It is reported that the bronze artifacts stained with the “bronze disease” contain multiple corrosion layers, and the chlorine-containing corrosion product is typically located in the inner layers adjacent to the matrix. Physical methods can only often remove the chlorine-containing corrosion product on the surface of the objects; however, it is particularly difficult to involve the chlorine element deep in the corrosion layer. The sodium sesquicarbonate method is a common and safe treatment for the stabilization of “active” bronze artifacts. Since the 1970s, most of the literature has mainly focused on providing a brief introduction to the method, with limited in-depth research and discussion. In this study, taking the stabilizing process by the use of sodium sesquicarbonate for an early Shang Dynasty bronze Jue as a case study, and combining analytical measures such as Raman spectrometry and ion chromatography, we reviewed and evaluated the range of application, efficacy and rule of chloride release process of the method. After a soaking treatment for 158 days and a subsequent tracking observation for 65 months, no fresh corrosion product was found, indicating excellent results. The study shows that in the absence of constant temperature and humidity control, the sodium sesquicarbonate method is still an effective method for stabilizing serious corroded bronzes, especially for large-scale outbreaks of the “bronze disease”. It is crucial to monitor chloride concentration to determine the replacement time of the solution and endpoint of the stabilizing treatment. Generally, the stabilization process can be divided into three stages:the first wash would extract approximately 73% of the total; the second wash about 19% and the third wash the remaining 8%. At the last stage, the chloride ion concentration in the solution decreased to lower than 50 mg/L. The average daily chloride ion concentration was maintained at about 3 mg/L, and the change in chlorine removal was small, indicating that the stabilizing treatment could be complete. Additionally, samples were meticulously collected from the “active” rust on the surface layer and those in the inner layer, which contained either amorphous or nanocrystalline SnO2. Employing Raman spectrometry, we tracked both the transformation process and extent of the “active” rust. The results show that the “active” rust in the inner layer was more difficult to transform than that in the surface layer, and frequent replacement of the washing solution could reduce the possibility of new phases being formed. Dechlorination transformation is a rather complex process that requires further in-depth research in the future. The present study takes the practical conservation treatment for cultural relics as a case study, and in comparison with previous literature, it has extensions in two aspects:first, it tracked and monitored the entire dechlorination process, and evaluated the dechlorination efficacy at each distinct stage for the method; second, it traced and detected the phase transition process of the “active” rust to summarize the chlorine removal degree at different depths of corrosion layers. This study provides valuable insights into the prediction of replacement time of the solution and endpoint of the stabilizing treatment with the sodium sesquicarbonate method.
Bronze ware, having high historical, artistic and scientific value, plays an important role in metal cultural relics. However, due to the characteristics of natural materials themselves and the influence of the burial environment, most bronze artifacts unearthed from archaeological sites or cemeteries have been seriously degraded and corroded. The corrosion of bronze ware can be divided into two types—harmful rust and harmless rust; the latter is relatively stable and harmless to bronze artifacts. However, “powdery rust” is severely detrimental to bronzes, and can make the bronzes crumble and rot, thereby seriously affecting the life of the relics. Furthermore, powdery rust also has a certain spreading power, which can cause damage to the bronze relics themselves and the neighboring ones. Therefore, we attempted for the first time to introduce low-temperature plasma technology as a means of removing powdery rust from bronzes. Bronze samples with powdery rust on the surface were prepared by electrochemical corrosion. Then the samples were treated with low temperature plasma in a reducing atmosphere. Comparison of data of bronze samples before and after the treatment shows that the low-temperature plasma technology could effectively remove chloride ions in the powder rust. This study could provide a new idea and method for the conservation of bronze relics.
Maintaining appropriate temperature and humidity to minimize the negative effect of the environment are basic requirements for the preventive conservation of cultural relics in museums. Because of regional climate differences, sometimes adopting a uniform standard is neither necessary for relics nor energy efficient. This paper discusses a risk assessment method for controlling temperature and humidity in the museum environment of bronze relics. A quartz crystal microbalance was used to carry out reactivity monitoring experiments of the simulated bronze material, to thus obtain corrosion data. Machine learning algorithms were used to establish a highly accurate corrosion rate prediction model of the simulated bronze material under specific temperature and humidity conditions. Then, using the corrosion rate as a quantitative indicator of risk, a continuous environmental temperature and humidity risk standard was proposed for four classes. Because of a difference in atmospheric pressure, a relative humidity conversion factor was added to expand the scope of its application. A graphical user interface tool was designed to make it easier to utilize. Based on the proposed assessment standard for museum temperature and humidity risk of bronze relics, a conservator could control the temperature and humidity according to local conditions while maintaining the risk level.
Many bronzes have been unearthed from Han tombs in Guangzhou, but their corrosion and mineralization are very serious. In order to study the mechanisms of corrosion, metallographic analysis, SEM-EDS, XRD of corrosion products, soil pH measurement and soluble salt determination were used. Typical samples extracted from bronze ware were analyzed with regard to three aspects:matrix, corrosion products and soil. The results show that there are two kinds of materials in eight of samples, namely seven pieces of lead-tin bronze and one piece of tin bronze. The mechanical properties of the alloy were not the best because of the fluctuation of the ratio of lead to tin. According to the microstructure of the matrix, most of the samples were cast microstructure. The dark field demonstrated that the mineralization corrosion process was from inside to outside. The XRD results show that all lacked copper chloride. The content of chloride ion in the burial environment of Guangzhou is not low compared with other areas in China, but the acidity is the strongest. The lead and tin in the ternary alloy were anodes relative to copper. There were a lot of intercrystalline and grain boundaries in dendrites and the potentials of different phases were different. There were a lot of inclusions and much shrinkage, the chemical composition was not uniform, the impurity potential of the alloy was higher than that of the matrix, and a lot of microcathodes and microanodes were formed. However, the main corrosion process of bronze vessels unearthed from Han tombs in Guangzhou area was H+ corrosion. The acidic soil environment, due to the abundant groundwater in Guangzhou, as well as abundant oxygen and carbon dioxide make copper and lead ions move outwards continuously, which results in serious corrosion and mineralization of bronzes.