Ochre was often used as a vivid red paint in ancient rock art known as pictographs. Despite its broad use throughout human history and a modern focus on how the artistic symbolism is interpreted, little research exists on the paint itself and how it was produced. Now, scientists are using electron microscopes to understand how ochre paint was created by hunter-gatherers in North America to produce rock art located at Babine Lake in British Columbia.

Brandi Lee MacDonald, David Stalla, Xiaoqing He, Farid Rahemtulla, David Emerson, Paul A. Dube, Matthew R. Maschmann, Catherine E. Klesner, Tommi A. White. Hunter-Gatherers Harvested and Heated Microbial Biogenic Iron Oxides to Produce Rock Art Pigment. Scientific Reports, 2019; 9 (1)

DOI: 10.1038/s41598-019-53564-w

Red mineral pigment use is recognized as a fundamental component of a series of traits associated with human evolutionary development, social interaction, and behavioral complexity. Iron-enriched mineral deposits have been collected and prepared as pigment for use in rock art, personal adornment, and mortuary practices for millennia, yet little is known about early developments in mineral processing techniques in North America. Microanalysis of rock art pigments from the North American Pacific Northwest reveals a sophisticated use of iron oxide produced by the biomineralizing bacterium Leptothrix ochracea; a keystone species of chemolithotroph recognized in recent advances in the development of thermostable, colorfast biomaterial pigments. Here we show evidence for human engagement with this bacterium, including nanostructural and magnetic properties evident of thermal enhancement, indicating that controlled use of pyrotechnology was a key feature of how biogenic iron oxides were prepared into paint. Our results demonstrate that hunter-gatherers in this area of study prepared pigments by harvesting aquatic microbial iron mats dominated by iron-oxidizing bacteria, which were subsequently heated in large open hearths at a controlled range of 750 °C to 850 °C. This technical gesture was performed to enhance color properties, and increase colorfastness and resistance to degradation. This skilled production of highly thermostable and long-lasting rock art paint represents a specialized technological innovation. Our results contribute to a growing body of knowledge on historical-ecological resource use practices in the Pacific Northwest during the Late Holocene.

Figure 1
Figure 1 - Context of Boling Point and Babine Lake Rock Art. (a) Sketch drawings of section of Boling Point pictograph, adapted from Mohs and Mohs; (b) area of rock art panel shown in photographs and where sample was taken; (c,d) photograph of Boling Point rock art panel in color (c) and digitally enhanced with D-Stretch (d), Babine Lake (courtesy of G. Keddie, Royal BC Museum); (e) map of British Columbia, highlighting area of study; (f) extracted fragment of Boling Point pictograph from Mohs and Mohs survey, from the Royal British Columbia Museum archive (Vancouver, Canada).