Feasibility Exploration and Research Examples of On-Site Metallographic Inspection Methods in the Analysis of Bronze Artifacts—A Case Study of Ming Jiajing Bronze Lions and the Shang Bronze Tripod Vessel with Cicada Designs

Abstract

This study explores a new microdestructive on-site metallographic inspection technique for analyzing metal artifacts. In the current archeometrical work, the metallographic analysis of metal artifacts requires mechanical sampling, which not only damages the integrity of the artifacts but also brings cold working effects to the metallographic structure during the sampling process, making the information inaccurate. This study designed a set of detailed on-site metallographic inspection methods for bronze artifacts, including grinding, sealing, polishing, etching, replicating, cleaning, and other steps. After verifying its safety through simulation experiments, the method was applied to several precious bronze artifacts, including two Ming Dynasty bronze lions from the Xi’an Beilin Museum and a Shang Bronze Tripod Vessel with Cicada Designs from the China Bronze Ware Museum. The metallographic findings show that the in situ metallographic technique can flexibly and accurately reveal the metallographic texture and process information of each localized part of the bronze artifacts, e.g., the heat-affected zone of the coins on the surface of the Ming Dynasty bronze lions proved the casting-inlay process, and the different heat texture of each foot of the Shang Bronze Tripod Vessel with Cicada Designs proved the chronological sequence of its two historical restorations. This study provides a novel approach to the process analysis of bronze artifacts, a method that can provide significant advantages in analyzing the processing techniques of precious and intact artifacts.

1. Introduction

In conventional technological archeology, the analysis of metallic artifact processing techniques typically relies on mechanical sampling methods [1]. The collected samples are sent to the laboratory for processes such as embedding, grinding, polishing, and etching, enabling observation of metallographic structures through an optical microscope [2]. This allows researchers to identify structural characteristics, analyze crystalline types, and reverse-engineer manufacturing techniques to uncover historical production information [3].

Despite its excellent photo quality, traditional laboratory metallographic inspection has significant limitations. Sampling complete bronze artifacts often damages uncorroded or mineralized areas, along with the corrosion layer [4]. Even when restricted to less critical edge areas, traditional sampling still causes irreversible damage. Furthermore, edge-area structures may not be representative due to usage or external forces during sampling and analysis. Additionally, bronze vessels often have distinct technological features in different parts, leading to varied metallographic structures. These factors collectively challenge the representativeness and accuracy of the data obtained using traditional methods for analyzing bronze artifacts.

In the field of industrial manufacturing, quality inspection technicians frequently employ an on-site metallographic inspection technique to conduct metallographic structure inspections on large, immovable metal components [5], such as bridges and pipelines. The objective of these inspections is to evaluate the quality of the welding and the distribution of stress characteristics [6]. However, this method used in industrial quality inspection causes significant wear to the surface of the samples, and such a method obviously cannot be directly applied to the scientific analysis of cultural relics.

Previously, scholars have conducted preliminary explorations in the field of on-site metallographic inspection technology for cultural relics. For example, Beisenov from Russia used on-site metallographic inspection technology when scientifically analyzing an iron axe from the Tasмoлинcкий (Tasmola) burial site [7]. However, there are still shortcomings in the practical application of this technology, namely that the area of abrasion damage to the cultural relics themselves is relatively large, and the safety of the residual etching solution has not been experimentally verified, which is not conducive to subsequent conservation efforts.

This article will present an experimentally verified on-site metallographic inspection technology. This method, which is designed to ensure the safety of cultural relics, has significantly reduced the degree of damage to the relics during inspection by combining technological innovation together with practical optimization. Furthermore, this article will demonstrate the application effects of this method through a series of examples, further proving its feasibility and superiority in the field of scientific analysis of cultural relics.

2. Background

2.1. Fundamentals of Classical Metallography Theory

Since the 20th century, metallographic analysis has been widely applied in archeological research and has since become an indispensable component of technological archeology. This technique not only reveals the material composition and manufacturing processes of metallic artifacts, enriching the historical information derived from archeological discoveries but also provides irreplaceable guidance for conservation and restoration efforts [1,2].

Metallographic analysis relies on differences in reflectivity between structures to create contrast. However, the uniform reflection of light on polished surfaces obscures structural details, necessitating an etching process to enhance contrast [8]. Common etching methods include chemical etching, electrolytic etching, and thermal etching, among others. These techniques exploit variations in corrosion resistance to selectively modify the sample surface, thereby highlighting microstructural features [9].

In the field of technological archeology, chemical etching is the most widely used method. This involves immersing or applying a chemical reagent to the polished sample surface (or wiping it on), followed by a designated period of reaction to reveal the metallographic structure. The etching process is characterized by complex chemical and electrochemical reactions. Due to differences in physicochemical properties, the grains, grain boundaries, and phases within metallic materials dissolve at varying rates. Regions with lower potential (anodes) dissolve faster, forming pits, while regions with higher potential (cathodes) dissolve more slowly [10,11].

