Cette étude par Jal et al. explore deux méthodes non invasives pour détecter des peintures préhistoriques dissimulées sous des dépôts de calcite : la susceptibilité magnétique et la spectroscopie de réflectance du visible à l’infrarouge. Testées d’abord en laboratoire puis dans la Grande Grotte d’Arcy-sur-Cure, ces approches permettent de détecter des pigments devenus invisibles à l’œil nu. La susceptibilité magnétique se révèle particulièrement efficace pour les pigments riches en fer, tandis que la spectroscopie apporte une information minéralogique plus large. Les résultats ouvrent ainsi des perspectives pour localiser et cartographier des peintures pariétales cachées sans retirer mécaniquement la calcite qui les protège. PDF
ABSTRACT: Detecting cave paintings, protected but hidden beneath calcite layers, is an important step in complet- ing the inventory of prehistoric art, yet it poses significant challenges. The mechanical removal of cal- cite is often invasive; image enhancement is often of limited effectiveness. This study investigates two non-invasive, complementary approaches to detect these hidden paintings: magnetic susceptibility and visible-to-infrared reflectance spectroscopy (400 to 2500 nm). Reflectance measurements give detailed insights into pigment composition, while magnetic susceptibility measures the magnetic properties of materials and is particularly effective in detecting iron-based pig- ments such as hematite and goethite. The two methods were first tested in the laboratory on a limestone slab painted with various pigments and covered with calcite speleothem masks of varying thickness. This setup was a proxy for prehistoric cave paintings covered by opaque calcite speleothems. Both reflectance spectroscopy and magnetic susceptibility were used to assess pigment detectability. Results showed that magnetic susceptibility could detect iron-based pigments beneath thin calcite layers (up to 7 mm in lab- oratory conditions), while reflectance spectroscopy analysis identified the spectral differences between all the materials tested, although the signal decreased with speleothem thickness. These findings were then validated in field tests at the Grande Grotte (Arcy-sur-Cure, France) where three prehistoric red ochre paintings covered by calcite were analysed. Both techniques were suitable for non- invasive detection: magnetic susceptibility was more effective for iron-based pigments, while reflectance spectroscopy provided broader mineralogical information. While these methods are not yet fully operational for field use, this preliminary study shows that it should be possible to use both magnetic susceptibility and reflectance spectroscopy probes to detect and delineate paintings hidden (and protected) beneath opaque calcite layers.