Chapter 5 – Climatologie du milieu souterrain à Lascaux : d’une étude globale à la microclimatologie des parois
p. 121-142
Texte intégral
5.1 Introduction
1Lascaux Cave, discovered in 1940 in Montignac, in Dordogne, in the Vézère Valley, is a UNESCO World Heritage Site. In 1963, Lascaux Cave was closed by an administrative decision of the Minister of Culture, A. Malraux. A long period of study followed, which permitted the cave to return to a stable state in which the prehistoric paintings could be preserved, a situation that was to last for several decades. Many scientists (J. Bauer, C. Delamarre-Debouteville, P.-P. Grassé, M. Hours, A. Moyse, G. Laporte, M. Lefèvre, M. Magat, H. Moureu, J. Pochon, H. Schoeller, A. Vandel, J. Wyart, L. Pontier, L. Comolet, J. Vouvé, etc.) have contributed to this commission and are the source of the solutions that have been retained to date. In 2001, new concerns arose concerning the preservation of the cave; a new committee for the conservation of the cave, appointed by the Minister of Culture and Communication, inquired as to whether the subterranean climate responsible for the good preservation conditions maintained up to now had been destabilized.
5.2 Presentation of the cave
2Lascaux is a natural cave that forms part of the upper fossilized system of a karst that is largely filled with a sandy-clay deposit (cf. chapter 4). The penetrable network (fig. 45) is essentially composed of three rectilinear galleries, and is of modest proportions; the cave, from the entrance door to the end of the Right Diverticulum, measures approximately 150 m in length, and 80 m from the current entrance to the Axial Gallery. The total length of the known subterranean system is less than 250 m. The accessible volume of the system is 3300 m3 ± 500 m3, of which 320 m3 is the Hall of the Bulls and 1300 m3 is the Apse and Nave11.

FIG. 45 – Plan of the cave.
© N. Aujoulat / ministère de la Culture et de la Communication
3We can make a morphological distinction between an axial section that comprises the Hall of the Bulls and the Axial Gallery and a lateral section that comprises the Passageway, the Nave, the Apse (an enormous chamber with a domed roof), which leads to the Shaft of the Dead Man and its system (large joint), and finally the Mondmilch Gallery extending into the Chamber of the Felines. We must add to this the machine room (technical area) that was dug below airlock 1 following the clearing of the debris cone at the entrance during the modification work in the cave (fig. 54).
4Taking the geometry of the cave into account, the elements of the subterranean system are at varying depths, as are the decorations: in the Hall of the Bulls the paintings are between 10 and 12 m below the current exterior ground level, in the Right Diverticulum between 13 and 15 m below and in the Axial Gallery between 10 and 15 m below. The bottom of the Shaft in the Chamber of the Felines (the deepest penetrable section) is situated between 24 and 25 m below ground level.
5The vertical section of the galleries covers an area of between 1 m2 in the Chamber of the Felines and 80 m2 in the Hall of the Bulls; this latter chamber has a maximum height of 6 m. As is evident from the longitudinal cross-sections taken, the cave descends from the entrance to the bottom of the Axial Gallery on one hand and from the Hall of the Bulls until the entrance of the access to the Chamber of the Felines on the other, at the end of the Mondmilch Gallery.
6In the Hall of the Bulls and the Axial Gallery, the stone is frequently covered with cauliflower-like carbonated concretions. Traces of paint have been applied to this rough surface (fig. 46). In the lateral section (Right Diverticulum), the very soft rock is practically bare at the level where a horse2 is depicted (fig. 47). It is both painted and engraved as is shown by traces of pigments preserved at the bottom of the fissures and crevices in the stone33 (fig. 48). Because the support is composed of stone with poorly joined detritic and biodetritic elements, the surface is powdery and has altered over time.

FIG. 46 – Panel of the Axial Gallery, named the “Falling Cow Panel” by Norbert Aujoulat. The pigments were pulverized onto a very gritty support. Start of a frieze of small, horizontally aligned horses.
© N. Aujoulat / ministère de la Culture et de la Communication, CNP

FIG. 47 – Depiction of a horse, painted and engraved, located in the Passageway. The back is highly curved, and the eyes and both ears are well incised. The animal has two lines on the back and a forked symbol on the stomach. The mane is marked by fine, parallel hatches (brush-like), applied to the neck. The support has some small, white calcite pustules that are particularly dense near the hind legs; some loss of pigmentation has affected the horse’s shoulder and flank; the stomach outline and one of the front outlines is affected by this phenomenon. As is also the case for a large part of the painted depictions that existed in this gallery, some of the paint traces from this figure have disappeared following the erosion that took place on the ceiling and walls due to the actions of air currents before the cave was discovered. This photo illustrates the fragility of the walls in the Passageway and Apse sections, as well as the risks encountered when pulsated air circulation ensured the regeneration of the subterranean atmosphere from 1958 to 1963, in order to facilitate tourist visits.
© N. Aujoulat / ministère de la Culture et de la Communication, CNP.

