Research on monitoring the chloride release process and evaluating the dechlorination effect of the sodium sesquicarbonate method for the stabilization of bronze disease
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 diseaseSodium sesquicarbonateRaman spectrumNanocrystalline SnO2Endpoint of dechlorination
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