In Situ Ultraviolet Fluorescence Spectroscopy for the Characterization of Translucent Calcite Coatings over Prehistoric Cave Art.

The conservation of prehistoric cave art involves complex taphonomic processes such as the formation of calcium carbonate coatings. Often invisible to the naked eye, these deposits are essential for understanding the evolution of the artworks and monitoring their preservation. While few noninvasive techniques probe the concretion layers, this work explores the potential of ultraviolet fluorescence to characterize calcite veils in the Chauvet cave, an Upper Paleolithic site displaying some of the oldest drawings known to date. Here, we show this technique is capable of distinguishing limestone from speleothem calcite, despite their similar crystallographic nature, thanks to organic impurities embedded in the carbonate matrices. A custom-built installation set up in the harsh conditions of the cave enabled ultraviolet fluorescence spectra to be measured at working distances of up to 2.5 m. Data collected in situ were processed according to a suitable fluorescence model, providing stratigraphic information on the translucent calcite layer covering the drawings and highlighting significant spatial heterogeneity in the concretions. Additionally, we observed localized orange emissions suggesting local thermal alteration of calcium carbonate by hot charcoals.