FIG. 48 – The entire Apse is covered with more than a thousand painted and engraved figures. This very high concentration of mostly engraved representations is now enhanced by only few remaining zones of color. A careful examination of the wall reveals a labyrinth of overlapping complete or partial animal depictions that have undergone significant natural degradation. The engraved traces are deeply incised into the fine-grained, sandy limestone, on which we can distinguish the elements at the maximum magnification. The paint, on the other hand, remains only in the crevices, fissures and between the grains of the support due to a slow disaggregation of the surface layer of the rock over several millennia. The weak natural cementing of the detritic and biodetritic elements of the Upper Cretaceous limestone facilitated this mechanism of degradation.
© Cabinet Guy Perazio.
7To retain only what is essential for understanding our comments below, we shall consider that the cave is of modest proportions, is situated in the heterothermal zone of the soil, and that it is at least partially ectothermal.
5.3 Before the discovery
5.3.1 Rock falls
8Figure 49 shows the cave opening before it was visited by the artists. A first rock fall from the rocky canopy, probably following climate change or seismic activity according to A. Glory (Leroi-Gourhan, Allain 1979: 45-74), would have taken place during the Paleolithic period. It is on this ground surface that Paleolithic humans entered the cave to decorate it. A second rock fall followed by filling with a sandy clay mixture and calcite sealed the cave, and nobody else appears to have visited it until September 12, 1940 (fig. 50).

FIG. 49 – Schematic longitudinal section of the cave before it was visited by the artists around 18,000 BP. Plan de la grotte :
© N. Aujoulat /ministère de la Culture et de la Communication, CNP.

FIG. 50 –Schematic, longitudinal cross-section of the cave before its discovery on September 12, 1940. Plan de la grotte :
© N. Aujoulat / ministère de la Culture et de la Communication, CNP.
5.3.2 Groundwater (temporary)
9The fissured limestone in the ceiling of the cave is home to temporary flows, with the main opening being in the ceiling of the entrance porch. It is fair to suggest that, before the discovery of the cave, water from the aquifer seeped into the pile of rocks at the entrance and disappeared into the Right Gallery. This flow of water invaded the large plunge pools in the Hall of the Bulls, the Passageway, and then the Nave up to the threshold of the access to the Chamber of the Felines. In the Hall of the Bulls, the “rivulet” pressed against the right wall, and the left part was spared (Leroi-Gourhan, Allain 1979: 53); in the Passageway, the flow went along the left-hand wall of the gallery.
10We can imagine that the water could have been colder or hotter than the air in the cave, depending on the season. An increase in flow resulting in a significant peak rise in the water level—meaning that it was clearly distinct from that of the continuous emptying of the stratum— could lead in winter to a significant decrease in the temperature of the stone and air, and an increase in temperature at the end of the summer. This corresponded to relatively cold water (rain, melted snow, water that had crossed cold ground) reaching the opening of the stratum in winter, and in summer, hotter water that had circulated in ground that was warmer than in winter. Since the external climate cycle could not be reproduced exactly from one year to the next, the hot or cold periods could be more or less early or late, changing their impact on the climate balance in the cave over time.
11Our hypothesis is supported by current findings. As we can see in figures 51 and 52, the impact of the groundwater temperature is considerable: up to 1°C. The thickness and physical properties of the ground enclosing the cave have barely changed since the Paleolithic period. Only the pluviometric system and the vegetation covering the soil above the cave might have changed (Leroi-Gourhan, Allain 1979: 46). The flows were usually greater than they are currently and, as we have pointed out, the groundwater covered the floor in the Hall of the Bulls, the Passageway and part of the Nave, thus providing a large surface for exchanges with the atmosphere of the subterranean environment. In short, the groundwater that invaded the cave played a major role in its climatic balance, sometimes beneficial, cooling the soil to encourage condensation on the low points and drying the air in the cave, and sometimes harmful, rapidly reheating the air in the cave and creating water vapor that might explain the damage to the paintings on the right wall of the Hall of the Bulls (fig. 53), as well as its condition (highly altered limestone?).

FIG. 51 – Impact of temporary water flows in the cave on the air temperature (measurements by J. Marsal).
© P. Sin / université Bordeaux 1, laboratoire Ghymac.

