Although the damage observed is extensive, the following factors may facilitate the conservation of the sandstone: The main salts present are gypsum and calcite; since these salts are not hygroscopic, they do not contribute to an increase in the moisture content of the stone. The salt content is moderate to medium compared to the total porosity of the stone, and the salts are concentrated in the outer 5-10 mm. Sacrificial mortar layers are able to displace the evaporation zone, simultaneously displacing the accumulation of salts from the stone surface into the rendering. With time, some extraction of the present salts can be expected, although gypsum and calcite are quite insoluble.
Because neither gypsum nor calcite are hygroscopic, the moisture of the stone is determined primarily by the amount of seeping water and condensation from the air. Sorption isotherms, therefore, give an indication of the appropriate conditions for conservation. Figure 3 shows that when the relative humidity is below 75%, the amount of absorbed water is less than 1 weight %, which is low enough to obtain a sufficient penetration depth of stone consolidants. For this reason, it is absolutely necessary to monitor the climate in the grotto and wait for these appropriate conditions. Yet treatments can be carried out only after a long dry period. Karsten pipe measurements can help determine whether uptake-and-penetration depth of a stone consolidant is sufficient (Wendler and Snethlage 1989).
In the near future, conditions appropriate for the application of a stone consolidant can be expected for only the upper parts of the grotto. The treatment, therefore, will be carried out in two or three steps. The upper areas, which are already sufficiently dry, will be treated first. Then, after the removal of the floor sediments and the control of the seeping water with drainage tubes, the lower walls will be allowed to dry out for several years. When dry, the remaining areas of the grotto can also be treated.
Acknowledgments
The investigations reported in this chapter were financially supported by the Federal Ministry of Research and Technology, Germany; and the Ministry for the Preservation of Cultural Property of Shaanxi Province, People's Republic of China.
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