Characterizing Trace Elements in Mummies

Researchers recently used micro-particle induced X-ray emission (µ-PIXE) and micro-ion beam induced luminescence (µ-IBIL) spectroscopy to analyze trace metals and dust particles in coatings from 16 Egyptian mummies from French museums, with a detailed focus on one human mummy housed at the Boulogne-sur-Mer Museum.¹ Vanadium and nickel measurements revealed that its bitumen originated from a previously unidentified source, unlike others linked to the Dead Sea. The study also identified Egyptian blue pigment and airborne particles, with distinct distributions on inner and outer surfaces of linen strips.¹ These patterns suggest exposure to two separate dust events in southern and northern Egypt. Elevated mercury levels indicate undocumented 19th-century preservation treatments using mercury salts to protect the mummy from biological degradation.¹

To learn more about the study’s findings, Spectroscopy sat down with lead author Didier Gourier, Professor at Chimie ParisTech-PSL.

How did the complementary use of µ-PIXE and µ-IBIL spectroscopy enhance your ability to characterize trace elements and particle distributions in the mummy samples?

A 3 MeV proton penetrating a few 10s of micrometers into matter interacts in various ways with the atoms along its path. Most of these interactions occur with the electron shells of these atoms, yielding different information depending on which types of electrons are involved in the interaction. Internal electrons are highly sensitive to the nuclear charge, and thus to the atomic number Z of the elements. The X-ray emission resulting from their excitation (PIXE spectroscopy) allows us to identify and quantify elements heavier than sodium (Z = 11). Since mummies and their coverings (balms, linen strips) are purely organic in nature (Z ≤ 8), the detection of heavier elements, whether in trace amounts or not, on a PIXE mapping allows for the identification of trace metal impurities in the organic matter and of mineral particles even when their size is much smaller than the diameter of the proton beam (approximately 30 mm).

However, the detection and quantification of elements such as silicon (Si), aluminum (Al), calcium (Ca) or iron (Fe), among the most abundant on Earth, is not sufficient to determine the nature of the minerals present on the mummy, because PIXE spectroscopy does not provide information about the environment of the probed atoms. It is the outer electrons (valence electrons) that can provide this information because their energy is highly sensitive to the ionization state of the emitting species, to the nature and number of surrounding atoms, and to the symmetry of the system.

In this case, the photons emitted (IBIL spectroscopy) have energies ranging from the near-UV to the near-infrared (NIR). However, it is precisely because this luminescence is highly variable and sensitive to the atomic environment that it generally cannot, on its own, identify the mineral particle responsible for the emission. In contrast, the combination of PIXE and IBIL mapping most often allows for the precise identification of these particles.

Consider the example of a PIXE mapping showing the presence of a microparticle containing both Si and Al, thus an aluminosilicate, trapped in a bioorganic matrix containing also sodium (Na), potassium (K), Ca, and Fe impurities. If this microparticle exhibits an emission band in the NIR region around 650–770 nm, we may conclude that this luminescence is because of Fe(III) impurities in an alkali feldspar (albite NaAlSi₃O₈), which can be verified by measuring the Si:Al ratio, which must be equal to 3. Because Fe is one of the most abundant elements on Earth, Fe(III) impurities are ubiquitous in samples taken from mummies, but only those that emit in the NIR allow them to be localized in feldspars.It is thanks to this m-PIXE/m-IBIL combination that we were able to detect the presence of abundant microparticles of feldspars, calcite, and also cuprorivaite (Egyptian blue) in the mummies’ bandages and embalming matter.

Could you elaborate on how vanadium (V) and nickel (Ni) quantification serves as a geochemical fingerprint for identifying bitumen provenance?

Vanadium (V) and nickel (Ni) are two metal elements that are not essential for life and are therefore present only in trace amounts in the human body and in the animal and plant substances used to prepare mummies. V and Ni are also present in trace amounts in the oceans, where they are soluble in the V(V) and Ni(II) states. When decomposing organic matter from plankton settles to the bottom of an oxygen-poor sea, the dissolved V(V) and Ni(II) ions are carried down with it as they form complexes with organic molecules such as humic acids, thereby enriching the organic sediment accumulating on the seafloor with these metals. Under reducing conditions of the seafloor, soluble V(V) is reduced to insoluble V(IV), which further enriches the sediment with vanadium by extracting the dissolved vanadium from the water. Thus, different Ni:V ratios of the organic sediment reflect different reducing conditions on the seafloor. As it accumulates in sediments, buried organic matter undergoes chemical transformation over millions of years to form oil, a heavy fraction of which constitutes the bitumen (or asphalt) used by ancient Egyptians. Trapped within this fossilized organic matter, V(IV) and Ni(II) ions form highly stable complexes—vanadyl and nickel porphyrins—by binding with porphyrins derived from the breakdown of planktonic chlorophyll. Thus, the V content, as well as the Ni:V ratio of the bitumen, closely reflect the biological and redox conditions prevailing in ancient seas, and are markers of different sources of bitumen.

Bitumen from the Dead Sea was a valuable material prized in ancient Egypt, and one that was relatively easy to access. Because it is lighter than water, large quantities of bituminous material seeping from geological faults at the bottom of the Dead Sea solidify into large blocks of bitumen that float on the surface. Their high vanadium content and relatively low Ni:V ratio can serve as a geochemical marker for this source of bitumen. The presence of V and Ni with a specific Ni:V ratio not only indicates the presence of bitumen in the embalming material but also provides clues as to its origin. If this origin is identified (for example, through a specific Ni:V ratio or other molecular markers), the absolute values of the V content can be used to determine the percentage of bitumen present in the embalming matter.

Your findings suggest that the bitumen used in one mummy originated from an unknown source rather than the Dead Sea. What analytical evidence led to this conclusion, and what are the broader implications for understanding ancient trade or material sourcing?

All of the mummy embalming matter we analyzed in this study contain varying amounts of bitumen mixed with other components of plant and animal origin. In all but one case, the bitumen is rich in vanadium and sulfur, with a low Ni/V ratio corresponding to that of Dead Sea bitumen. The human mummy that stands out from the others exhibits two distinctive characteristics. First, the bitumen covering this mummy is pure and unmixed with other plant or animal components, as confirmed by organic analysis and electron paramagnetic resonance (EPR).² On the other hand, this bitumen contains less V and as much Ni as Dead Sea bitumen. Thus, it has a significantly higher Ni:V ratio, but it also has a much higher sulfur content. Taken together, these chemical characteristics do not match those of bitumen sources from Red Sea coasts of Egypt, Palestine, or other sites in the Middle East. A detailed analysis of all Middle Eastern sources could help identify the origin of the bitumen from this particularly unusual mummy.

From a historical perspective, a supply chain may be active or not, depending on the prevailing geopolitical situation at the time. For example, a military conflict could temporarily cut off the supply of bitumen from the Dead Sea, making it necessary to find an alternative source. This is somewhat similar to the situation we are currently facing with the ongoing conflict in the Middle East.

References

1. Gourier, D.; Anduze, O.; Lemasson, Q.; et al. 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.