FIG. 52 – Impact of temporary water flows in the cave on the air temperature (automatic measurements).
© P. Sin / université Bordeaux 1, laboratoire Ghymac

FIG. 53 – Right wall of the Hall of the Bulls. This protome of the sixth Great Bull is highly degraded. It was traced on a surface exposed to alteration processes. A segmented trace is all that remains on a surface that is highly modified.
© N. Aujoulat / ministère de la Culture et de la Communication, CNP.
5.4 The crisis of 1963
12Since the cave was discovered, there have been two successive main stages in the conservation of this site of cultural heritage: one, from 1947 to 1963, was completely dominated by the motivation to adapt the cave for visitors; the other, from 1963 to now, is characterized by a scientific approach to the problems, with the monument’s conservation becoming the priority. It is worth remembering that, in spite of everything, conservation was not forgotten in the first stage. For example, we can quote M. Secondat, who wrote on 2 October 1947, “Since the cave has been open, we have found a change in its internal hygrometric state and precautions will have to be taken to avoid these unique frescos from being changed in the long term.” (Leroi-Gourhan, Allain 1979: 31). We can also quote Y.‑M. Froidevaux: “In order to ensure that there is adequate protection, the Architecture Department has established a committee comprised of specialists, prehistorians, geologists, physicists, biologists, hygienists and various technicians. Its mission is to determine the dangers and to study preventive measures. Scientific monitoring of the new conditions in which the cave found itself and of any developments in the condition of the paintings has therefore been organized. Since then, microclimatological studies have been carried out and spectrophotometric measurements of the paintings taken regularly and carefully analyzed under the supervision of Mr. Noetzlin, a member of the committee, who will establish a synthesis of the different forms of research as well as providing technical monitoring of the implementation of the installation. After the first few years of observation, no deterioration could be detected since the process of change can only be evaluated over a much longer period. However, the examination of the conditions created by visits, which are growing in popularity each year, aroused great fears, since they combined elements that are favorable to an acceleration of the problems.” (Froidevaux 1960).
5.4.1 Adapting the cave for visits: the regrettable effect of the use of machinery to regenerate the subterranean atmosphere
13During the period from 1947 to 1963, visitor conditions were reconsidered and adapted to visitor numbers, which rapidly went far beyond what had been predicted.
14Immediately after the discovery, the idea of protection (using the techniques of the time) became evident: administrative protection by means of classification as a historic monument and the implementation of means to ensure its conservation. The Historic Monuments Service thus implemented a program taking into account the preservation of the paintings and their presentation to the public under the leadership of Y.‑M. Froidevaux, Chief Architect of Historic Monuments (1948).
15This program (fig. 54) included the installation of a stairway covering the rock fall and leading to the Hall of the Bulls, and the creation of two airlocks at two different levels that would, according to their designer, make it practically impossible for hot air to enter from the outside, as this was the most dangerous threat to the cave. Finally, modifications necessary for the evacuation of infiltration water furnished by the groundwater table after periods of bad weather were made. Measures were also taken to facilitate the circulation of people in the cave and for lighting—for both visits and studies.

