In a monument such as a concrete burial vault with the same evaporatory atmosphere as a cave (Fig. 4), the saturated or capillary water content of the walls' stone or mortar (or other porous or permeable archaeological elements) migrates toward the interface where it eventually evaporates (see Fig. 5, walls M1 and M2). This phenomenon sometimes includes chemical precipitates and often entails loosening of mortar, joints, and other coatings. It is also often accompanied by flaking, powdering, and bleaching of the colored areas, which may be made up of crushed and compressed brick. In Figure 5, the effects (bleaching) of water evaporation are shown coming from the walls (M1 or M2) through the floor.
The next three examples represent cases in which the water-vapor pressure of the wall surfaces is lower than that of the surrounding air. In A and A' (cave or rock shelter), the water vapor of the subterranean atmosphere condenses on the surfaces and/or the calcite formed on the vaults of an enclosed space. Continual condensation over the years (Fig. 2) has created a constant supply of water, thousands of microscopic droplets that enlarge and merge. Methods used to obtain visual images and automatic processing of these data make it possible to identify these droplets, especially when they appear on a painted support (Fig. 6). Depending on the calcitic micromorphology, the interaction of these droplets with each other may lead to water flow. This, in some cases, leads to leaching loss and dispersion of organic (wood charcoal) or mineral pigments in wall paintings. Preventive measures are essential if the wall paintings and rock art are to be conserved. It is also important to note that in situations where the wall is homogeneously wet, without flowing, the effect of the film of water increases the visibility of the painted layers.
Case B is an example of an urban archaeological site with wall constructions and overlapping flooring built on fine silt, sandy clay, and coarse gravel alluvial deposits that are extremely close to the middle level
Figure 6
Parietal condensation droplets on a painted support, Lascaux.
A Controlled Subterranean Environment: The Cave of Lascaux
Of the alluvial water table. This site has been reinforced by the construction of a ring of walls and also has been covered over by a concrete slab.
The surrounding archaeological excavation remains active for eight hours per day Continuous recordings of the ground temperature and subterranean air and wall temperatures, as well as the air hygrometry, make it possible to calculate the imbalance between the water-vapor pressures in real terms. An air-extracting pump may be employed to dry the air, if necessary. In cases where condensation is developing (Fig. 4), it is possible to follow the moisture content of the wall surface (in this case, of a vault) by mapping the visual observations of the state of the site. Observations over time show increases and growth in the droplets that form on the recesses of the horizontal vault (Fig. 7a, b). These droplets lower the energy barrier necessary for the formation of a liquid-vapor interface. Small, separate droplets increase in size at the expense of the vapor, then merge; when they have reached their maximum size, they fall off and soak the ground, as well as structures and archaeological objects. The cycle of evaporation from the ground and condensation on the vault leads to destabilization of poorly adhered materials.
Since its discovery in 1940, the cave of Lascaux has been subject to a number of adverse circumstances, making it a very complex model (Fig. 8a, b). Situated at a depth of 0-25 m below ground (Fig. 9), this cave highlights the problems of conserving original works of art and necessitates a definition of the ideal subterranean climate (Figs. 10-12). The restoration of the original atmosphere is one of the main aims of the conservation program (Vouve et al. 1983).
From 1958 to 1963, a mechanical forced-air system was installed in the machine room for regulating the cave environment to account for the increasing number of tourists. The result was a system designed to adapt the subterranean climate of the cave to the visits.
Figure 7a, b
Cartography of the evolution of the condensation droplets on the horizontal vault of an archaeological site shown (a) on 25 March 1992 and (b) on 28 March 1992.
Figure 8a, b
State of the access of Lascaux (a) in 1940, the year of its discovery; and (b) in 1963.
A
B
Extensive scientific study preceded the installation of the regulating equipment (Malaurent, Vouve, and Brunet 1992:319-32). The research monitored the following parameters (Fig. 9):
Temperature of the air inside the cave at twenty-four specific points
Temperature of the rock inside the cave at nineteen specific points
Readings of the maximum and minimum external temperatures, and rainfall data from a meteorological station Assessment of the development of water from the phreatic water table intercepted by the entrance porch Assessment of the temperature at the intake and outlet vents of the primary cooling system
Assessment of the temperature at the intake and outlet vents of the two secondary cooling systems
Regulation of the output of the primary and secondary cooling systems
Readings of air and water-vapor pressure taken with psy-chrometers from several strategic points within the cave Readings of CO2 content in the enclosed spaces and pits
Figure 10
Painting in the Salle des Taureaux, Lascaux, showing an animal drawn on limestone and concretion.
Figure 9
Map of Lascaux cave with main points where measurements were taken.
• Visual assessment of condensation on sensitive surface areas inside the cave
• Visual assessment of all the decorated surfaces
• Photographic assessment of pinpointed trouble spots
The air-conditioning system has two closed circuits: circuit A (at a temperature of 5 °C), linked with the external cooling units; and circuit B (at 7 °C), which is internal. These two circuits are connected by exchanger D (Fig. 13). Exchangers C provide negative kilocalories (cold) from about June to December. Thus, controlling seasonal variations caused by the removal of part of the original screen, which acted as a natural exchange regulator between the Salle des Taureaux and the central branch of the cave, resulted in relatively good conservation conditions for the paintings (Fig. 9).
Monitoring of the rock in the Salle des Taureaux (chamber of the bulls) has demonstrated the need to control seasonal factors. Because of the slow progression of thermal waves in the ground during the winter period, the rock in this area is influenced by the air coming from more exposed areas, such as the machine room, which is closer to the exterior. Cooling air circulating from the cave's vault network to the top openings in the machine room produces water condensation (from one to several liters of water per day) on the radiators (the coldest points in the system). The cooled air, which is now drier, then returns to the decorated vault network via the bottom openings in the machine room (Fig. 13).
In June, the cave generally appears to be in a stable condition, and the temperature of the rock surface begins to fall naturally with the onset of winter. However, the premise is that the water-vapor level in the
Figure 11
“Chinese horse” drawn on cauliflower deposit.
Figure 12
Frieze of deer drawn on limestone.
Machine room air is higher than in the Salle des Tauraux. In order to avoid any condensation on painted walls, the cooling system is turned on. This helps stabilize the water-vapor pressure of the air and prevents condensation on the walls (Fig. 14).
Figure 13
Lascaux, simplified diagram of the air circulation in the cave. The cross section shows the accelerated convection controlled by machinery between the Salle des Taureaux and the machine room. Scale drawing showing the dynamic of airflow in the rooms and corridors: A = primary cooling system; B = secondary cooling system; C = thermic exchangers; D = cylindrical thermal exchanger; E = infiltration water; F = air circulation by natural convection.
Figure 14
Diagram showing the pressure of water vapor expressed in millibars in the Salle des Taureaux: curve 1 = partial pressure of water vapor in the air; curve 2 = partial pressure of the counterbalancing water vapor of the surface of the rock on the left-hand wall; curve 3 = partial pressure of the counterbalancing water vapor of the rock on the vault. A comparison of curves 1 and 2 shows that there is no possibility of water vapor condensing, for example, on the rock near the drawing called “Unicorn.”
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