Painted and engraved rock art is situated at the interface between rock and air, and it is affected by the physicochemical mechanisms that develop there. Thermal exchanges with the interface (rock-water, rock-air, air-water) and dynamic exchanges between the exterior and subterranean environments cause water and air migration into underground caves that, in turn, produce condensation or evaporation. In limestone caverns, all these mechanisms have both favorable and adverse effects on calcite formations.
The mechanical stability of a cave's inner walls and decorations (as well as the surfaces of most archaeological objects) is dependent essentially on humidity level. A sudden and significant dehydration of the surface will impair the cohesion of pigments and mortar. In the same way, significant condensation will form a permanent water film, which fosters the development of harmful bacteria and algae. Condensation trickling down a wall's surface eventually leads to deterioration and loss of paintings and decoration.
Given these conditions, safeguarding the subterranean environment with its art and artifacts requires nothing less than actual atmospheric control, with tourist visits limited to five persons per day
Evaporation-Condensation and Conservation of Subterranean Environments
Only a few environmental parameters can be measured with certainty:
Temperature of the external air relative humidity of the external air atmospheric pressure of the external air temperature of the air inside the space relative humidity of the air inside the space atmospheric pressure of the air inside the space temperature of the inner wall surfaces
The air mass of a cavity of temperature Ta, in contact with the inner wall of temperature Tp, is subject to the physical laws of condensation and evaporation. Thus, when the air's water-vapor pressure is greater than its saturated water-vapor pressure (at the same temperature as the wall surface), a film of water will appear as a result of condensation. Characteristics of this wet film vary, depending on the air mixture at the wall surface, the makeup of this surface, and its porosity. Unfortunately, these factors are almost impossible to calculate. Likewise, if the air's water-vapor pressure is lower than the saturated vapor pressure (at the same temperature as the wall surface), water will evaporate from the wall. How will this condensation and evaporation affect the support surface?
In climates where rainfall lasts from a few weeks to several months of the year, the water exchange (vapor-liquid) within the heart of the rock and at the interface with the atmosphere leads invariably to deterioration. The problem of evaporation-condensation and the response this causes on the interface is the main issue here (Stefanaggi, Vouve, and Dangas 1986; Malaurent, Vouve, and Brunet 1993).
In each of the following examples, droughts, intense rain, or other exceptional weather patterns—as well as structural work—are at the root of the microclimatic processes. These conditions cause an imbalance between the water-vapor pressure in the air and at the interface, resulting in either a physical or a physicochemical reaction on decorated and undecorated surfaces. In the following case studies of moisture imbalances,
A and A' refer to decorated caves and shelters; B refers to archaeological sites (Fig. 1).
The first three examples present cases in which the water-vapor pressure at the carbonate (or similar type) wall surfaces is greater than the more balanced pressure of the surrounding air.
Figure 2
Diagram showing evaporation-condensation phenomena in the case of a cave; analysis was carried out between 1989 and 1990.
Figure 1
Caves (A), decorated shelters (A'), and covered archeological sites (B) all have their own particular microclimate: 1 = calcareous fissures; 2 = impermeable calcareous clay;
3 = sand infilling; 4 = paintings; 5 = sandy clay alluvial deposits; 6 = archeological objects; 7 = alveolar concrete vaults;
8 = water table.
World History
