6.1.1 Dry-wet cycle
From the daily average humidity variation in Niche of Sakyamuni Entering Nirvana (Chen et al., 2004), the reduction time (9:00-21:00) of humidity and maintenance time (21:00-9:00) at high humidity in a day was about 1:1. The sandstone samples were air-dried (9:00-21:00) and placed into a container filled with distilled water (21:00-9:00). This process represented the completion of a cycle, and seven cycles were set as one period, as shown in Fig. 5.