A Study on the Mechanism of Salt Damages on the Mural Paintings of Mogao Grottoes

摘要

The salt damages of the mural paintings were closely related to the types and movements of salt,environmental and meteorological conditions,etc.The main dissolvable salts in Mogao grottoes resulted in the salt damages of the mural paintings were sodium chloride (NaCl) and sodium sulfate (Na2SO4) and the crystallization of salts was the main destructive force in the process of salt damages [1～10].In this paper,the isothermal solubility of the ternary system a+/Cl-,SO42---H2O at five characteristic temperature namely 308K,298K,288K,278K,268K had been studied and their phase diagrams were determined,the crystallizing dynamics of NaCl and Na2SO4 in the ternary system Na+/Cl-,SO42---H2O and the crystal structure were investigated.The results showed that the solubility of NaCl was not sensitive to the temperature and its crystallization was mainly occurred with the evaporation of H2O.On the contrary,Na2SO4 was very sensitive to the temperature and its crystallization was easily taken place with the decrease of temperature besides with the H2O evaporation.NaCl and Na2SO4 exhibited different behaviors in the process of salt crystallization.The dominant role between NaCl and Na2SO4 was alternated at the mass ratio of 1:3～6 (for example,WNa2SO4/W NaCl=1:3 at 298K).The results indicated that Na2SO4 was the main factor in the process of salt damages with the fluctuation of temperature when the content of Na2SO4 in the system was exceeded 30%.Based on those results,the permitted-critical relative humidity for the protection of the mural paintings corresponding to various content of salts were obtained.We speculated an initiation and developing mechanism of salt damages in the mural paintings.For the NaCl crystal was very hard and its granularity was smallness and uniformity,it was speculated that the appearance of salt damages on the mural painting would be mainly in point-focused bleb form.On the contrary,Na2SO4 crystal (mainly existed in the form of Na2SO4·10H2O) was very loosen,easily be effloresced and supersaturated,so we speculated that the deterioration in the mural painting resulted from Na2SO4 crystals would be in the form of extensive area,powdered and porous crisp,strongly damaging force and the deterioration appearance on the mural painting appear mainly in the form of hollow inside and base-ground powdered porous loose.