From fragments to completeness, through it you can actually see the scenes from thousands of years ago?

In the process of human civilization, the emergence and use of metals have played an important role and influence, significantly improving the survival ability of human beings in nature [1]. Copper is an important metal element. The development of artificial copper smelting technology has played a vital supporting role in the birth of Chinese civilization. Bronze is the earliest metal material widely used by humans in many aspects of production and life. The use of bronze ware made Chinese civilization from the Xia, Shang and Zhou Dynasties to the Qin and Han Dynasties reach a powerful and prosperous period in history [2-3]. Among the Chinese bronze ritual instruments, the bronze bell is one of the representatives of this prosperity. The bronze bell is a percussion instrument [4], which is often unearthed in groups. It started as early as the Western Zhou Dynasty. However, the development of the bell remained an indispensable type of percussion instrument until the Han Dynasty, and its evolution process has obvious characteristics of the times [5].

Since the founding of the People's Republic of China, China's archaeological work has developed rapidly and achieved fruitful results. However, due to the influence of being buried underground for many years and different burial environments, a large number of bronze artifacts were mostly in bad condition when they were unearthed. Therefore, in order to better analyze the historical and cultural information of bronze artifacts and reveal their historical, artistic and scientific value, it is urgent to protect and restore bronze artifacts in a scientific way[6].

A large number of different types of bronze artifacts have been unearthed in Jiangxi Province, my country. For example, the bronze bells unearthed from the tomb of Liu He, the Marquis of Haihun in the Western Han Dynasty, which was rated as one of the "Top Ten Archaeological Discoveries in China" in 2015 and one of the "Top 100 Archaeological Discoveries in China in the Past 100 Years" in 2021 [7], as many as 10 bronze bells were unearthed in the musical instrument library of the Northern Coffin, breaking the "convention" of only 5 bronze bells unearthed in the Western Han Dynasty. In order to better explore the archaeological value and scientific significance of the bronze bells unearthed from the tomb of Liu He, the Marquis of Haihun, this article summarizes the restoration process of one of the more severely damaged bronze bells, in order to explore the role and value of scientific protection and restoration of bronze artifacts based on multidisciplinary cooperation.

1. Overview of Cultural Relics

The bronze bell (Figure 1) was unearthed in 2015 from the musical instrument storehouse of the northern storage coffin of the tomb of Liu He, the Marquis of Haihun of the Western Han Dynasty in Xinjian District, Nanchang City, Jiangxi Province. The instrument was heavy and was broken into 10 pieces when it was unearthed. Except for one piece that was completely detached from the dance part to the drum part and another piece that was almost completely detached, the other pieces were all detached from different parts such as the seal part and the drum part. There were also different degrees of deformation between the splicing of the pieces. On the seal part of one side of the bell, there was also a horizontal crack about 17 cm long, which was deformed left and right in the plane direction and returned to the other side. The widest opening of the crack was 3 mm. There are patterns on the bell. The middle part is decorated with a circle of tile patterns, the lower part is decorated with a circle of protruding belt decorations, and the part between the belt decorations is decorated with coiled dragon patterns, which are spirals above, and the upper part of the spirals is engraved with 绹 patterns. The middle part of the dance is decorated with a circle of string patterns near the bell, and the front and back parts of the dance are decorated with coiled dragon patterns. The coins are engraved with spiral lines, and the coins and seal bands are separated by string patterns. The space between the coins is decorated with coiled dragon patterns. The seal bands are decorated with coiled dragon patterns. The middle of the coins on both sides of the bell is decorated with coiled dragon patterns.

Figure 1 The bronze bell before restoration

2. Detection and analysis of bronze bells

2.1 Sampling information

Following the principle of minimal intervention, the bronze bell was sampled, and the sampling sites were all from its damaged parts. A total of 2 samples were taken, sample yz-1 was taken from the damaged part of the seal part, and sample yz-2 was taken from the inner wall of the vessel. The sampling details are shown in Figure 2.