The microstructure of bronze casting provides an illustrative example. When observing a properly etched sample under a microscope, under appropriate etching conditions, it becomes evident that the tin-rich phase at the grain boundaries and in the dendrites dissolves before the copper, forming depressions in these areas. When light enters these depressed areas, due to the strong scattering effect, these areas appear dark or black. In contrast, the high-copper phase that solidifies first within the grains dissolves to a lesser extent, resulting in a smooth and flat surface with a diminished scattering effect on light. This gives rise to a bright copper-yellow color, as illustrated in Figure 1 and Figure 2. This difference in color and morphology provides a crucial basis for the identification and analysis of the internal microstructure of metallic materials.

2.2. The Advantages and Disadvantages of Laboratory Metallographic Inspection Methods

In conventional technological archeology, the metallographic analysis of metallic artifacts typically relies on mechanical sampling methods, where samples are extracted for laboratory-based metallographic analysis. Despite the advanced sample processing equipment and mature methodologies of laboratory metallographic analysis, which enable the clear presentation of the microstructural characteristics of metallic materials and the capture of high-resolution metallographic images, this approach has inherent limitations that are difficult to overcome.

To elaborate, this method faces significant challenges in practical applications, with the most prominent issue being the physical damage caused to cultural relics during the sampling process. Common sampling tools such as pliers or cutting wheels inevitably leave marks on the samples. When using pliers for sampling, although the extent of damage can be controlled to some degree, the sample surface may still exhibit cold working marks due to the operational process, as shown in Figure 3 and Figure 4. These marks can interfere with subsequent test results, and it is often impossible to determine whether these cold working marks originate from ancient manufacturing and usage or from the modern sampling process. In contrast, while cutting wheels can reduce the impact of cold working marks, they result in significant material loss, as shown in Figure 5 and Figure 6. Moreover, since the surfaces of metallic artifacts are often covered by an oxide layer, additional handling is required during the sampling process, further increasing operational complexity and causing considerable physical damage to the artifact.

Additionally, sampling principles restrict the researchable areas to inconspicuous parts of bronze artifacts and other relics. This limitation makes it difficult to fully and accurately obtain specific technological features such as localized processing or heat treatment, thereby affecting the understanding of the overall technological techniques employed in the artifact’s creation.

To address these issues, it is necessary to explore non-destructive or minimally invasive detection technologies, such as portable microscopes and advanced on-site analysis tools. Such innovations not only enhance the ability to study ancient artifacts but also align better with the ethical requirements of cultural heritage preservation and archeological research.

2.3. Theoretical Basis of On-Site Metallographic Inspection Technique

On-site metallographic inspection does not require pre-sampling. This method involves the direct grinding, polishing, and etching of the sample, followed by the use of a metallurgical microscope to observe the metallographic images on-site. Compared to the traditional method of sampling and sending the samples to a laboratory for metallographic testing, the on-site metallographic inspection method offers higher accuracy and intuitiveness, with less damage to the sample and lower costs.

Nevertheless, in accordance with the tenets of cultural relic protection, when this method is employed for the metallographic inspection of bronze cultural relics, two principal factors must also be taken into consideration: Firstly, the area of physical damage to the relics must be strictly controlled, with any such damage limited to an extremely small range. Secondly, during the examination process, any new corrosion factors [12] that could lead to further deterioration of the relics must be avoided.

The damaged area of cultural relics can be precisely controlled by selecting smaller grinding and polishing tools, while the control of corrosion factors should be carefully assessed and verified for the residual issues of etchant.

(1)

Metallographic etching reaction.

In the process of etching copper alloys, the chemical reagents most frequently employed often comprise a combination of multiple components. This article presents an overview of the reaction mechanism of the most commonly used tin–bronze chemical etching reagent.

A common ratio for a ferric chloride hydrochloric acid alcohol solution is 5 g of ferric chloride, 5 to 30 mL of hydrochloric acid, and 100 mL of alcohol (or acetone). In long-term scientific research, an investigation into the chemical concentrations available on the market revealed that a solution comprising 5 g of ferric chloride, 5 mL of hydrochloric acid, and 100 mL of anhydrous ethanol demonstrated optimal etching efficacy and operational reliability when employed in the etching of ancient tin–bronze samples. Moreover, the products generated by the reaction of this etching solution with copper–tin alloys are all soluble compounds.

This etching solution primarily undergoes the following chemical reactions with a copper–tin alloy:

$$\mathrm{C}\mathrm{u}+2{\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}_3\to {\mathrm{C}\mathrm{u}\mathrm{C}\mathrm{l}}_2+2{\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}_2$$

(1)

Cu is oxidized by FeCl₃, forming Cu²⁺ and dissolving in the solution.

$$\mathrm{S}\mathrm{n}+2{\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}_3\to {\mathrm{S}\mathrm{n}\mathrm{C}\mathrm{l}}_4+2{\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}_2$$

(2)

Sn is oxidized by FeCl₃, forming Sn²⁺ and dissolving in the solution.

