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Carbon Dating Study Day

I’ve been on another “Behind the scenes study day”, again organised by my favourite archaeological travel company, Andante Travels. This time it wasn’t a museum I visited, but rather the Oxford Radiocarbon Accelerator Unit of the Research Laboratory for Archaeology and the History of Art, otherwise known as the Oxford University carbon dating laboratory. The day was hosted and run by two very enthusiastic post-docs from the research lab, who clearly relished passing on some of their knowledge and love of the subject. One of them in fact was so engrossed in his subject that whenever someone asked him a particularly interesting question, he gave a little jump of excitement! I went on the study day with my sister’s boyfriend, who is also an engineer with an interest in history. My sister decided that she would spend the day pottering around the shops and museums in Oxford, but joined us for lunch, and I was impressed at how flexible and accommodating the hosts were at making her feel included over lunch.

I found the day fascinating. It was a mixture of lectures on the theory of radiocarbon dating, case-studies (including the Oxford contributions to the recent Richard III project, and dating the Turin Shroud – the latter clearly mediaeval according to their measurements) and lab visits. We saw both the “wet chemistry” labs where the samples are meticulously prepared, and the particle accelerator where the C14/C12 ratio is measured. I found it really interesting hearing the researchers talk about the massive efforts they go to to get accurate dates, and the huge numbers of potential variables they need to take into account. That included such things as the diet of the individual whose bones they might be dating (a diet high in fish makes their radiocarbon age look older than it should really be – an issue they had with Richard III who had a very high-status diet rich in fish) to the impact of the Industrial Revolution and atomic bomb tests in the 1950s on the amount of C14 in the atmosphere.

I personally was interested in the mathematical modelling which allows the researchers to take into account known archaeological information about sequences to more precisely date linked groups of items. Treating a group of related objects as a whole rather than as completely independent items allows them to reduce the uncertainty of dates potentially from hundreds of years down to just a few dozen years. One example that the researcher gave, as it was his own special area of interest, was dating a sequence of artefacts from Ancient Egypt. He knew which pharaoh was reigning at the time each artefacts (mostly papyrus) was produced, and the order in which the pharaohs reigned is well known from King-Lists on the walls of some Egyptian temples. In many cases the rough length of a reign is known too, as jubilees are frequently documented in the records. What isn’t known is the exact dates for each pharaoh. But by modelling a Bayesian network to include all of the prior information about reign order and duration, and propagating the raw C14 dates through that network, they were able to come up with a pretty tight chronology. To me, this approach was really interesting – I’ve worked closely for years with a group of Bayesian statisticians, who would naturally approach the problem this way. I found it surprising that it was considered such a completely new and revelatory approach in archaeology – it does make you wonder how many more breakthroughs could be made with a more inter-disciplinary approach to a problem.

It felt most peculiar being back in Oxford again. The three of us stayed overnight in a B&B, and went out for dinner to Brown’s, an Oxford institution which I used to frequent when I was a student (particularly when I had a boyfriend with a job who could afford to take me out to dinner!) Although in many ways Oxford has changed a lot over the last 20+ years since I was a student, I found that if I didn’t think too hard about where I was going, my feet just automatically took me there!