Quantifying Edge Sharpness on Stone Flakes: Comparing Mechanical and Micro-Geometric Definitions Across Multiple Raw Materials from Olduvai Gorge (Tanzania)

Journal of Archaeological Method and Theory https://doi.org/10.1007/s10816-022-09596-0 Quantifying Edge Sharpness on Stone Flakes: Comparing Mechanical and Micro‑Geometric Definitions Across Multiple Raw Materials from Olduvai Gorge (Tanzania) Alastair Key1 · Tomasz Bartkowiak2 · Danielle A. Macdonald3 · Patryk Mietlinski2 · Bartosz Gapinski2 · Ignacio de la Torre4 · W. James Stemp5 Accepted: 25 November 2022 © The Author(s) 2022 Abstract In line with engineering research focusing on metal tools, techniques to record the attribute of ‘edge sharpness’ on stone tools can include both mechanical and micro-geometric approaches. Mechanically-defined sharpness techniques used in lithic studies are now well established and align with engineering research. The single micro-geometrically-defined technique—tip curvature—is novel relative to approaches used elsewhere, and has not explicitly been tested for its ability to describe the attribute of sharpness. Here, using experimental flakes produced on basalt, chert, and quartzite sourced at Olduvai Gorge (Tanzania), we investigate the relationship between tip curvature and the force and work required to initiate a cut. We do this using controlled cutting tests and analysis of high-resolution microCT scans. Results indicate cutting force and work to display significant dependent relationships with tip curvature, suggesting the latter to be an appropriate metric to record the sharpness of lithic tools. Differences in relationship strength were observed dependent on the measurement scales and edge distances used. Tip curvature is also demonstrated to distinguish between the sharpness of different raw materials. Our data also indicate the predictive relationship between tip curvature and cutting force/work to be one of the strongest yet identified between a stone tool morphological attribute and its cutting performance. Together, this study demonstrates tip curvature to be an appropriate attribute for describing the sharpness of a stone tool’s working edge in diverse raw material scenarios, and that it can be highly predictive of a stone tool’s functional performance. Keywords Edge Geometry · Cutting Performance · Focus Variation Microscopy · Edge Profile Curvature Analysis · MicroCT · Basalt, Chert, Quartzite * Alastair Key Extended author information available on the last page of the article 13 Vol.:(0123456789) A. Key et al. Introduction Edge sharpness is one of the most important attributes influencing the functional performance of cutting technologies. It is key to determining the force and energy (work) required for a cut (fracture) to be formed in a worked material, and the level of material deformation created while doing so. This relationship is known to apply equally to metal and stone cutting edges in modern and archaeological contexts, and has been demonstrated in diverse worked material contexts (Atkins, 2009; Key, 2016; Reilly et al., 2004). Understanding of a relationship between edge sharpness and stone tool cutting performance is commonplace in lithic archaeology (e.g., Jones, 1980; Tryon et al., 2005; Dewbury & Russell, 2007; Machin et al., 2007; Braun et al., 2008; Bebber et al., 2019; Stemp et al., 2019; Lin & Marreiros, 2021), with references from as early as the nineteenth century (Lartet & Christy, 1875; Prestwich, 1860). Indeed, the intuitive, often common-sense, connections between the formal properties of an object and the individual interacting with it mean that functional considerations—such as the relationship between sharpness and cutting performance—are widely considered within Palaeolithic research, an example of which is the influential typological list developed by F. Bordes (1961). These perceptions are typically formed through a process of analogical reasoning or the experimental use of replica stone tools. Analogical reasoning has a long history of informing form-function relationships in stone tool technologies (Key & Lycett, 2017a; Pettitt & White, 2013), including when it comes to edge sharpness. We are as dependent today on hand-held cutting tools as we were during the Palaeolithic, and fundamental principles, such as the impact that edge dulling (reducing sharpness) has on modern metal cutting tool performance, can be readily transferred to stone technology. With the growth of experimental archaeology and the use of replica stone tools to provide referential frameworks for understanding artefacts (Eren et al., 2016; Lin et al., 2018; Outram, 2008), our senses further reinforce these analogical links. Indeed, perceptions of ‘effort’ and cutting ‘ease’ as we use stone tools inform us about which aspects of their morphology may be influencing their cutting performance. In the present context, this means that as replica stone tools are used for extended durations and cutting edges start to dull, there are changes to the relative performance characteristics (c.f., Schiffer & Skibo, 1997) that become perceptible to tool users (e.g., increased working forces). Together, this has contributed to the widely held understanding that edge sharpness would have been important to stone tool users in Palaeolithic, ethnographic, and historical contexts. Such is its prominence, the requirement to maintain a sharp working edge is the functional selective pressure underpinning important theoretical frameworks emphasising edge ‘resharpening’ and/or ‘rejuvenation’ (e.g., Kuhn, 1990; McPherron, 1999; Iovita, 2010; Eren et al., 2013; Morales & Verges, 2014; Buchanan et al., 2015; Schimelmitz et al., 2017; Maloney, 2019), or explaining the heat treatment of stone tool raw materials (Domanski & Webb, 2007; Key et al., 2021; Rick & Chappell, 1983). Ethnographic and experimental accounts have even detailed how much ‘use’ is required prior to edges becoming 13 Quantifying Edge Sharpness on Stone Flakes: Comparing… blunt enough to require resharpening. Weedman’s (2006) account of Gamo hide workers in Ethiopia, for example, details hafted stone scrapers used during hide preparation to require resharpening after an average of 281 ‘scrapes’. Notably, the impact of sharpness on tool performance can also include ergonomic interactions that require edges to be dulled to improve safety and ‘ease’ of handling. Usually this takes the form of ‘backing’ or intentional blunting at the point of interaction between the hand and the gripped portion of the tool (Delpiano et al., 2019; Parush et al., 2015; Tringham et al., 1974). There are even indications that the earliest stone toolmaker hominins would have been aware of the benefits of using sharper and more durable stone edges (Braun et al., 2008; Key et al., 2020). Even Kanzi, the bonobo (Pan paniscus) trained to flake and use stone tools, was observed testing the relative sharpness of flake edges using his tongue (Schick et al., 1999; Toth et al., 1993). Despite the likely importance of edge sharpness to past populations, direct investigation of this attribute on lithic (...truncated)