Influence of solar radiation on the surface loss of the painted statues in Grotto No. 3 of Maijishan Grottoes, China

The embedded hygrothermal environment of cultural heritages (solar radiation, temperature, humidity, etc.) plays a significant role in their conservation and survival, especially in the case of outdoor ones. The surface loss of painted statues is often related to fluctuations in the surface temperature caused by the ever-changing solar radiation; however, the quantitative relationship between them remains unclear. To clarify, in this paper, hygrothermal monitoring and numerical simulation were conducted on painted statues with an age of over 1400 years in the World Heritage Maijishan Grottoes. The results showed that the ruined eaves have led to significant differences in solar radiation intensity and surface temperature between the upper and lower statues, with maximum values of 598.7 W/m2 and 13.6 °C, respectively, corresponding to surface loss rates of nearly 15% and 50%. Under the influence of direct insolation, the thermal stress on the statue surface increased with decreasing height, resulting in a difference of 10.8 kPa between the highest and lowest points. Taking the tensile strength of clay (10 kPa) as a threshold of thermal stress, the yearly number of events exceeding this threshold at the upper and lower rows were 2819 and 3173, respectively. Overall, direct insolation has a significant effect on the temperature changes on painted statue surfaces, thus leading to greater thermal stress and greater degrees of surface loss. For heritages in open or semi-open environments, it is necessary and effective to evaluate their preservation conditions based on the threshold and then propose suitable conservation methods for preventive protection. •Two vertically adjacent rows of statues in a semi-open grotto show varied loss rates.•Inadequate protection of ruined eaves lead to vertical distribution of thermal stress.•Solar-induced thermal stress is one of the main reasons for surface deteriorations.•A suggested early warning threshold of temperature differences (6.0 °C) is proposed.•Protective facility is needed and effective for heritage exposed to direct insolation.