A natural varnish was discovered on 37,000-year-old horse paintings in Chauvet Cave using ultraviolet light

In Chauvet Cave, in the Ardèche gorge of southern France, researchers set up a tripod on the steel walkway in the Hillaire Chamber and ran a carbon fiber pole out toward the Horses Panel.

Artists drew those horses in charcoal about 37,000 years ago. A lens at the pole's tip sat 2 inches (5 centimeters) off the rock and lit it with ultraviolet light. The operator, meanwhile, stayed up to 8 feet (2.5 meters) back on the walkway.

What came back showed a translucent skin of calcite over parts of the black lines. The team called it a natural varnish and plotted the spots where it shows on the panel. Still, the survey cannot say how thick it is.

Steel walkways, 1994 footprints

Jean-Marie Chauvet, Éliette Brunel and Christian Hillaire got into the cave on Sunday, December 18, 1994, according to the French Ministry of Culture, and on Christmas Eve they went back and put a plastic strip down over the path they had used.

The steel walkways follow that strip today. Per the ministry's page on the cave layout, they are 2 feet (60 centimeters) wide. In the main chambers, they run for 1,214 feet (370 meters). Crews also widened the old entrance and fitted it with a heavy reinforced door.

There are animal tracks and human footprints on the floor, so everyone works from the walkways only. The paper puts the walls 3 to 20 feet (1 to 6 meters) away from them. At the panels in this study, the gap is 5 to 10 feet (1.5 to 3 meters).

At that range the team ruled out X-ray fluorescence and Raman spectroscopy, the standard pigment tools. Neither can tell bedrock limestone from calcite left later by dripping water anyway, as both are calcite.

A probe on a carbon pole

First author Yohan Boulard works at LAMS, the Laboratory of Molecular and Structural Archaeology of Sorbonne University and the CNRS in Paris. He designed the instrument with Thiéry Guillou and Philippe Walter, the corresponding author.

Light came from a 1.3-watt OSRAM LED at 365 nanometers. It went up the pole through one of seven fibers in a 10-foot (3-meter) bundle, and the other six carried the glow back to an Ocean Optics USB4000 spectrometer, with a 24-volt battery for power and a Python program on a laptop to log each reading.

Dyed papers left under the beam did not fade. So the team took the light as safe for the walls. Humidity in the chamber ran at about 100 percent, at 55 degrees Fahrenheit (13 degrees Celsius).

In the lab, calcite sampled from Chauvet for earlier work glowed with one blue peak, near 450 nanometers. However, limestone from near the site showed that peak and another near 580.

When the team dissolved the calcite in hydrochloric acid, the blue stayed in the liquid. That tied it to organic matter from the surface ecosystem.

Sixty spectra, 47 on the horses

Carole Fritz, of LAMS and the CREAP center in Toulouse, led the cave's scientific team. She also supervised the study with Walter and Maguy Jaber. The group took 60 spectra in all, 47 of them on the Horses Panel.

Over the black lines the signal dropped to as little as one-twentieth of what bare rock gave. Charcoal soaks up ultraviolet light without glowing, so any light at all off a line meant something was lying over it.

The lip of the central horse gave almost only the blue calcite peak. Bare limestone beside it gave both peaks, and the jaw, where the charcoal covers the rock only in part, fell in between.

The authors pointed out that thick crusts can hide paintings but a clear one can "behave like a natural varnish." For example, at Arcy-sur-Cure's Grande Grotte in the Yonne department, an opaque calcite crust hid the Paleolithic paintings. They stayed hidden until restorers rubbed it off.

Uranium-thorium dates on such crusts were used to claim that Neanderthals made cave art in Spain more than 64,000 years ago, and a long list of rock art specialists rejected that reading in a 2020 paper in the Journal of Human Evolution.

Lines that stopped the flowstone

The Alcove of the Lions, in the same sector as the Horses Panel, gave a different picture. Water got in there after people left the cave, and it left flowstone, calcite laid down in sheets where water runs over rock.

The team recorded eleven spectra in the alcove. On the left, flowstone pours out of a horizontal crack that crosses a horse's head. Over that head, the blue glow was bright enough to show in the photographs.

Farther down, on a horse's breast, the calcite grew on bare rock and stopped at the charcoal lines. Those lines gave no calcite signal at all.

The team took this as a sign that the charcoal affected how the crystals formed. Earlier work on iron and manganese oxide pigments, by contrast, found no such link.

Seeping water also coated the Signs Panel in Chauvet's Red Panel Chamber, according to the ministry, and the same mineral builds the stalagmites that rise from cave floors.

Traces of heat beneath some lines

Some charcoal marks had orange patches just under them in the ultraviolet photographs, and the spectra from those spots peaked near 580 nanometers.

The team burned Scots pine at 752, 932 and 1,292 degrees Fahrenheit (400, 500 and 700 degrees Celsius) to make test charcoal, because the cave's own charcoal came from that tree, as an earlier study in Antiquity found. Its tar glowed blue.

The team also heated limestone and calcite samples to 932 degrees Fahrenheit (500 degrees Celsius) for one hour or 24 hours. Afterward, they glowed orange. The same orange showed up near torch smears, the marks people left on the wall when they revived a torch.

The authors suggested that some lines were drawn with charcoal that had "hot, possibly burning tips," and they wrote that this is not direct proof.

At Font-de-Gaume, another French cave, charcoal from black drawings has been used for dating, to between 13,000 and 16,000 years ago.

Thickness the method cannot measure

There is no thickness for the varnish in the paper, in millimeters or anything else. Nobody made calcite layers of known thickness to calibrate against. The authors also wrote that grown layers like the cave's would be very hard to copy.

The blue signal depends on both the thickness of the calcite and how much organic matter it holds. In the Alcove of the Lions, flowstone and thin veils formed in different ways. So nobody can compare the numbers there one to one.

The burning-charcoal idea still needs testing. For example, studies could look at how far the charcoal burned. They could also test how heat changes clays with more or less iron oxide. Also, the survey covered only one sector of the cave, the Horses Sector.

The authors called the method "a promising tool" for deciding how decorated caves are protected and who gets in. For now, the readings taken from the Hillaire Chamber walkway show a translucent calcite layer over the horses.

The full study was published in the journal ACS Omega.

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