几种罕见的黄铜钱币锈蚀产物的识别与形成原因探析

Identification and interpretation of unusual corrosion products formed on brass coins

我国从明中期开始使用黄铜钱币,一直到清末。过去,对古代铸币的科学研究主要集中于合金成分分析和铸造工艺两方面,对锈蚀产物和锈蚀机理的研究主要针对青铜铸币。黄铜的合金元素中较青铜多锌元素,其腐蚀产物与腐蚀行为较青铜有一定的差别,也较青铜更为复杂,然而现有的文献关于此方面的研究匮乏,特别是对于大气保存环境下黄铜钱币的锈蚀产物及锈蚀成因的研究基本空白。本研究通过多种分析手段包括X射线荧光分析仪(XRF)、扫描电镜能谱仪(SEM-EDS)和显微激光拉曼光谱仪(Raman)对国家博物馆馆藏一批传世明清各时期的黄铜钱币合金组成,不同颜色的锈蚀产物等开展科学分析。研究结果表明,嘉靖、隆庆和万历年的黄铜铸币,为Cu+Sn+Pb+Zn四元合金,平均含量Cu 66.4%,Sn 6.8%,Pb 10.4%,Zn 15.6%,并含有微量Fe,不含As。泰昌、天启和崇祯年间及清代各时期的黄铜铸币,主要为Cu+Pb+Zn三元合金,并含有微量Fe、As和Sn。这批黄铜锈蚀产物与常见的青铜锈蚀产物不同,最典型的是钱币表面蓝绿色和白色锈蚀物,经Raman和SEM-EDS分析并结合相关文献初步确认蓝绿色锈蚀物为一种含铜、钠离子的有机酸盐,理论化学式为NaCu(CO_(3))(CH_(3)CO_(2));白色锈蚀物采集到三种不同的拉曼谱图,但均无标准拉曼图与之匹配,综合多种分析结果并结合保存环境可进一步推测三种不同物相的白色锈蚀物主要为含有Zn、Cu阳离子的碳酸盐或有机酸盐,且Zn元素占主导地位;此外,通过Raman光谱确认两种罕见的同时含Cu、Zn阳离子且化学计量式为Cu_(3)Zn(OH)6Cl_(2)的同分异构体,矿物名为herbertsmithite和kapellasite。文中同时对蓝绿色锈蚀产物的成因进行了探讨。结合保存环境推测锈蚀产物中的乙酸根应来源于库房木质文物柜架所挥发的有机酸,钠和碳酸根则可能为入库前钱币流通时带入的污染造成的。本研究最主要的拓展有以下两方面:一是以黄铜为研究对象,通过拉曼分析鉴别出几种新发现或罕见锈蚀物,进一步补充完善有关黄铜锈蚀物种类的数据库;二是深入探讨木质挥发物对金属文物腐蚀的影响作用,为后期采取针对性的预防性保护措施和相关主题研究提供参考借鉴。

Brass coins were used in ancient China from the mid-Ming Dynasty to the late Qing Dynasty.In the past,scientific research on ancient coins mainly focused on alloy compositions and casting techniques,but the main research object of corrosion products and mechanisms was bronze.Brass contains more zinc than does bronze,so the corrosion products and behavior of brass are different from and more complicated than those of bronze.However,there is a lack of research on this aspect in the existing literature,and the research on corrosion products and behaviors of brass in the atmospheric environment is less.In this study,three analytical techniques,such as X-ray fluorescence(XRF)spectroscopy,scanning electron microscopy-energy dispersive spectroscopy(SEM-EDS)and Raman spectrometry were used to detect alloy compositions and corrosion products with different colors of some Ming and Qing brass coins collected in the National Museum of China.The results show that 1)the brass coins of the Jiajing,Longqing and Wanli periods of the Ming Dynasty are Cu+Sn+Pb+Zn alloys with average contents of 66.4%Cu,6.8%Sn,10.4%Pb and 15.6%Zn,with traces of Fe but no As and_(2))the brass coins of the Taichang,Tianqi and Chongzhen periods of the Ming Dynasty and various periods of the Qing Dynasty are mainly Cu+Pb+Zn alloys with trace amounts of Fe,As and Sn.The brass corrosion products are different from the bronze patinas,and many of them are newly found or rare.The typical corrosion products are blue-green and white.Based on the analyses and related literature,it is preliminarily confirmed that the blue-green one is an organic acid salt containing copper and sodium ions,with its empirical chemical formula being NaCu(CO_(3))(CH_(3)CO_(2)).Three Raman spectra of white corrosion products were collected,but no standard Raman spectrum could be matched with them.In combination with various analytical results and the preservation environment,it can be further inferred that the white ones with three different phases are mainly carbonate or organic acid salts containing Zn and Cu cations and that Zn is the dominant element.In addition,two rare isomers,herbertsmithite and kapellasite,both having the stoichiometric formula,Cu_(3)Zn(OH)6Cl_(2),were identified using Raman spectroscopy.In addition,the formation of the corrosion products is also discussed in this paper.In combination with the storage environment,it is deduced that the acetate in the blue-green corrosion product could come from the organic acid volatilized from wooden cabinets,and the sodium and carbonate may be due to the contamination before storage.This paper has two main aspects:the first is to take brass as the research object and to identify several newly found or unusual corrosion products using Raman spectroscopy,which could further supplement and improve the database of brass rust species;the second is to deeply investigate the impact of wood volatiles on the corrosion of metal collections,which could provide a reference for the subsequent adoption of targeted preventive conservation measures and related research.