重庆地区露天石刻早期封护材料老化产物及其影响研究

摘要

重庆市市委会办公大楼旧址前有一组露天存放的清代砂岩石狮子,表面覆盖着黑色硬壳状物质,发生大面积脱落,起翘和卷曲.为了揭示黑色硬壳状结构组成,研究其形成过程及对文物产生的影响,利用环境扫描电子显微镜(SEM),X-射线衍射仪(XRD),X-射线荧光光谱仪(XRF),傅里叶变换红外光谱仪(FTIR)和X-射线光电子能谱(XPS)对黑色硬壳及文物表层砂岩样品进行了检测分析研究.结果发现:黑色硬壳断面Mapping元素分布图显示其分为底漆层,中间层和表层,系石刻早期封护层的老化产物;中间层含有立德粉(硫化锌和硫酸钡),黑色外观源于表层中含铅颜料(铅白)变色形成黑色硫化铅及树脂碳化所致;红外光谱与光电子能谱显示出黑色硬壳中含有强的羟基(—OH)特征峰,说明封护层中有机物老化后形成了大量羟基,从而增强了自身亲水性,造成易吸水溶胀与干燥收缩情况,导致大面积脱落,起翘和卷曲现象;黑色硬壳起翘和卷曲部位与下层石刻表面之间形成了易于积水的微空隙,能够聚集雨水中的有害物质,造成石刻表层岩石发生腐蚀,例如黑色硬壳背面及下层岩石表面中高含量硬石膏(CaSO4),经生水化作用后转化为石膏(CaSO4·2 H 2 O),发生体积膨胀造成岩石表面松动和酥粉.因此,当露天文物表面上封护层已老化时,及时地进行清除是十分必要的.%A pair of hand carved sandstone lions produced in Qin Dynasty lie in the front of the government site of Chongqing city covered by a layer of black crust with missing parts,and cocked or curly edges.To explore the structural and compositional properties of the black crusts and reveal its influence on the surface of stone sculpture,scanning electron microscopy (SEM),X-ray diffraction (XRD),X-ray fluorescence spectrometer (XRF),Fourier transform infrared spectroscopy (FTIR)and X-ray photoelectron spectrum (XPS)were applied to analyze the samples of black crust and stone base.The conclusion indicated that the black crust was originated from the weathering of prior protective coating including three chief layers:the undercoat,the in-termediate coat (mixed by CaSO4 and ZnS)and topcoat observed by elemental Mapping.Among them,the black crust was chief-ly attributed to a discolored process from white basic lead carbonate [(2PbCO3 ·Pb(OH)2 )]to black galena (PbS),and the car-bonization of organic material in the protective coating.The strong —OH peak identified by both FTIR and XPS analysis on the black crust demonstrated an increasing tendency of —OH in the carbonization of the protective coating which made it more hy-drophilic and easy to swell when it is damp and shrink when it is dry,so as to cause a large area of peeling,cocking and curling. Most importantly,water would be detained in those spaces between black crusts and the surface of sandstone which could accu-mulate the corrosion resulting in some damages on the carved sculpture.For example,a high concentration of anhydrite (CaSO4 ) was detected by SEM/EDAX and XRD in the back of black crust and the surface of sandstone,which can be transformed into gypsum by hydration with volume expansion causing the pulverization of sandstone.Therefore,when encountered with the aged protective coating on the outdoor stone cultural relics,it is important to remove them timely avoiding the further corrosion in the micro spaces between black crusts and the below sandstone.