Given that the etching solution and its reaction products with a copper–tin alloy are both soluble in an ethanol solution, theoretically, wiping the etched area with anhydrous ethanol can effectively remove the etching solution and reaction products. Nevertheless, in order to guarantee comprehensive removal and to avoid the introduction of new corrosion factors [13], an oxidation sealing treatment can be conducted for subsequent processing. With this method, residual chlorides [14,15] can be converted into stable chlorine gas, which then escapes, thereby removing chloride residues.

Therefore, by combining physical cleaning with anhydrous ethanol and chemical treatment with oxidation sealing treatment, a more comprehensive and reliable strategy can be formed for removing the residue of etching solutions and their reaction products.

(2)

Hydrogen peroxide oxidation sealing method.

Hydrogen peroxide (H₂O₂) is a strong oxidizing agent and can be used for the treatment of copper rust.

The standard potential diagram for copper is:

$$\begin{gathered} {\mathrm{C}\mathrm{u}}^{2+}\stackrel{+0.158\mathrm{V}}{\to }{\mathrm{C}\mathrm{u}}^{+}\stackrel{+0.522\mathrm{V}}{\to }\mathrm{C}\mathrm{u} \\ {\epsilon }_R-{\epsilon }_L=0.522-0.158=0.364>0 \end{gathered}$$

(3)

Therefore, Cu+ can disproportionate into Cu²⁺ and Cu in solution.

$$2{\mathrm{Cu}}^{+}\rightleftharpoons {\mathrm{Cu}}^{2+}+\mathrm{Cu}$$

(4)

At 298 K, the equilibrium constant for the disproportionation reaction, $\mathrm{K}=\left[{\mathrm{C}\mathrm{u}}^{2+}\right]/{\left[{\mathrm{C}\mathrm{u}}^{+}\right]}^2=1.4\times {10}^6$, indicates a significant reaction trend. Furthermore, the standard reduction potentials for other species involved are as follows:

$${\epsilon }_{{\mathrm{C}\mathrm{l}}_2/{\mathrm{C}\mathrm{l}}^{-}}=+1.358 \mathrm{V}$$

(5)

$${\epsilon }_{{\mathrm{H}}_2{\mathrm{O}}_2/{\mathrm{H}}_2\mathrm{O}}=+1.770 \mathrm{V}$$

(6)

$${\epsilon }_{{\mathrm{C}\mathrm{u}}^{2+}/{\mathrm{C}\mathrm{u}}^{+}}=+0.159 \mathrm{V}$$

(7)

According to the aforementioned electrochemical principles, H₂O₂ can oxidize Cl⁻ to Cl₂ and convert Cu⁺ to Cu and Cu²⁺, achieving the purpose of removing Cl⁻ and Cu⁺ [16].

This method offers several advantages, including ease of operation, convenient reagent acquisition, substantial Cl⁻ removal efficacy, and minimal impact on the original appearance of cultural relics. It is a commonly used method for treating harmful rust on bronze artifacts. Due to its ability to remove chlorine, this method is particularly suitable for removing Cl⁻ from local areas caused by etching solutions after the on-site metallographic inspection of bronze artifacts.

Based on the theoretical foundation, the authors designed a practical on-site metallographic inspection technique that is highly feasible for real archaeological work on bronze artifacts. Its safety was then verified through simulation experiments. This technique has achieved ideal results in the technological analysis of the Ming Dynasty Jiajing period bronze lions at the Xi’an Beilin Museum and the Shang Dynasty Bronze Tripod Vessel with Cicada Designs at the China Bronze Ware Museum. The following section will provide a detailed account of the implementation process and operational specifics of the aforementioned technique.

3. Materials and Methods

3.1. Materials

The materials analyzed in this article include the following: four modern cast bronze samples used for simulation experiments, the Ming Jiajing Bronze Lions at the Xi’an Beilin Museum, the “Jiajing Tongbao” coins (嘉靖通宝), and the Shang Bronze Tripod Vessel with Cicada Designs at the Chinese Bronze Ware Museum.

To meet the requirements of analytical work, this study conducted on-site metallographic inspection and analysis on representative areas of the aforementioned cultural relics, with a total of nine positions being tested. The specific criteria for the selection of these testing positions and their locations will be elaborated in subsequent discussions.

3.2. Experimental Instruments and Methods

3.2.1. On-Site Metallographic Inspection

Using the MSD9198-type handheld metallographic microscope, from Murzider, Donguan, Guangdong, China, the specific sample processing method can be found in Section 3.4. On-site metallographic inspection was performed directly on the surface of cultural relics, so the examination area cannot guarantee complete flatness; therefore, some metallographic photos have undergone depth-of-field synthesis processing.

3.2.2. X-Ray Fluorescence Spectrometer (XRF)

Surface cleaning of artifacts was conducted using anhydrous ethanol to effectively remove surface impurities [17]. Subsequently, a handheld XRF analyzer [18] (model Bruker TRACER 5i, from BRUKER, Bremen, Germany, with operating parameters set to 40 kV voltage, was used to measure each sample for 30 s, and the Ancient Bronze3 analysis mode was selected) to conduct a thorough analysis of the chemical composition of all artifacts before the surface metallographic inspection. To ensure the accuracy of the data, each sample area was measured three times, and the resulting averages and predicted values are compiled in

Table 1. SEM-EDS composition analysis results of simulated experimental samples.

