Mechanical properties of white clay used as wall painting substrate material: measurement of strength and modulus of elasticity of simulated substrate material of wall paintings Hiten at Kondo, the main hall at Horyu-ji Temple

Preserving cultural artifacts while minimizing the energy consumption and costs associated with environmental control is crucial. This often requires predicting artifacts degradation caused by temperature and humidity. Here, mechanical damage can be predicted by comparing stress and damage criteria. Although the constitutive coefficients and damage criteria of various materials have been extensively studied, the mechanical properties of white clay remain unknown. This material is a crucial component of various artifacts including some important cultural artifacts and national treasures of Japan. Thus, this study aims to identify the strength and elastic properties of simulated white clay mimicking the substrate of the wall paintings Hiten at Kondo, the main hall at Horyu-ji Temple. Following existing literature, we created the simulated white clay by combining clay, paper fiber, and rice glue. We fabricated 36 distinct specimen patterns by varying the equilibrium humidity and material mixing ratios. We measured the tensile strength using splitting tensile tests and, compressive strength, Young's modulus, and Poisson’s ratio using compressive tests. The tensile strength, compressive strength, Young’s modulus, and Poisson’s ratio ranged from 0.0785–1.17(MPa), 0.358–3.67(MPa), 0.0394–0.274 (GPa), and 0.10–0.44(-), respectively, with variations depending on the equilibrium humidity and ratio of material mixing. We also formulated the results as functions that depend on the equilibrium humidity and material mixing ratios. These findings can be used to predict stress and damage to targeted wall paintings and to inform the preservation and restoration of cultural artifacts containing white clay.

Graphical Abstract