Introduction

As a significant center of ancient civilization of China, plenty of the Warring States period bronzes were unearthed in Chengdu Plains. The bronzes play an important role of “Bashu Culture”. More than half of the unearthed bronzes are weapons [1, 2].

Apart from the weapons, bronze vessel, served as an important part of burial objects in Bashu district, contributing significantly to the study of funeral regulation, as well as the organization of ancient social order. Bronze vessels were excavated from 134 tombs out of 559 in Bashu district, up to 252 pieces. Different type of vessels were unearthed, bronze vessel “Mou” (鍪) was found in 122 tombs, mainly distributed among Chengdu Plains and Eastern Sichuan ridge-valley area [3]. Bronze vessel “Mou” was excavated in central China, originated from Chengdu Plains and spread over surrounding areas. Sichuan Province owns the largest number and clearest series of such archaeological findings. Bronze “Mou” was placed together with other cooking vessels in the tombs. Traces of burning were found in the bottom of the vessel, residues of animal bones were found inside the vessel, indicating the vessel was used for cooking. Bronze vessel “Mou” was buried with regular number in the tombs of certain level, proving it was also treated as ritual vessel [4, 5]. Typological study of Mou indicates its type varies in time and region from Qin Dynasty to the Warring States.

During the excavation of Pujiang ship-shaped graves in 2016, several bronze “Mou” with sever corrosion were unearthed together with other bronzes. It would be of interest to study the compositional and metallurgical feature of the unearthed vessels, as well as the characterization of corrosion products. Archaeologists and conservators are puzzled by such problems that need to be answered by systematic scientific studies. Different scientific methods have been applied to the research of bronzes, for a better understanding of their alloy ratio, metallurgical characterization and corrosive behavior [6,7,8]. Studies on the development of society, technology and military have been conducted by archaeologist according to the period and category of the unearthed bronzeware [9]. Technical analysis of the bronzes in Bashu district has been concentrating on unearthed weapons, revealed bronze production in the ancient Chengdu Plains [10,11,12], as well as special craft adopted during manufacturing [13]. Few study of bronze vessel “Mou” has been reported.

The unearthed bronze vessels in Pujiang County are served as a precious material for a better understanding of the bronze manufacturing technology of Chengdu Plains in the Warring States period. Technical methods, such as OM, SEM-EDS, XRD and Raman Spectroscopy were adopted for the research, for a better understanding of compositional and metallurgical characterization of the bronze vessel, as well as the identification of the corrosion products.

Archaeological context

Located in the southwest of Sichuan Basin in China, Pujiang County (80 km to Chengdu City) experienced the excavation of 60 tombs from the late Warring States period in 2016 (Fig. 1). The tombs are divided into three different types according to their sizes with the length up to 9-m, 6-m and 3-m, respectively. Several ship-shaped coffins were found with the length from 4 to 7 m and the width from 0.9 to 2 m. The coffin was made by curving the wood into the shape of the ship. More than four hundred ancient artifacts were unearthed, including 12 seals with the interpretation of “Bashu”, 2 eyebeads, lacquerwares, potteries and bronzes. The number of bronzes exceeds 45% of the artifacts. The bronzes consist of different categories, including vessel, weapon, tool, coin and ornament. Bronze vessel Fu (釜) and bronze vessel Mou (鍪) were identified. Unfortunately, most of the bronzewares are broken and are suffering from severe corrosion. For the first time, the character “Cheng Du”, carved on a bronze spear in tomb M32, was identified in Chengdu area. This is potentially indicative of a recognition by local citizens in the late Warring States period [14].

Fig. 1
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Map of the excavated site

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The ancient citizens lived along river, therefore buried in a ship-shaped coffin with bronzes, lacquerware and pottery was usual through the Warring States period [15]. Another ship-shaped coffin was excavated in Pujiang in 1998 with 51 pottery, bronzeware and woodware [16]. There are different opinions about the origin of “Ba” and “Shu”, the culture varies a little bit though they are adjacent, which also reflects by the unearthed bronzes in ship coffins [17].

Material and methods

Material

Twenty five unearthed bronze vessels (bronze “Mou”) dated to the late Warring States period were adopted for the research. 51 samples, both from the matrix (26 samples) and the corrosion part (25 samples) of each bronze vessel, were carefully collected under microscope to ensure its full structure, and therefore are considered to be representative of the vessel. The size of the collected samples was strictly controlled to minimize the adverse impact for restoration (less than 0.5 × 0.5 cm). Basic information is shown as Table 1. PJ 19 and PJ 21 comes from the top and bottom of the matrix of M11:1. The size and the weight of each vessel were measured accordingly. The bronzes are in poor condition, most of which are suffering from corrosion and incompleteness.

