铅丹、朱砂及其混合物紫外光致变色的电化学及光谱研究

Electrochemical and spectroscopic study of the photodegradation of minium,cinnabar and their mixtures

铅丹(Pb3O4)和朱砂(HgS)是古代壁画和彩绘上常用的红色颜料,它们的变色是文物保护工作者、策展人和艺术史学家长期关注的问题。但由于其所处环境复杂,影响因素众多,这些颜料的变色机理的研究较为困难。为了探究紫外光照对这些颜料的影响,采用电化学方法和控制环境的紫外老化模拟实验研究了铅丹、朱砂及其混合物的光降解过程,并利用拉曼光谱和X射线光电子能谱(XPS)对实验样品进行分析。电化学实验中铅丹受405 nm激光照射出现还原电流,朱砂出现氧化电流。紫外老化实验确认了铅丹在光照下的还原反应,反应产物为碳酸铅(PbCO3)和碱式碳酸铅[Pb3(CO3)2(OH)2]。因此,光照会引起铅丹发生还原反应,引起朱砂发生氧化反应,两种颜料表现出典型的半导体性质。另外,光化学反应动力学结果显示,掺杂朱砂可能会加快铅丹的光降解过程。

Minium(Pb3O4)and cinnabar(HgS)were commonly used as red pigments on paintings in ancient times.The discoloration of these red pigments has been a long-term concern for conservators,curators and art historians.However,it is difficult to study the degradation mechanism of these historical materials because of their long time exposure to the environment and complex decaying factors.To investigate and monitor the process of photodegradation of minium,cinnabar and their mixtures,an electrochemical setup with a pigment-modified graphite electrode was used.It was confirmed that light irradiation of minium induced a reductive photocurrent,while an oxidative photocurrent was detected for cinnabar.In this work,the photochemical reaction of the mixed pigment of minium and cinnabar was determined to be a reduction process.Both pure and mixed pigments having experienced ultraviolet aging were analyzed by Raman and X-ray photoelectron spectroscopy(XPS).Minium was partly converted to cerussite(white lead)and hydrocerussite after UV light aging.The reaction kinetics results show that the addition of cinnabar into minium might accelerate its photodegradation reaction.