Grinding Charcoal for Rock Painting: A Methodological Framework for Particle-Size Analysis in Rock Art Micro-Samples

Abstract

Research on the materiality of rock art has shifted attention from images as finished products to the technical and social processes involved in their making. Within this framework, pigment grinding is a crucial but still methodologically elusive aspect of painting production, despite its potential to inform technological choices, labor investment, and variability in painting practices. In this paper, we propose an experimental and archaeometric approach to the study of charcoal grinding in rock art micro-samples. By combining experimental archaeology, optical microscopy, and particle analysis, we develop a model linking charcoal particle area to grinding time. We apply this method to study black rock paintings from the Oyola archaeological site, in northwestern Argentina, where charcoal was identified as the main black pigment. The results show that microscopically measurable differences in particle areas make it possible to distinguish varying degrees of grinding among paintings that are macroscopically and chemically similar. Beyond the local case study, our findings demonstrate that pigment granulometry can provide access to otherwise invisible dimensions of rock art production, complementing the existing knowledge of technical variability, painting events, and the historical processes through which painted surfaces were made and transformed.