In addition to the day-by-day coin removal we've also finally reached the stage of cleaning the gold jewellery which is incredibly exciting. Before Christmas we were lucky enough to have a visit from Pieta Greaves and Jenni Butterworth, two of the people who have been working on the amazing Anglo-Saxon Staffordshire hoard. We had asked them over so we could pick their brains about all they've learnt on that project. In particular we wanted to find out what sort of research we might be able to commission on the metalwork and organic remains to find out as much as we can about our artefacts.
Talking with them about this was extremely helpful to us and we're drawing up a research programme now from the lessons we've learnt. A lot of the discussion was about preserving evidence as we work and this is now dictating how we will clean the jewellery. As scientific techniques constantly improve it is becoming possible to find out importance evidence about the past from increasingly tiny traces of preserved material. The importance of this for our cleaning and conservation work is that where thirty years ago it would have been acceptable to clean the gold in one go by immersing it in an acid we would now no longer do it as the cleaning process would wipe out all the traces of material that could be used for analysis.
The use of acid also has implications for the study of the metal of the jewellery itself. No historic gold jewellery is actually pure gold. It is always alloyed with other metals like copper and silver. This becomes obvious when we see all the hoard jewellery togther, as the gold colour of each piece varies from bright yellow to rose gold to pale silvery gold. If we agressively use an acid to remove corrosion from the pieces' surfaces the acid will remove some of this copper and silver and the surface layer will therefore be artificially higher in gold and lower in the other metals if it is analyzed. This would give a false impression of the metal in the whole object.
To avoid these problems the conservators on the Staffordshire hoard used a technique of manual cleaning, where they actually physically pick and pry off the corrosion and dirt from the surface of the objects. Of course the difficulty of this technique with gold objects is that their surfaces are soft and very easily scratched. The ingenious solution that they came up with to prevent this happening is to use thorns as the cleaning tools! Specifically to use Berberis thorns, about 10-20mm long, fixed in small hand tools. The thorns are great because they are hard enough to push off the dirt and corrosion but soft enough not to scratch the gold.
The Staffordshire team were kind enough to send over a packet of the thorns a couple of weeks ago and I was therefore able to try it myself on two of our torques. These are ones without exposed torque interiors where there might be valuable scientific evidence lurking still wet inside. (We are sampling and analyzing these organic rich ones before cleaning them fully). I started very slowly, doing all the work under a stereo microscope so that I could do very fine work. The technique worked well, if very slowly. It took me about twenty five hours before I felt I'd removed everything I could from the first one. One advantage of working this way is that I could "go around" any traces of organic material or unknown staining on the surface, thus preserving those areas. One I'd finished this I was still concerned that I'd been unable to remove all the overlying corrosion products from the surrounding coins. A thin green "glaze" was still present in some areas that just wouldn't come off with the thorns. I discussed this with Pieta again and fortunately she confirmed that a light wipe over with a formic acid solution on small cotton wool buds followed by a good rinsing would do the trick without affecting the organics or metal composition. I tried this and found that it was very effective and did all the remaining cleaning after just a minute or two's exposure. Having learnt the knack by then I must have been more confident in cleaning the second torque because that one took me less than a day.
The cleaned torques look beautiful and have now revealed all the detail on their surfaces that we could not see properly before. In addition to the deliberate decorations we can see evidence of the long and busy life the torques had before burial. Both of them show scratch and dent damage all over their surfaces from when they were worn by their owners, the wear being worst just where you would expect it from rubbing again clothes or from where they'd rest on a hard surface. On one of the torques there are also small splits in the gold surrounded by rust staining. We belive this happened after the burial when ground water made the torque's central iron core rust and expand, splitting the gold skin.
Back on the hoard itself we have continued to make new discoveries. When we removed almost all the visible jewellery in November I had thought that we'd reached the end of that layer and that there were predominantly ingots left underneath but over the last few days we have discovered a new gold torque, a mysterious silver semi-circle over twenty millimeters deep, a small length of chain and a silver bracelet so who knows what we will find next. I'll let you know when we do