Vermiculations in painted caves: New inputs from laboratory experiments and field observations

Vermiculations are aggregates of small particles commonly found on cave walls. They are a major concern for the conservation of painted caves, as they can potentially alter valuable prehistoric cave paintings. A previous rheological study of fine sediment deposits on cave walls revealed that this material can undergo a solid-to-liquid transition triggered by variations in the chemical composition of the water film on the wall. Such a transition could occur at the origin of vermiculations by allowing the sediment to flow under low mechanical stress. In this work, we provide quantitative information on the conditions leading to this transition and show the importance of the chemical composition of the water film on the cave walls. A complete understanding of the phenomenon will, however, require more field information. This includes monitoring of the evolution of vermiculations, for which we have developed a dedicated observation protocol. Based on the combination of photogrammetry and a geographic information system we were able to precisely map the walls of the Hall of Bulls in Lascaux cave from past and future photographs. To better understand the vermiculation process, pictures need to be taken regularly, and the chemical composition of the thin water film covering cave walls needs to be analyzed with a similar time step. The correlation between the evolution of vermiculations, the humidification phases of the walls, temperature changes and the chemical monitoring of the water film should shed new light on conditions triggering vermiculations.