Figure 2 Sampling diagram of bronze bell

2.2 Experimental instruments and methods

2.2.1 Metallographic microscope

In order to understand the production process of the bronze bell, the sample was subjected to metallographic analysis. According to the standard metallographic sample preparation method, after inlaying, grinding and polishing, it was etched with a 3% mass fraction of ferric chloride hydrochloric acid alcohol solution, and the metallographic structure of the sample was observed with a metallographic microscope, and metallographic photos were taken. The metallographic microscope instrument model is: ZEISS Zeiss Axio Imager.M2m.

2.2.2 Scanning electron microscope energy dispersive spectrometer (SEM-EDS)

In order to explore the chemical composition and alloy technology of the bronze bell, it was analyzed by scanning electron microscope energy dispersive spectrometer (SEM-EDS). The scanning electron microscope model is: ZEISS Zeiss EVO MA 25; the energy spectrum analyzer model is: Oxford Instruments; the analysis software is: INCA; the experimental conditions are: the acceleration voltage during the test is 20 keV, and the spectrum acquisition time is 60 s.

2.3 Results and Discussion

The metallographic structures of samples yz-1 and yz-2 taken from the bronze bell are copper-tin α solid solution dendrites, a large number of (α+δ) eutectoid structures are interconnected into a network, Pb particles are densely distributed, and some Pb particles are large in size, which is a Pb-Sn bronze casting structure (Figure 3). Therefore, it is inferred that its processing technology is casting. There are three core support marks on the bell, which is suspected to be the first casting of the bell, and then the bell is placed on the position of the inner core of the bell body, and the mold soil is dug around the bell to connect with the body. There are traces of joining at the lower end of the bell on the inner side of the bell. After casting, the bell is tuned, and there are traces of tuning on the inner side of the bell.

Figure 3 Metallographic structures of samples: (a) yz-1; (b) yz-2

The backscattered electron images and corresponding spectra of the two samples yz-1 and yz-1 taken from the bronze bell are shown in Figures 4 and 5. Similarly, SEM-EDS analysis shows that the bronze bell is a Cu-Sn-Pb ternary alloy bronze, and no S, As, Fe and other elements are found. The specific results are shown in Table 1.

Figure 4 Backscattered electron image (a) and energy spectrum (b) of sample yz-1

Figure 5 Backscattered electron image (a) and energy spectrum (b) of sample yz-2

Table 1 SEM-EDS analysis results of bronze bell (mass fraction, %)

SEM-EDS analysis shows that the bronze bell is a ternary alloy of Cu-Sn-Pb, with a high Sn content (>15%). The addition of Sn and Pb can lower the melting point of the material, increase hardness, and improve mechanical properties. The addition of an appropriate amount of Pb can also increase the fluidity of the Cu liquid, which is conducive to casting exquisite patterns [8]. The reasonable alloy ratio of this bronze bell shows the maturity of bronze alloy technology.

In addition, the metal matrix of this bronze bell is well preserved, and in the subsequent protection work, special tools such as expanders and jacks can be used to reshape and repair the cultural relic. However, it should be noted that the Sn content of this cultural relic is high, and the matrix of the cultural relic is relatively brittle, so the strength of the reshaping should be controlled appropriately to avoid the bronze from breaking during the reshaping.

3. Protection and Restoration of Bronze Yong Bells

3.1 Preparation of targeted protection and restoration plans

When compiling the protection plan for the bronze bell, it is necessary to focus on the assessment of the historical value, scientific value, and artistic value of the bronze bell, and work together with cultural relics restoration personnel, archaeological experts, and art history experts to draw conclusions about the value assessment of the bell by combining archaeological archives and documents, and the current status analysis information of cultural relics. At the same time, it is necessary to analyze and investigate the damage of the bell. Due to limited conditions, the naked eye observation method is used to find out that the body of the bell is incomplete, broken, and deformed, and a damage map of the bell is drawn based on this.

