Revisiting lithic edge characterization with microCT: multiscale study of edge curvature, re-entrant features, and profile geometry on Olduvai Gorge quartzite flakes

Abstract

The angle and shape of lithic tool edges have long been used as a method of inferring prehistoric tool function. However, accurately measuring and characterizing edge angles of lithic tools is notoriously difficult. Studies using goniometers, calipers, or morphometrics often rely on twodimensional representations of edges. Furthermore, there have been limited attempts to quantitatively document stone tool edges based on surface metrology, most notably edge curvature. In this study, we use microCT to capture models of the complex geometry, or 'freeform' surfaces, of experimental quartzite flakes from Olduvai Gorge (Tanzania) to document and mathematically calculate edge curvature using multi-scalar length-scale analysis. Through this analysis, we also explore the quantification of re-entrant [overhang] features on lithic edge cross-sections. On lithic tools, especially coarse-grained rocks with complex surface topographies such as quartzite, traditional techniques for edge angle measurement are incapable of capturing re-entrant features on edge profiles. Here we present the first archaeological study that addresses the measurement of these re-entrant features, using novel length-scale and curvature analysis methods of calculating edge angles for complex freeform surfaces. With these new methods for measuring edge angles, we can consider the impact of complex geometry, including re-entrant features, on the function of lithic tools in the past.

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

“…D Demonstrates different measurement scales, and in turn frequencies of 'steps' used by the algorithm to calculate tip curvature (not to scale) (see also: Fig. 7 in Macdonald et al [2022]) a cut at low force and deformation', with this being a widely observed definition of sharpness (Schuldt et al, 2016: 13).…”

“…Following Macdonald et al (2022), first we performed microCT scanning of each edge to acquire its 3D geometry (Fig. 2).…”

“…For the curvature calculations, 21 profiles were extracted from the demarcated 10 mm segment along the edge at 0.4-0.5 mm intervals. We calculated the curvature of each profile as a function of the location and scale following the technique proposed in Macdonald et al (2022). In brief, our applied method employs Heron's formula to calculate the inverse of the radius as a function of the scale of observation on an edge profile.…”

“…Macdonald et al (2020) later used focus variation microscopy on replica chert microliths used to harvest wheat, measuring edge curvature over multiple scales and identifying that maximum edge curvature increases as the duration of a tool's use increases. More recently, microCT scans of quartzite flakes from Olduvai Gorge (Tanzania) were used to record the complex edge geometry of this notoriously irregular crystalline raw material (Macdonald et al, 2022). This included the first formal analysis of lithic 're-entrant' features, where the complex surface structure of quartzite often leads to overhang features invisible to optical 3D scanning techniques.…”

“…This included the first formal analysis of lithic 're-entrant' features, where the complex surface structure of quartzite often leads to overhang features invisible to optical 3D scanning techniques. Moreover, Macdonald et al (2022) applied a revised edge curvature algorithm, where equally spaced points along a scanned edge profile are used to fit a triangle from which curvature is calculated.…”