Scientists from Sorbonne University and France’s National Center for Scientific Research (CNRS) have developed a noncontact method for detecting a thin, translucent calcite coating on the approximately 37,000-year-old paintings in France’s Chauvet Cave. Using ultraviolet fluorescence spectroscopy from several meters away, the researchers identified mineral layers that are invisible to the naked eye and found evidence that prehistoric artists may have used hot or burning charcoal, altering the rock beneath their drawings.
Multiple collapses sealed Chauvet Cave during the Upper Paleolithic, helping preserve hundreds of paintings and sculptures depicting rhinos, mammoths, lions, horses, bison and other animals. The decorations, which date back about 37,000 years, provide a rare view of the early development of art.
Artworks in underground environments remain extremely fragile. Their condition depends on a delicate balance of environmental factors, creating significant challenges for the management of famous archaeological sites and decisions about public access.
“Our methodology is very similar to a forensic investigation,” said the study’s lead author, Dr. Philip Walter. “We will examine all the evidence to decipher the techniques of prehistoric artists and understand how paintings have been preserved through time.”
Detecting calcite without touching the paintings
The researchers’ method uses ultraviolet (UV) fluorescence. A UV beam is directed at the cave wall, and the weak light emitted in response is analyzed.
Chauvet Cave’s floor is exceptionally well preserved and contains animal and human footprints. Visitors are therefore restricted to narrow passageways, leaving scientists between 1 and 6 meters from the painted panels—too far away for many standard analytical instruments.
To work at that distance, the team built a spectroscopic device around a 3-meter bundle of optical fibers mounted on a carbon-fiber pole, a 365-nanometer ultraviolet LED and a battery pack. The researchers used the system to collect measurements from as far as 2.5 meters away in the humid cave, where the temperature was 13 degrees Celsius.
The method works because the cave’s limestone bedrock and the calcite deposits that form on its surface fluoresce differently under UV light, even though the two materials are nearly identical chemically. Organic impurities trapped in each material give them distinct spectral signatures.
The charcoal pigment used in the drawings absorbs UV light rather than emitting it. On the black lines in the horse panels, however, the researchers found that most of the detected light came from calcite rather than limestone. They interpret this as evidence that a thin calcite layer formed over the drawings.
Such a layer could function as a natural varnish, the researchers suggest, although it could eventually obscure the artwork if it became thick and opaque.
Evidence of how the cave art formed
In the lion alcove, water entered the cave after human occupation had ended and produced flows of calcite. Several charcoal lines followed the boundaries of these flows. In some areas, calcite formed on bare rock but not on pigmented surfaces, suggesting that the drawings influenced how the mineral crystallized.
Dr. Walter and his colleagues also detected an unexpected orange glow beneath some charcoal marks and around torch marks. In laboratory tests, only heated limestone and heated calcite produced a similar orange signal; tar and degraded charcoal did not.
The researchers therefore believe that some of the lines were made with hot charcoal, possibly still-burning charcoal, which altered the underlying rock.
“These results demonstrate that careful interaction between humans and geology has enabled these works of art to be preserved for tens of thousands of years,” the researchers said.
They added: “We expect this technology to have applications ranging from forensics to monitoring biofilms on the International Space Station. This project is already underway.”
An article describing the results was published this month in ACS Omega. Paper
Johan Boulard et al. 2026. “On-site ultraviolet fluorescence spectroscopy for characterization of translucent calcite coatings on prehistoric cave art.” ACS Omega 11 (40): 60980–60990. doi: 10.1021/acsomega.6c05061.
Source: www.sci.news
