NUMERICAL ANALYSIS OF THE INFLUENCE OF HYGROTHERMAL VARIATION ON SALT WEATHERING OF A BUDDHA STATUE CARVED INTO A CLIFF

－Study on the conservation of a stone Buddha carved into a cliff at Motomachi PART1－

Abstract

 Motomachi Sekibutsu, which was registered as a national historic site in 1934, is the representative stone Buddha statue carved into the cliff in Oita city, Japan. It is always affected by the ground water through the cliff because it cannot be separated from the cliff, and thus there has been a great concern in its moisture-related problems such as salt damage and growth of bryophytes.
 As an onset to proposing a conservation environment that is the most appropriate to suppress the salt damage, this paper is dedicated to environmental measurement to obtain the essential data set and numerically quantifying influences of the heat and moisture behavior in the cliff on potential salt damages mainly at and near the surfaces of the main Buddha statue, which has been suffering from salt precipitation.
 Sodium sulfate (Na₂SO₄) is focused on in this paper as it is considered to have been the most problematic at this site. In discussions, we deal with two phenomena, (1) crystallization due to water evaporation and/or solubility changes and (2) thenardite-mirabilite cycles that are assumed to be dependent on the temperature and humidity.
 We analyze simultaneous heat and moisture transfer in the two-layered cliff that consists of soil and tuff, the latter of which includes the stone Buddha. Two steady state and two unsteady state analyses are carried and their results are compared, in order to quantify the influence of the room temperature, humidity, solar radiation and the rainfall. The main results are described below.
 1. As a result of the ground water level that is considered at about 1m below the ground level, a high water content is maintained deep in the cliff; the moisture content can get much lower due to evaporation at and near the surface of the stone.
 2. When the moisture content at the surface is low, significant vaporization of water occurs inside of the statue where the moisture content changes drastically.
 3. The moisture content inside of the statue fluctuates throughout the year. The moisture content near the surface is increasing from summer to autumn, and decreasing in the rainy season and from winter to spring.
 4. The direct solar radiation and a rapid absolute humidity decrease by opening the door of the shelter can accelerate vaporization of water in the stone near the surface, even when the moisture content is low at the surface.
 5. It is in autumn to winter that salt damage near the surface can be the most significant because of an overall decrease in solubility of Na₂SO₄ due to a low temperature, a local increase of solar radiation incident on the stone Buddha, and the rapid absolute humidity decrease by opening the door of the shelter.
 6. The phase change of Na2SO4 occurs at the shallow areas in the internal corner; in the external corner, it occurs not only at the shallow area but also deep in the stone.
 7. With the current shelter, at the knee in the external corner, thenardite precipitates in autumn to winter and dissolution of thenardite often occurs in winter to spring. Thus, a particular attention would be recommended to be paid to salt damage in these seasons.
 8. A high air-tightness and ventilation would lead to more frequent thenardite-mirabilite cycles.