Découvertes de tracés inédits par imagerie hyperspectrale dans la grotte ornée paléolithique Mayenne-Sciences (France)

Rock art research requires precise redrawing, which began in the late 19th century to authenticate and disseminate Palaeolithic paintings and engravings. Currently, redrawing methods have evolved towards digital photography and computer-assisted analysis, revealing details invisible to the naked eye. However, accurate documentation still requires specific equipment and meticulous image processing. Image processing software is essential for enhancing the contrast of photographs and revealing subtle details, making it a crucial tool for the analysis of rock art. Rock art studies are now benefiting from technological advances in imaging. Among such technologies, hyperspectral imaging (HSI) has emerged as a powerful tool for the analysis of ancient paintings, disclosing details that remain invisible to the naked eye. This cutting-edge technology is particularly effective for complex applications, capturing hundreds of extended spectral bands. These bands, which are both contiguous and finely sampled, can extend well beyond the visible spectrum. HSI creates a detailed spectral signature for each pixel, forming a hypercube or three-dimensional data cube. This hypercube consists of a series of images, each image corresponding to a specific spectral band captured by the imager.
The research presented here applies HSI to Palaeolithic parietal art in the Mayenne-Sciences cave (Mayenne, France), whose drawings and engravings have been dated to the Gravettian period, between 25 800 and 27 700 calBP. The main objective was to determine what additional information HSI could provide over traditional rock art analysis methods, particularly in the visible and very near infrared spectrum. An additional challenge involved testing the application under difficult conditions: total darkness requiring appropriate lighting, high humidity (almost 99%) and the presence of clay particles that could infiltrate the equipment. Using an HSI Specim IQ imager, we acquired hyperspectral images under artificial lighting of various parietal figures, mainly drawn with black pigment on limestone concretions. This non-invasive method has enabled the discovery of new figures, clarification of pigment composition, and shed new light on the techniques used to create these works of art. The findings from the Mayenne-Sciences cave demonstrate the effectiveness of hyperspectral imaging (HSI) not only in complementing visible images but also in detecting previously unknown paintings and signs. Using advanced mathematical transformations to reduce the dimensionality of the data, we were able to extract and differentiate the pigments used, revealing previously unknown paintings and drawings. A charcoal-drawn bison, which had been heavily altered, appeared almost exactly as it had been drawn by the Palaeolithic artist. In addition, two horses appear to have been drawn several times, with analysis revealing at least two layers of different pigments. Finally, the discovery of a new triangular sign with rounded edges (a sign that was found in two caves nearby) is of particular importance. The results also indicate that HSI is highly effective in enhancing pigment contrast in the visible range and providing additional discriminating information in the near-infrared spectrum. The potential to re-evaluate and refine the interpretation of prehistoric artworks through the use of advanced imaging technologies such as HSI is therefore considerable.
The study emphasises the importance of integrating these technologies into the study of rock art and opens up prospects for their application to other archaeological sites. Regarding future prospects, this work emphasises the necessity for further exploration of HSI applications in other rock art sites, while considering the specific challenges associated with each environment. The ongoing improvement of hyperspectral imaging technologies and the adaptation of methodologies to varying field conditions could unveil new research opportunities in the study of prehistoric art. Moreover, the integration of SWIR (Short-Wave Infrared) HSI, or VNIR (Visible and Near Infrared) combined to SWIR point spectroscopy, alongside other in-situ analytical techniques such as X-ray microfluorescence and Raman spectrometry, has the potential to enrich our insight. Furthermore, when coupled with laboratory analytical techniques that necessitate sampling, such as “invasive” methods (mass ­spectrometry and scanning electron microscopy), a more comprehensive understanding of pigments and painting techniques may be facilitated. Such an approach would not only refine our interpretation of these ancient practices but also illuminate artistic traditions in the Palaeolithic era. In addition to its scientific benefits, the hyperspectral imaging study of the Mayenne-Sciences cave enhances our perception of rock art as a crucial element of humanity’s cultural heritage. This research reveals previously unseen details and provides new interpretations of prehistoric works of art, contributing to a better understanding of the history of art. It also underlines the importance of preserving these irreplaceable testimonies to human creativity.