In 2008, archaeologists excavating a medieval refuse pit in Mainz, Germany, discovered a heavily corroded pendant likely made in the late 12th century. But they were loath to open the pendant to find out what might be inside, lest they damage an already fragile artifact. Now technology has come to the rescue. Researchers from the Technical University of Munich scanned the pendant using neutron tomography, among other methods, and discovered it contained bone splinters—likely religious relics, i.e., the purported bones of saints. The findings were published in the interim meeting of the International Council of Museums-Committee for Conservation (ICOM-CC) Metals Working Group.

Neutron tomography works much the same way as X-ray and gamma-ray imaging methods, except it uses a neutron beam. A target object is shot with a beam of radiation, and some parts interact with the sample while others pass through. The ones that pass through collide with an imaging target to create what's known as an attenuation pattern—essentially an image of the interior of the sample. Neutron tomography is not as sensitive to the density of materials as X-ray and gamma-ray imaging, and unlike those methods, neutrons interact strongly with very light elements like hydrogen. So some things easily visible with neutron imaging may be challenging or impossible to see with X-ray imaging (and vice versa).

The techniques can be complementary and are especially useful for imaging archaeological or paleontological artifacts because they don't damage or destroy the original object. For instance, in December 2021, researchers combined X-ray microtomography—which involves using X-rays to make cross-sections of a physical object—and neutron tomography to create a highly detailed 3D model of a 365 million-year-old ammonite fossil from the Jurassic period, revealing internal muscles that have never been previously observed. Among other findings, they observed paired muscles extending from the ammonite's body, which they surmise the animal likely used to retract itself further into its shell to avoid predators.

The gold-plated copper pendant in Mainz measures just 2.4 inches (6 centimeters) high and wide and is in the shape of a quatrefoil (a shape common in traditional Christian symbolism). The front and back are enameled using a technique known as champlevé, which involves carving or etching troughs into the surface of a metal object and then filling them with porcelain enamel. The uncovered portions are gilded, a common practice in medieval times. One side depicts Jesus, with four evangelists pictured in the four rounded ends. The other side features 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 all the elements present. And infrared spectroscopy revealed a small sample of beeswax. However, "We couldn’t just open the trailer and look inside,” said Matthias Heinzel, a restorer at Leibniz-Zentrum für Archäologie (LEIZA), part of the Technical University of Munich. “The object and, above all, the locking mechanism have been severely damaged by centuries of corrosion, and opening it would mean destroying it irrevocably.”

Using neutron imaging preserved the pendant while revealing five small reliquary packages of silk and linen holding bone splinters. Heinzel et al. identified individual elements of the sample by triggering them with a gamma-ray technique called prompt gamma activation analysis (PGAA). “We can’t say whether or not these bone splinters are from a saint and, if so, which one,” said Heinzel. “Usually relic packages contain a strip of parchment indicating the name of the saint. In this case, however, we unfortunately can’t see one."

The now-fully restored pendant is currently on display at the Mainz State Museum.