Table 1 Basic information of collected samples
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Methods

Microscopic analysis

Observation of the samples was conducted by two microscopes (Axio Scope A1 equipped with ICCS optical system, Zeiss Ltd., Germany and a three-dimensional digital microscope VK-X250, Keyence Corporation, Japan) with the magnification from 50 to200 × and 20 to 200 ×.

Metallographic analysis

The metallographic observation was conducted on 26 samples collected from the matrix of the vessels. Samples were embedded by a metallographic inlaying machine (CitoVac, Struers Ltd., Denmark), ground and polished by a Tegramin-20 automatic grinding and polishing equipment (Struers Ltd., Denmark) according to the standard metallographic procedures to achieve the cross-section of the samples. A polarizing microscope (Axio Scope A1, Zeiss Ltd., Germany) was used to observe the cross-section before and after etching with aqueous ferric chloride alcohol solution with the magnification from 50 to 200 ×. All the solvents were purchased from Sinapharm Chemical Reagent CO., analytical grade. The collected samples were observed directly under microscope to record its corrosion product. SEM-EDS analysis was conducted with a FEI Verios 4G SEM coupled with an Oxford X-max 20 EDS analyzer to obtain secondary electron (SEM) images and backscattered electron (BSE) images for phase studies. The embed samples with cross-section were analyzed before etching. Samples were analyzed with10–20 kV acceleration voltage and 8–9 mm working distance.

Compositional analysis

SEM–EDS analysis was conducted with a FEI Verios 4G SEM coupled with an Oxford X-max 20 EDS analyzer was also hired for the characterization of chemical composition. Matrix (cross-section) and corrosion samples were analyzed with10–20 kV acceleration voltage and 8–9 mm working distance. Industrial copper reference sample was used for calibration and optimization before analysis. Mapping scanning was adopted during element analysis by EDS in order to minimize the detection error. The area with Pb particle and corrosion were avoided while selecting the detection zone. For the EDS analysis of inclusions and special phase, dot scanning was preferred for accurate results. Each sample was analyzed for three times, the elemental results detected were averaged and normalized.

XRD was performed by a X’Pert Pro MPD diffractometer (Philips, Netherlands) equipped with a Cu-Ka radiation source (λ = 0.15418 nm) in the range of 5°–95°, with a tube voltage of 40 kV and a current of 200 mA at a scan rate of 10°min−1. Corrosion samples were ground into powder for analysis. XRD spectra was analyzed and matched by JADE.

A Renishaw inVia RM200 Raman spectrometer coupled with microscope was used for analysis. Measurements were performed using an argon gas laser at 532 nm with the range 100–3000 cm−1. The detection temperature is 25 °C with the humidity lower than 50%. The spectral resolution was 0.6 cm−1. The laser power was approximately 0.5 mW, which ensured that good quality spectra was recorded. Corrosion samples were placed directly under microscope for the selection of certain corrosion product before detection (Fig. 2).

Fig. 2
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Distribution of the tin concentration in bronze vessels

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Results and discussion

Analysis of the matrix

The SEM-EDS analysis of the matrix indicates the bronzes are Cu–Sn–Pb trinary alloys, the proportion of Cu, Sn and Pb differs from 65–85%, 5–22% to 2–17%, respectively (Table 2). The data has been treated with main elements for comparison. It has been generally recognized tin bronze refers to a copper-based alloy containing 2% tin or over, a Cu–Sn–Pb ternary alloy contains both tin and lead at the level of 2% or more [18, 19]. Therefore, the bronzes excavated from Pujiang are tin–lead bronze. Compared with the published data of the Warring States, the composition of the bronze vessels is basically Cu–Sn–Pb, different from the Cu–Pb or Cu–Sn binary alloys found in bronze weapon and ornament. The proportion of Sn concentrates between 10 and 18%, which is in accordance with the bronze vessels in Baishoulu, Shu State [11]. The cemetery of Xindu, Majia, 100 km northeastern to Pujiang, is a late Warring States-period tomb in Bashu district. The ratio of Sn of Majia bronze vessels various from 6 to 20%, with an 8.7% found in bronze Mou [20]. Therefore, the Warring States bronze vessel in Chengdu Plains and surrounding areas are made to a specific designed composition. The variation of the Sn concentration might have a relation with the tomb owner’s social status [21]. The lead sources of the vessels are still unclear, which could be reported by further studies to explain the bronze flow in ancient time.