FIG. 54 – Schematic longitudinal cross-section of the cave in its current configuration, practically unchanged since the work of Y.‑M. Froidevaux in 1963 (Aujoulat 2004: 32).
Plan de la grotte : © N. Aujoulat / ministère de la Culture et de la Communication, CNP.
16In 1956 and 1957, it was found that the temperature and hygrometry of the air could no longer be kept constant due to the emission of water vapor and heat by the visitors, the emission of heat from the lighting, and significant thermal and hydric exchanges with the exterior. Increased carbon dioxide levels and a reduction in oxygen in the subterranean air were also observed. Since the risk of damage to the decorated walls—through crumbling of the wall or the proliferation of small calcite crystals on the surface of the paintings, leading to the formation of an opaque veil, etc.—had been noted, a certain number of measures were recommended: maintaining stable conditions of temperature at 14°C, of hygrometry (in the vicinity of 95% to avoid any condensation on the walls) and of the composition of the subterranean atmosphere (oxygen content greater than or equal to 20%). These measures were also meant relieve the respiratory discomfort experienced by many visitors during the peak period. A machine to regenerate the subterranean atmosphere was studied; it was meant to provide air-conditioning in the true sense of the term, as well as to remove the carbon from the air. The choice of 14°C as the air temperature was the result of measurements taken in the cave, which had unfortunately been greatly influenced by the visitors, and by the misconception that the natural temperature in the cave had to be constant all year round.
17The machinery, known as «Froidevaux», was installed in 1958. The regular distribution and circulation of air in the cave (by means of blowing at a speed of 4.5 m s-1) were ensured by ducts (blower channel and return channel) and vents located in the floor, whose effect on the decorated walls we might well question with hindsight. This involved significant earthworks and rock drilling, and in particular the removal of the debris cone at the entrance. The conditioned air was blown into the Hall of the Bulls and the Nave, and taken up at the bottom of the Right Diverticulum and the bottom of the Axial Gallery. The equipment was fitted with an automatic control to trigger its operation when the number of people present in the cave reached five.
18The goal to be attained, according to Y.‑M. Froidevaux and fixed by the program, which was to be prepared for the disturbance caused by the permanent presence of sixty occupants an hour, could thus largely be exceeded. In effect, after the adjustments made throughout the first year of its operation, the cave was able to receive the maximum number of visitors permitted by its geometry and the need for monitoring; nearly 100,000 visitors had access to the Lascaux cave in 1960.
19An obvious remark should be made: the initially strict instructions for the operation of this machinery were very quickly disregarded44. All the conditions that would result in the situation that forced the closure of the cave in 1963 were gradually established: by 1960, algae had developed in the Axial Gallery and the Hall of the Bulls. This algae was on the surface of the stone but also between the calcite crystals that made up the natural covering of the walls in the Hall of the Bulls and the Axial Gallery. Cultures in a solid environment showed Cyanophyta, Chlorophyceae and Xanthophytes. Some other findings indicated the presence of lesser bacteria and fungi (Laporte, Lefèvre 1970).
5.4.2 Priority given to conservation
20The alarming rate of the development of green stains in the axial part of the cave led to its closing by a Ministerial decision in April 1963. In early May 1963, the Commission for the Scientific Study and Preservation of Lascaux Cave defined the main directions for investigation and consideration. These were:
– to reveal the origin of the green stains, based on the work of J. Bauer and J. Pochon (Bauer, Pochon 1967);
– to make a statement regarding the way to manage the cave during this research;
– to define a system for visits. A few years later, a young researcher, P.‑M. Guyon, gave the following view of the work accomplished between 1965 and 1966 in the framework of the Scientific Committee for the study of the cave: “The decision by the Ministry of Culture to close the cave was imposed by the rapid, uncontrolled development of algae on the painted walls. This “green disease”, which has since been cured, was the consequence of pollution caused by too many visitors and an inadequate air conditioning system. At the instigation of the Conservateur of Historic Monuments, a small team of scientists comprised essentially of the biologists M. Lefèvre, J. Pochon and G. Laporte began studying this problem. Dr. J. Bauer of the Faculty of Medicine at Marseille made systematic observations of the painted walls in order to monitor the chromatic development of the algae zones during the treatment (1963-1964). While we found that the areas contaminated by algae regressed, Dr. J. Bauer’s macrophotographic studies revealed that the calciferous support of the Lascaux paintings was changing.”
21One of the questions raised at the time (1963) was whether the millennia- old balance amply proven by the state of preservation of the paintings had been destroyed by the modifications made to facilitate visits and, if this were the case, what the original balance might have been.
22A climatological study of the cave began in 1963 under the direction of H. Schoeller, but it quickly became evident that the frequency of observations, as well as the precision and reliability of the measuring instruments, were inadequate. They were unable to establish the necessary correlations between the various interacting parameters. In order to study the climatic context of the cave, a scientific protocol was based and is still based on the following data:
– measurement of air and stone temperature inside the cave;
– measurement of external climatic parameters (temperature, humidity, atmospheric pressure, pluviometry);
– measurement and monitoring of the development of groundwater flows intercepted at the entrance porch;
– measurement of the partial pressure of water vapor in the air at several marker points;
– measurement of the carbon dioxide content in different parts of the system;
– visual monitoring of several condensation indicators at the most sensitive points in the cave;
– monitoring of the temperature of liquid at the entrance to and exit from the primary cooling circuit and the secondary circuit, and regulation of cooling circuit flows;
– visual monitoring of the condition of all decorated walls;
– photographic monitoring of key indicators.
23In report no. 3 of February 28, 1966, P.‑M. Guyon gives a reminder of the committee’s new objective: “The main objective is to ensure that the parameters governing the balance of the physical chemistry of the walls are kept constant.” This aim had been defined by the same committee on 15 May 1965. In a report written by P. Michel in 1966, the committee’s wishes of March 21, 1966 are clearly recalled: “To create a cooling system, to refresh the air to ensure that CO2 is evacuated, and to reflect on the plunge pool problem.”
24In his report entitled Climat et Conditionnement de la Grotte [= Climate and Conditioning of the Cave], L. Comolet writes: “The Scientific Committee, whose task it is to study the different problems posed by the preservation of Lascaux Cave, initially had extremely fragmentary information that was often very imprecise. A considerable effort was made to take samples for temperature, saturated vapor pressure, CO2 content, and anemometer speeds. The obligation to take precise measurements was not imposed immediately. The term “keeping parameters constant” involved recreating the natural thermodynamic conditions of the cave by seeking to maintain the carbon dioxide levels at a very low value and the humidity levels at a very high value compatible with the absence of condensation. It was during this research that the particular physiognomy of the cave gradually became clear, but it took tens of thousands of measurements; accurate measurements, taken by hand or by direct reading since the recorders were usually not precise enough. By analyzing the causes of these problems, we found that it was necessary to do the following, all at the same time:
