Tuesday, 4 December 2018

Laser ablation analysis of Viking silver

One of the main barriers to analysing silver objects is getting permission to sample destructively. Understandably, museum curators are often reluctant to allow researchers to drill or otherwise remove metal samples from their precious objects for analysis in the lab.

Other means of analysing silver objects that don’t leave any visible damage are therefore of great interest. Analysing objects by laser ablation, which is coupled to a mass spectrometer and can be used for both lead isotope and trace element analysis, offers one such method. Critically, unlike XRF and other analytical methods that measure only a layer of a few micrometres, laser ablation gets below the surface of an object to measure the bulk alloy. This ensures that the results we get aren’t affected by silver surface enrichment: an issue known to affect Viking silver. 

Laser ablation has the same high accuracy trace-element/lead isotope capabilities and low detection limits of solution-based analysis (requiring destructive sampling), and has returned precise and reliable results for both types of analysis in recent work.  Most importantly, the small diameter of the laser ablation spot is about the diameter of a strand of hair. This means that the method is not visible to the naked eye, allowing for the characterisation of museum artefacts that would otherwise be barred from analysis. Since it requires no sample preparation, laser analysis is also cost- and time-efficient. So, win, win. 

A huge gold silver neck-ring/ torc from the Bedale hoard. This item
is far too large for the laser ablation chamber. Fortunately for me,
a small fragment from the ring also survives, and could be used.
So far, there have been very few analyses of silver via laser ablation methods. But a few weeks ago, I analysed some of the Bedale hoard objects this way with Dr Simon Chenery at the British Geological Survey (for a nice video of the Bedale hoard, see here). We managed to analyse 18 of the 37 silver items in one working day (not bad going at all). A constraint was fitting the items within the laser ablation chamber (measuring about 10 by 10 by 2 cm). There was no way that the enormous silver torc from the hoard, seen in the picture above, was going to fit!  But by careful arranging of the other items, we fit several ingots in one analytical session. By having them all facing inwards in a clock-like manner, we ensured that the laser didn’t have to move about too much as it jumped from one object to the next. 

Ingots from the Bedale hoard snuggly positioned on the laser ablation tray
Each object was ablated in three different areas, for 150 seconds. The laser beam quickly penetrated the surface layers, suggesting that the effects of surface modifications were not deep (interestingly, this is a different result to that found on medieval silver coins). If you strain your eyes, you can see this in the way that the peaks recorded in the concentration depth profiles for first 5 seconds or so of analysis quickly reduce to a fairly stable level. 

Live results. The laser beam quickly penetrated the
surface of the silver ingots. 
The results were extremely interesting. Since we already have lead isotope ratios for these objects and have also analysed the entire hoard via portable XRF (the surface-only method), we now have lots of different data points for the same objects. This is starting to reveal interesting patterns regarding the likely sources of silver, as well as the limitations of the XRF method - the main method of analysis in the past. More on this in later posts....

A big thanks to Simon Chenery at the BGS, as well as the fantastic curators at the Yorkshire Museum: Andy Woods, Lucy Creighton and Emily Tilley.

Monday, 9 April 2018

PODCAST: Dawn of England: Viking Archaeology

A 20-minute podcast for BBC Radio about the Vikings in England: why were they so successful, and what is the value of the Watlington hoard for assessing the reign of Alfred?

Tuesday, 30 January 2018

Tracing Viking loot

During the ninth century, Vikings launched sustained raids on the Continent. Their aim was to acquire portable wealth: gold and, especially, silver. By all accounts, they were remarkably successful. So why does so little Viking loot survive in the archaeological record?

The sums raised by the Vikings through bribery, theft and extortion were enormous, especially when raiding intensified in the mid ninth century. Surviving documentary sources tell us that Vikings on the Somme extracted 5,000 lbs. of silver in 860. Two years later, another group on the Loire gained 6,000 lbs. Exact sums are not always mentioned, but a reasonable estimate is that raids on the Continent earned the Vikings a total of 7 million Carolingian pennies (or c. 30,000 lbs of silver). In today’s money, that’s somewhere between 85 and 170 million British £ (maths courtesy of a 10th century Anglo-Saxon law code, and reckoning on the, rather variable, modern price of sheep).