Table 2 Chemical composition of the bronzes
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A method has been established to infer whether the bronzes in a certain region are primary alloys or contain recycled metal by considering the shape of the distribution of the tin concentrations within the assemblage. Thus a unimodal approximately normal distribution of tin centered around 10–20% indicates the assemblage is likely to be made of a primary alloy [22]. The distribution of tin concentration of bronze Mou peaks at 16, indicating recycled alloy was not adopted in this case.

Metallurgical observation of the cross-section presents typical casting microstructure, further processing during manufacturing didn’t appear according to the metallographic analysis. Different from the weapons in ancient Chengdu Plains, the vessels were cast without secondary processing [12]. Part of the metallographic observation was shown in Fig. 3, presenting a dendritic microstructure. The (α + δ) eutectoid was uniformly distributed among the dendritic microstructure. Lead inclusions shows random distribution [12, 19]. The bronze samples are suffering from heavily mineralization, PJ59 presents layered structure with little bronze matrix remains. Samples of PJ17/PJ23/PJ29 and PJ41 are nearly in the same condition. Small holes and cracks, as well as dendritic segregation, appear in both matrix and corrosion product. A few blue-grey inclusions are visible, shaped as small and big particles, diffusely distributed among δ solid solution and outside(α + δ)eutectoid. For other samples with less mineralization, small holes and cracks are concentrating among the corrosion layer.

Fig. 3
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Metallographic analysis of the matrix

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BSE images of the samples are attached as Additional file 1: Fig. S1. Element distribution of each microstructure was detected, taking PJ59 as an example (Fig. 4 and Table 3): Two different metallographic phases, α phase (Spot-1) and (α + δ) phase (Spot-2) were observed based on the BSE mode of SEM, while white lead particle was visible (Spot-4). The proportion of Cu and Sn differs in different phases. The dark particle in Spot-3, observed as blue-grey inclusions in metallographic analysis, contains Cu and S. Apart from fully mineralization samples, sulphide was frequently identified.

Fig. 4
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SEM image and element distribution image of PJ59. a Total view, b spot 1–3, c spot 4

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Table 3 The chemical composition of PJ59
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Analysis of the corrosion products

Different corrosion products are shown in Fig. 5, appear as yellow, black, orange, green, grey and red mineral. The microscopic observation indicates the corrosion products exist in the form of smooth and dense layer with cracks.

Fig. 5
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Photo of corrosion samples. a M1:1, b M8:8, c M49:16, d M11:1, e M41:4, f M16:6

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Seven embedded matrix samples with full structure were analyzed by SEM-EDS for a better understanding of corrosion status. Layered structure of the corrosion products was obviously noticed under microscope, presented as matrix layer and corrosion layer with unclear boundary. Corrosion appears in the matrix layer as well. Apart from Cu, Sn and Pb, elements such as Si, P, Fe and As were identified in the corrosion layer, indicates mineralization starts during corrosion. The content of Cu decreases from inner layer to the surface layer, demonstrating corrosion starts from the surface. The main element in the corrosion layer is Sn, followed by Cu, which is the characteristic of “tin-rich” corrosion layer [23]. “Tin-rich” corrosion layer, often considered appears in the bronze mirror or weapons with a Sn concentration exceeds 17%, was found in the bronze vessels with the Sn concentration varies from 12 to 17% in this case. Two different corrosion layers were observed in PJ17, shown as red corrosion layer and green corrosion layer from inside to the outside (Fig. 6). The composition differs in each layer according to the elements detected by EDS. Cu, Sn and Pb were detected in zone 1. Elements such as Cu, Sn, Pb, S, Fe, Si and As were found in zone 2. Cu, Sn, Pb, Si, Fe and Al were identified in zone 3. According to the previous research, the corrosion products consists of outer layer (mineralized layer) and inner layer (altered zone). The outer layer possesses a stable Cu/Sn ratio with element from burial environment. The inner layer has a higher proportion of Cu, and couldn’t be observed in every sample [24,25,26]. Fortunately, element Cl wasn’t detected by EDS, indicates that copper hydroxychlorides, which will cause “bronze diseases”, didn’t appear in the corrosion products [27].

