Mineral FIngerprinting of Cylinder Seals – A Non-destructive Raman Spectroscopic Approach

Cylinder seals are among the most iconic and oldest portable rock-based written artefacts of ancient Mesopotamia/Near East, used for thousands of years to mark ownership and protect what was sealed when rolled over a soft material like clay. But what are these barrel-shaped carved historical objects actually made of, and what can their material analysis tell us about the ancient world? To answer this, Raman spectroscopy was applied as a non-invasive non-destructive analytical tool to study 30 Mesopotamian cylinder seals held in the collection of the Museum für Kunst und Gewerbe (Hamburg, Germany), spanning from the late 4th to the second half of the 1st millennium BCE. In addition to determining the mineral constituents of the seals, the chemical compositions of the identified minerals were quantitatively extracted directly from the Raman spectra.

Previously well-established relationships for several mineral groups between the crystal chemistry and Raman spectral parameters, including peak positions, widths, and relative intensities, were used to demonstrate that: (i) the Mg content in calcite-based seals, along with the Mg, Fe²⁺, and Ca contents in a pyroxene-made seal, can be determined with a relative error as low as 2%; (ii) the Mg and (Fe+Mn)²⁺ contents at the octahedral crystallographic sites in talc-based seals, as well as the Mg and Fe²⁺ contents at the same sites in antigorite-based seals, can be quantified with a relative error of 5%; (iii) for chlorite-based cylinder seals, the content of octahedrally- Mg, Fe²⁺, Al and tetrahedrally-coordinated Si, and Al can be estimated within relative errors of 15–20%. Furthermore, we highlight the limitations of conventional gemological classification for cylinder seals, particularly when the dominant mineral phases include oxides, iron sulfides, and phyllosilicates such as talc, antigorite, and chlorite-group minerals. Finally, we establish that the integration of classical art historical analysis, archaeological context, and detailed material characterization constitutes a robust framework for advancing provenance studies, reconstructing ancient trade routes, detecting forgeries, and identifying restored or substituted components.