No.	Sample No.	Method of Processing	Main Element Content (Wt%)
			Cl	Fe	Cu	Sn	Pb	O	Other	Summation
1.1	①	-	-	0.05 ± 0.08	77.45 ± 0.40	14.10 ± 0.23	7.41 ± 0.33	0.75 ± 0.19	0.23	100.00
1.2	①	etching	3.57 ± 0.08	1.45 ± 0.10	72.21 ± 0.41	13.19 ± 0.22	5.01 ± 0.31	4.37 ± 0.26	0.20	100.00
1.3	①	C2H6O + H2O2	-	-	70.37 ± 0.42	12.26 ± 0.22	10.82 ± 0.32	6.16 ± 0.22	0.39	100.00
2.1	②	-	-	0.06 ± 0.09	71.19 ± 0.39	19.65 ± 0.25	7.91 ± 0.32	1.05 ± 0.21	0.14	100.00
2.2	②	etching	3.70 ± 0.08	1.54 ± 0.10	67.43 ± 0.39	18.89 ± 0.25	5.69 ± 0.32	2.03 ± 0.22	0.72	100.00
2.3	②	C2H6O	1.23 ± 0.07	0.03 ± 0.09	71.35 ± 0.39	19.40 ± 0.26	6.13 ± 0.33	1.51 ± 0.21	0.36	100.00
3.1	③	-	-	-	70.26 ± 0.38	20.40 ± 0.25	8.19 ± 0.32	0.96 ± 0.20	0.19	100.00
3.2	③	etching	2.93 ± 0.09	1.22 ± 0.1	68.37 ± 0.39	20.06 ± 0.26	5.10 ± 0.31	2.00 ± 0.22	0.32	100.00
3.3	③	C2H6O	1.06 ± 0.07	0.04 ± 0.09	71.11 ± 0.40	20.48 ± 0.26	5.66 ± 0.33	1.41 ± 0.22	0.23	100.00
4.1	④	-	-	0.07 ± 0.08	90.54 ± 0.41	6.27 ± 0.21	1.97 ± 0.29	0.87 ± 0.17	0.28	100.00
4.2	④	etching	6.18 ± 0.12	1.16 ± 0.09	83.53 ± 0.40	5.76 ± 0.20	1.80 ± 0.29	1.27 ± 0.17	0.30	100.00
4.3	④	C2H6O + H2O2	-	-	86.55 ± 0.40	5.84 ± 0.18	3.55 ± 0.30	3.72 ± 0.19	0.34	100.00

[19,20].

3.2.3. Scanning Electron Microscopy and Energy Dispersive Spectroscopy Analysis

The sample was polished and then observed with a TESCAN VEGA-3XMU scanning electron microscope (SEM) from TESCAN, Brno, Czech Republic, equipped with an OXFORD INCAX-ACT X-ray energy dispersive spectrometer (EDS), from Oxford, UK, for detailed examination of the micro-morphology and in-depth analysis of the chemical composition.

The copper alloy standard sample HPCu-5N (Chinese National Standard, ≥99.999% Cu) was used for instrument calibration. Optimal calibration, within the range of 99.5% to 100.5%, was performed before each test to ensure accurate results. Given the potential phenomenon of elemental segregation in copper artifacts [2], six representative areas were selected for each sample to determine the elemental content in detail, and the average value was ultimately taken as the representative composition of the sample.

The test conditions are as follows: a tungsten filament electron gun is used as the excitation source, a backscattering probe is used for signal acquisition, the excitation voltage is set at 20 kV, the scanning time is 60 s, and the working distance is 17 mm. In the raw data obtained, the total elemental content is controlled within the range of 95% to 105%, all values have been normalized, and the composition content is expressed as a mass fraction, ensuring the accuracy and comparability of the data.

3.3. Verify Simulation Experiment

Generally, the etching process for laboratory metallographic inspection methods follows these steps: clean the polished sample → wipe with alcohol → etch → rinse with running water → wipe with alcohol → dry with a hair dryer. There are three specific methods for etching: ① immersion method; ② drop etching method; ③ rubbing etching method.

In the field of technological archeology, the overwhelming majority of samples are small fragments embedded in resin. In practical work, the etch method is widely employed due to its high feasibility and ease of observation. The operator is required to wear acid- and alkali-resistant gloves, hold the sample with one hand, ensure that the polished surface is facing upwards, and, with the other hand, use a dropper to add a small quantity of the etchant to the polished surface of the sample, ensuring that the entire polished surface is covered. Once the optimal etching effect has been attained, the etching process is terminated and the subsequent steps, including rinsing and drying, are initiated in a systematic manner.

