Tag Archives: metallurgy

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! Continue reading


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.

‘Cooked gold’ in tenth-century Barcelona coinage: a likely correction

One of the advantages of doing scholarship on the Internet, insofar as one can, is supposed to be that you can update and correct your work. Those who like this idea seem to believe that one would never put any of one’s projects down and move on, but be happy to update them forever, rendering them forever unreliable as citations, and in general you may guess that I don’t agree that this should be the future.1 All the same, sometimes one does find something that makes one’s work look likely to be wrong and then there seems little point in not using this outlet to make that public. The unlucky victim this time is my article, “Currency change in pre-millennial Catalonia: coinage, counts and economics” in Numismatic Chronicle Vol. 169 (London 2010 for 2009), pp. 217-243, and specifically the bit of it where I discuss a particular usage of the documents from around Barcelona in the late tenth century, prices given in auro cocto, ‘cooked gold’.2 Here’s what I said in the article:

“The use of bullion was becoming more common, and the increasing incidence of qualifications like ‘bono placibile’, and in the case of the foreign mancuses, ‘chocto’, literally ‘cooked’, ‘burnt’, suggest that its standard was frequently a matter of concern.

“The term ‘chocto’ is worth a brief digression. This apparent testing or melting may have been because of a variety in standards of the gold dinars that were reaching Barcelona from various mints in al-Andalus and, probably, beyond. The origin of individual dinars is only specified in later documents, when the bulk of coin in use must have been such that such testing would have been impractical. At this early stage foreign coins may have been converted on arrival into bullion of a known standard. It is hard to read the term ‘chocto’ as referring to anything other than melting; destructive assay methods would hardly have been used on so large a scale and would, in any case, have left no minted coin with which to pay the required price.62 It may therefore be that the coins were being reminted into local versions of the mancus.63 When the supply of Islamic mancuses began to dry up in 1020, a moneyer by the name of Bonhom began to mint local ones that circulated for many years.65 The paucity of finds of imported coin of an earlier period might be explained by such a practice.”

”    63 See A. Oddy, ‘Assaying in Antiquity” in Gold Bulletin 16 (1983), pp. 52-9. I am grateful to Marcus Phillips for bringing this useful paper to my attention.
”    64 On local manufacture of mancuses elewhere see L. Ilisch, “Die imitativen Solidi mancusi. ‘Arabische’ Goldmünzen der Karolingerzeit’ in R. Cunz (ed.), Fundamenta Historiae: Geschichte im Spiegel der Numismatik und ihrer Nachbarwissenschaften. Festschrift für Niklot Kluüßdorf zum 60. Geburtstag am 10. Februar 2004 (Hanover, 2004), pp. 91–106.
”    65 On the mancuses of Bonhom and Eneas, see [Anna M.] Balaguer, Història [de la moneda dels comtats catalans (Barcelona, 1999)], 53-5 and [Miquel Crusafont, Anna M. Balaguer and Philip Grierson, Medieval European Coinage, with a Catalogue of the Coins in the Fitzwilliam Museum, Cambridge, 6: the Iberian Peninsula (Cambridge, 2013), pp. 78-81]….”

This was a sticky bit when I wrote it and looking back now the problems are even more evident. Poor-standard coins should have been more concerning once there were more of them, so why would the people of Barcelona have adopted an expensive reminting process before that point but then abandoned it? I provided an answer to this but I don’t like it, and the fact that the Bonhom mancuses survive but my notional earlier ones don’t could be just coincidence—and this whole article was after all about coins we probably don’t have—but it doesn’t make the theory any more likely. Still, in the light of what I knew it seemed like a workable answer. But then, on New Year’s Eve 2014 (because I know how to have a good time) I was reading up on the scientific study of Byzantine gold coinage for the All That Glitters project, and I found Robert Halleux getting all Greek and quoting a papyrus that contains ancient instructions for the testing of gold, in French translation which I translate as follows:

“If you want to purify gold, melt it anew or heat it, and if it is pure it keeps the same colour after being put in the fire, pure like a piece of money. If it appears more white, it contains silver; if it appears ruddier and harder, it contains copper and tin; if it is black, but pliable, it contains lead.”3

Not content with that, Halleux then quotes a [Edit: thanks to Gary for the corrected source here]letterthe Natural History of Pliny the Younger as well: “aurique experimentum ignis et, ut simili colore rubeat ignescatque et ipsum”, which is an oddly-cut quote that makes me think M. Halleux’s Latin was perhaps not so smart as his Greek in 1985. His citation certainly wasn’t, as I can find no sign of this text in Pliny, but Part of it, however, appears to mean, “gold tested in flames, both so that it shines and burns with the same colour and…”.4 Whatever M. Halleux was actually quoting, This just seems much more likely to be what is going on in my documents, testing by fire in a non-destructive way rather than actually remelting. In that case, however, it seems much less likely that the coins would have been restruck, so the Bonhom mancuses probably were the first local ones made in Barcelona.