– avoid drying wet surfaces too quickly, unless there was a risk of a calcite veil forming;
– avoid water condensation, especially in the presence of CO2, unless there was a risk of the surfaces and paintings corroding. The air currents also had to remain quite low in the vicinity of the walls, so as not to:
25– dry them when they were wet;
– degrade them by wind erosion when they were dry. In order to reduce undesirable interactions and uncontrollable parasitic effects to a minimum, the following modifications were gradually realized:
– channeling infiltration water from the ceiling of airlock 1;
– thermal insulation of the doors at airlocks 1 and 2;
– installation of an airlock at the entrance to the Right Diverticulum, creating a clear separation between cave systems I and III;
– elimination of carbon dioxide gas by means of suction using a duct opening into the Shaft of the Dead Man (maximum flow 50 to 80 l·mn-1);
– removal of the air-conditioning machine and installation of two large condensation surfaces composed of radiators covered by cold water to organize and regulate convection movements.
26In the arrangement that was finally adopted, we constituted an independent set of machinery comprising system I (Hall of the Bulls and Axial Gallery) linked to two rooms: the machine room where the condensation surfaces were located, and airlock 2, which acted as a volume plug. Between the latter and the Hall of the Bulls there is a wall with openings that can be regulated to allow the direction and level of gas movements to be determined.”
5.4.3 Experiments
27The very small air movements, of no more than a few dozen cm s-1, are practically only convective in origin; the physical properties of the air (temperature, composition) are slightly variable from one point in the cave to another.
28The direction of these air movements was revealed by P.‑M. Guyon and A. Dupuis by making, positioning and observing expanded polystyrene floats comprised of a cross-sail fixed to cork plugs moving on a tub (a recycled, cleaned and painted oil can) half-filled with water containing 5% formalin to avoid the formation of algae. When affected by the air current, the floats moved from one side of the can to the other. A small metal wire tightened on the four sides of the can prevented the floats from getting stuck on the walls because of the surface tension of the water. The position of the floats merely had to be changed at regular intervals to obtain the direction of air flow at that place. Some one hundred of these indicators were put in the cave on metal gallows in the areas making up the right-hand sections of the cave. These very sensitive floats were disturbed by the column of hot air created by the observer, so it was necessary to keep a distance of at least 5 m. The floats settled after 10 minutes.
29The speed of these air flows was then evaluated by releasing inflated, ballasted helium balloons. The speeds measured ranged from 5 to 20 cm s-1 in the Hall of the Bulls, and 50 cm s-1 in the large joint. Some tests have been conducted using feathers attached to wires. The results appear to be less clear than those obtained using the balloons for air displacement to the ceilings but they have been used to interpret air displacement on the floors.
30The use of small, inflated, suitably ballasted helium balloons is a delicate operation in Lascaux Cave, since the movements are small and the column of hot air produced by the observer significantly affects the measurement. On the other hand, P.‑M. Guyon constructed an expanded polystyrene revolving stand that allowed speeds of approximately 0.1 to 1 m s-1 to be measured. The same author measured air flows of 1 000 m3 h-1 crossing the high runners in the machine room, which allowed the speed in these runners to be estimated at 0.15 m s-1. All these observations have allowed us to define the convection cells and compartmentalize the cave into zones or “systems”. Two natural ventilation zones are more or less independent: system I (Hall of the Bulls and Axial Gallery) and system III (sandy rooms, large joint, Shaft, Apse, Nave and the Mondmilch Gallery). In system II (Passageway), which links the other two systems, the movements were very small, or absent. System II is composed of a gallery descending from the Hall of the Bulls with a floor gradient of about ten meters. It was separated from the Hall of the Bulls by the creation of an airlock, called the “Bauer Airlock”, which has since been removed.
31Zone III is also a set of steps descending sharply to the right of the edge of the Shaft of the Dead Man. Set III is physically shared by two sections through the insertion, in the second term of 1966, of flaps and a trapdoor at the edge of the Shaft.
32The air movements can be caused by exchanges with the outside:
– either through a difference in density between the exterior and interior air;
– or through variations in atmospheric pressure.
33Since the cave is of a descending type, all possible communication with the outside is at a much higher level than in the galleries. Because of this, only heavy, cold air with low vapor pressure can enter the cave and dispel lighter, hotter air with higher vapor pressure from the cave. This phenomenon can only be observed (when the doors of all the airlocks are open) in winter. The exchanges of air using this process are so slight as to be non measurable.
34The air flows caused by a relatively rapid difference in atmospheric pressure are negligible due to convection movements. For example, for a total cave volume (visitable system and system connected by joints) of 30 000 m3 (measurement taken using a nitrogen trigger) and for a variation in atmospheric pressure of 2% in twenty-four hours, the difference in the volume of the air in the cave is 600 m3. Taking as our hypothesis the fact that all the air in question passes through a gallery with a cross-section of 10 m2 (for example, the average cross-section of the Passageway), the speed would be less than 1 mm·s-1.
35For the record, we shall mention the test done by Prof. H. Schoeller: “A first experiment, intended to direct our research, was conducted on June 9, 1964 at 11 a.m. At the bottom of the Mondmilch Gallery, we ignited a very small quantity of damp hay. I was in charge. The smoke produced went up to the top of the Gallery and into the upper sections of the Nave and Apse. From there, it went into the Shaft opening and along the corridor to the large joint as far as the sandy rooms. A very small amount went into the Right Diverticulum or Passageway, but stopped a few meters before the Hall of the Bulls and eddied. At 3 p.m., the smoke had spread to the top of the rooms. No smoke reached the Hall of the Bulls or the Axial Gallery.” (Schoeller 1965).
5.4.4 The solution adopted
36Depending on the seasons, the walls and the air were more or less damp, which could be harmful to proper conservation of the prehistoric paintings and engravings. For six months a year (according to the work done by H. Schoeller in 1964 (Schoeller 1965) and based on the condition of the cave) it was possible for subterranean air vapor to condense on the decorated walls; for the remainder of the year the evaporation process could take place. These two phenomena were linked to variations in the surface temperature of the stone and in the subterranean air produced by the phase difference of the thermal surface waves (fig. 55).

