Bronze production in the Ancient Chengdu Plains: A diachronic metallurgical perspective on a separate cultural region

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Bronze production in the Ancient Chengdu Plains: A diachronic metallurgical perspective on a separate cultural region Haichao Li a,b , Zhiqiang Zuo c , Jianfeng Cui d , Jianbo Tian c , Yingdong Yang c , Li Yi c , Zhiqing Zhou c , Jianan Fan b,∗ a

Archaeomaterials Research Laboratory, Sichuan University, Chengdu 610065, China Department of Archaeology, School of History and Culture, Sichuan University, Chengdu 610065, China c Chengdu Municipal Institute of Cultural Relics and Archaeology, Chengdu 610015, China d School of Archaeology and Museology, Peking University, Beijing 100871, China b

a r t i c l e

i n f o

Article history: Received 14 July 2019 Accepted 14 November 2019 Available online xxx Keywords: Chengdu Plains Elemental compositions Microscopic observations Lead isotope ratios Diachronic study Bronze production

a b s t r a c t As one of the most important separate cultural regions in ancient China, the Chengdu Plains provide a unique example to explore the development of bronze production. We analysed 37 bronze objects from the Western Zhou period to the Tang Dynasty in the Chengdu Plains for elemental compositions, microstructure, and lead isotope ratios. We referred to bronze data from the Sanxingdui sacrificial pits to compare results that demonstrated that most samples from the Chengdu Plains were Cu-Pb-Sn alloys with variable tin and lead content. The alloy technique used in the Warring States period bronzes, which were uncovered in the Baishoulu cemetery, differed from the techniques used in other samples. Casting is the major technique used for all types of objects in different periods. The only cold-worked and annealed sample found so far was a Chu-style vessel. Different lead sources and a possible single copper source were constantly used in local bronze production from the Western Zhou Dynasty to the Warring States period (more than 600 years). Some coins in Han and Tang Dynasty potentially also used these same lead sources. Comparison with Sanxingdui bronze suggests that both lead and copper sources of Sanxingdui bronzes are different from the later metal sources. We therefore propose that the Sanxingdui bronze might not have been locally made or was made with outside materials. Our study suggests that diachronic study on bronze production could provide clues to solve more archaeological questions other than the development of bronze production. © 2019 Elsevier Masson SAS. All rights reserved.

1. Introduction and research aims 1.1. Introduction The Chengdu Plains are situated in the western portion of the Sichuan Basin in China (Fig. 1). This area is referred to as the land of abundance because of its fertile soil and temperate climate. This area has been home to important civilisations in China and has been the centre of the Sichuan Basin since ancient times. The region has developed into a separate geographic and cultural area, as it is surrounded by mountains and is blocked off from access. This region is useful for studying the development of metallurgy due to this disconnectedness. In the Chengdu Plains, bronze objects recovered from Sanxingdui sacrificial pits are the most famous [1]. The discov-

∗ Corresponding author. E-mail address: [email protected] (J. Fan).

ery of many bronze masks, statues, trees, and other objects unique to the Chengdu Plains suggests that the Sanxingdui developed a characteristic bronze culture in China. The dating of these Sanxingdui sacrificial pits corresponds with the early period of the late Shang Dynasty (14th to 12th centuries BCE), as the Dynasty’s capital was in the Central Plains’ Yinxu site [2] (Fig. 1). Several additional bronze civilisations developed along the Yangtze River during the same time period, including the Ningxiang bronzes in Hunan province and the Xingan bronzes in Jiangxi Province [3–6] (Fig. 1). The interactions between the Yangtze River civilisations and the Central Plains Dynasty define that time period. The Jinsha bronzes eventually succeeded the Sanxingdui ones, but these latter bronzes were of diminished quantity and quality. Several bronze types eventually vanished; however, the richness of Jinsha is indicated by its uncovered gold and ivories [7–10]. The Shierqiao culture dominated the primary period of the Jinsha site, from the 11th to 8th centuries BCE, and corresponded with the end of the late Shang to the beginning of the Spring and Autumn

https://doi.org/10.1016/j.culher.2019.11.005 1296-2074/© 2019 Elsevier Masson SAS. All rights reserved.

