How to distinguish red coloring matter used in prehistoric time? The contribution of visible near-infrared diffuse reflectance spectroscopy

Résumé En

Although the main prehistoric color used for paintings is red, knowledge of this coloring matter often boils down to saying that it is “ochre.” However, the red coloring matter of Prehistory is numerous and may have been the subject of various preparations, mixtures, or even alterations. Understanding the use and transformation of coloring matter raises questions about the technical processes but also about the supply strategies of these ancient societies. In the case of analysis of solid archaeological remains, we can access the petrography, mineralogy and chemistry of these ferruginous rocks. But, when it is about deposited powder, the means of investigation become limited. We therefore propose to test the complementarity of spectro-radiometry, a non-invasive method that allows us to obtain a spectral signature of the material whatever its mode of preparation. From six geological reference samples chosen for their color (from red to yellow) and for their mineralogical composition, spectra in the visible and near-infrared were recorded under several experimental conditions and several modes of preparation of the matter, using two spectro-gonio radiometers. It is then possible to discriminate these different coloring matter on the basis of their spectral signature and to understand the link with their mineral composition.