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

Abstract

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.

From RGB camera to Hyperspectral imaging: a breakthrough in Neolithic Rock Painting analysis

“…The ICA decomposition on VNIR HSI data may also detect pigments under another paint layer where this layer is thin enough to be translucent in the very near infrared (> 700nm), where most chromophore mineral pigments are less absorbing (see e.g. Chalmin et al 2021). In addition to uncovering parts of an underlayer gure, this ability of HSI may also be to 'clean' gra ti drawn over Neolithic painting, such as in the example of panel #6.…”

“…Extending to the near infrared (SWIR: Short-Wave InfraRed, 1000-2500 nm) might also improve the separation between paints and rock and should also provide additional information especially on the composition of pigments and on the taphonomy of the rock wall (see e.g. Chalmin et al 2021, Defrasne et al 2022), but the logistic (and cost) of SWIR HSI instruments is even heavier, in particular in weight and energy consumption, which should be considered for remote eld operations.…”

“…Each painting pigment has its unique composition and texture and therefore re ects sunlight according to its characteristic spectral signature over the electromagnetic spectrum (see e.g. Chalmin et al 2021). However, rock paintings undergo physical, chemical, biological and/or anthropic alterations that alter their visibility, i.e.…”

“…Because of both the increased spectral range and the number of measurement channels, HSI can provide a larger amount of information that may allow us to identify the screened materials, and separate them from the underlying rock, based on their chemical composition rather than only their remaining perceived visible colors. As an example, the spectrum of a very thin layer of red pigment painted on a at slice of brown carbonate rock compared to the VNIR spectrum (Visible-very Near InfraRed: 400-1000 nm) of the rock itself (Chalmin et al 2021) is shown in Fig. 2.…”

From RGB camera to Hyperspectral imaging: a breakthrough in Neolithic Rock Painting analysis

“…Each painting pigment has its unique composition and texture and therefore re ects sunlight according to its characteristic spectral signature over the electromagnetic spectrum (see e.g. [42]). However, rock paintings undergo physical, chemical, biological and/or anthropic alterations that alter their visibility, i.e.…”