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Fig. 1. Map with locations of the Chengdu Plains and sites in other regions.

periods in the Central Plains (roughly approximate to the Western Zhou Dynasty). The Western Zhou Dynasty built a vassal-state system and distributed hundreds of vassal states throughout the Central Plains and its surrounding areas [11]. However, the relation between Western Zhou Dynasty and the power in the Chengdu Plains is unknown. In the next development state, the Chengdu Plains became the geographical and political centre of the Shu state. The beginning date of Shu state is uncertain; it is widely accepted that it already existed in the Warring States period (475–221 BCE) [2]. The majority of the uncovered remains in the Chengdu Plains belong to the Warring States period, when vassal states became increasingly independent and conquered each other. The bronze objects from this period were mainly weapons such as swords, dagger-axes, and spears. In the war in 316 BCE, the Qin state conquered the Shu state and subsequently established the first unified empire. After this, the Chengdu Plains developed into the central dynasties’ administrative region and the bronze objects—including coins, practical vessels, and tools—became significantly less important. The majority of archaeometallurgical studies on the Chengdu Plains’ bronzes have utilised elemental and lead isotope analyses to focus on the Sanxingdui and Jinsha bronzes [12–18]. However, the existing research has yet to establish a consensus regarding the provenance of these objects because the region’s metallurgical features are unclear. Moreover, several existing analytical results are not ideal because the archaeological background is unclear or misunderstood. The Jinsha bronzes provide the best example of these contradictory findings. Numerous scholars have analysed them; for instance, Jin et al. published elemental and lead isotope data on the Jinsha bronzes [16], and Xiao et al. [17] and Xiang et al. [18] both also presented archaeometallurgy results. This is problematic because all analysed bronze objects were uncovered in a disturbed ditch, indicating that information on the objects’ original context was lost. These scholars assumed that these bronze objects belonged to the Shierqiao period; however, the Jinsha site included remains from periods both earlier and later than the Shierqiao period. Therefore, these analysed objects might belong to several different periods. Regarding the Warring States period bronzes, researchers have only published data on the major elements from several tombs [19]. Therefore, there is still a lack of basic knowledge on this period’s metallurgy. No metallurgical study has been conducted on any period later than the Warring States period.

1.2. Research aims The development of bronze production in the Chengdu Plains is an important topic to address because of the region’s isolation. It is necessary to investigate the outside influence on the region’s bronze production. How did the alloying technique develop over time? Were the same ore sources used consistently? Answering these questions will improve archaeological research in Southwest China and provide a new metallurgy research model in China. Over the past 10 years, we collected bronze samples from the Shierqiao culture, Spring and Autumn periods, Warring States period, and Han (202 BCE to 220 CE) and Tang Dynasties (618–907 CE). The collection of these samples enabled us to conduct a diachronic study about the development of bronze production in the Chengdu Plains. In this paper, we present new analytical data on these collected bronze samples and conduct comparisons with the existing, published data. 2. Archaeological background We included bronze samples from three sites in the Chengdu Plains. The dates for these three sites cover nearly the entire Bronze Age and some of the historical period. We discuss each site’s archaeological context below. The Jinsha site is in northwest Chengdu City. The primary remains uncovered at the Jinsha site are as attributed to the Shierqiao culture, which dates from the end of the late Shang period to the beginning of the Spring and Autumn period in the Central Plains (11th to 8th century BCE). Several prestige goods have been discovered, including bronze, gold, jade, ivory, and pottery. It is one of the greatest archaeological discoveries from the 21 centuries of Chinese history. Most of these prestige goods were unearthed from the central area of the Jinsha site, known as the sacrificial area (Fig. 2a). This area is a rectangular foundation 125 m long and 90 m wide. This area (5895 m2 in total) was excavated from 2002 to 2005. The excavators revealed numerous remains, including pits and house foundations [7–10]; the bronzes analysed in this study were uncovered here. The Baishoulu cemetery is in Chengdu City’s downtown area (Fig. 2b). During 2015 and 2016, 2800 m2 were excavated. Many tombs and other remains discovered in this cemetery belong to three different phases. The first phase was discovered with the

