Material Profiling of Mesopotamian Cylinder Seals by Raman Spectroscopy

Abstract

Raman spectroscopy has been used to non-invasively analyse 30 Mesopotamian cylinder seals dated from the end of the fourth to the last half of the first millennium bce, kept in the collection of Museum für Kunst und Gewerbe (Hamburg, Germany). Besides the identification of the mineral constituents of the seals, the chemical compositions of the corresponding minerals were quantitatively determined from the Raman spectra. Using previously established relationships between the crystal chemistry and Raman-peak positions widths and/or relative intensities for various mineral groups, we demonstrate that (i) the Mg content in calcite-based seals as well as the Mg, Fe²⁺ and Ca contents in pyroxene present in one silicate seal can be determined with a relative error of 2%; (ii) the ^MMg and ^M(Fe + Mn)²⁺ contents in talc-based seals as well as the ^MMg and ^MFe²⁺ contents in antigorite-based seals can be quantified with a relative error of 5%; and (iii) in the case of chlorite-based cylinder seals the content of ^MMg, ^MFe²⁺, ^MAl, ^TSi and ^TAl could be estimated with relative errors of 15%–20%. We show that a classical gemological description of cylinder seals is insufficient, particularly when the most abundant mineral phases are oxides, iron sulfides and phyllosilicates, such as talc, antigorite and chlorite-group minerals. We demonstrate that combining classical art historical studies with archaeological evidence and detailed material profiles can advance provenance studies, reveal trade roots, unravel forgeries and distinguish replaced parts.