After the above analysis of the bronze bell, the process of its targeted protection and restoration plan is shown in Figure 6.

Figure 6 Flowchart of the targeted protection and restoration plan for the bronze bell

3.2 “Chloride ion” detection

The main corrosion problem of bronze artifacts is "bronze disease". "Powdery rust" is its specific manifestation. "Powdery rust" is a light green powdery corrosion product. Once it appears, it will continue to produce new rust, which will harm the long-term preservation of bronze artifacts[9]. Therefore, before protecting and repairing bronze artifacts, it is necessary to conduct a necessary assessment of the disease status and corrosion degree of the artifacts based on the data of analytical testing. The objects of scientific testing include the bronze body, rust products and other attachments. The method adopted is "chloride ion" testing. Its chemical methods mainly include silver nitrate or mercuric nitrate titration, ion chromatography, potentiometric titration, etc.[10]

The "chloride ion" detection method used for this bronze bell is silver nitrate titration. This traditional detection method is easy to operate and uses relatively simple instruments and equipment. The specific steps are: First, observe the suspected "powdery rust" in the bell and carefully take rust samples. Sampling should follow the principle of minimum intervention in cultural relics, that is, sampling should be carried out on the rusted fragments or unimportant locations of the bell. These sampling points must not only meet the needs of disease detection, but also be representative enough to reflect the disease situation of the entire bell. A total of 3 samples were taken for detection, namely ① powder and block, green, inner wall of the fragment; ② block, dark green, inner wall of the fragment; ③ powder, light yellow green, drum of the bell.

Secondly, dissolve the removed rust samples in 6~8 mol/L HNO3, and after they are completely dissolved, add 0.1 mol/L AgNO3 solution. If the solution is clear and no white precipitate is generated, it means that the sample does not contain chlorine. If a large amount of white flocculent precipitates in the solution, it means that the sample contains "powdery rust" components.

After testing, no white precipitate was found in any of the three samples, indicating that the bronze bell did not contain harmful rust, that is, it did not suffer from "bronze disease".

3.3 Cleaning and rust removal

Scientific inspection and preliminary cleaning of bronze artifacts can determine their type and estimate the expected state of conservation. During the cleaning process, physical cleaning methods are traditional rust removal methods, which are generally relatively safe and convenient, easy to master, and have a wide range of applications. Specific methods include: manual rust removal, laser cleaning[11], ultrasonic cleaners[12] and sandblasting.

This bronze bell was manually rust-removed during physical cleaning. The specific steps are as follows: It can be seen from observation that there are patterns on the surface of the bell, but since the surface and inside of the vessel are covered by a layer of attachments, the attachments need to be cleaned. First, use a scalpel to remove large pieces of rust on the surface. For the rust on local decorative parts, use a bamboo knife and a scalpel to gently remove it to reveal the patterns.

3.4 Shaping and welding

Bronze artifacts were squeezed and deformed due to the influence of the burial environment when they were unearthed, and they need to be corrected through shaping to restore their original shape. Shaping, also known as orthopedics, is to restore the deformed part to its original shape by applying a force in the opposite direction of the deformation[13]. The main methods of bronze shaping include hammering, twisting, prying, propping, sawing and heating. At the same time, with the development of science and technology in recent years, in order to meet the needs of bronze artifact restoration, some new types of shapers and orthopedics have begun to appear, such as round shaping barrels, F-type clamps, G-type clamps, etc.[14]

This bronze bell was broken into 10 pieces, each of which was deformed. Based on this, the specific shaping process was as follows:

(1) Use inner and outer annular rings to support each other and reshape the deformed rim and belly of the upper part of the vessel to restore the vessel to its original shape;

(2) Clamp with G-type clamp and F-type clamp, continue to fix, wait for a few days and then adjust the size of F-type clamp. Use jack to support the inside of the oval short diameter of the edge and adjust the bolt of jack. The short diameter of the object is obviously supported outward. Use tape measure to measure the corresponding diameter. It can be seen that the size is basically close (Figure 7);

Figure 7: Bronze bell shaping process

(3) As the copper material of the vessel is relatively good, it needs to be corrected to prevent rebound;

(4) Wait for a few days before making some minor adjustments. Use a hose clamp to fix the cracked part of the object and continue to use a jack to support the inside.