A Carolingian coin of Lois the Pious, minted at Melle, western France between 822 and 840. It was found in Louth, Lincolnshire - one of a growing number of single finds of Carolingian coins from England. PAS 'Find-ID' F6C6E1

Despite this, only around 200 Carolingian coins are recorded within Scandinavia. Even more incredibly, these coins date mainly to the 820s and 830s, indicating that they arrived in Scandinavia before the high point of raiding in the middle of the ninth century.  Carolingian coins are also found in England and Ireland, both as single finds and in Viking-Age hoards. But even here they are not especially numerous, even in hoards deposited soon after the main events, in the ninth century. The recently-discovered Watlington (Oxfordshire) hoard, deposited in the late 870s, contains only 2 Frankish pennies out of a total of c. 200 coins. We’re talking a trickle, rather than a flood.

A selection of items and coins from the Watlington hoard, deposited in the late 870s. It contains just 2 Carolingian coins out of c. 200 coins - the remainder of which are 9th century Anglo-Saxon issues. One of the Carolingian coins is the larger-than-average coin in the top centre. They were slightly heavier than contemporary Anglo-Saxon coins.

So, what happened to all the Continental coins? Even if we accept that the Vikings sometimes received other goods (cattle, grain, wine) in place of silver, and spent a portion of this wealth (on land, for instance), we would still expect a significant proportion of coins to end up back in Scandinavia or in other areas settled by Viking groups in the ninth century. Do the written sources exaggerate the amount of European wealth that ended up in Viking hands? Or did the Vikings simply melt down most of the coins to make silver ingots and rings (which were, after all, easier to transport)?

Because the act of melting down the silver effectively masks its origins, the only way to answer this question is through metallurgical analysis. This has only rarely been undertaken for Viking silver. But I recently had the opportunity to analyse the silver objects (ingots, rings and a brooch) and contained in the Bedale, Yorkshire hoard, probably deposited around c. 900 AD. Together with Jane Evans at the British Geological Survey, I sampled all 37 silver artefacts from the hoard, obtaining lead isotope data for each item. This data gives an isotopic signature of the small amount of lead that is present in the silver. It can be compared with the isotopic ratios of lead in Carolingian pennies, and other likely source coinages, to identify relationships between the coins and the silver artefacts. It has the potential to reveal where the silver used to make ingots and rings originally came from.  

This is work in progress (you can see a short video about it here, and at the Vikings: Rediscover the Legend exhibition, currently on at Southport Atkinson centre). I'm going to write future blog articles about how the analysis as a whole sheds new light on the question of where Viking loot ended up. But here I wanted to share one, particularly exciting result. 

The two Hiberno-Scandinavian (ie. Irish-Viking) objects in the Bedale hoard seem to have lead isotope ratios that match closely those of coins produced at Melle in western France: a major Carolingian mint in the ninth century. The results would seem to suggest that they were made by melting down Carolingian coinage, perhaps coinage that was brought to Dublin in the years before c. 900. This is interesting, because the Cuerdale hoard, deposited c. 905-10 in Lancashire, north-west England, contains an unusually large number of Carolingian coins (about 1000), many of which come from western France and may be the result of a Viking raid on Aquitaine in 898. Much of the Cuerdale hoard silver came from Ireland - what we may be seeing is an influx of Carolingian coins into Dublin or the Irish Sea region shortly before the turn of the tenth century.  

A Hiberno-Scandinavian broad band arm-ring from the Bedale hoard. Its lead isotope ratio is consistent with 
that of coins produced at Melle in western France, suggesting it was made from melted-down Carolingian pennies. Photo copyright Yorkshire Museums.
In this case, then, it seems likely that silver coins obtained by the Vikings on the Continent were taken to Britain or Ireland where they were either preserved as coins or melted down into ingots and rings that the Vikings could store, trade and/ or wear as appropriate.  We are beginning to unravel the history of Carolingian coins once they passed into Viking hands. Would silver rings and ingots from Scandinavia reveal the same link with Melle?