Investigative experiment on in-situ low-temperature crystallization induction time measurement for Dunhuang Murals, considering porosity factors

Crystallization induction time is a critical parameter in assessing the salt crystallization process, which profoundly influences the degradation rate of historical mural heritage. This understanding compels us to delve into an in-depth analysis of how various environmental factors specifically impact crystallization induction time. Therefore, In this study, we present a comprehensive analysis of the evolution of pore volume resulting from diurnal temperature cycles, along with the determination of solution supersaturation and associated crystallization pressure in subsurface pores. Initially, we examined the impact of increase number of cycles to the crystallization induction time of Na2HPO4·12H2O. Through field experiments employing a hot-cold alternating pattern, we established a relational model between the number of cycles and the crystallization induction time under a temperature gradient. Subsequently, we thoroughly investigated the accumulation phenomenon of crystal seeds within the pores. Our findings reveal that crystals tend to aggregate and grow at the initial location of the first crystallization, resulting in a reduction in the induction time. This phenomenon indicates that the presence of crystal seeds accelerates the process for crystallization, thereby increasing the risk of deterioration to historical murals. Additionally, this study delves deeply into the micro-morphology and porous structures of the samples, employing both qualitative and quantitative approaches to thoroughly explore how these factors synergistically influence the crystallization process. This study sheds light on the salt crystallization behaviors in in-situ mural environments, considering the evolution of pores, and provides valuable insights for the preservation and conservation of historical murals.