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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.

Although prehistoric sites frequently contain numerous fragments and traces of many different kinds of colouring matter, intensive study of this type of archaeological remains began only recently. Such studies, aimed at determining how raw materials formed and changed over time, and how they were transported by the groups of humans who used them, are extremely valuable as they reveal shared strategies, that is, cultural traditions and the spaces in which they developed. The scope of this paper focusses on the description of the main geological contexts in which ferruginous colouring materials form and are found. In the framework of a collective research program called Pigmentothèque (iron- and manganese-rich rocks and minerals library), geological surveys are conducted taking into consideration the geological settings in which colouring materials are present and using a common record and sampling methodology which is followed by petrophysical, mineralogical and chemical analyses based on a shared procedure and vocabulary. In order to go beyond descriptions based solely on colour and chemical composition, we describe the great variety of iron-rich materials that can be used to obtain colouring matter. This diversity in the formation and evolution of iron-rich materials must be taken into account when trying to understand past humans’ choices of raw materials, their provenance and the anthropogenic and natural modifications they have undergone. We also describe criteria for recognising cohesive remains of colouring matter during archaeological excavations, so these artefacts can take their place alongside other mineral resources in helping improve our understanding of past societies.

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