Gold mancus of Count-Marquis Ramon Berenguer I of Barcelona (1035-76)

The Bonhom mancuses are themselves vanishingly rare, however, and there seem to be no pictures of them on the web, so, here’s a slightly later Barcelona mancus struck under Count-Marquis Ramon Berenguer I of Barcelona (1035-76), from a Cayón sale of 2009

Admittedly we still have no more sign of the actual Andalusi mancuses in the area than we do my hypothetical ones, but at least we know that the Andalusi ones did exist and that the Barcelona documents were reacting to coins we have from elsewhere.5 I don’t think it does anything serious to my overall argument in my article, either, but this alternative reading of the ‘cooked gold’ in those documents is good reason to scotch what was always one of my weaker suggestions. So let it be noted, I disavow my old idea, and I now think that that ‘cooking’ was no more than a light flame-grilling to see what colour the coin turned.

1. Compare David Parry, “Burn the Boats/Books” and Jo Guldi, “Reinventing the Academic Journal”, both in Daniel J. Cohen and Tom Scheinfeldt (edd.), Hacking the Academy: new approaches to scholarship and teaching from the digotal humanities (Ann Arbor 2013), pp. 15-18 and 19-24, both fixed texts of what were originally online presentations archived here, with Alex Sayf Cummings and Jonathan Jarrett, “Only Typing? Informal Writing, Blogging, and the Academy” in Kristen Nawrotzki & Jack Dougherty (edd.), Writing History in the Digital Age (Ann Arbor 2013), pp. 246-258.

2. J. Jarrett, “Currency change in pre-millennial Catalonia: coinage, counts and economics” in Numismatic Chronicle Vol. 169 (London 2010 for 2009), pp. 217-243 at pp. 234-235.

3. R. Halleux, “Méthodes d’essai et d’affinage des alliages aurifères dans l’Antiquité et au moyen âge” in Cécile Morrisson, Claude Brenot, Jean-Pierre Callu, Jean-Noël Barrandon, Jacques Poirier & Halleux, L’or monnayé I : purification et altérations de Rome à Byzance, Cahiers Ernest Babelon 2 (Paris 1985), pp. 39-77 at p. 40:

“Si vous voulez purifier l’or, fondez à nouveau ou chauffez, et s’il est pur il garde la même couleur après la mise au feu, pur comme une pièce de monnaie. S’il paraît plus blanc, il contient d’argent ; s’il paraît plus rude et plus dur, il contient du cuivre et de l’étain ; s’il est noir, mais mou, il contient du plomb.”

The text of reference here is Halleux’s own, R. Halleux (ed.), Papyrus de Leyden, papyrus de Stockholm : fragments de recettes. Texte établi et traduction (Paris 1981), within which the bit here cited is Papyrus Leyden X 43, but it ought also to be locatable in Earle Radcliffe Carey (trans.), “The Leyden papyrus X: an English translation with brief notes” in Journal of Chemical Education Vol. 3 (New York City 1926), pp. 1149-1166.

4. Halleux, “Méthodes d’essai”, p. 40, citing Pliny, Natural History XXXIII 59, which you can see for yourself with a slightly more comprehensible text here.

5. On the absence of actual mancuses in finds from Catalonia, see Miquel Barceló, “L’or d’al-Andalus circulant als comtats Catalans entre 967 i 1100: un or vist o no vist?” in J. M.Gurt & A. M. Balaguer (edd.), Symposium Numismatico de Barcelona I (Barcelona 1979), pp. 313-327; on the chronology of the documentary mentions see Pierre Bonnassie, “Nouveautés linguistiques et mutations économico-sociales dans la Catalogne des IXe-XIe siècles” in Michel Banniard (ed.), Langages et Peuples d’Europe : cristallisation des identités romanes et germanique. Colloque International organisé par le Centre d’Art et Civilisation Médiévale de Conques et l’Université de Toulouse-le-Mirail (Toulouse-Conques, juillet 1997), Méridiennes 5 (Toulouse 2002), pp. 47-66.

This post was written with the aid of The Bevis Frond’s White Numbers (Woronzow 2014), which has made it much more pleasant to pull together.