Migration, Distribution, and Crystallization of NaCl and Na2SO4 Solutions in Three Different Media

Salt damage is one of the most common and serious diseases in silicate cultural relics. In this research, low-field nuclear magnetic resonance (low-field NMR), automatic high-speed X-ray microtomography imaging, polarized light microscopy, and ultra-depth of field microscopy were applied to investigate the migration, distribution, and crystallization of NaCl and Na2SO4 on the surface of hydrophilic media, glass capillaries, and porous SiO2 materials, respectively. The results show that these two salts have different crystal growth behaviors in the same medium. NaCl grows in a granular form on the surface of hydrophilic medium and generally crystallizes outside the glass capillary tube, whereas Na2SO4 grows in a circular ring and always crystallizes inside, and some bubbles can be seen clearly in the hydrophilic medium. Meanwhile, different from NaCl, which is mainly concentrated on the upper surface of SiO2 sample, the migration of the Na2SO4 solution is distributed in the whole sample, and crystals accumulate on the interior of the sample surface. The different crystallization behaviors of salts are speculated to be related to damage conditions such as efflorescence and mural blisters in silicate cultural relics.