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Fig. 2. (a) Photograph of artefacts found in the sacrificial area at the Jinsha site; (b) photograph of the Baishoulu cemetery; (c) photograph of the Xindu site.

unearthing of 56 Warring States tombs; its unearthed bronze objects included axes, swords, spears, dagger-axes, knives, saws, stamps, and coins. The second phase is defined by the Han Dynasty (202 BCE to 220 CE); during this period, bronze production decreased, and the bronze object types differed from earlier phases. Most bronze items uncovered from the Han Dynasty were coins. The third phase falls between the Tang Dynasty (755–907 CE) and the Five Dynasties (907–960 CE); 26 brick-constructed tombs belonged to this third period. Similar to the Han Dynasty, the majority of bronze objects from this period were coins. The Xindu site is in northern Chengdu (Fig. 2c). It was a typical Shierqiao culture site. Archaeologists discovered residential remains like pits. During a 2012 excavation, professionals unearthed two bronze objects, dating to the Western Zhou period. 3. Materials and methods To study the development of bronze production in the Chengdu Plains, we collected bronze samples from different locations in the Chengdu Plains, including 18 objects from the Warring States period tombs, four Han Dynasty coins, and three Tang Dynasty coins from the Baishoulu cemetery; two Western Zhou period objects from the Xindu site; and 10 objects from Jinsha site (Fig. 3). The date of the 10 Jinsha objects lasted from the Middle and Late Western Zhou Dynasty (900–800 BCE) to the Middle Spring and Autumn period (700–600 BCE). To observe this period, we did not discuss the Jinsha objects in a more detailed chronological framework. We collected 37 bronze objects to analyse, from the Xindu site (1046–771 BCE) to the Jinsha site (900–600 BCE) and to the Warring States period of the Baishoulu site (475–221 BCE). These samples create a nearly complete chronology to study the bronze production from 1046 to 221 BCE, the focus of this study. Additionally, this period witnessed the greatest development of bronze production

not only in the Chengdu Plains but also in other regions of China. These bronze objects shared several similar features. First, they were all recovered from Chengdu—the centre of the Chengdu Plains since the Bronze Age. Therefore, they are the most representative samples to discuss local bronze production in the Chengdu Plains. Second, all these bronze objects show local styles. Most Jinsha samples came from local-style accessory fragments; the vessels were also decorated with local motifs. The bronze weapons, such as triangle dagger-axes and swords in the shape of willow leaf, and vessels, such as bowls and cooking pots, were typical local bronzes. The origins of the Han and Tang Dynasty coins are complicated, considering they may have been exchanged from other parts of China. Since no other bronze materials were available for analysis, we used only these for reference. Bronze production after the Warring States period will be considered in future studies. Table 1 displays detailed information on these samples. We analysed the major and trace elements and the lead isotope ratios and microstructures for most samples (two Xindu site samples had no lead isotope data) to study bronze production based on alloying techniques and ore sources. Each sample was cut into two separate parts for mounting and dissolution. The samples for mounting were cold mounted in epoxy resin and then ground and polished with a Struers Tegramin-20 polish-grinding machine. A Nikon LV-100 polarising/metallographic microscope was used to observe the samples before and after etching with alcoholic ferric chloride solution (FeCl3 ) [21]. For the samples for dissolution, we first removed the samples’ corrosion and contamination and then dissolved the samples in aqua regia, diluted to 100 mL with deionised water. A Leeman Labs Prodigy inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to measure elemental compositions. The working conditions were as follows: RF power of 1.1 kW, argon plasma gas flow rate of 20 L/min, and nebuliser gas at 20 MPa. We

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Fig. 3. Photographs of typical bronze objects analysed.

