Quantitative identification of flaking techniques using Bulb Surface Angle (BSA) derived from 3D surface data

Abstract

Identifying flaking techniques used in blade production is a fundamental challenge in lithic analysis because morphological attributes often overlap among different production methods, limiting the reliability of traditional classifications. This study evaluates a quantitative approach based on the bulb surface angle (BSA) and vertical interval (VI) derived from three-dimensional surface data. The analysis is based on 613 experimentally produced blades manufactured by skilled knappers using four flaking techniques: hard-hammer direct percussion, soft-hammer direct percussion, indirect percussion, and pressure flaking. BSA and VI were measured within a standardized three-dimensional coordinate system and used to examine the distributional structure of each technique. The results show that blade production techniques form distinct distributional patterns in BSA–VI space. Logistic regression analyses incorporating both BSA and VI demonstrated particularly strong discriminative performance for pressure flaking and indirect percussion, achieving balanced accuracies of 97.5% and 96.1%, respectively, whereas direct percussion exhibited substantial overlap and remained difficult to differentiate. In particular, discrimination between hard- and soft-hammer direct percussion was limited. These findings suggest that BSA captures technological variation associated with modes of force transmission and fracture propagation, rather than serving merely as a geometric descriptor of flake morphology. The combination of BSA and VI provides a reproducible framework for quantitatively evaluating variability in blade production techniques and offers a new approach for investigating technological variation in archaeological blade production.