All That Glitters, Experiment 5

Fittingly in some ways, given the distressing news of the last post, this post takes me back to Birmingham (which continues to happen, with a trip there on Wednesday coming that I will delight in telling you about before long if all goes to plan…). In fact, this is the last of the posts promised in my second Chronicle round-up, which means that we are now progressed in the story of my academic life to December 2015… It doesn’t look a lot like blogging progress, but let’s ignore that and instead tell the next part of the story of my project to zap Byzantine gold coins with X-rays, All That Glitters.

Bruker S8 TIGER XRF analyser open for business

The maw of the S8 TIGER XRF analysis machine in the Department of Chemistry, University of Birmingham, already much featured in these posts

It’s getting a little silly now to re-summarise the project every time I do one of these posts, however far apart they may be, so I’ll invite you to look here for the premise and just say where, by December 2015, the project had got up to. In brief, we had started from a belief that we might be able to find out about sources of metal for the Byzantine coinage and how those changed and maybe why by analysing them using a technique known as X-Ray Fluorescence (XRF). We got money to investigate this possibility in April 2015, and either before that or thereafter moved through the following developmental steps:

  1. finding out that the lightweight, energy-dispersive kit that we had hoped to use just wasn’t going to get the information we needed;
  2. finding out that the big, stationary, wavelength-dispersive kit we had to use instead (by kind courtesy of the Department of Chemistry, University of Birmingham) would get us the best results only on its longest cycle, cutting the number of things we could test in the time we could pay for down considerably;
  3. finding out that the results we were getting apparently included quite a lot of invisible surface deposits that seemed most likely to be leftover soil;
  4. discovering that, against all expectations, cleaning the coins in acetone actually made this problem worse, if anything;
  5. deciding, along the way, that we could not, as we had hoped, test different areas of coins for comparison of homogeneity either, because the results were just too darn variable to interpret;
  6. establishing that despite all these limitations, we could still distinguish between mint practices sometimes, but that only in the most difficult of cases was this telling us anything a competent numismatist couldn’t have seen by thmselves;
  7. and, although this was my colleague Dr Rebecca Darley, not myself, presenting these initial findings at the International Numismatic Congress in Taormina and at the Joint British Museum/Institute of Archaeology Seminar at University College London.1
  8. At the former of these presentations we got some pushback from the numismatists whose work we were implicitly questioning, which was understandable, but in the latter we got lots of pushback from one or two archaeometallurgists who felt that we were not people properly trained to do such work and that in fact it was pointless, which I saw as one of those ivory-tower problems; people are out there doing such work badly anyway, so would you rather just let them publish it and be accepted or shall we aim to do at least a bit better?2 Admittedly, we were having trouble doing much better, but that was what we now set about solving…

On 17th December 2015, therefore, three of us brought our test set of coins back to the Department of Chemistry, but this time with a difference. We’d already tried cleaning the coins in acetone, as said, so we had decided that we needed to try harder. But how hard should you try to clean a relatively soft precious-metal object of considerable value? Thankfully, this was a question that the team working on the Staffordshire Hoard had already faced, and since I’d been able to talk with one of them earlier in the year, we had a kind of answer, which was, berberis (or barberry) thorns: tough enough to shift surface dirt, soft enough not to scratch the metal!3 So before the test, Maria Vrij, by now in post succeeding me as Interim Curator of Coins at the Barber following my move to Leeds, had taken the coins and more acetone down to the Barber’s most suitable room for the purpose and, with the windows wide open, had laboriously worked over their surfaces with thorns under a magnifying glass.4 I can only say that this made me very glad to have moved jobs before this could have become my task, and I remain very grateful to Maria for doing it, but of course the real question was, what difference did it make? And the answer was, sadly, ‘a bit’: the levels of presumably-surface material that shouldn’t really be in the coins (calcium, silicon, potassium, aluminium) dropped, but were not gone.

A gold solidus of Emperor Heraclius struck at Constantinople in 613-616, Barber Institute of Fine Arts B2762, in a WD-XRF sample cup

A gold solidus of Emperor Heraclius struck at Constantinople in 613-616, Barber Institute of Fine Arts B2762, in its WD-XRF sample cup for testing

This was, in many ways, not the answer we wanted, as with so many of the findings thus far. We would much rather not have had to use the big, fixed machine to which the coins had to be brought, rather than one of the portable ones we could have taken to other collections; we would rather have been able to use a shorter test cycle and thus test more things in the time we had; we’d rather not have had to clean the coins at all; but if we had to clean the coins, we’d rather it had been possible just with a wash and a rub in acetone, not with hours of picking at them with thorns with your face close over a bath of solvent. If we had (and by we, I really mean Maria, sorry Maria), to do all that, however, we’d at least have liked it to produce good results. What it actually produced, however, was only measurably less bad results, which was not the exciting scientific conclusion for which we might have hoped. But it might be a bit more like actual science, and sadly, it’s a lot more like real life; messy, never quite sorted out, but still interesting…