measured 13 elements: Sn, Pb, Fe, Co, Ni, As, Zn, Sb, Se, Te, Ag, Au, and Bi. Three samples (BW2, BW9, BW17) were heavily corroded and their elemental results are only for reference. We measured the lead isotope ratios with a VG Elemental multicollector–inductively coupled plasma mass spectrometer (MC-ICP-MS) (VG Elemental Axiom, Thermo Fisher Scientific Inc., USA). The relative errors of the 207Pb/206Pb, 208Pb/206Pb, and 206Pb/204Pb ratios were <0.01%, 0.01%, and 0.1%, respectively. We used an SRM981 international lead isotope standard to calibrate the spectrometer. We re-measured the standard for every set of 6–8 sample measurements. 4. Results and discussion 4.1. Alloying technique development Table 2 illustrates the elemental compositions for the 37 bronze samples. We present the data, excepting the corroded samples, graphically to conduct a diachronic study (Fig. 4). Different scholars have proposed different standards of defining alloying [20]. In this study, we chose the traditional 2% cut-off as the standard to determine alloying types. Among the 37 samples, five, five, three, and 18 samples were made of pure copper, Cu-Sn, Cu-Pb, and CuSn-Pb alloys, respectively. We believe the three corroded samples were originally Cu-Sn-Pb alloys although they are heavily corroded.

The remaining three samples were Han Dynasty coins and contain more than 2% lead and tin. Moreover, antimony and silver concentrations were extremely high in the coins. The antimony and silver content were between 4.795 and 5.224 and 1.112 and 3.854 wt%, respectively. The coins show a totally different alloying type from the other objects. Overall, the tin and lead composition varied from 0.039 and 42.512 and 0.099 and 33.264 wt%, respectively. From the Western Zhou to Spring and Autumn period, all of the above alloys were seen. Tin and copper did not show a clear pattern. In the Warring States period, 72% bronze objects were Cu-Sn-Pb alloys; the alloying technique seems to have been more developed. Still, the composition of tin and lead was highly variable (Table 2). Three Han Dynasty coins showed a special alloying type with high composition of antimony and silver. The other three Han and Tang Dynasty coins included two Cu-Sn-Pb and one Cu-Pb alloys. A much larger coin sampling might reveal an alloying pattern. The present data demonstrated that the coin-production alloying was not consistent (Table 2). Fig. 4 presents a scatter plot of Pb versus Sn and illustrates the Sanxingdui bronze data. This scatter plot demonstrates that most samples belonged to two groups. One included the Warring States bronzes from the Baishoulu cemetery and had the higher lead content, and the other included most of the analysed and Sanxingdui bronzes (Fig. 4). Although the lead content difference is clear, it is still difficult to propose that there were diachronic changes in the

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Table 1 Context and type of the analysed bronze objects. Object’s code

Context number

Type

Date

BW1 BW2 BW3 BW4 BW5 BW6 BW7 BW8 BW9 BW10 BW11 BW12 BW13 BW14 BW15 BW16 BW17 BW18 BH1 BH2 BH3 BH4 BT1 BT2 BT3

M21:2 M1:2 M22:1 M2:1 M58:6 M22:3 M14:2 M10:16 M60:1 M21:1 M58:2 M22:2 M12:1 M22 M1:3 M60:4 M58-2 M58 BSL93 BSL94 BSL95 Y2 M7:1 M39-1 M39-2

Knife Axe Axe Axe Vessel Dagger-axe Dagger-axe Sword Sword Sword Sword Sword Spear Vessel Knife Axe Unknown Accessory Coin Coin Coin Coin Coin Coin Coin

Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Warring States (475–221 BCE) Han Dynasty (202 BC to 220 CE) Han Dynasty (202 BC to 220 CE) Han Dynasty (202 BC to 220 CE) Han Dynasty (202 BC to 220 CE) Tang Dynasty (618–907 CE) Tang Dynasty (618–907 CE) Tang Dynasty (618–907 CE)

XD1

2012CXC TN2E2

:1

Unknown

Western Zhou Dynasty (1046–771 BCE)

XD2

2012CXC TN2E2

:2

Unknown

Western Zhou Dynasty (1046–771 BCE)

JS1

I8106

Fragment

Middle and late Western Zhou Dynasty (900–800 BCE)

JS2

IT7114, IT7113-T7014, IT7013

Fragment

Middle and late Western Zhou Dynasty (900–800 BCE)

JS3

IT7013(7113)/IT7014(7114)

Unknown

Middle and late Western Zhou Dynasty (900–800 BCE)

