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.

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