However, in the on-site metallographic inspection of metal cultural relics, due to limitations in conditions and considerations for the protection of cultural relics, the impact of the etchant should be minimized. Therefore, the swabbing method becomes the preferred etching technique. This method involves using a pointed cotton swab to apply the etchant, and then only swabbing the central area of the polished surface. Once the required effect is achieved, a cotton swab soaked in anhydrous ethanol is immediately used to dilute and remove the residual etchant from the surface. Subsequently, a new cotton swab soaked in anhydrous ethanol is used to continue cleaning and wetting the etched area, with strict precautions taken throughout to avoid contamination of the etchant beyond the polished area of the relic. Additionally, a cotton swab soaked in a 3% hydrogen peroxide solution is used to oxidize and seal the etched area to remove any residual chlorides.

Given that bronze artifacts may face the risk of chloride [12,21] contamination during on-site metallographic inspection due to the use of etching solutions, this research aims to experimentally verify the effective removal effect of chloride from the etching solution by cleaning with anhydrous ethanol and using an oxidation-sealing treatment in order to provide a more scientific and safe solution for the protection and metallographic analysis of bronze artifacts.

Two sets of modern cast bronze samples were prepared, each set consisting of two samples, as illustrated in Figure 7a. Samples ② and ③ constituted a single set. Following the etching of the central area, they were cleaned exclusively with anhydrous ethanol. Samples ① and ④ were the other set, and after etching the central area, they were first cleaned with anhydrous ethanol and then with 3% H₂O₂ for 5 min, with the solution being replaced every 1 min to maintain a fresh solution. The etching solution consisted of 5 g ferric chloride, 5–30 mL hydrochloric acid, and 100 mL alcohol. SEM-EDS chemical composition analysis was performed on the experimental samples at three stages: before etching, after etching, and after cleaning. The results are presented in Table 1.

The SEM-EDS chemical composition analysis revealed the presence of Cl element residues in the etched areas of all four samples following metallographic etching. This phenomenon is also evident in the SEM images (c) and (f). After cleaning with anhydrous ethanol, the Cl element residues on the surfaces of the samples ② and ③ were significantly reduced, and the SEM images could not identify the large residues of the etching solution. Nevertheless, EDS was still able to detect a minimal quantity of Cl residues. Following the cleaning of the etched areas of samples ① and ④ with alcohol and the subsequent oxidizing cleaning with a H₂O₂ solution, no Cl residues were identified.

The results indicate that the cleaning method involving anhydrous ethanol washing followed by oxidation with 3% H₂O₂ can effectively remove residual chlorides from the etching solution containing ferric chloride, hydrochloric acid, and alcohol components after metallographic etching. This metallographic etching and cleaning method can be safely applied to the on-site metallographic inspection of bronze cultural relics.

3.4. The Specific Operation of On-Site Metallographic Inspection Technique

Following the theoretical and experimental verifications outlined above, standardized procedures have been established as a specific operational guide for the on-site metallographic inspection technique of bronze cultural relics in our practical work.

Step I: Grind the oxidized layer on the surface of the bronze artifact until the bronze body is exposed. In this step, a handheld electric grinder equipped with a 1000-grit polishing needle is used to polish in circular motions. As shown in Figure 8I.

Step II: Apply a temporary protective material to the exposed bronze body and allow it to set. In this step, a fast-drying epoxy resin is employed as the temporary protective material. As shown in Figure 8II.

Step III: The areas where the aforementioned temporary protective material has been applied to the bronze body are then polished. In this phase of the process, a handheld electric grinder with a conical wool felt polishing head is employed. Subsequently, polishing paste is applied to the designated inspection area. The pointed end of the polishing head is then utilized to execute circular motions for polishing. It is essential to remove a portion of the provisional protective coating and continue polishing until the surface of the inspection area is free of discernible scratches (the grit size of the polishing paste is 2.5 μm). As shown in Figure 8III.

Step IV: Conduct an etching treatment on the polished region of the bronze body to obtain the sample for examination. In this step, a cotton swab with a pointed end is employed for the application of the etching solution, which is used for the etching treatment. The etching solution is composed of 5 g ferric chloride, 5 to 30 mL hydrochloric acid, and 100 mL alcohol. As shown in Figure 8IV.

Step V: Place the sample to be inspected under a metallographic microscope for observation to achieve on-site metallographic inspection. As shown in Figure 8V.

Step VI: Use anhydrous ethanol to wipe the eroded area, then wipe with 3% H₂O₂ for 5 min, with the solution being replaced every 1 min to maintain a fresh solution. Use anhydrous ethanol to wipe and remove the remaining temporary sealing material. As shown in Figure 8VI.

Note: It should be noted that this method cannot achieve a completely non-destructive metallographic analysis of metal artifacts. Depending on the specific sample situation, this on-site metallographic inspection method may leave a tiny copper-exposed spot on the surface of the object. Although such a copper-exposed spot is safe and stable after treatment in Step VI, some objects may still require restoration due to museum exhibition requirements. Our common practice is to collect the rust powder from the artifact itself during the grinding process in Step I, and then use a safe and stable sealing material to reattach the rust powder to the copper-exposed spot after completing Step VI.