JS4

IT6613-6714

:748

Fragment

Middle and late Western Zhou Dynasty (900–800 BCE)

JS5

IT6611-6712

:1-1

Vessel

Late Western Zhou Dynasty – early Spring and Autumn period (800–700 BCE)

JS6

IT6611-6712

:1-3

Vessel

Late Western Zhou Dynasty – early Spring and Autumn period (800–700 BCE)

JS7

IT7111

:5

Fragment

Late Western Zhou Dynasty – early Spring and Autumn period (800–700 BCE)

JS8

IT7107

:3

Fragment

Late Western Zhou Dynasty – early Spring and Autumn period (800–700 BCE)

JS9

IT8307

:11

Vessel

Middle Spring and Autumn period (700–600 BCE)

JS10

IT8307

:9

Fragment

Middle Spring and Autumn period (700–600 BCE)

:25 :5 :6

Fig. 4. Scatter plot of Pb versus Sn of the bronze samples from the Baishoulu cemetery, Jinsha site, Xindu site, and Sanxingdui sacrificial pits.

alloying technique used in the Chengdu Plains because we analysed different object types. While most Warring States bronzes in the Baishoulu cemetery were weapons, the Han and Tang Dynasty bronzes in the Baishoulu cemetery were coins; the Jinsha bronzes were mainly vessels and fragments; and most Sanxingdui bronzes

were vessels, masks, and decorations. It is possible that these differences (both in type and date) contributed to the different alloying techniques. Fig. 5 shows the microstructure pictures of the different period samples. The Jinsha samples all showed a dendritic microstructure

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Table 2 Results of ICP-AES analysis and ‘Copper Groups’ analysis of Baishoulu bronze samples (wt%). Object’s code

Copper Groups

Sn

Pb

Fe

Co

Ni

As

Zn

Sb

Se

Te

Ag

Au

Bi

BW1 BW2 BW3 BW4 BW5 BW6 BW7 BW8 BW9 BW10 BW11 BW12 BW13 BW14 BW15 BW16 BW17 BW18 BH1 BH2 BH3 BH4 BT1 BT2 BT3 XD1 XD2 JS1 JS2 JS3 JS4 JS5 JS6 JS7 JS8 JS9 JS10

7 7 4 4 4 7 7 4 7 4 7 4 4 1 4 4 7 7 12 12 12 12 7 7 7 4 7 7 4 4 4 7 7 7 1 7 6

12.780 42.512 12.912 11.330 12.498 17.418 17.800 11.293 27.427 12.313 19.225 5.254 17.296 1.605 14.118 14.300 22.154 16.874 4.865 4.704 8.300 0.609 8.561 9.913 8.484 1.040 0.040 5.711 0.039 0.342 1.385 3.305 1.853 8.197 2.063 10.088 6.650

1.185 7.566 6.253 8.602 2.588 5.890 8.744 1.952 33.264 2.433 5.046 1.784 0.173 0.622 5.329 2.898 7.195 9.539 11.267 10.256 10.576 23.894 20.616 11.335 4.879 3.010 1.370 13.646 0.206 0.248 0.731 15.368 22.913 7.338 0.099 7.919 5.613

0.444 0.289 1.474 0.199 0.238 1.000 0.170 0.481 0.458 0.561 0.222 0.219 0.354 0.052 0.162 0.358 1.627 0.348 2.753 2.550 0.856 1.236 1.441 0.646 1.408 0.090 0.010 0.204 0.119 0.042 0.106 0.108 1.756 0.070 0.214 0.188 0.220

0.000 0.000 0.000 0.001 0.001 0.001 0.001 0.000 0.001 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.001 0.001 0.003 0.002 0.002 0.002 0.002 0.007 0.005 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.002 0.002 0.001 0.001

0.002 0.002 0.004 0.005 0.005 0.008 0.005 0.004 0.002 0.002 0.005 0.002 0.001 0.001 0.004 0.007 0.021 0.006 0.013 0.007 0.004 0.004 0.004 0.011 0.007 0.000 0.010 0.002 0.001 0.002 0.001 0.001 0.002 0.004 0.002 0.002 0.002