FIG. 55 – Difference between the rock temperature (left scale) on the right wall of the Hall of the Bulls and the average outside air temperature (right scale) measured under a weather shelter above the cave.
© P. Sin / université Bordeaux 1, laboratoire Ghymac.
37The solution adopted was an air-conditioning system based on the thermosiphon principle proposed by P.‑M. Guyon and then implemented and updated by successive engineers who worked in the cave (P. Michel, P.‑L. Gavet, G. Salvagnac, G. de Guichen, J.‑M. Py). When there were no visitors, the air conditioning in the cave was realized by small, simple heat exchangers situated behind the high wall openings in the machine room (runners). This principle worked until the 1990s, since the air temperature in the cave became increasingly hot as the depth increased. We will return to this fact in the next section.
38In order to limit exchanges with the outside and to insulate Airlock 2 from the Airlock 1 zone, subjected to great variations in external temperature, two isothermal partitions were built. The diagram showing the principle of climate regulation adopted at the time is given in figure 56.

FIG. 56 – Diagram of the climate assistance principle defined by P.‑M. Guyon. Plan de la grotte :
© N. Aujoulat / ministère de la Culture et de la Communication, CNP.
Reprise : © P. Sin / université Bordeaux 1, laboratoire Ghymac.
39Thanks to the use of aerodynamic exchanges that take place naturally using convective cells between the axial part of the cave and the machine room, there is limited possibility for ambient water vapor to condense on the painted walls. On the other hand, this is encouraged at the level of the cold surfaces (batteries) put in place outside the section that is to be protected, in the machine room. Therefore, on average, from June to December of the year in progress, it is possible to regulate the thermal and hydric properties of the subterranean air to adjust and improve the conditions. Such intervention does not affect the system outside the cave and adapts the ambient conditions to those of the stone.
5.5 The crisis of 2001
5.5.1 Reminder of the facts
40The white mold appeared shortly after some major construction was carried out in the machine room. As was pointed in the cave condition report drawn up in May 1995 by Jean-Michel Geneste, the Curator of the cave, the dilapidated state of the air-conditioning equipment required changes to be made, with replacement by equipment that was theoretically identical.
41Well before the crisis of 2001, a number of studies had already raised doubts about the imagined stability and invulnerability of the cave. Scientific documents (measurements, observations, etc.) confirmed what had already been remarked thirty-eight years before (1963 crisis): that there was the very little room for maneuver in order to ensure optimal conservation conditions (Brunet et al. 1997).
5.5.2 Analysis of the climatic situation
42Understanding the mechanisms responsible for the dynamic equilibrium of the cave is complex. The cave is in a situation that evolves naturally; since 1963, the monitoring of this evolution has shown that for six months a year there was a possibility that the subterranean air would condense on the decorated walls, and the rest of the time the evaporation process was possible. These mechanisms, triggered by changes in temperature on the surface of the stone and in the subterranean air, can cause corrosion phases or the deposition of calcite as a result of the presence of carbon dioxide.
43The goal remains the same as before: to restrict the opportunities for condensation by reducing the differences between the partial pressure values of the water vapor of the air and the calculated values of saturating partial pressure, corresponding to the temperature of the stone, in order to reduce the risk of condensation on its surface. The use of aerodynamic exchanges that occur naturally by means of convection between the axial part (Hall of the Bulls and the Axial Gallery) and the machine room meant that a system could be installed to regulate the atmosphere (cf. § 5.4.4). It restricted the opportunity for water vapor to condense on the decorated walls; instead, condensation was stimulated artificially in the machine room. Specifically, this circulation— natural or artificial depending on the season of the year— ensured that the air was cooled and excess humidity was lost through contact with the heat exchangers. Depending on the period, this loss of humidity also occurred upon contact with the stone and the ceiling of the machine room, which collects water from the temporary aquifer stratum situated above the cave. Under these conditions, the seasonal climate disruption detected by H. Schoeller and P.‑M. Guyon could be overcome.
5.5.3 Showing climate change
44The analysis of the data for the years 1965-2004 (Malaurent et al. 2006) shows that for all the measuring points there was a decrease in the average temperature until the end of 1980, followed by a regular increase (fig. 57). This trend can be explained by the evolution of the outside temperature over the same period. Depending on the measuring points, the temperature changes are more or less chaotic. The deepest points correspond to the smoothest evolution curves.