3.5. Examples of On-Site Metallographic Inspection

3.5.1. The Ming Jiajing Bronze Lions Inlaid with Coins

The Xi’an Beilin Museum displays a pair of large bronze lions from the Ming Dynasty, now known as the Ming Jiajing Bronze Lions. The lion on the north side is a mother lion treading on a child, while the one on the south side is a father lion treading on a ball. Both bronze lions bear inscriptions that read “Made in the tenth month of the thirty-eighth year of the Jiajing era of the Great Ming, on an auspicious day, by the Inner Office of the Qin Mansion (大明嘉靖三十八年十月吉日秦府内典)” and “The original place of the Director of Dietary Affairs Ning wang is Zhourong Village, Lintong County, Xi’an Prefecture (膳正宁王原系西安府临潼县周荣里人)” indicating that the lions were cast in the thirty-eighth year of the Ming Jiajing era, which corresponds to 1559. They were initially placed in front of the Duanli Gate, the Southern Gate of the Inner City of the Qin Mansion in Xi’an, and were supervised by the Ning wang. The position of the food manager (典膳正) is an official title. During the Ming Dynasty, each imperial clan’s mansion (王府) had an Office of the Steward (长史司), under which there was an Office of Dietary Affairs (典膳所). It was staffed with one Director of Dietary Affairs (典膳正), initially ranked as the sixth rank, later downgraded to the eighth rank. The Director of Dietary Affairs (典膳正) was responsible for the sacrifices, hospitality, and meals of the prince and his consort. Zhourong Village in Lintong County, Xi’an Prefecture, was the native place of Ning wang.

It is known that there are 13 large-scale bronze lions [22] from the Ming Dynasty still existing in China, most of which are gate guardians for imperial palaces, royal mansions, or large temples. Among these, the Ming Jiajing Bronze Lions are one pair. However, among these numerous lions, the Ming Jiajing Bronze Lions have a very special feature: visible coin pattern decorations all over the bodies of both lions. Bronzeware with coin patterns was widely popular in the Han Dynasty [23], but the craftsmanship of the coin pattern on this artifact is the first of its kind, which was made with the actual currency in circulation at that time—“Jiajing Tongbao (嘉靖通宝)” [24].

It should be noted, however, that the coins in question are situated in highly visible locations on the surface of the Bronze Lions. Sampling required to explore the manufacturing techniques would inevitably result in significant damage to the artifacts. Such an approach is not endorsed by the national cultural relics management authorities and is not aligned with the conservation principles espoused by museums. Consequently, the utilization of on-site metallographic inspection techniques represents an optimal solution. This approach enables the effective acquisition of information regarding the craftsmanship of the Bronze Lions while limiting the impact on the cultural relics.

The final imaging results are shown in Figure 9c,f, where it can be seen that the metallographic structure at the edge of the coin exhibits clear signs of being heat-affected [25]. The results indicate that the metallographic structure of the coins on the surface of the Bronze Lions is composed of large α solid solution grains, with Sn and Pb between the grains, and no signs of cold working are observed. It is a distinct brass structure that has been heat-affected post-casting.

To demonstrate the changes in its metallographic structure after heating, this study also conducted the same metallographic and alloy composition tests on three “Jiajing Tongbao (嘉靖通宝)” coins with the same inscriptions as the Bronze Lions, using the same instruments and methods. The metallographic structure and alloy composition of the three coins were consistent, as shown in Figure 9i and

Table 2. Analysis results of the lions inlaid with coins and Jiajing Tongbao.

Name of the Artifact	Sampling Location	Metallographic Structure	Component Content/%					Alloy Material	Manufacturing Process
			Cu	Sn	Pb	Zn	Other
Female lion	Surface coins	The middle part of the α solid solution still has a dendritic morphology, with intracrystalline segregation not being very obvious. A large number of polygonal island-like (α + δ) eutectoid and free-state lead particles are distributed between the crystals. Near the edge of the coin, the α solid solution exhibits the equiaxial structure.	64.6 ± 0.16	8.8 ± 0.12	15.9 ± 0.23	9.70 ± 0.05	1.0 (Fe, Ag, Sb, et al.)	Cu–Zn–Sn–Pb	Post-casting heat-affected
Male lion	Surface coins	The middle part of the α solid solution still has a dendritic morphology, with intracrystalline segregation not being very obvious. A large number of polygonal island-like (α + δ) eutectoid and free-state lead particles are distributed between the crystals. Near the edge of the coin, there is a distinct area where equiaxed crystals have become significantly smaller. The α solid solution dendrites exhibit clear intragranular segregation, with a small amount of lead particles distributed between the grains, and no obvious eutectoid structure is observed.	72.2 ± 0.17	10.1 ± 0.13	6.7 ± 0.14	9.70 ± 0.05	1.20 (Fe, As, Sb, et al.)	Cu–Zn–Sn–Pb	Post-casting heat-affected
Coin 1	-	α solid solution dendrites with significant intragranular segregation, a small amount of lead particles distributed between grains, and no obvious eutectoid structure observed.	70.62 ± 0.16	6.00 ± 0.11	6.09 ± 0.14	16.41 ± 0.06	0.89 (Fe, As, et al.)	Cu–Zn–Sn–Pb	Casting
Coin 2	-		68.05 ± 0.15	6.32 ± 0.10	7.63 ± 0.10	17.30 ± 0.07	0.69 (Fe, As, et al.)	Cu–Zn–Sn–Pb	Casting
Coin 3	-		69.42 ± 0.15	6.73 ± 0.12	5.51 ± 0.09	17.67 ± 0.07	0.67 (Fe, As, et al.)	Cu–Zn–Sn–Pb	Casting

. The alloy structure of the Jiajing Tongbao (嘉靖通宝) coins, which were cast without secondary heating, consists of fine α solid solution dendritic segregation, with no obvious second phase between the grains but very clear dendritic segregation within the grains. These results are consistent with previous research findings on the Jiajing Tongbao (嘉靖通宝).

