NUMERICAL EVALUATION OF THE INFLUENCE OF SALT DAMAGE BY IMPROVEMENT OF THE SHELTER

－Study on the conservation of a stone Buddha carved into a cliff at Motomachi PART 2－

Abstract

 In this study, we clarified how the shelter suppresses salt damage at Motomachi Sekibutsu, that is stone Buddha directly carved into a cliff at Motomachi, by calculating the hygro-thermal environment in the shelter. By numerical analysis, we modeled the cliff including the stone Buddha in two dimensions and the wall of the shelter in one dimension, and analyzed the heat and moisture behavior. In addition, we built a combining model to calculate the temperature and humidity in the shelter, and we verified the validity about the analytical results by reproducing the hygro-thermal environment in the shelter during the period from November 2014 to October 2017. We quantitatively estimated the risk of salt precipitation and salt damage by the hygrothermal properties in the stone Buddha, assuming that the amount of salt precipitation is proportional to the amount of evaporation from the statue and salt damage occurs during when salt(sodium sulphate) changes its phase. By using those models, we divide the renovation of the shelter into three elements: improvement of the thermal insulation, air tightness and shielding of solar radiation. The main results obtained by numerical analysis are described below.

 1. By the renovation of the shelter, the amount of evaporation from the surface of the stone Buddha is suppressed by all the elements.

 2. Focusing on the seasonal change in the amount of evaporation, it is possible to suppress the evaporation in summer, but difficult in winter except for the thermal insulation renovation.

 3. In the study on the phase change of sodium sulphate, since the relative humidity in the shelter is higher than before the renovation and the hygrothermal property of the surface of the statue have become unfavorable to Thenardite, it is considered that the number of phase change about sodium sulphate is reduced. In other hand, if only we improve the air tightness, temperature in the shelter is kept high and it is easy for sodium sulphate to change its phase into Thenardite, so it may not be possible to say that the risk of salt damage has been generally suppressed.

 4. When all the renovations are carried out (i.e. after the renovation), it is considered that the amount of salt precipitation and the the number of phase change about sodium sulphate are significantly reduced, so the risk of salt damage is suppressed.

 Future issues are as follows. In Motomachi Sekibutsu, as a result of renovation of the shelter, the relative humidity in the shelter has become more humid throughout the year. In such a high humidity environment, there is a high risk of mold growth and we concern about the influence on the health of the visitor. In addition, as long as there is a huge water supply source such as groundwater, it is difficult to suppress the evaporation from the statue completely. Therefore, since salt is gradually accumulated in the stone Buddha, it is considered that the precipitation salt cannot be suppressed completely. In particular, the latter problem cannot be solved only by controlling the hygrothermal environment in the shelter, so we will start developing a technology to remove the salt accumulating in the statue. Furthermore, it is necessary to consider the preservation and exhibition method of Motomachi Sekibutsu based on the health risks of visitors and the preservation of the statue.