Mechanical properties of wall soils for predicting damage to the substrate of Hiten wall paintings in the Horyu-ji Temple main hall caused by humidity fluctuation: Measurements with mock-up materials

Safe and efficient conservation of cultural artifacts requires preventing artifacts deterioration and energy-saving environmental control. To achieve this, predicting deterioration caused by environmental conditions is necessary. Predicting the mechanical damage caused by humidity fluctuations necessitates knowledge of the mechanical properties of cultural artifacts materials. Although the mechanical properties of several artifacts have been extensively studied, no investigations have focused on the soils underlying wall paintings. This study aims to clarify some mechanical properties of the upper- and middle-coat soils serving as the substrates for Hiten wall paintings at Horyu-ji Temple. Mock-up materials were prepared, and splitting tensile and uniaxial compressive tests were performed. Simultaneously, specimens with various equilibrium humidities were tested to clarify their humidity dependency. The tensile and compressive strengths, Young's modulus, proportional limit, and Poisson's ratio of the upper-coat soil were 0.103–0.239 MPa, 1.16–2.55 MPa, 0.115–0.209 GPa, and 1.10–2.49 MPa, and 0.152, respectively. Moreover, the humidity-induced strains for the upper- and middle-coat soils were measured, and the moisture expansion coefficients were approximately 1240 and 2337 μST/−, respectively. The results of this study provide vital data for the conservation of the wall paintings and contribute to a deeper understanding of wall soil properties.