陕西历史博物馆藏汉代“华仓”瓦当表面析出物的分析研究

Study of crystalline efflorescence on a Han Dynasty eaves tile in the collection of Shaanxi History Museum

陶质文物表面析出物中含有机酸盐的现象在国内鲜有报道。本研究采用离子色谱仪(IC)、扫描电子显微镜-能谱仪(SEM-EDS)、X射线衍射仪(XRD)、傅里叶变化显微红外光谱仪(FTIR)、超景深显微镜(DM)对一件馆藏汉代“华仓”瓦当表面析出物进行了表征和检测。结果表明,瓦当表面析出物主要由少量的二水硫酸钙和大量有机酸钙盐[Ca_(3)(CH_(3)COO)_(3)Cl(NO_(3))_(2)·6H_(2)O]组成,两种盐的形成与瓦当的制作过程、埋藏环境和灰尘有关,有机酸盐的形成还与馆藏环境中的乙酸有关。这种析出物不仅影响瓦当表面的真实文物信息,还会进一步腐蚀瓦当本体,建议从展柜中取出瓦当进行脱盐处理,存放在无酸的环境中。本检测分析为这件瓦当的保护修复方案的制定提供理论依据,也为类似文物样品的分析提供借鉴。

Tiles are building materials commonly used in ancient Chinese architecture.Efflorescence may be produced on eaves tiles after years,which not only affects the true cultural relic information on the tile surface but also further damages eaves tiles.In 2022,crystalline efflorescence was found on a Han Dynasty eaves tile in the collection of Shaanxi History Museum.It was unearthed from Jingshicang Ruins in Huayin,Shaanxi Province,with a diameter of 16.1 cm and a convex string pattern inside the side wheel.Two characters of“Hua”and“Cang”were written in the seal script on the tile surface.The Chinese-character tile of the Han Dynasties provides physical materials for studying the ideological consciousness of the Han Dynasty people,as well as social changes,economic development status and calligraphy and seal carving art of the Han Dynasties.Identification of the crystalline efflorescence,or its components,may reveal the causes and sources of the efflorescence,and appropriate measures may only be determined when such information is known.Therefore,the crystalline efflorescence on the surface of the eaves tile was identified.First,super depth-of-field microscopy(DM)was used to observe the tile surface and crystalline efflorescence.The results show that there were salt crystals,powdering and flaking on the tile surface,in the form of fluffy whiskers.Both the yellow-white cluster crystals and a white needle-shaped substance formed by the parallel arrangement of small fibers were present.Ion chromatography(IC),scanning electron microscope-energy dispersive spectrometry(SEM-EDS),X-ray diffraction(XRD)and infrared spectrometry(FTIR)were used to analyze the chemical composition of crystalline efflorescence.The results show that two major compounds were found:gypsum(CaSO_(4)·2H_(2)O)and thecotrichite[Ca_(3)(CH_(3)COO)_(3)Cl(NO_(3))_(2)·6H_(2)O].Based on the analytical results of the crystalline efflorescence,we also investigated the sources and causes of the two salts.These included:1)the production process and raw materials for tiles;2)some reports about many water-soluble ions in soil and underground water in Huayin;3)determination of the concentrations of various ions in the storeroom dust samples from Shaanxi History Museum;4)determination of the concentration of acetic acid vapors inside showcases to determine any relationship between the pollutant concentration and observed damage;5)monitoring of the relative humidity inside showcases over one year to provide information about conditions in which damage occurred.The results of investigation show that the formation of two salts might be related to the production process,burial and storage environments of tiles.Eaves tile is generally made of clay,and gypsum(CaSO_(4)·2H_(2)O)is one of the common impurities in clay,so the raw material for making tiles itself contains calcium sulfate dihydrate.In addition,there is SO_(2)-SO_(3)gas present in the majority of kilns firing bricks and tiles,which reacts with calcium carbonate in clay to generate calcium sulfate.However,calcium sulfate cannot be completely decomposed within the firing temperature range of general bricks and tiles,and calcium sulfate was also detected in the XRD testing of the tile body mentioned earlier.Therefore,when the tile is fired,it already contains calcium sulfate.Some reports have revealed the presence of many water-soluble ions from the soil and underground water in Huayin,such as Ca^(2+),Na^(2+),SO_(4)^(2-),Cl^(-)and HCO_(3)^(-).These ions originate from burial,where the salts accumulated as absorbed underground water evaporated.Therefore,soil and underground water in burial sites are one of the sources of these salts.The higher concentrations of Ca^(2+)、SO_(4)^(2-)、Cl^(-)and NO-3 in the storeroom dust samples confirm that the storage dust is a source.As it was directly exposed to the atmosphere,the porous tile could absorb dust,which was converted to soluble salts to produce large crystallization stress during sharp changes in temperature and humidity.The results of investigation also show that the formation of this organic salt might be related to acetic acid in the museum environment.The higher concentrations of acetic acid was found in the showcase,and the origin of the acetate ions in the salt was thought to be acetic acid vapors from materials of the showcases.Also,the relative humidity inside the showcase was measured over one year:the RH inside the showcase varied within a range of 34%-85%;from June to October,the humidity was above 70%and the conditions were much more humid.According to the studies of thecotrichite,it happens not only in humid RH conditions but also in“normal”museum RH conditions(40%-60%).Through the above analysis,it is supposed that a small amount of acid deposited on the tile and was mixed with the salt solutions(calcium chloride and nitrate)already present,causing the formation of this complex acetate salt.It effloresced and caused damage onto the tile body.Therefore,it is recommended that the eaves tile be removed from the showcase,desalinated and stored in a well-sealed,acid-free environment.There are few reports in China on the phenomenon of organic salts in the surface precipitates of cultural relics.This study provides a theoretical basis for the establishment of the conservation and restoration program for this tile,and also provides a reference for the analysis of similar heritage samples.