A closer look at Roman pottery

Well, it’s been a few weeks since my last post to this blog, and perhaps you’d thought I’d disappeared. Far from it!

On getting home, I managed to get my good night’s sleep, but then it was time to play catch-up with all the work (some boring, some interesting) that didn’t get done whilst at Daresbury, and apply for time at the DIAMOND light source. I have also been taking a preliminary look at our results, and wow! It looks like we can draw some interesting conclusions.

The first thing to do with our data was to distil it down into manageable chunks – this means that what looks like a lot at the time (we filled several DVD’s with data!) actually reduces to a few pictures and graphs – but this means we can now start to see the wood for the trees.

Here’s a little taster – but before I start, a small caveat. These are very early results, and the analysis may change as they are prepared to be submitted to scientific journals. This is important, as anything which goes to these journals is subjected to review by other scientists working in the same area, so they have to be right (or at least, our conclusions have to be justified, which is not necessarily the same thing!). So, this is what we think is going on at the moment, but this may change as we work things out.

Remember the blue pigment I showed you a picture of a couple of posts ago? Well, here’s what things look like if we zoom in more closely with better imaging. From left to right, we have a colour optical image of the region we’re looking at, what we see in green light after x-ray irradiation and what we see in infra-red light. (The images are all the same size, about 0.8x0.8 mm)

Paul Hatherley
Colour optical image of rocks

If we compare the pictures carefully, we see that the green light comes from the clear grains, and the infra-red from the blue (an exception is the grain near the centre, which seems to be giving green light – but look carefully – there looks to be a clear grain attached to it!). Remember, the green is from sand, and the infra-red from Egyptian Blue.

If we now look at how the light emitted changes as we change the x-ray energy, we get an idea of how the different minerals absorb x-rays, which tells us something about how the atoms are arranged. Here’s what we found if we target x-rays which interact strongly with the silicon in sand and Egyptian Blue. The green curve is what we get if we look at the green light (sand) and the red, if we look at the infra-red (Egyptian Blue).

Paul Hatherley
Graph showing X-ray results

Don’t worry about the scales etc. The important thing is the appearance of the curves. The green curve shows two clear, sharp dips (don’t worry about the deep, broad dip). These show that the silicon atoms are arranged in an orderly, crystalline way. These dips are absent though in the red line (Egyptian blue), showing the silicon is less ordered – in other words, it looks more like a glass, which has no definite structure! This is a very telling result, since some other work indicates crystals of Egyptian Blue are present. Do we have a mixture then? Even more intriguingly, does this result tell us something about how the Egyptian Blue in this specimen was made?

Hmm. Perhaps how Egyptian Blue was made would make a good post – watch this space...

Paul.