Improving Reconstructions of Ancient Artifact Histories

Recent studies have examined analyzing trace elements in mummy remains.^1,2 These studies have helped shed light on the burial practices, as well as the conservation history, of these mummies. Didier Gourier, who is Professor at Chimie ParisTech, is exploring this issue with his team.

Previous interview segments with Gourier concentrated on why micro-particle induced X-ray emission (µ-PIXE) and micro-ion beam induced luminescence (µ-IBIL) spectroscopy were ideal techniques to analyze old wrappings of mummies and what these techniques could tell us about the conservation history of them. In the final part of our interview with Gourier, he discusses how conservation treatments can be identified through the use of µ-PIXE and µ-IBIL spectroscopy.

The detection of mercury suggests 19th-century conservation treatments. How were you able to distinguish these later interventions from the original embalming materials?

Cinnabar, a mercury sulfide (HgS), was prized in ancient Egypt for its intense red color. Because of its rarity, it was primarily reserved for symbolic or prestige purposes. This is why the presence of significant amounts of mercury in samples from a human mummy may suggest that cinnabar was used in the mummification process. However, we have not observed any correlation between mercury and sulfur concentrations, which rules out the presence of cinnabar. On the other hand, we have identified the presence of mercury chloride and mercury bromide, two highly toxic compounds unknown in ancient Egypt. Mercury chloride (HgCl₂), known as “corrosive sublimate,” was first prepared during the Middle Ages by alchemists, but since bromine was not discovered until 1826, mercury bromide (HgBr₂) was synthesized shortly thereafter, between the late 1820s and the 1830s. Therefore, we can rule out the use of these mercury salts in the preparation of the mummy. HgCl₂ was used in medicine until the early 20th century to treat syphilis and certain skin diseases, but also as a preservation agent in biological collections to delay the degradation of specimens. However, no such past uses are known for HgBr₂, perhaps because of its limited availability and high cost, so that its presence on the mummy may seem quite surprising. Given that when HgBr₂ was first synthesized, the mummy was in the possession of a Parisian doctor, one might imagine that this scientifically minded collector was aware of the latest advances in chemistry, and used mercury bromide in addition to mercury chloride to attempt to delay the mummy’s deterioration, particularly from insect damage. Anyway, it was this mercury-contaminated mummy that was acquired by the Boulogne-sur-Mer Museum in 1837.

How can the integration of microanalytical techniques like µ-PIXE and µ-IBIL contribute to more accurate reconstructions of ancient artifact histories and conservation practices moving forward?

Because of their age, many heritage objects retain some of their mysteries. For example, we want to understand how they were made and the origin of the materials they are composed of. These objects are often altered, and it is important to understand the mechanisms of degradation to control them and ensure their protection. Some may have undergone undocumented restorations in the past, and, more importantly, certain historical objects may simply be forgeries. This is why combining multiple types of spectroscopy and associated imaging techniques in a single analysis significantly increases the amount of chemical information available, and thus the chances of revealing the secrets of cultural heritage objects. The analytical methods using an air-extracted proton microbeam offers a number of advantages. First, this type of analysis is non-invasive and can be performed directly on an object of any size. Since the proton beam can scan the surface, all types of radiation emitted at a given impact point can be recorded simultaneously. The only limitations are the size of the scanned surface—and thus the duration of the analysis—and the number of detectors that can occupy the space around the object and the beam exit window, which determines the number of different spectroscopic techniques that can be used simultaneously.

In addition to interacting with the electron shells of atoms (m-PIXE and m-IBIL), which account for the vast majority of interactions, protons can also interact with atomic nuclei. For heavy chemical elements (high Z), the particles are pushed away by the nucleus via Coulombic interaction (Rutherford backscattering, RBS). In the case of light elements (Z < 11), protons penetrate the nuclei and induce nuclear reactions, emitting gamma photons (Particle induced g-ray emission, PIGE) or other nuclear particles (generally referred to as Nuclear Reaction Analysis, NRA).

To take a simple and illustrative example, in 1923 the Louvre Museum in Paris acquired a blue glass head dating from Egypt’s 18th Dynasty, likely a representation of the young Tutankhamun (c. 1336–1327 BCE). This iconic object was on display in the museum for 86 years, until ion beam analysis revealed that it was a forgery. Among other things, PIXE analysis showed that the glass making up the head had been opacified using lead arsenate, a technique unknown to the ancient Egyptians, who used calcium antimoniate (Ca₂Sb₂O₇). Opacification with lead arsenate was invented in the 17th century in Venice. PIGE analysis also revealed the presence of fluorine, a chemical element unknown to the ancient Egyptians, indicating that the glass was artificially aged using hydrofluoric acid, a technique dating from the late 19th century.

References

1. Gourier, D. Tracing the Post-Mortem History of Egyptian Mummies Using Nuclear Microprobe Analysis of Trace Metal Elements and Mineral Dust Particles. J. Anal. At. Spectrom. 2026, 41, 385–403. DOI: 10.1039/D5JA00339C
2. Dutoit, C.E. ; Binet, L. ; Fujii, H. ; et al. Nondestructive Analysis of Mummification Balms in Ancient Egypt Based on EPR of Vanadyl and Organic Radical Markers of Bitumen. Anal. Chem 2020, 92, 15445–15453. DOI: 10.1021/acs.analchem.0c03116.