How vivid-red rock art paint was created


Rebecca Pool

Thursday, November 21, 2019 - 15:00
Unravelling rock art: EDS detail of a red pigment particle embedded in the calcite layer. [MacDonald et al., Scientific Reports]
Microanalysis of ancient rock art pigments has shed new light on how ochre paint was created by hunter-gatherers in North America to produce pictographs.
SEM, TEM and other methods revealed that the paints were prepared by heating microbial biogenic iron oxide - an orange-brown sediment - to between 750°C and 850°C to initiate colour transformation to vivid red.
Researchers heated a single grain of ochre and watched the effects of temperature change under an electron microscope at MU's Electron Microscopy Core facility. [University of Missouri]
"It's common to think about the production of red paint as people collecting red rocks and crushing them up," highlights Brandi MacDonald from Anthropology at the University of Missouri. "Here, with the help of multiple scientific methods, we were able to reconstruct the approximate temperature at which the people at Babine Lake were deliberately heating this biogenic paint over open-hearth fires.”
“So, this wasn't a transformation done by chance with nature,” she adds.
A piece of rock art found at Babine Lake, representative of the rock art that was analysed in the study. [University of Missouri]
Ochre, one of Earth's oldest naturally occurring materials, was often used as a vivid red paint in ancient rock art known as pictographs across the world.
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.
With this in mind, MacDonald and colleagues set out to characterise the pigment used to produce rock paintings at Babine Lake, British Colombia, using a combination of scanning electron microscopy, micro-Raman spectroscopy, and X-ray diffraction.
Brandi MacDonald is an assistant research professor in the Archaeometry Laboratory at the University of Missouri Research Reactor (MURR). She also holds a joint appointment in the Department of Anthropology at the College of Arts and Science. [University of Missouri]
Initial investigations suggested that pigment comprised microfossil sheaths of the biomineralizing bacterium, Leptothrix ochracea, that had been thermally treated.
Intrigued by this unexpected discovery, MacDonald and colleagues subjected freshly-collected L. ochracea to a series of experiments to evaluate the potential for heat treatment in the past.
Control samples were either heated in a furnace or using in-situ SEM, with both these and ancient pictograph fragments then analysed using SEM, TEM, X-ray EDS, and superconducting quantum interference device magnetometry.
A series of digital micrographs illustrating thermally-induced colour change of L. ochracea. Samples were heated in oxidizing atmosphere in a muffle furnace, held for 3 h each at (a) Untreated, (b) 200 °C (c) 400 °C (d) 600 °C (e) 800 °C (f) 1000 °C. Scale 1 = mm. [MacDonald et al., Scientific Reports]
Analyses pointed to a sophisticated use of iron oxide produced by L. ochracea, with the samples' nanostructural and magnetic properties indicating the paint was prepared using carefully controlled pyrotechnology.
SEM-EDS hyperspectral map of rock art sample. (a) Cross section EDS detail of a red pigment particle embedded in the calcite layer. Scale = 25 µm. (b) EDS map detail of pigment particle showing concentrations of iron and phosphorus. [MacDonald et al., Scientific Reports]
“Our results demonstrate that hunter-gatherers in this area of study prepared pigments by harvesting aquatic microbial iron mats dominated by iron-oxidising bacteria, which were subsequently heated in large open hearths at a controlled range of 750 °C to 850 °C,” writes MacDonald in Nature Scientific Reports. “This technical gesture was performed to enhance colour properties, and increase colourfastness and resistance to degradation.”
“This skilled production of highly thermostable and long-lasting rock art paint represents a specialised technological innovation,” she adds. “Today, engineers are spending a lot of money trying to determine how to produce highly thermostable paints for ceramic manufacturing or aerospace engineering without much known success, yet we've found that hunter-gatherers had already discovered a successful way to do this long ago."
Research is published in Scientific Reports.
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