1. The former of these papers is now published, in fact, as Rebecca Darley, “All that glitters…: the Byzantine gold solidus, c. 300-1092″, in Maria Caccamo Caltabiano (ed.), XV Internationa Numismatic Congress, Taormina 2015: Proceedings (Rome 2017), II, pp. 982-985. A cite for the latter would be Rebecca Darley, “What does the science mean? Interpreting metallurgic analysis of Byzantine gold coinage”, unpublished paper presented at the British Museum/Institute of Archaeology Joint Seminar, University College London, 15th December 2015.

2. It seems mean to point fingers, but once it’s being cited it is probably fair game and, on the basis of our experiments, I might raise questions about Rasiel Suarez, “A Metals Analysis of Silver Roman Imperial Coins using X-Ray Fluorescence Spectroscopy”, online here, whose precision just seems impossible with the equipment he used despite his checks (which were not carried out against a standard), and one would like at least to be able to ask more questions about the methods and reproducibility of the tests in Monica Baldassarri, Gildo de Holanda Cavalcanti, Marco Ferretti, Astrik Gorghinian, Emanuela Grifoni, Stefano Legnaioli, Giulia Lorenzetti, Stefano Pagnotta, Luciano Marras, Eleonora Violano, Marco Lezzerini and Vincenzo Palleschi, “X-Ray Fluorescence Analysis of XII–XIV Century Italian Gold Coins” in Journal of Archaeology (2014), pp. 1–6, online here. Note that we are not the only researchers wondering about things like this, by now: see also V. Orfanou and Th. Rehren, “A (not so) dangerous method: pXRF vs. EPMA-WDS analyses of copper-based artefacts” in Archaeological and Anthropological Sciences Vol. 7 (Basel 2015), pp. 387–397, DOI: 10.1007/s12520-014-0198-z, and E. S. Blakelock, “Never Judge A Gold Object by its Surface Analysis: A Study of Surface Phenomena in a Selection of Gold Objects from the Staffordshire Hoard” in Archaeometry Vol. 58 (Chichester 2016), pp. 912–929, DOI: 10.1111/arcm.12209.

3. See ibid.!

4. Of course, she is no longer Interim, but now actually properly Curator of Coins, and much better at it than ever I was, despite the acetone fumes!


9 responses to “All That Glitters, Experiment 5

  1. Perhaps you were hoping for too much from acetone, especially since your pollutants weren’t organic. Have you tried the various preparations used for cleaning proteins and whatnot from surfaces? Google immediately yielded this.

    • We’d chosen acetone at the outset as the cleaner least likely to damage the coins or to mess with our results, but you’re demonstrably right that it wasn’t up to this. Surely proteins are also organic, though? Our suspicion was that we’re dealing with aluminium carbosilicates, though I will freely admit that I might as well have read that off a cereal box for all that I understand the deep chemistry of the term. I’m not sure a protein remover would get something so mineral off, though, would it?

  2. Come to think of it: Birmingham, cleaning, surfaces. A search turned up this.

    • Now that is something we should maybe have found out ourselves, thankyou. We did contact some people in Chemical Engineering (who were less interested than Chemistry), but that was before we’d realised cleaning would become so important. If we expand on this project ever, I shall have to see if he’s interested in lending expertise. Thankyou again!

  3. An awful lot of experimental science involves just buggering on. It’s tricky, I suppose, when you are loath to risk damaging the “substrate” on which your dirt resides. Have you any duplicates of low historical value that you could use for preliminary suck-it-and-see experiments?

    I am reminded of a childhood song based on an advert:

    Shine your buttons with Brasso
    It’s only three-ha’pence a tin.

    • That kind of experiment with disposable coins has been done, in fact, although not with the present generation of XRF machines. Most importantly, though, it’s almost always done with copper-alloy, and on one memorable occasion silver, coins. No-one has so far been willing to mess about with gold in this way. And, unfortunately, when you’re dealing with X-rays, other metals aren’t a very good model for something as dense as gold. Lead tokens might be somewhere close, but they’re never preserved uncorroded and there’s almost nothing one could clean them with that wouldn’t ruin them…

  4. Density? A bit of depleted uranium might do. Or less teasingly, how about tungsten?

  5. Great insights there, thanks a lot

  6. Pingback: All That Glitters, Experiment 6 and final | A Corner of Tenth-Century Europe

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