Assessing the viability of portable Raman spectroscopy for determining the geological source of obsidian

Abstract

Obsidian is a naturally occurring volcanic silicate glass produced when highly viscous felsic lava cools rapidly through the glass transition temperature, not allowing sufficient time for crystal growth. Obsidian was a popular raw material in prehistory because its amorphous and isotropic nature means that little force is required to produce conchoidal fractures of predictable shapes and sizes and it is easily flaked or knapped into desired shapes. A class of obsidian artefacts known as ‘stemmed tools’ has been found in locations across lowland Papua New Guinea and has been roughly dated to the early and middle Holocene periods (ca. 10,000–3400 BP). The rarity of obsidian in the natural world, the large size, fragility, brilliance, and distinctive shapes of these objects, as well as their manufacture by craft specialists, suggest that the tools were highly valued in the past. This inter-disciplinary project aims to determine when and how prehistoric obsidian stemmed tools were used to structure social relations and if they defined status differences. This involves studying the history, typology, technology, function, and geological sources of the stemmed tools and comparing this information with other highly worked stone tools from the major obsidian sources of West New Britain and Manus. Raman spectroscopy and multivariate analysis have been previously used by the authors to discriminate between three major Pacific obsidian sources, an important first step in tracking the movement of stemmed tools through social networks. This paper assesses the viability of using a portable Raman spectrometer (EZRaman-i, 785 nm, ∼6 cm −1 spectral resolution) for identification of the geological source of obsidian through the comparison of spectra collected from 65 samples of known provenance using both a portable and laboratory-based instrument (Renishaw inVia, 785 nm, ∼1–2 cm −1 spectral resolution). The paper demonstrates that results achieved with the portable instrument are similar to those obtained with a laboratory-based instrument.