Time takes its toll: Detection of organic binder media in ochre paints with visible near-infrared and short-wave infrared reflectance spectroscopy

Abstract

Organic binder media in ochre rock art have so far resisted in-situ spectroscopic analysis. In this study visible near-infrared and short-wave infrared (VNIR-SWIR) spectroscopy was investigated for its usefulness in characterising experimental paints made with ochre and local (Weld Range, Western Australia) organic materials. VNIR-SWIR spectroscopy is a proven technique for the detection of mineral molecular compounds. Historic art conservators have successfully detected organic binder materials in 500-year old Renaissance art with near-infrared (NIR) spectroscopy. In this study, the chemical morphological spectral features of organic materials in newly made ochre paints, and UV accelerated 100-year aged ochre paints, were investigated. The analysis revealed complexities that would affect the detection of organic materials as anthropogenic in rock art. The first of these concerned the nature of the organic materials. There were significant deteriorative spectral changes caused by reactions initially with hematite (red ochre), and additional changes caused by accelerated UV light, resulting in remnant features different from the raw materials. These would render current references and databases useless for identification. The spectroscopic identification of organic binders in old rock art may depend on such remnants. Secondly, the composition of naturally occurring red ochre included organic materials as natural constituents, confounding the potential to differentiate those added by humans. Finally, this study has challenged the assumption that all organic materials, in particular animal fats, might function as binders to chemically adhere pigment. This paper presents a novel approach to the characterisation and identification of the constituents of ochre and organic paints, using VNIR-SWIR spectroscopy.