In 2008, while excavating a medieval rubbish pit in Mainz, Germany, archaeologists discovered a heavily corroded pendant that was probably made in the late 12th century. But they were reluctant to open the pendant to see what might be inside, lest they damage an already fragile artifact. Now technology has come to the rescue. Researchers at the Technical University of Munich scanned the pendant using neutron tomography, among other things, and discovered bone splinters in it – presumably religious relics, i.e. alleged bones of saints. The results were published at the Interim Meeting of the Metal Working Group of the International Council of Museums-Committee for Conservation (ICOM-CC).
Neutron tomography works similarly to X-ray and gamma-ray imaging, except it uses a beam of neutrons. A beam of radiation is fired at a target, and some parts interact with the sample while others pass through. Those that pass collide with an imaging target to create what’s called an attenuation pattern — essentially an image of the inside of the sample. Neutron tomography is not as sensitive to material density as X-ray and gamma-ray imaging, and unlike these methods, neutrons interact strongly with very light elements such as hydrogen. Some things that are easily visible with neutron imaging can be difficult or impossible to see with X-ray imaging (and vice versa).
The techniques can complement each other and are particularly useful for imaging archaeological or paleontological artifacts as they do not damage or destroy the original object. For example, in December 2021 researchers came together X-ray microtomography— which uses X-rays to create cross sections of a physical object — and neutron tomography to create one highly detailed 3D model of a 365 million year old ammonite fossil from the law Period revealing inner muscles never seen before. Among other things, they observed paired muscles extending from the ammonite’s body, which they think the animal likely used to retreat further into its shell to evade predators.
The gold-plated copper pendant in Mainz is only 6 centimeters high and wide and has the shape of an a quatrefoil (a form common in traditional Christian symbolism). The front and back are enamelled using a technique known as champlevé, in which depressions are carved or etched into the surface of a metal object and then filled with porcelain enamel. The uncovered parts are gilded, a common practice in the Middle Ages. One side shows Jesus with four evangelists in the four rounded ends. The other side shows Mary surrounded by four female saints.
The team first analyzed the surface using a combination of micro X-ray fluorescence and Raman spectroscopy to identify any elements present. And infrared spectroscopy showed a small sample of beeswax. “But we couldn’t just open the trailer and look inside,” said Matthias Heinzel, restorer at the Leibniz Center for Archeology (LEIZA), part of the Technical University of Munich. “The object and especially the locking mechanism are badly damaged by centuries of corrosion, and opening them would mean irreversible destruction.”
Using neutron imaging, the pendant was preserved, while five small silk and linen reliquary packages containing bone fragments were uncovered. Heinzel et al. identified individual elements of the sample by triggering them using a gamma ray technique called Prompt Gamma Activation Analysis (PGAA). “We can’t tell if these bone splinters came from a saint, and if so, which one.” said Heinzel. “Typically, packages of relics include a strip of parchment inscribed with the saint’s name. In this case, unfortunately, we can’t see any.”
The trailer is now fully restored to see currently in the Mainz State Museum.
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