(5) After the reciprocating tool is operated, the object returns to its original state.

After the bronze bell fragments have been shaped and restored to their original shape, they need to be welded. The "tin soldering method" is a traditional welding method. The advantage of this method is that the welding can be adjusted in time [15]. Before welding, it is necessary to first determine whether the fracture and crack parts of the object have metallic properties. If the copper object still has a high metallic property, it can be welded. The welding operation of this bronze bell is as follows: dip a brush in zinc chloride as flux and apply it to the welding area, use a 300 W electric soldering iron with Sn to weld the fracture, and grind the welding area smooth after cooling (Figure 8).

Figure 8 Bronze bell welding process diagram

Neutralize with 3.5 times sodium carbonate solution

Since zinc chloride was used as flux at the fracture of this bronze bell, chloride must remain in the fracture gap. If it is not removed, it will cause secondary damage to the bronze. Therefore, the object was immersed in a 5% sodium sesquicarbonate solution to neutralize the chloride ions remaining on the object in the zinc chloride during welding. After 2 hours, the object was taken out and washed with distilled water until the pH test paper tested the solution to be neutral, and then forced to dry. After immersion and neutralization, the rust on the surface of the bell was stable and easy to preserve.

3.6 Sustained release and sealing

Slow release is an important protective step to prevent or delay further corrosion of bronze artifacts. Currently, commonly used corrosion inhibitors include BTA, AMT, PMTA, etc. [16]. Sealing is to apply a layer of sealing agent on the surface of the cultural relic after restoration, which is very helpful for the maintenance of restored bronze artifacts. Common sealing agents include various waxes and acrylic resins [17].

The area where this bronze bell is preserved belongs to the subtropical monsoon climate, the climate is hot, and due to the limited conditions of the warehouse, the old equipment, and limited funds, it is difficult to achieve constant temperature and humidity, so the bronze bell must be protected by emergency, that is, slow release and sealing. Due to the large size of the artifact, the corrosion slowing and sealing are both done by brushing.

Corrosion inhibition: Use a brush to dip 2% BTA anhydrous ethanol solution and evenly brush the inner and outer walls of the bronze bell, wait for natural drying, and after the ethanol is fully developed, brush it a second time, continue to dry naturally, and after the ethanol is fully developed again, observe whether white needle-like substances appear. If so, wipe the white substances with anhydrous ethanol, and then repeat the brushing solution for the third time.

Sealing: Similar to the corrosion inhibition method, use a 2% by mass Paraloid B72 acetone solution to seal the bronze bell. Use a brush dipped in the solution to apply the sealant to the inner and outer walls of the object, wait for natural drying (30 min), apply a second time and wait for the object to dry naturally.

3.7 Distressed (according to color)

In order to meet the artistic requirements of cultural relics, China has had the practice of aging and repairing bronze cultural relics since ancient times. Specifically, various methods are used to cover and modify the traces of repair welding and other processes, so that these traces are naturally connected with the overall artifact and are highly artistic. The effect of aging bronze artifacts is closely related to the environment in which the aging work is carried out. Only when the color is aged under natural light on a sunny day can it meet the requirement of not being biased in any environment[18].