This evidence indicates that the inlay technique employed for the coins on the surface of the Bronze Lions should be categorized as a non-mechanical hot inlay process. This is very similar to the copper-inlay [26] technique that briefly became popular during the Eastern Zhou period [27] and then gradually disappeared. This casting-inlay technique involves creating various shapes from red copper sheets and placing them inside the mold cavity before casting. As the bronze is poured, the higher melting point of red copper allows the red copper sheets to remain intact within the object during the cooling and solidification process of the bronze, thus forming distinctive red copper decorations. Later, the copper-inlay technique gradually matured, evolving into a semi-inlay method that uses thinner sheets of red copper in conjunction with a supporting nail structure for inlaying.

The metallographic observation results of the Ming Jiajing Bronze Lions are also consistent with the logical craftsmanship. However, it is noteworthy that the “Jiajing Tongbao (嘉靖通宝)” coins are made of brass, which has a lower melting point, and no traces of supporting nails were observed on the back of the coins. Therefore, the specific details regarding the inlaying technique of the “Jiajing Tongbao (嘉靖通宝)” coins need to be confirmed after further simulation experiments.

Summary

Through observation, it was determined that the surface of the Ming Jiajing Bronze Lions’ coins’ metallographic structure was formed by post-casting heating, without any traces of cold working. The heat-affected zone at the edge of the coins indicates that the coin patterns on the surface of the lions were inlaid using the casting-inlay method with actual currency of the time, “Jiajing Tongbao (嘉靖通宝)”. The examination of the Ming Jiajing Bronze Lions demonstrated that the on-site metallographic inspection method can be employed to observe the local metallographic structure of large bronze artifacts with minimal damage to the surface oxide layer, thus providing a valuable tool for the study of local craftsmanship. Furthermore, the intrinsic benefits of on-site metallography allow for the clear and precise representation of directional microstructural details, such as the heating direction at the observation location, which is challenging to achieve through metallography after sampling.

3.5.2. The Shang Bronze Tripod Vessel with Cicada Designs

This artifact was unearthed in May 1970 at the Zhou Tomb in the southern suburb of Baoji, and is a second-grade cultural relic of the Shang Dynasty. The artifact is 25 cm high with an inner diameter of 15.67 cm, depth of 12.2 cm, and weight of 3116 g. The Shang Dynasty bronze tripod was used for both cooking and ritual purposes. According to the existing data [28], most round tripods from the Shang Dynasty exhibit burn marks on their bottoms. This cicada-pattern tripod is no exception, as its bottom and two feet clearly show signs of burning. These burn marks indicate that the tripod was subjected to heating during its use. Heating leaves thermal traces in the metallographic data, which reflect post-casting heat treatment. Therefore, when selecting positions for subsequent metallographic testing, it is necessary to compare the microstructural differences between the heated and unheated areas. This comparison will provide more accurate information about the manufacturing process and reveal additional details about the artifact’s usage history.

The Bronze Tripod Vessel has multiple traces of repairs on its body. To facilitate differentiation, the three legs are marked as A, B, and C, as shown in Figure 10 and Figure 11. Leg A is integral to the body of the Bronze Tripod Vessel and has intact decorations on its surface, Leg B is half restored historically, and Leg C is entirely restored historically. Given the high status of the Bronze Tripod Vessel and the integrity of its shape without any obvious breaks, destructive sampling is not appropriate.

To explore the differences in the techniques used during the historical restoration of Leg B and Leg C, on-site metallographic inspection was conducted at the locations marked in Figure 12b,d,g,i. Prior to the analysis, the selected areas were polished, and then a handheld X-ray fluorescence spectrometer (XRF) was used to analyze the composition of the polished areas. The specific data are shown in

Table 3. Analysis results of the Shang Bronze Tripod Vessel with Cicada Designs.

