From golden emperor to filled Buddha

A hollow bust made from pure gold, discovered in a Roman sewer pipe. It was already a sensation in 1939 when assistants at a dig in Avenches, in the Swiss canton of Waadt, uncovered the delicately wrought image of the emperor Marcus Aurelius. Since then, the priceless object has served archaeologists and art historians as an important source for research into the Roman history of Switzerland. But scholarship and science did not stop there: In 2006, the bust came from the Roman Museum in Avenches to the Paul Scherrer Institute. Here the golden emperor was expected to give up his last secret: How were the artisans of the time able to create a hollow likeness of Marcus Aurelius's head, neck, and shoulders in such a way that no seams between joined gold plates were visible? Eberhard Lehmann of PSI had the technology that should make it clear: imaging with neutrons, which so to speak transilluminate the metal. Lehmann knew, however: Here we can only produce data. The archaeologists have to interpret it.

Both the specialists in cultural heritage and the PSI experts in advanced imaging techniques are thrilled with how productive their collaborations are. For that, however, both sides needed the will to approach each other. Marie Wörle, head of the Laboratory for Conservation Research at the Swiss National Museum, appreciates the approach of the scientists at the neutron source: We and the PSI physicists have a similar attitude towards work, she finds. On both sides a very great openness prevails with respect to new topics and collaboration with external researchers.

Neutron images show where corrosion strikes

Art historians, archaeologists, and conservation researchers such as Wörle profit from the insights provided by imaging at the PSI neutron source SINQ: In neutron images the Spanish figure of a violinist from the year 1920, fashioned from wood and cased in lead, disclosed hidden corrosion: The lead corrodes outward from the inside, because organic acids escape from the wood. A second example are historic brass instruments: So they can still be played as well as preserved, Wörle found out -  thanks to the neutrons - at which spots pockets of moisture form during use. In another joint project, researchers from the Swiss National Museum, Bern University of the Arts, and PSI are currently investigating a conservation technique for ancient objects; the neutron images show if the corrosion treatment is effective to the desired depth.

Neutron beams reveal what lies inside metal objects without destroying them. That's because the neutrons penetrate materials where any X ray beam - the classic means for nondestructive testing - will fail. This pleases the researchers as well as the museum staff. The point is to get a glimpse of an object's inner life, explains Myriam Krieg, head of the Laboratory for Conservation and Restoration at the Roman Museum in Avenches. One such object is an armlet with a bulging hollow space that had originally opened on a hinge. Experts in Switzerland's Roman period know that a coin was sometimes kept in such cavities, perhaps as a talisman. The piece of jewelry in question had corroded shut, however, over the centuries. The archaeologists would have liked to pry the capsule open to get a look inside, Krieg says. For the art restorer, this was inconceivable. Instead, she turned to PSI: The examination at SINQ showed that there was no coin in the hollow space - but rather several little metal beads and a substance yet to be identified. Sometimes a new insight raises further questions.

Sitting together at a table and discussing the results with each other often leads to amazing insights, Krieg says. One of the latest finds that she had analysed at PSI was a cosmetic toolset made of brass that Roman residents of Avenches would have taken with them to the thermal springs. Among the items hanging on this ring, as if on a Swiss Army knife, were blades for the removal of body hair and small spoons for cleaning the ears.

For scholars of culture like Krieg, the atmosphere at PSI is a new experience: Deeply impressed -  nearly awe struck is how she describes her feelings the first time she entered the hall of the neutron source SINQ: Strict safety precautions, contamination tests, and concrete walls behind which the neutron beam is aimed at ancient artefacts and other objects to be studied – such a setting is rather unfamiliar for people who go about their work in the comparatively plain atmosphere of a museum.

A research laboratory "like a fallout shelter"

Michael Henss, a PhD art historian and expert in Asian art, describes his first impression even more dramatically: You get the feeling that you're coming into a fallout shelter. Through a colleague, Henss had heard of the machine that could help see through metal. That was exactly what he was looking for: For several centuries in Tibetan and Chinese Buddhism, hollow statues made from bronze, brass, and other copper alloys were filled with consecrated offerings meant to breathe life and religious meaning into them. There was a whole palette of possibilities including medicinal herbs, flowers, exotic woods, scrolls with mantras -  that is, Buddhist religious formulas -  and more. The ritual proceeds according to strict rules. A massive metal plate seals the base of the Buddha statue. To open it would mean an immediate desecration. The neutron imaging technology, however, brought the hidden contents of the statue to light without this. What represented no surprise for the Asia expert was, in contrast, new for the PSI scientists. To actually see what's inside the sculptures still impressed Henss, too. Out of this cooperation grew follow on projects of PSI with Henss as well as numerous museums and private collectors of Buddhist art and even, in 2014, a symposium at the Rietberg Museum in Zurich. Henss is enthusiastic about it: That is really a rare and fascinating collaboration between natural science and cultural scholarship.

The neutron images have finally made it possible get to the bottom of Marcus Aurelius's secret too: With their help, the archaeologists of the Roman Museum in Avenches were able to discern that the metal bust, only about one millimetre thick, was probably crafted from a single gold plate. And that's not all: In the neutron images, the archaeologists could even identify individual hammer strokes made in Roman times by the bust's creators and thus could reconstruct the artful technique by which it was produced.

Text: Luise Loges