FIG. 57 – Measured trend in rock temperature in the cave at different depths below ground: ceiling of the Hall of the Bulls (TVR), at a depth of 10 m, and bottom of the Axial Gallery (DFR), at a depth of 20 m.
© P. Sin / université Bordeaux 1, laboratoire Ghymac.
45As a reminder, hot air that is superimposed onto cold air stratifies, while cold air on hot air causes convection movements due to the difference in density. These movements are naturally maintained when the air reheats upon contact with the ground and cools again upon contact with the ceiling. To illustrate our theory, we can examine two measuring points in more detail, selected in the axial part of the cave: the ceiling of the Hall of the Bulls (HB) and the bottom of the Axial Gallery (AD).
46Examination of the temperature curves calculated in the soil (fig. 58) at different depths (10 m, 15 m and 20 m) using the Fourier equation and temperature curves measured at two typical points of the cave (fig. 57) shows between 1950 and 2004, the respective changes in this temperature. We can deduct that, from 1965 to 1981, the subterranean thermal conditions related to the propagation of external heat waves were favorable to the establishment of an air circulation system based on convection between the bottom of the Axial Gallery and the Hall of the Bulls when the surface temperature of the stone was greater than the temperature of the air. Consequently, since the air in the deepest parts of the cave is on average hotter than that of the air in the highest parts, it rises towards the exterior, and convection is therefore well maintained. Since 1981, this convection system has been affected by the impact of external climate changes. In the curves in fig. 57, this is shown by the inversion of the temperatures in the cave. The air in the deepest sections is now colder than the air in the highest parts, resulting in a stratification of the air.

FIG. 58 – Trend in the temperature measured at the surface at the Gourdon weather station and calculated theoretical trend of the temperature at different depths in the ground between 1950 and 2004.
© P. Sin / université Bordeaux 1, laboratoire Ghymac.
47From 1970 to 1981, the temperatures measured in the stone (fig. 57) at the bottom of the Axial Gallery (DFR) were higher than those measured on the ceiling of the Hall of the Bulls (TVR). Since this time, the temperatures on the ceiling of the Hall of the Bulls have been higher than those in the Axial Gallery. This phenomenon is responsible the increasingly unfavorable conditions for convection to the machine room. It has consequently affected the regulation of the subterranean climate as it was defined by the researchers of the scientific committee in 1965. We can again refer to the experiment by H. Schoeller (cf. § 5.4.3), realized during a similar period to that of today, and which seems to confirm our conclusion.
48From figure 59, we obtain the following information:
– from 1965 to 1981, the temperature gradient in the cave was consistent with operation scheme defined by the committee in 1963; in winter, starting from a depth of 10 m, the temperatures increased from the surface downwards: machine room (MR) < Airlock II < Hall of the Bulls (HB) < Axial Gallery (AD); in summer this order is maintained artificially by deliberately lowering the air temperature in the machine room (fig. 56);
– since 1981, the evolution of theoretical and measured temperatures at 10 m, visible in figures 57 and 58, shows that the temperatures in the deeper sections of the cave are becoming lower than the average temperature at a depth of 10 m (the thermal inertia of the floor increases as the thickness of the floor increases). The natural temperature gradient is inverted. This phenomenon is independent of the artificial regulating system in the cave; it exclusively related to the outside climate.
5.6 Partial conclusion
49In the current climatic context, the consequences for preservation are far from negligible. Of the two major risks—risk of water vapor condensation and risk of degradation of the walls—let us look more closely at the former: the effect of water condensation on the prehistoric decorations. The wall at the bottom of the Axial Gallery is invaded by water droplets at the end of winter. In April 2001, practically the whole body of the Great Bull was covered with drops (fig. 60). The origin of this phenomenon can be attributed to the disruption of the natural exchanges in the subterranean microclimate. Since then, it is essentially the floors and the lower parts of the Axial Gallery that have been damp. We must remember that the air-conditioning machinery was designed to meet a demand corresponding to a need in accordance with the subterranean environment of the time. The slow natural drift of external parameters therefore has repercussions for the subterranean climate and therefore on the management of the cave. In the current climatic situation, the heat regulation system in use is, at best, only effective for the Hall of the Bulls.

