Shining Light on Dark Matter: Advancing Functional Analysis of Obsidian Tools with Confocal Scanning Microscopy

Journal of Archaeological Method and Theory (2025) 32:32 https://doi.org/10.1007/s10816-025-09700-0 RESEARCH Shining Light on Dark Matter: Advancing Functional Analysis of Obsidian Tools with Confocal Scanning Microscopy Fiona Pichon1,2 · Juan José Ibáñez Estevez1 · Laurence Astruc3 · Bernard Gassin4 · Amelia Rodríguez Rodríguez5 · Carlo Lugliè6 Accepted: 22 January 2025 / Published online: 18 February 2025 © The Author(s) 2025 Abstract Over the past decade, confocal microscopy has increasingly been employed to examine changes in stone tool surfaces and has proven to be an accurate technique for quantifying use-wear texture. Promising results have emerged from characterizing Polish formation on experimental and archaeological flint tools. Recent studies also highlighted the potential of confocal microscopy for analyzing tools made from reflective materials, such as quartzite. In this paper, we investigate the capability of confocal microscopy to discriminate use-wear on obsidian quantitatively. We examine whether confocal microscopy and 3D texture analysis can correctly classify several worked materials that are challenging to differentiate using the optical standard method of use-wear analysis. For cutting activities, we include butchery, de-skinning a fresh hide from grease and meaty tissues, cutting tanned leather, harvesting domestic ripe cereals, harvesting semi-green wild cereals, and sawing wet limestone. As for scraping activities, we explore discriminating differences among tools used for working dry hide, dry antler, soaked antler, fresh bone, softwood, fresh reeds, and wet limestone. Our results demonstrate that these worked materials can be confidently identified in experimental tools. While other relevant factors affecting use-wear texture, such as the intensity of use or post-depositional alterations, need to be controlled before employing the method on archaeological materials, our research suggests that the quantitative approach can enhance the standard method of use-wear analysis, providing unprecedented precision for identifying worked materials in obsidian tools. Keywords Obsidian Tools · Surface texture analysis · Confocal microscopy · Quantification · Experimental archaeology Extended author information available on the last page of the article Vol.:(0123456789) 32 Page 2 of 63 F. Pichon et al. Introduction Understanding the function of prehistoric lithic tools is crucial for reconstructing the behaviors and activities of past human societies. Reliable interpretations of the function of archaeological tools rely on comparisons with an experimental use-wear reference collection, allowing to identify specific traces resulting from the processing of different organic and inorganic materials using modern stone tool replicas. Traditional use-wear studies, alongside residue analysis, have relied on qualitative observations of use-wear patterns using low- and high-power reflected light microscopy, scanning electronic microscopy, and digital microscopy. This involves examining fractures, edge-rounding, edge-scarring, polish, and striations on tool surfaces (e.g., Tringham et al., 1974; Anderson, 1980; Keeley, 1980; Odell, 1981; Anderson-Gerfaud, 1983; Mansur-Franchomme, 1983; Vaughan, 1985; Knutsson, 1988; D’Errico, 1988; Yamada, 1993; González Urquijo & Ibáñez Estévez, 1994; Ollé & Vergès, 2008; Van Gijn, 2010; Rots, 2010; Borel et al., 2014; Marreiros et al., 2020; Gendron et al., 2024). Over the past two decades, there have been numerous attempts to integrate methods for quantifying use-wear on stone tool surfaces, aiming to address issues regarding the accuracy and subjectivity inherent in qualitative approaches while promoting reproducibility and data sharing. These studies hypothesize that microwear is characterized by different surface textures depending on the contact material, which can be measured quantitatively on the stone tool to identify the type of material worked. Several methods have been used for use-wear quantification, such as digital image analysis, defining gray-scale levels of polish brightness for different contact materials (e.g., Grace et al., 1985; Grace et al., 1987; Knutsson, 1988; Grace, 1989; Bietti et al., 1994; González-Urquijo & IbáñezEstévez, 2003; Sferrazza, 2023), optical interferometry (Dumont, 1982; Astruc et al. 2008; Borel et al., 2021), rugosimetry (Anderson et al., 1998; Beyries et al., 1988), atomic force microscopy (Kimball et al., 1998; Kimball & Kimball 1995), and laser profilometry (Stemp, 2014; Stemp & Stemp, 2001, 2003; Stemp et al., 2008, 2009). During the last decade, along with the standardization in areal definitions, confocal microscopy has proved to be a promising technique for surface texture quantification of lithic use-wear (e.g., Álvarez-Fernández et al., 2020; Evans & Donahue, 2008; Evans & Macdonald, 2011; Ibáñez et al., 2014; Macdonald, 2014; Stemp & Chung, 2011; Stemp et al., 2013; Stevens et al., 2010; Zhang et al., 2024). Specifically, confocal technologies have been successfully developed to distinguish different contact materials and stages of wear development from roughness analysis using 3D ISO surface parameters on experimental flint tools (Ibáñez & Mazzucco, 2021; Ibáñez et al., 2016, 2018). Also, sickle gloss texture analysis has been employed in recent studies to identify the types of plants that were harvested in Epipalaeolithic and Neolithic contexts in SW Asia (Ibáñez et al., 2021; Mazzucco et al., 2022; Pichon et al., 2021, 2023). Variations in the gloss that developed on experimental sickles used for cutting ripe domesticated cereals, semi-ripe cultivated wild cereals, green wild cereals, reeds, and grasses were identified and compared with sickle gloss on archaeological tools, Shining Light on Dark Matter: Advancing Functional Analysis… Page 3 of 63 32 demonstrating the progressive changes toward ripe harvesting of cereals alongside the domestication process. While the usefulness of confocal microscopy and roughness texture analysis to quantify use-wear on flint tools is well demonstrated, its application to non-flint lithic raw materials, such as quartzite and obsidian, is still in its early stages of development (Pedergnana et al., 2020a, 2020b; Stemp et al., 2013, 2018). Further research is needed to assess the method’s reliability for tools made in rocks other than flint. In this paper, we present the first application of confocal microscopy and 3D surface texture analysis on experimental obsidian tools, addressing the specific challenges posed by this vitreous, highly reflective silica-rich volcanic glass. Obsidian tools hold significant importance in archaeological contexts worldwide due to their unique properties and essential role in prehistoric societies (Tykot, 2021). This volcanic glass was often transported over vast distances, sometimes spanning hundreds or even thousands of kilometers from its original geological sources (Tykot, 2021). Obsidian analyse (...truncated)