Birmingham, a machine called the S8 TIGER. I am only just able to describe the difference between these two analytic methods: in so far as I can, it’s to do with what is being used to pick up the energy given off by the things you’re bombarding with x-rays. The WD machinery includes crystal collimators that are sensitive to certain wavelenths of that energy, which therefore get picked up better, where the ED machines, which measure only in terms of intensity of signal, simply wouldn’t see such things among the massive gold return, as we had surmised. The WD machine also scans its samples in a vacuum, which eliminates interference from the air.

Bruker S8 TIGER XRF analyser open for business

The mouth of the TIGER yawning wide, with five sample cups waiting and one under analysis

On the other hand there are also problems with the WD machinery that don’t exist with the ED kit. For us the first of those was simply access; it’s nothing to do with the actual machinery except in so far as it’s immovable, but because we had to take the coins to the kit rather than vice versa, that meant arranging transport and insurance even on campus, and the transport repeatedly went wrong, which cut into our experimental time a lot. But, also, the ED kit works with narrow beams focussed on points; the WD machine scans its samples in masks such as the one below, of which the two sizes relevant to us were 5 mm and 8 mm, and those were therefore the only area sizes that we could analyse. Importantly, this also precluded examining coins at their edges or over piercings, because the sample has to fill the exposed area completely. This also highlights a problem with both ED and WD methods: non-homogeneity. If for some reason your coin had an odd tiny lump of platinum on its surface, say, the ED machinery would either miss it (in which case you’d never know) or find it and report a massive platinum signal (which would be misleading for the coin’s overall composition). The WD machinery, however, would factor it into the average, so that you wouldn’t necessarily realise that it was a coherent inclusion rather than a component of the main alloy. So there was plenty to worry about even if the machine worked perfectly.

Emperor Heraclius just visible on one of his solidi of Constantinople loaded behind an 8 mm mask for analysis in the Bruker S8 TIGER

Emperor Heraclius just visible on one of his solidi of Constantinople loaded behind an 8 mm mask for analysis

Anyway, we had our goals clear for this test. The first was to get our hands on the machinery and find out what the operational considerations in any further planning were, the results of which you sort of see in the musings above. Here I have to acknowledge the tremendous help and general goodwill of Dr Jackie Deans, official keeper of the TIGER, and Dr Adrian Wright, who had first let us involve the Department of Chemistry in the project and had helpful things to say whenever he dropped in. Our second priority was to run the same ten coins around which we’d built our first experiment on the S8 TIGER and see how the results differed from those on the ED kit. And as it turned out, our third one was to determine how we wanted to use the S8 TIGER, because as Jackie explained to us, it could analyse at three levels, a 2-minute cycle that would probably get us no more data than the ED machinery had, an 8-minute one which should do the job, and an 18-minute one which was the very most data it could gather. Adding 10 minutes to each analysis was obviously going to limit the number of coins we could actually analyse in any given timeframe, so we really rather needed to know whether or not it was worthwhile.

Gold solidus of Empress Eirini at Constantinople set up for analysis in a Bruker S8 TIGER XRF analyser

The rather different visage in gold of the Empress Eirini, likewise cruelly cut down to 8 mm of glaring royalty

And so what did we find? Well, this machine certainly had more to tell us. We were now getting returns in terms of many elements, at concentrations of down to parts per thousand or even less. This ineluctably meant a decrease in gold concentration reported, because there was now simply more data to fit into the percentages, but the overall picture of lots of gold, not much silver and less copper was still very apparent in the reported figures. What we hadn’t expected, and had now to deal with, was that copper wasn’t usually the third most detected element, and sometimes silver not the second: instead, we were seeing lots of calcium, silicon and sometimes aluminium beating them out. It seemed a priori unlikely that these were original metallic components of the coins in these quantities. That in turn implied that these elements had got into, or much more likely onto, the coins since striking, be that from use, preservation or anything else that might have happened to them. But, whatever they were, they also seemed to be more consistently detected on the long cycle than the medium-length one, meaning that we were going to need to use the long analysis to have any chance of consistent findings. So now we had two difficult questions to answer in setting up Experiment 3: firstly, what could we get done with less than half the scans that we might have hoped to do in any given day of experiments, but secondly, when we did, could we determine whether these results were merely contamination or do anything about that if they were? And these were things which we attempted to address in the New Year, so I’ll stop here for now.