Investigations of alteration phenomena of Palaeolithic rock art : The case of the large cave of Arcy-sur-Cure ( Yonne, France, 24000-28000 BP )
Résumé
Palaeolithic rock art belongs to the most ancient artistic representations of human beings. Famous examples are the Altamira and Lascaux paintings. These paintings are conserved under very exceptional conditions. Basically, the pigments are composed of iron oxides or ochre for red and yellow paints, and charcoal and manganese oxides for black colors. Many studies have already been devoted to the characterization of the employed pigments, their treatment prior to application and the artistic techniques (Baffier et al. 1999, Pomiès et al. 1999, Chalmin et al. 2003 and 2004). However, rock art is today threatened by numerous alteration phenomena, among them the formation and interaction with calcium carbonate concretions. The cave “Grande Grotte” of Arcy-sur-Cure (24500-28000 BP, Yonne, France) with prehistoric paintings is particularly concerned by this problem. This cave is generally known for its impressive calcite speleothems as stalactites and stalagmites. Important calcite concretions also cover the walls. Basically, two types of calcite have been observed on the walls: translucent yellowish layers and opaque white or grey layers. Other calcite types are present in the lakes of the cave (floating calcite rafts at the surface of the lake and soft calcite at the bottom of the lake). Prehistoric figures were discovered in 1990 after a cleaning of the dark walls, fouled during several centuries of visits, of this karst cave with an acidic solution,. In the past, the presence of the calcite layers on the walls had a protective effect of the prehistoric paints. Today, prehistorians try to identify the ensemble of prehistoric representations in the cave. So, identification and restoration of a part of the cave paintings has been done by removing the opaque white calcite layer by mechanical abrasion while keeping another underlying translucent layer, in order to evidence and to study the paintings (Girard et al., 2001).