0.019 0.013 0.004 0.024 0.008 0.004 0.016 0.006 0.009 0.003 0.021 0.004 0.003 0.001 0.009 0.023 0.031 0.014 0.119 0.113 0.098 0.178 0.036 0.062 0.010 0.040 0.070 0.024 0.002 0.001 0.004 0.015 0.051 0.026 0.002 0.007 0.183

0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003 0.002 0.000 0.000 0.001 0.000 0.002 0.003 0.001 0.003 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.001 0.000 0.003

0.111 0.437 0.018 0.047 0.067 0.304 0.137 0.050 0.331 0.056 0.135 0.006 0.076 0.016 0.073 0.000 1.766 0.218 5.224 4.967 4.795 0.099 0.358 0.623 0.140 0.070 0.110 0.144 0.003 0.011 0.025 0.096 0.122 0.303 0.015 0.106 0.243

0.000 0.000 0.000 0.000 0.052 0.024 0.000 0.000 0.000 0.000 0.112 0.046 0.000 0.000 0.094 0.064 0.005 0.000 0.023 0.054 0.000 0.025 0.002 0.000 0.000 0.000 0.000 0.015 0.023 0.010 0.027 0.005 0.015 0.144 0.011 0.026 0.004

0.033 0.027 0.030 0.029 0.027 0.022 0.021 0.031 0.022 0.025 0.011 0.018 0.064 0.002 0.029 0.000 0.036 0.018 0.044 0.012 0.051 0.020 0.010 0.029 0.030 0.040 0.040 0.016 0.015 0.010 0.015 0.018 0.012 0.000 0.000 0.001 0.006

0.121 0.591 0.170 0.260 0.197 0.313 0.293 0.169 0.480 0.359 0.321 0.241 0.153 0.016 0.249 0.114 0.303 0.306 2.828 3.854 1.112 0.364 0.177 0.486 0.207 0.230 0.170 0.311 0.196 0.207 0.214 0.376 0.439 0.149 0.091 0.120 0.039

0.002 0.003 0.000 0.000 0.002 0.002 0.000 0.000 0.004 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.002 0.001 0.041 0.000 0.000 0.024 0.002 0.004 0.001 0.002 0.005 0.012 0.006 0.008 0.001

0.037 0.055 0.055 0.058 0.049 0.038 0.038 0.035 0.040 0.050 0.059 0.044 0.047 0.003 0.046 0.055 0.000 0.033 0.072 0.071 0.094 0.021 0.057 0.081 0.042 0.090 0.050 0.193 0.015 0.025 0.008 0.039 0.045 0.002 0.004 0.032 0.056

with lead and Cu-S inclusions, suggesting casting [21,22] (Fig. 5: JS1, JS2). Sample JS1 showed signs of being heated after casting (Fig. 5). Many factors could contribute to this phenomenon, including ritual firing, which was mostly seen in Sanxingdui [25], or burning a cooking vessel. However, JS1 is a small fragment of uncertain type, making it difficult to interpret. From the Xindu site, XD1 showed ␣ grains with Cu-S inclusions and was also cast. Among all analysed samples, there was no sign of cold-working technique until the Warring States period. The microstructures of the Warring States samples in the Baishoulu cemetery included two types. The first is a microstructure with equiaxed grains, annealing twins, and slip lines, suggesting coldworking and annealing techniques [21,22] (Fig. 5: BW5). The second is a dendritic microstructure with Cu-Sand lead inclusions and (␣ + ␦) eutectoid, which is typical casting microstructure (Fig. 5: BW6, BW14). The cast bronze objects included both weapons and vessels. The only cold-worked and annealed object was a vessel rarely seen in the Shu state but typical of Chu-style vessel [23]. Moreover, many vessels in the Chu state also showed signs of cold working [24]. Therefore, both the style and technique belong to the Chu state in the middle Yangtze River. It is unknown whether it was imported or made locally by Chu craftsmen. All coin samples of the Han and Tang Dynasty showed typical casting microstructures (Fig. 5: BH1, BT2). Describing the development of alloying techniques through time or different techniques applied to different object types remains difficult. It is necessary to collect more data before reaching a conclusion on alloying techniques. However, we did observe that bronzes from each period were measured with lead and tin, suggesting that the Chengdu people recognised very early the

