🧠 FEMTO MAGAZINE: High tech for the Louvre

It may be the oldest writing system in the world: Starting around 3300 BC, the Sumerians in Mesopotamia immortalised themselves using a script called cuneiform, by pressing a stylus into soft clay. This technique was later adopted by other cultures, such as the Babylonians, Assyrians and Persians – until it was finally supplanted by more modern forms of writing about 2000 years ago.

Forever unopened

However, not every document reached its destination – and some remained unopened forever. Today, archives and museums hold thousands of such sealed letters. As interested as the experts are in studying their contents, they do not want to open the old treasures – after all, this would mean destroying them. Yet the X-ray machines that museums have been using up to now cannot penetrate the clay envelopes in order to read the contents of the tablets inside. This is because such machines produce comparatively low-energy radiation. While these X-rays are adequate for analysing paintings and have the big advantage that proper radiation protection is comparatively easy to implement, the low-energy radiation is simply not strong enough to penetrate the thick clay envelopes. That requires a higher-energy X-rays with noticeably more “penetrating power”.

Christian Schroer’s group is now making a fresh, more technically sophisticated attempt. It has constructed a mobile X-ray device that uses high-energy radiation to look inside the clay letters and reveal their contents. This work is being conducted within the Cluster of Excellence “Understanding Written Artefacts” at Universität Hamburg, comprising experts from various northern German research institutions who are taking a closer look at the evolution and function of written documents from all over the world. Schroer’s team has been working particularly closely with the Hamburg Assyriologist Cécile Michel.

“The Sumerians are now thought to have first dried the actual clay tablets,” says Schroer. “Then they wrapped the envelopes around the tablets fairly loosely, leaving a narrow air gap between the two.” The idea is that this air gap will provide the necessary contrast in the X-ray image, allowing the tablet to be distinguished from the envelope and its contents to be deciphered.

3D images

In the laboratory at the Centre for X-ray and Nano Science (CXNS), Schroer’s colleague Ralph Döhrmann points to the device. Its design is modular, consisting of several segments that can be assembled to form a complete unit. “All the individual parts are portable,” explains the engineer. “That way, the device can be transported up stairs and down narrow corridors in a museum.” Once assembled, the unit is about the size of an overflowing shopping trolley – and, like a shopping trolley, it can be pushed around on wheels. All the individual parts were manufactured and tested in DESY’s workshops, and the programs that control it were developed and written by Schroer’s team. The heart of the device is a special X-ray tube that focuses the radiation onto a spot that is only 20 to 40 micrometres across. A micrometre is one thousandth of a millimetre. The X-rays illuminate a sample holder on which the clay tablet is mounted. “This holder consists of finger-like grippers, which can be used to gently clamp the tablet,” explains Döhrmann. “It can be rotated and moved in all directions.” As a result, the tablet can be scanned from every angle – the device is a tomograph, which produces three-dimensional images. An X-ray detector records the radiation passing through the sample, and the data ends up on an onboard server and hard drive. Depending on the dimensions of the tablet – the largest are the size of a brick – screening can take two to three hours, in extreme cases even a day.

The biggest challenge in designing the tomograph is that it should deliver as much power as possible in the smallest possible space, and yet remain absolutely safe. “Due to the high energy of the X-ray tube, it is not at all easy to screen the outside from the radiation, ensuring radiation protection and keeping the operators safe,” explains Döhrmann. The problem is that, on the one hand, the tungsten–aluminium plates used as shielding need to be thick enough to reliably contain the radiation. On the other hand, they must not be too thick, otherwise the unit would become too heavy for mobile use – it should not weigh much more than 300 kilograms in total.

Laboratory testing

Using a large laboratory tomograph, the experts were able to demonstrate some time ago that the method does work in principle. The object used at the time was a “replica” clay envelope with clay contents. “We were able to separate the inner part from the outer part,” says Ralph Döhrmann. “Afterwards, we managed to print a 3D model of these test clay tablets using the tomography data.” Similar tests are to begin soon with the new, compact model. The first place to use the mobile tomograph will probably be the world’s most famous museum: the Louvre in Paris. Some 50 sealed clay letters are waiting there to be deciphered.

“I could imagine that the experts at the Louvre will be very reluctant to relinquish the device,” says Schroer.

“Because it can be used to study many other works of art and cultural treasures.” Ideas for this already exist, including analysing the spines of historical books. These often used to be made from waste paper. If X-ray scanners could reveal what was written on these documents, which were considered superfluous at the time, some surprising insights might emerge. “We have already carried out some initial tests at DESY and found that documents written with ferrous inks can indeed be identified,” Schroer says.

Anyway, the new mobile X-ray tomograph does not have to remain the only one of its kind. Admittedly, its development was complicated and took over two and a half years. But the hardware costs are manageable, so it should be perfectly possible for a company to manufacture a small-batch series of the devices. “Once people see that the tomographs work,” says Christian Schroer, “I think there will be a strong demand for them.”