The Obsidian Sieve: an Excel‑Based Tool for Provenance Studies

Abstract

This study presents the Obsidian Sieve, an Excel‑based tool designed to provide a simple, rapid, and flexible first‑step provenance assessment for Central Mediterranean obsidians using only major and minor elements. Its development was grounded in a systematic review of provenance studies that report quantitative concentrations of major and‑minor oxides/elements, including chlorine (Cl), still relatively uncommon in the literature despite its high discriminant potential. Seven bivariate systems traditionally employed in obsidian characterization were re‑evaluated through a unified statistical workflow combining Kernel Density Estimation, Mahalanobis distance, 97.5% confidence ellipses, and quantitative measures of cluster overlap. This comparative analysis demonstrates that the Na2O vs Cl diagram provides the clearest and most stable separation among the four main Italian sources, yielding the highest Discriminant Power Index and uniquely displaying non‑overlapping confidence fields. These results form the statistical basis for defining conservative Na2O‑Cl windows that underpin the Obsidian Sieve. The tool assigns archaeological specimens to Monte Arci, Palmarola, Pantelleria, or Lipari with high specificity, while flagging atypical or ambiguous cases as Unknown, thus directing research towards further analysis with more advanced geochemical techniques. Because the Sieve is transparent, editable, and easily adaptable, users can extend its capabilities by incorporating additional published datasets, refining source windows, or testing alternative discriminant bivariate plots. In this way, the Obsidian Sieve provides not only a practical screening instrument but also a flexible framework for cumulative improvement as new analytical data become available.