importance of these two materials. Casting played a major role in all sampled periods. 4.2. Ore sources used in different periods We measured trace elements and lead isotope ratios to study the possible ore sources used in the Chengdu Plains. We utilised published data on the Sanxingdui bronzes’ trace elements by Ma et al. [25] and lead isotope by Jin [26] as references to represent the late Shang period bronzes in the Chengdu Plains. Table 3 presents the lead isotope ratios for the bronzes we analysed. Some scholars have posited that the lead is introduced by copper ores when the lead composition is between 50 ppm and 4% [27–29]. Stech argued that determining the intentional mixture of metals or impurities in ores when the element’s composition is less than 5% is difficult [30]. Since only one sample contained lead (4.88%) between 4% and 5%, both standards are applicable in this study; we chose the 4% standard. Twelve of the 35 measured samples fell below the threshold, between 0.1% and 2.9%. The remaining 23 samples’ lead composition varied from 4.88% and 33.26%. We compared two sets of LIA data, and most data clustered in the same areas (Fig. 6c and d). The samples with Pb ≥ 4% were distributed in a relatively larger area, indicating the lead sources. For the samples with Pb < 4%, it is still difficult to reach a conclusion. Fig. 6a and b shows the graphical results of LIA data. Interestingly, the Jinsha and Warring States samples in the Baishoulu cemetery were clearly plotted in different areas, suggesting that different lead sources were used. However, one Warring States sample was distributed in the Jinsha area. Coin distribution is complicated. At least two Han and one Tang Dynasty coins might be

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Fig. 5. Photomicrograph of the analysed samples.

made with the Warring States’ and Jinsha lead sources, respectively, suggesting that lead sources of Jinsha and Baishoulu Warring States samples were potentially used in later periods. Two samples fell out of the discussed areas (Fig. 6a and b). One is a Warring States knife with high radiogenic lead content. The knife might have been re-melted from the Sanxingdui bronzes, which also showed high radiogenic lead. The other was a Tang Dynasty coin, whose different lead source might indicate that was exchanged from other regions. Compared to the Sanxingdui LIA data, the differences are stark. Most of the Sanxingdui samples showed high radiogenic lead and the ratio of 206 Pb/204 Pb was greater than 20. There was no overlap with the analysed samples in this study except for the Warring States knife. We did not present the comparison graphically

because plotting the hugely different sets of data would make it difficult to distinguish the newly analysed data. The high radiogenic lead was quite unique and mostly found in the Late Shang period (14th century to 1046 BCE). In the capital of Late Shang Dynasty Yinxu, as well as in indigenous cultures along the Yangtze River, such as Sanxingdui, Ningxiang, and Xingan, high radiogenic lead was common [26]. The Sanxingdui LIA data were only published together with thirteen elemental data, four of which had lead content greater than 4%. At least some of the high radiogenic lead data indicated lead sources. Therefore, Sanxingdui bronze objects must have used lead of different source from the Jinsha and Baishoulu objects. Table 2 presents the trace elements for all the analysed samples included in this study. Arsenic, antimony, silver, and nickel

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Table 3 Results of lead isotopic ratios of Baishoulu bronze samples. Object’s code

208

Pb/204 Pb

BW1 BW2 BW3 BW4 BW5 BW6 BW7 BW8 BW9 BW10 BW11 BW12 BW13 BW14 BW15 BW16 BW17 BW18 BH1 BH2 BH3 BH4 BT1 BT2 BT3 JS1 JS2 JS3 JS4 JS5 JS6 JS7 JS8 JS9 JS10

40.960 38.164 38.492 38.529 38.971 38.455 38.452 38.921 38.509 38.355 38.340 38.405 38.854 38.397 38.445 38.493 38.363 38.221 38.816 38.842 38.642 38.729 38.141 38.498 36.855 37.780 37.644 38.362 38.442 38.019 37.908 38.724 38.374 37.703 38.774

