Detecting pigments of cave paintings hidden beneath calcite layers: The potential of magnetic susceptibility and visible-to-infrared (400-2500 nm) reflectance spectroscopy

• Iron-based pigments in prehistoric rock paintings may be masked by calcite layers. • Magnetic susceptibility and reflectance spectroscopy (400–2500 nm) are non-invasive. • We propose MS and RS techniques associated with advanced statistical analysis. • Laboratory experiments detect iron-bearing pigments beneath up to 7 mm of calcite. • Prehistoric rock art is detected under 3 mm of calcite on a cave wall by MS and RS. Detecting cave paintings, protected but hidden beneath calcite layers, is an important step in completing the inventory of prehistoric art, yet it poses significant challenges. The mechanical removal of calcite 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 pigments 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 laboratory 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.