As you might have gathered from previous blog posts, we are suckers for machines that whirr and ping, even if we are not quite sure how and why they work. I have mentioned before that the chert tools from our sites vary enormously. Some are made from very smooth black chert, others of nasty granular grey stuff that must have been a real challenge to work and the rest are of all qualities of raw materials in between these two extremes. For his BSc dissertation Carl is trying to identify groups of raw materials, to see how many different chert and flint sources were used by Late Neolithic and Early Bronze Age people in Lancashire. He is comparing chert and flint tools from two sites, the 2012 and 2013 excavations at the New Laund enclosure and another, similar sized, assemblage from the platform cairn at Moseley Height, near Burnley.
He has spent the last few months in the relatively low-tech environment of our artefact study room sorting the raw materials into groups based on how they look and feel. This is the flint, which he grouped into seven separate categories, and there were eight distinguishable types of chert. The question, of course, is whether these visual differences actually mean that the rocks came from different sources. One possible way to answer that question is to look at the trace elements in the rock, different geological sources, formed under different conditions, should include different trace elements.
To do that he needed to take himself and the stone tools down to the analytical suite and use our portable X-Ray florescence analyser on them (it’s not just ours, I hasten to add, this is where being in the same school as a lot of analytical chemists comes in very handy). Here are Carl and Clare looking very technical next to the pXRF rig, this is the same tool that Clare used for her MSc thesis on Californian rock art pigment.
Once the lid is in place it is bombarded with radiation and the resulting fluorescence is measured (I’m simplifying slightly so as not to display my shameful ignorance of the physics). All the heavier elements in the periodic table fluoresce at a distinctive wavelength and so, if you run the process for long enough, you can build up a full spectrum of what proportions of each element are present in each sample. Importantly, you get this without doing anything more destructive to the finds than zapping them with X-rays for about 30 seconds. The reason the analyser is shaped like a Captain Kirk-era phaser is because, being portable, you can take it out onto archaeological sites and run the process there. This means that awkwardly fixed and heavy things, like Clare’s rock art, can be sampled in the same way. It also means that artefacts in museum collections can also be sampled without them having to be loaned to us.
And so, after 30 seconds or so we can see that there is a lot of silica in our rocks, who knew. More usefully, we can also see smaller spikes for all the trace elements. Carl sampled sixty odd pieces this afternoon, four bits from each of the groups of different looking rocks he identified in the artefact study room. The preliminary results are interesting. All the flint is very similar, regardless of the differences in colour. However, the chert varies enormously. Even pieces that look the same have very different chemical signals. He now has to go away and think about what all this might mean.