the letter m


I’ve been obsessing recently about the letter m.

Last week as part of our AHRC funded Metamorphoses project, Sarah Craske and myself took a copy of Ovid’s Metamorphoses from 1735, to the Surface Analysis Labs at the University of Surrey for analysis with Energy-Dispersive X-ray Spectroscopy.

In the lab, we carefully cut out a small sample of the book and placed into a aluminium sample holder ready for analysis. Now, I would have cut out a practical rectangular piece (I’m the scientist) but Sarah chose a more aesthetically pleasing circle (Sarah’s the artist), and we went ahead with this. We ended up then, with a small circle of nearly 300 year-old paper with the word “nondum” at its centre (again this was Sarah’s choice)

As preparation for its analysis, the paper had to be coated with a layer of elemental gold of just 0.7 nanometres in depth. For reference, gold leaf is typically around 0.1 micrometres in depth, and thus ~140-times thicker.

So we placed the ancient paper circle into the sputtering machine. At an appropriate state of vacuum, a swirling deep purple (maybe that’s where the got their name from) plasma flashed into existence, and danced briefly over our paper. When the paper was removed from the device, it had the faintest golden sheen to it, and I’m guessing that it had gained a little bit in value because of it diaphanous coating of gold.

For the analysis itself, our fragile and ancient paper disk was placed into a JSM-7100F Field Emission Scanning Electron Microscope (£500,000 worth of hardware) and its acceleration voltage ramped up to 5.0 kV.

The principal behind Energy-Dispersive X-ray spectroscopy (EDX) is that a high-energy beam of charged particles (electrons in our case) is focused into the sample of interest. The energy of the incident beam is such that it can eject an electron in an atom of the sample from the inner shell, creating an electron hole where the electron once was. Finally, an electron from an outer, higher-energy shell will then move in to fill this hole, and because it is more energetic than the hole that it now has to occupy, it releases this difference in energy as an X-ray. These X-rays are unique and indicative for each individual element, so that when they are measured, they reveal the identity of the element that they came from.

The machine then began to slam electrons into our ancient paper at speeds approaching that of light itself. Remarkably, as we watched the results of the high-energy electron collisions, and the collected X-rays from these accumulate on the computer screens before us, the letter m, the last letter of “nondum” began to form. Unlike our own eyes, which see the letter m because of the way that the ink interacts with light, the machine sees the letter for what it is really is, that is a small area and design made nearly 300 years ago from the element carbon.

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