Researchers have detected a nearly invisible layer of calcite covering some of the prehistoric paintings in Chauvet cave, in southern France. The cave’s artwork dates back around 37,000 years, and the discovery provides new information about how the paintings have changed since they were created.
The study, published in ACS Omega, used ultraviolet fluorescence to identify translucent mineral deposits that are difficult to see with the naked eye. The researchers describe the thinnest coatings as a form of natural varnish. While some may offer protection, thicker or more opaque mineral crusts can obscure the images beneath them.
The research, led by Yohan Boulard and Philippe Walter with other specialists, examined several painted areas, including the Panel of the Horses and the Chamber of the Lions. The distribution of the calcite was uneven: some surfaces had thin coatings, others showed heavier accumulations, and some appeared to have none.
To distinguish the deposits from the limestone wall, the team combined ultraviolet fluorescence imaging with spectroscopic measurements inside the cave. Although both the rock and the later formations contain calcite, impurities in their structures produce different fluorescent responses. This made it possible to identify coatings that conventional methods such as X-ray fluorescence or Raman spectroscopy may struggle to separate from the underlying rock.
The researchers developed a specific, non-contact device for the work. Because Chauvet preserves prehistoric floors, human footprints and animal traces, researchers must use permanent walkways. A fibre-optic system allowed measurements to be taken with the operator up to 2.5 metres from the surface while the probe approached the wall without touching it. The campaign produced 60 fluorescence spectra under conditions of nearly 100% relative humidity and a temperature of about 13 degrees Celsius.
At the Panel of the Horses, the analysis revealed a translucent calcium carbonate film over some charcoal drawings. Spectroscopy detected signs of the deposits even in areas where ultraviolet images did not show them clearly. In the Chamber of the Lions, the team observed more visible mineral accumulations associated with ancient water infiltration. Calcite could also vary along a single painted line, illustrating the complex natural processes that have affected the cave walls.
The study also found orange fluorescence near certain charcoal marks. Tests on limestone, calcite subjected to heat, and materials produced by burning and degrading wood suggested that the signal was more consistent with heated mineral material than with combustion products, which mainly produced blue fluorescence.
The findings raise the possibility that some marks were made with charcoal whose ends were still hot or burning. Similar orange emissions were detected near certain torch marks on the walls. However, the signal was not present in every black line or possible torch trace, and the researchers stress that it is not direct proof of the technique used by the prehistoric artists. Further work will be needed to establish how combustion and heating affected the minerals and clays.
The main advance is the development of a non-invasive way to study mineral coatings on rock art. The method is semi-quantitative: fluorescence intensity depends on the thickness of the deposits as well as the type and concentration of fluorescent substances. It therefore cannot directly determine the absolute thickness of a calcite layer, but it can reveal differences between surfaces and provide information relevant to their stability and conservation in the UNESCO World Heritage cave.
Sources: elDiario.es.