Preliminary Analysis of Crystallization of Na_2SO_4 Solution in Silicate Cultural Relics

摘要

Currently, there is a lack of research on the migration and distribution of soluble salt solutions in the pores of fragile silicate cultural relics, especially the thermodynamic changes during crystallization, and the forces causing the destruction of cultural relics. Thus, this paper used infrared thermal imaging analysis and extended depth-of-field microscopy to observe the changes in temperature, morphology, migration, and crystallization rate of Na2SO4 solution - the most typical solution that causes damages to cultural relics - in the crystallization process and in different capillary pores. The results show that the formation of soluble salt crystal nuclei and subsequent crystal growth mainly occurred in the liquid phase region below the ambient temperature, and then the salt crystals in the deliquescent state became more compact due to the attractive force generated from liquid-phase water evaporation. Additionally, three stages of migration and crystallization of the salt solution in different capillary pores were found. Large-area crystals appeared in the capillary pore with a large diameter, and bubbles were generated. This is likely because the surface tension of the Na2SO4 solution decreased with increasing concentration, and the solubility of the air dissolved at the solid-liquid interface of the mouth of the capillary pore and the pressure in the capillary pore decreased. A self-designed double-layer contact surface internal pressure method and extended depth-of-field microscopy were used to carry out real-time online observation of the force applied on the contact surface during the crystallization of the salt solution. It was found that the airflow determined the direction of capillary movement of the solution, and water migration determined the laminar flow in the capillary pore. Salt crystallization played a role in distortion and shape-fixation of pores. Small spikes in the mass indicated that the expansion-contraction force generated by the salt solution during the crystallization process was in a state of constant change, and the degree of expansion was always slightly more significant than the change caused by contraction. Expansion forces dominated the surface of the sample. In this paper, the change in crystallization and the forces of the soluble salt solution were analyzed.