Fig. 6
figure 6

a Sample of PJ17, b OM images, c SEM-BSE image. c-1–c-3 EDS analysis of zone 1–3

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The bronzes were buried underground for thousands of years and were subject to corrosion due to the interaction with water, oxygen and carbon dioxide [28,29,30]. Micro-Raman spectroscopy and XRD were adopted to identify the chemical composition of the corrosion products. Cu2CO3(OH)2 (PDF#72-0075) and Cu2O (PDF#78-2076) are the most common minerals among corrosion products. Besides, SnO2 (PDF#72-1147) and SiO2 (PDF#87-2096) were identified in some samples by XRD detection (Fig. 7). SnO2 was frequently identified on the surface of ancient corroded bronzes. Analysis by SEM-EDS shows a significant higher content of Sn and a lower content of Cu in the corrosion layer compared to the matrix layer. Such patina was formed according to the “natural corrosion mechanism” with two key factors played important roles, i.e., high Sn content and the damp, wet and acid burial conditions [31], which is in accordance with previous discussion. The formation of SnO2 comes from the selective corrosion [32]. Tin oxides are easier formed than cuprous oxides [33]. The loss of Cu results in the enrichment of Sn on the surface [34].

Fig. 7
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X-ray diffraction pattern

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Tested samples by Raman spectrometer shown different minerals (Fig. 8). The presence of cuprite (Cu2O) can be concluded from spectra of reddish-brown minerals (Fig. 8-E), reveals well-resolved bands at 148, 218 and 629 cm−1 [35, 36]. The formation of cuprite layer starts from the early process (before burial) of bronze oxidation/corrosion by exposure to the open air. Such layer benefits the preservation of bronzes due to its dense structure [37, 38]. Green area (Fig. 8-D) with its characteristic absorption in 152, 183, 220, 272, 360, 432, 535, 721, 757, 1065, 1097, 1373, 1498 and 1593 cm−1 indicates the existence of malachite [39]. Apart from the cuprite (known as “noble patina”), basic copper carbonates malachite is typically part of the “vile patina”, which usually appears as overlapping secondary corrosion products [27, 40, 41]. The absorption bands of light blue area (Fig. 8-G) in sample PJ56 should be attributed to the vibration of Cu–O and SO4 in chalcanthite (CuSO4·5H2O) [42, 43], with aqueous sulfate tetrahedral oxyanion at 998 cm−1 and a complex set of overlapping bands observed at 143, 207 and 250 cm−1. Sulfating corrosion of copper alloys can be observed as pitting corrosion of surfaces exposed to fresh water. For chalcanthite to form at any Eh/pH combination, one must have very high concentrations of sulfate and copper [44]. Chalcanthite dehydrates over pH 2.5 as well as in atmospheric exposure [45]. The presence of chalcanthite indicates the burial environment owns acidity soil with abundant water.

Fig. 8
figure 8

Raman spectrum of the samples: C (Dark White area of PJ36), D (Green area of PJ20), E (Reddish-brown area of PJ2), F (White area of PJ2), G (Light blue area of PJ56)

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Peaks in 157, 219, 683, 842, 1055, 1373, 1471 and 1587 cm−1 (Fig. 8-F) are in accordance with the characterize peaks of PbCO3 [46]. The characterization peaks (Fig. 8C) also indicates the formation of PbSO4 [47]. The adding of certain amount of Pb during the bronzes manufacturing is one of the features among the Warring States bronzes. Pb distributes along dendritic crystal as isolated phase, shaped as sphericity and particle in bronzes. Suffering from the CO2, O2, H2O and sulphate in the burial environment, the isolated Pb is easily corroded, forming lead corrosion products [48, 49].

Conclusion

The Warring States bronze vessel “Mou” excavated from southwestern China, presents compositional and metallurgical characters which enriches our knowledge of ancient manufacturing. All the collected samples present typical casting microstructure, the (α + δ) eutectoid was uniformly distributed among the dendritic microstructure while lead inclusions shows random distribution. The SEM-EDS analysis indicates the bronzes are Cu–Sn–Pb trinary alloys, the proportion of Cu, Sn and Pb differs from 65–85%, 5–22% to 2–17%, respectively. The Warring States bronze vessel in Chengdu Plains and surrounding areas are made to a specific designed composition, recycling isn’t involved with the production of such vessels. The bronzes are buried in a seasonal environment with acid soil and abundant water. Corrosion happens during the interaction with oxygen, carbon hydrates and water. Layered structure of corrosion products was observed. Different corrosion products were identified as Cu2CO3(OH)2, Cu2O, SnO2, PbSO4, PbCO3 and CuSO4·5H2O. The findings help us gain insights into the bronze manufacturing of Bashu district in Warring States period, as well as the corrosion behavior in certain environment. Besides, most of the bronze “Mou” are corroded and broken, the study offers supporting for the conservators during restoration.