基于莫高窟成盐元素相关系探究壁画盐害作用机理

Study on the mechanism of salt damages on the mural paintings of mogao grottoes

壁画盐害与壁画的成盐元素种类、盐分迁徙活动以及环境气象条件等密切相关。莫高窟壁画酥碱病变的可溶盐主要为NaCl和Na2SO4，盐的结晶是破坏力的重要来源。本文运用经典溶液化学的研究方法，界定了三元体系NaCl-Na2SO4-H2O于308K，298K，288K，278K，268K的相关系，以此为基础阐释了壁画盐害的发生和发展规律，计算得到了安全含盐量和环境条件的临界数值。研究表明：NaCl的饱和溶解度对温度敏感度不大，它的结晶析出更多的是伴随着水分的蒸发而发生；Na2SO4的饱和溶解度对温度表现较为敏感，它的结晶析出，除了因溶剂水分的蒸发而发生发展之外，还将因温度的降低而产生，他们各自在壁画盐害的发生发展过程中有一定的行为差异，其主导作用的交替点大致在w Na2SO4/w NaCl=1：3～6左右，即当该体系中Na2SO4的质量比约达30％-15％时，NsaSO．将成为壁画盐害随洞窟温度变化而发生的主导诱因。三元体系NaCl-Na2SO4-H2O中NaCl和Na2SO4在5—35℃的介稳饱和溶解度的测定，取得了与相图相一致的结果。NaCl结晶物质地坚硬、粒度均匀，在壁域盐害表现形式上应以点状疱疹为主；Na2SO4结晶物主要以十水硫酸钠Na2SO4·10H2O的形式析出，质地疏松，易风化，易返潮，所导致的壁画酥碱病变有一定的反复性。Na2SO4的过饱和溶解度大，易富集，易浓缩，结晶区域较大，相应破坏面也较大，在壁画盐害的表现形式应以粉状酥碱为主，如壁画较大面积的空鼓、地仗层酥松等。

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 . In this paper, the isothermal solubility of the ternary system NaCl-Na2 SO4-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 NaCI-Na2SO4-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,W NaSO4/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.