FIG. 60 – Powerful black bull at the end of the Axial Gallery, accompanied by four yellow, horned heads and two silhouettes of red cows. At 3.71 m wide and 1.93 m high, the Black Bull at the end of the Axial Gallery is imposing. In the photo, the partial loss of pigmentation of the chignon might be the result of attacks by condensation water on the support over the years, which formed systematically between the horns. This phenomenon has now disappeared. According to Norbert Aujoulat, the entire animal was realized by pulverization.
© N. Aujoulat / ministère de la Culture et de la Communication, CNP
5.7 The immediate future
50A numerical simulation of fluid mechanics will be used to determine the optimal conditions for climate assistance and to suggest new solutions. This is the “Lascaux simulator”.
51Acknowledgements
52The authors would like to thank the Aquitaine Regional Director of Cultural Affairs, the Aquitaine Regional Curator for Historic Monuments, the Curator of the Aquitaine Regional Archaeology Service, the Director of the Laboratoire de Recherche des Monuments Historiques, the President of the Lascaux Cave Scientific Committee and everyone who has worked at Lascaux since its discovery.
53References
54Aujoulat 2004 : AUJOULAT (N.). — Lascaux : le geste, l’espace et le temps. Paris : Le Seuil, 2004.
55Bauer, Pochon 1967 : BAUER (J.), POCHON (J.). — Étude technique de l’évolution calcique des parois de Lascaux. Marseille : Institut de photographie scientifique et médicale, faculté de médecine, 1967. 76 f.
56Brunet et al. 1997 : BRUNET (J.), MALAURENT (P.), VOUVÉ (J.). — Lascaux, histoire d’un difficile sauvetage. Archéologia, 332, mars 1997, p. 24‑35.
57Froidevaux 1960 : FROIDEVAUX (Y.‑M). — Protection de la grotte de Lascaux : installation d’une ventilation conditionnée. Les Monuments historiques de la France, 4, oct dec. 1960. 5 p.
58Laporte, Lefèvre 1970 : LAPORTE (G. S.), LEFÈVRE (M.). — Au chevet de Lascaux : les travaux de la commission scientifique pour la sauvegarde des peintures rupestres de la grotte préhistorique. Bulletin de l’ordre des pharmaciens, 143, 1970. 39 p.
59Leroi‑Gourhan, Allain 1979 : LEROI‑GOURHAN (A.), ALLAIN (J.). — Lascaux inconnu. Paris : CNRS, 1979, 381 p. (supplément à Gallia préhistoire ; 12).
60Schoeller 1965 : SCHOELLER (H.). — Étude géologique, hydrogéologique et climatologique de la grotte de Lascaux pendant le cycle 1964‑1965. Rapport pour la première commission scientifique internationale de la grotte de Lascaux, univ. Bordeaux 1, 1965. 97 p.
61Malaurent et al. 2006 : MALAURENT (P.), BRUNET (J.), LACANETTE (D.), CALTAGIRONE (J. P.). — Contribution of numerical modelling of environmental parameters to the conservation of prehistoric cave paintings : the example of Lascaux Cave. Conservation and management of archaeological sites, 8, 2006, p. 1‑11.
Notes de bas de page
1 Claude Bassier (letter of 15/04/1966 ; document of 06/04/1966).
2 On the support, there are small white pustules of calcite that are particularly dense on the back legs; there is a loss of pigment on the shoulder and flank of the horse; the ventral outline and one of the anterior members are affected by this phenomenon. Part of the painted line of this figure, like many of the painted representations that existed in this gallery, was removed by erosion caused by the flow of air currents on the ceiling and walls before the cave was discovered.
3 The entire apse is covered with more than one thousand painted and engraved figures. This very high concentration of mostly engraved representations is enhanced by only a few colored areas. A detailed examination of the wall reveals an intermingling of complete or partial drawings of animals that have suffered significant damage by natural processes.
4 Oral information from J. Marsal.
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