Moisture control

The modern artificial dripline is usually of a silicone sealing compound which is applied with a pressure gun. The silicone must have good bonding ability, but remain removable without damage to the rock, it must have a high thermal stability and resistance to moisture and ultraviolet radiation, and it should be of an unobtrusive colour (the clear version is favoured in Australia). At sites visited by the public it may be more advisable to use coloured silicone to match the colour of the rock, because it has been found that tourists tend to pull the silicone threads off the rock (Gillespie 1983). The nozzle of the pressure gun is adapted to specific applications. Rock surfaces need to be cleaned of organic deposits before the silicone is applied.

This method of water control is based on the fact that a thin laminar surface flow can be broken at or slightly below a natural dripline on the outer roof of a rockshelter, where much of the moisture follows gravity and drips to the floor. Part of the flow may continue, especially in conditions of heavy rain, and this can be encouraged to drip to the floor by a low (c. 5 mm) barrier whose inside is sloped upwards. The preferred position of the dripline is on the outermost horizontal surface, just under the rim of the roof, where the water still has vertical flow velocity.

The extent of the use of this method in Australia is illustrated by the fact that, in one small region alone, the site managers have spent some $300 000 on such artificial driplines. The method has been introduced in other countries and will probably be adopted universally. However, it is most important to appreciate that many such driplines have been installed in unsuitable places (e.g. on vertical or inadequately sloped surfaces), and in such cases may in fact aggravate a conservation threat. It is therefore essential that the conservators using the method have been fully briefed concerning the placement of these barriers (Gillespie 1983). Other measures of diverting water flow include gutters (first used by M. Lorblanchet in Australia, in Glenisla Shelter, Grampians), small roofs and, in the case of caves with vertical entrances (sinkholes), surface channels or embankments (Bednarik 1988a).

Guttering has also been used in Australia to reduce excessive capillary action (e.g. at Mt Grenfell, NSW), in conjunction with modifications to vegetation to reduce retention of water (Walston and Dolanski 1976). Vegetation can also be introduced to soak up surface run-off above a site, especially a cave in a karst region (Bednarik 1991b). Deforestation can affect the hydrology of a site setting quite dramatically, for instance in Paroong Cave (South Australia) it resulted in occasional flash floods, one of which lowered the sediment deposit by some 80 cm. To arrest this trend I re-contoured the catchment area above the cave to reduce it by 98% and re-established the previous vegetation regime to ‘soak up’ the remaining precipitation (Bednarik 1988a). The area involved amounted to 18 150 m2. Such indirect modification of site conditions can be very successful and it illustrates how important it is to consider the entire site environment rather then just the most immediately apparent conservation problem. It also shows that some hazards are best tackled at their source, well away from the actual rock art. This applies not just to surface run-off, but also to gravitational water (percolating through the rock fabric above the art). There are no practical means to control it at the rock art panel, the source must be determined. One exam-ple of this is the spectacular work of the Indian Archaeological Survey, Bhopal office, at Auditorium Rock, Bhimbetka. The 40-m-high quartzite tower was scaled to locate ingress openings of rainwater which were grouted with waterproof cement. Similar work has been undertaken on a much smaller scale at various sites, such as Mt Grenfell, NSW.

While it is feasible, and often quite practicble, to control gravitational water, capillary moisture is virtually impossible to eradicate. Rising in porous rock, usually from an aquifer, it is probably the main factor in the formation of sandstone shelters at the base of cliffs (Lambert 1980). Moisture rises from the ground and the salt spalling it causes through wetting and drying cycles erodes the rock rapidly, but only above ground. The lack of exfoliation below ground, combined with the gradually rising sediment level leads to the formation of a sloping bedrock shelf and a receding inner wall, up to the height of the susceptible rock zone. The erosion process continues after rock art has been placed in the shelter, and short of preventing the aquifer from reaching the rock there is no realistic possibility of arresting the process. Attempts to control capillary moisture failed in an Australian project. However, if the moisture originates not from an aquifer, but from a moisture-buffered sediment, removal of such waterlogged material can be very effective in lowering moisture content in contiguous rock (Bednarik 1988a).

In theory, the deposition of condensation water is relatively easy to control, by modifying the climatic regime of the site. Since the dew point is determined by the difference between rock and air temperatures, the latter being higher, and a very high air humidity, one can either raise rock temperature or lower air humidity. The practical application of such principles is not so simple, however. Schwartzbaum (1985) advocates the installation of electric heating coils, and while this would be effective in some instances, it is obviously not feasible in most affected rock art sites. In some caves containing rock art the ultimate technological solution has been to control the entire climate of the cave by air-conditioning equipment such as that installed at Lascaux and Font de Gaume in France (Brunet et al. 1995). This means that temperature and air humidity are set at appropriate levels thought best for the rock art. There are, however, much less intrusive methods of achieving a level of control over adverse microclimatic conditions in limestone caves. Since most condensation occurs near the cave entrance, well-designed alterations to nearby vegetation, to the configuration or size of the entrance or to sediment deposits in it can all affect the microclimate. To some extent this also applies to rockshelters, although when planting vegetation near rock art, the fire danger must always be taken into consideration.

Another form of water damage, through freeze-and-thaw cycles, can result in the gradual reduction of rock mass to cryoclastic detritus. Controlled experiments in Scandinavia have shown that this can be significantly alleviated by covering rock pavements (Bertilsson and Magnusson 2000). A variety of materials, ranging from sediment to synthetic sheets such as mineral wool ground insulation, can be used for this purpose. Vegetation has also been reported to have some effect by reducing exposure. A second option is to attempt control of the moisture source, but this is probably not readily possible in most cases. Finally, the ultimate solution at open sites is to enclose them, but if the structure has no walls the effect might not be great. A fully closed building might eliminate freezing but would introduce a completely new environment, whose effect on the rock art may well be more deleterious than that of ice (see ‘Climate control’).