This bronze bell was aged in the traditional way. Shellac was used as an adhesive, and various mineral pigments were applied as colorants to the treated surface to make it blend in with the surroundings. That is, the welded parts of the bronze bell were aged according to the color, and mineral pigments were mixed with shellac, and the colors were applied by dotting, picking, and flicking. Since the rust color on the surface of the object is relatively small, only light green and earth rust, it is relatively easy to color. The coloring can be completed by using the usual coloring method and repeated coloring. After the coloring is completed, wait for it to dry completely, observe the overall color, and finally calibrate the color of the welding part of the object to make it overall coordinated.

3.8 Complete the file

After a series of steps, the bronze bell has completed the preliminary protection and restoration work, as shown in Figure 9. However, for each specific cultural relic, there should be a restoration file of its own, only in this way can we ensure the reasonable connection between the previous and the next work and better protect the cultural relics.

Figure 9 Photo of the restored bronze bell

The restoration archives include basic information on the restoration of the Yong bell, pre-restoration inspection and analysis data, restoration process logs, and preservation status records. The purpose of improving the archives is to facilitate subsequent cultural relics protection workers to master the restoration data on cultural relics through the restoration archives, understand the work and progress completed by predecessors, and formulate more reasonable and effective plans and measures for subsequent continuous maintenance work based on the data archives left behind.

4. Conclusion

The protection and restoration of this bronze bell is strictly based on the industry standard "Specifications for the Preparation of Protection and Restoration Plans for Metal Cultural Relics in Museums" promulgated by the State Administration of Cultural Heritage, and is based on the analysis and testing results, and scientific protection and restoration is carried out in accordance with the principles of minimum intervention, identifiability, reversibility, and compatibility. Before the specific implementation of the protection and restoration work, the background analysis of archaeological knowledge and the technical support of scientific analysis and testing were effectively combined, so that on the one hand, it was protected and restored in accordance with the more conventional bronze restoration procedures, and on the other hand, it was based on reality and treated according to the disease, and special shaping and welding methods were formulated for the special situation of the bronze bell.

The protection and restoration of cultural relics is complex and involves a wide range of fields. The protection and restoration of this bronze bell combines the wisdom of cultural relic archaeology, scientific and technological analysis, and restoration personnel. Starting from their respective professional perspectives, they consulted with each other and comprehensively raised and solved problems, and then formulated a targeted protection and restoration plan. It not only runs through the modern international cultural relic restoration concept, but also combines traditional cultural relic restoration technology, which is a tacit fusion of tradition and modernity.

The implementation of multidisciplinary cooperation is very helpful for the protection and restoration of bronze artifacts, can play a good scientific guiding role, and can smoothly extend the "life" of bronze artifacts.

**Acknowledgements:** The experimental part of this paper was greatly guided by Professor Xian Yiheng from the Key Laboratory of Cultural Heritage Research and Protection Technology of the Ministry of Education at Northwest University. I would like to express my sincere gratitude!

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About the Author:

Jiang Jing (1990-), female, was born in Nanchang, Jiangxi Province. She is a student majoring in the history of science and technology at Guangxi University for Nationalities. Her main research directions include scientific and technological archaeology and cultural relics protection, metallurgical history and metallurgical archaeology.

Li Wenhuan (1987-), male, from Dingxi, Gansu, is a librarian at the Jiangxi Provincial Institute of Cultural Relics and Archaeology. He graduated from the University of Science and Technology Beijing in 2015 with a major in the history of science and technology. His main research directions are: scientific and technological archaeology and cultural relics protection, metallurgical history and metallurgical archaeology. He participated in the on-site excavation of the tomb of Marquis Haihun in Nanchang, Jiangxi Province, and is currently in charge of the protection of cultural relics unearthed from the tomb. He has participated in the investigation and excavation of metal mines, jade mines and other sites in various periods many times, and has published many academic papers in journals such as "Cultural Relics", "Jianghan Archaeology" and "Southern Cultural Relics". Correspondence address: Jiangxi Provincial Institute of Cultural Relics and Archaeology, southeast of the National Defense Science and Technology Building, Innovation Road, Qingshan Lake District, Nanchang, Jiangxi Province 330029