Name of the Artifact	Sampling Location	Metallographic Structure	Component Content/%					Alloy Material	Manufacturing Process
			Cu	Sn	Pb	As	Other
The Shang Bronze Tripod Vessel with Cicada Designs	Leg A	The α solid solution twins and equiaxed grains show no significant intragranular segregation, with a small amount of polygonal island-like (α + δ) eutectoid distributed between grains. A few lead particles are distributed between grains, typical of a post-casting heat-treated structure, and due to the longest preservation time, a large number of grains have already been mineralized.	70.28 ± 0.15	18.24 ± 0.22	9.41 ± 0.23	0.63 ± 0.06	1.44 (Fe, et al.)	Cu–Sn–Pb	Post-casting heat-affected
	Leg C	Equiaxed grains of α solid solution show no significant intragranular segregation, with a small amount of polygonal island-like (α + δ) eutectoid distributed between grains. Spherical lead particles are also distributed between grains.	75.95 ± 0.20	8.37 ± 0.13	13.87 ± 0.21	0.81 ± 0.06	1.01 (Fe, et al.)	Cu–Sn–Pb	Post-casting heat-affected
	Leg B	The α solid solution dendrites show significant intragranular segregation, with large blocks of lead particles and a small amount of eutectoid structure distributed at the grain boundaries.	87.73 ± 0.18	0.26 ± 0.12	10.47 ± 0.19	0.78 ± 0.08	0.75 (Fe, et al.)	Cu–Sn–Pb	Casting
	Ear	The α solid solution dendrites show significant intragranular segregation, with a small amount of lead particles and a large amount of (α + δ) eutectoid structures distributed at the grain boundaries.	70.27 ± 0.16	17.84 ± 0.17	9.67 ± 0.20	0.40 ± 0.07	1.83 (Fe, et al.)	Cu–Sn–Pb	Casting

. Through comparative analysis of the differences between legs in metallographic detection and XRF component analysis, we can further reveal the differences in historical restoration techniques between the two, thus providing useful references for the protection and restoration of bronzeware.

The results indicate that Leg A is consistent with the alloy composition of the Bronze Tripod Vessel’s ear, confirming that it was cast as an integral part initially. Both Leg B and Leg C are supplemental parts added later, and there is also a significant difference in the alloy composition between B and C. Furthermore, both Leg A and Leg B exhibit distinct post-casting heat-affected microstructures, which further suggests that Leg B and Leg C were added as repairs at different times.

Given that Bronze Tripod Vessels typically underwent heating during use, the belly of this particular tripod also exhibits clear indications of burning. This suggests that after the repair of Leg B, the Bronze Tripod Vessel was used for heating. In contrast, Leg C should have been the last to be repaired. Following the addition of Leg C, there was no further use for heating.

These pieces of information provide valuable physical evidence for the study of ancient bronze restoration techniques. However, the specific dates of these repair activities still need to be determined through further scientific analysis, which is not the focus of this article.

Summary

Through analysis, it is inferred that the Shang Bronze Tripod Vessel with Cicada Designs had two legs that were restored and supplemented historically, and after one of the legs was restored, the tripod was still used for a period of time, which can serve as evidence of ancient restoration practices for bronzeware. The examination process of the Bronze Tripod Vessel demonstrated that for bronzeware that has undergone multiple modifications overall, this on-site metallographic inspection method can flexibly explore the craftsmanship differences in various parts with minimal damage to the artifact. Especially for high-level bronzeware like the Shang Bronze Tripod Vessel with Cicada Designs, which are precious, intact, and difficult to sample, on-site metallographic inspection can better showcase its advantages.

4. Conclusions

Through these analyses, this study explores the feasibility of on-site metallographic inspection methods in the detection of metal cultural relics, sharing a set of minimally invasive and flexible metallographic inspection methods in the field of technological archeology. It is of significant importance in alleviating the contradiction between the utilization and preservation of cultural relics. In the examples involved in this study, some technological details of the Ming Jiajing Bronze Lions from the Xi’an Beilin Museum and the Shang Bronze Tripod Vessel from the China Bronze Ware Museum were also discovered, providing examples for the application of on-site metallographic inspection methods in the technological analysis of metal cultural relics.

(1)

The coins inlaid on the Ming Jiajing Bronze Lions are the actual currency of the time, “Jiajing Tongbao (嘉靖通宝)”, using the casting-inlay method on the surface of the lions.

(2)

The Shang Bronze Tripod Vessel with Cicada Designs has two legs that were restored and supplemented historically. After the partial restoration of the feet, the Bronze Tripod Vessel was put back into use. After the full restoration of the feet, the Bronze Tripod Vessel was never heated and used again.

(3)

During the study of partial craftsmanship of large bronzeware, the on-site metallographic inspection method can observe the local metallographic structure of the object by only damaging a very small area of the surface oxide layer and accurately indicate the direction of heating and other directional structural details at the observation location.

(4)

When facing bronzeware that has been processed multiple times overall, the on-site metallographic inspection method can flexibly explore the craftsmanship differences of each part, and the damage to the object is minimal, making it applicable to complete objects that are not suitable for sampling.

In summary, this study provides a basic understanding of the feasibility of the on-site metallographic inspection technique in the detection of metal cultural relics and shares a set of practical on-site metallographic inspection methods and practical examples. However, there are still some issues that require further research, such as the feasibility and safety of this method in the detection of iron cultural relics, the technical details of the surface casting-inlay of coins on the Ming Jiajing Bronze Lions, and the implementation date of the historical restoration of the Shang Bronze Tripod Vessel with Cicada Designs.

5. Patents

Methods for on-site metallographic inspection of bronze relics, CN202410377838.6 (in Chinese).