207

Pb/204 Pb

15.888 15.515 15.638 15.617 15.715 15.569 15.588 15.715 15.668 15.562 15.556 15.575 15.701 15.581 15.579 15.601 15.541 15.519 15.674 15.680 15.631 15.632 15.502 15.613 15.275 15.514 15.354 15.522 15.579 15.459 15.369 15.583 15.593 15.415 15.623

206

Pb/204 Pb

20.650 17.618 18.106 18.118 18.619 17.855 17.952 18.550 18.017 17.715 17.754 18.294 18.532 17.900 17.936 17.956 17.749 17.570 18.650 18.692 18.269 18.212 18.038 18.085 16.617 17.444 17.527 18.161 18.309 17.889 17.804 18.511 18.117 17.489 18.524

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Pb/206 Pb

1.984 2.166 2.126 2.127 2.093 2.154 2.142 2.098 2.157 2.165 2.159 2.099 2.097 2.145 2.143 2.144 2.161 2.175 2.081 2.078 2.116 2.128 2.115 2.129 2.220 2.166 2.148 2.112 2.100 2.125 2.129 2.092 2.118 2.156 2.093

207

Pb/206 Pb

0.769 0.881 0.864 0.862 0.844 0.872 0.868 0.847 0.869 0.878 0.876 0.851 0.847 0.870 0.869 0.869 0.876 0.883 0.840 0.839 0.856 0.858 0.859 0.863 0.919 0.889 0.876 0.855 0.851 0.864 0.863 0.842 0.861 0.882 0.843

Fig. 6. Diagram of the lead isotope data of analysed objects. (a, b) Present data of different locations and date; (c, d) present data of different lead compositions.

were the most suitable elements for studying ore sources [31]. Silver and nickel often indicate ore characteristics, while arsenic and antimony often indicate ore types [32]. However, to conduct a diachronic study based on trace elements, a classification method was much needed for comparing data of different periods. Recently, the research team led by Mark Pollard proposed an Oxford system, which included metal chemistry, alloys, and

lead isotopes to study the circulation of copper and copper alloys [33,34]. The ‘Copper Groups’ method in this system was designed to study trace elements. Sixteen ‘Copper Groups’ were defined by the presence/absence (0.1% cut-off) of four trace elements above. The different groups provided a preliminary copper classification. To interpret different groups, sufficient archaeological study is needed. Instead of interpreting a single group, we focused on a

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Fig. 7. Illustration of the combination pattern of ‘Copper Groups’.

combination of different groups. If analysis showed that bronze objects of five different locations were all made from four types of material (Copper Groups 1–4), then it is likely that they were made in the same workshops, or the metal resources were managed and circulated by a central power (Fig. 7). Based on our study, we were surprised to discover that data of the Xindu, Jinsha, and Warring States bronzes in the Baishoulu cemetery all showed the same pattern—CG4(Ag) and CG7(Ag + Sb) were the two dominant groups (together constituting 80%–100%). The percentage of these two groups was similar (Table 4). Other groups, like CG1(clean metal) and CG6(As + Sb), constitute only a small proportion (Table 4). To understand what kind of metal sources the ‘Copper Groups’ indicates, we divided the data into two groups (Pb < 4 wt% and Pb ≥ 4 wt%). It is noteworthy that CG4 dominated the group Pb < 4 wt% while CG7 composed the largest part of the group Pb ≥ 4 wt%. Only one CG7 and three CG4 samples were in groups Pb < 4 wt% and Pb ≥ 4 wt%, respectively. The difference between

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CG4 and CG7 was the presence of antimony. The antimony was likely introduced by lead, contributing to CG7. Therefore, CG7 might have been influenced by lead. For CG4, most samples were pure copper and Cu-Sn alloys. The addition of tin did not seem to change the ‘Copper Groups’. The CG4 probably indicates copper source. Table 4 shows that the Chengdu Plains copper source, CG4, remained the same from the Western Zhou to the Warring States period. For the copper source to remain the same over this extended period probably indicates that a local source was used. However, CG7 was also constantly present. This group was influenced by lead, but the lead sources used varied over time. ‘Copper Groups’ and LIA data did not agree. Our hypothesis is that different lead sources used in this period might share a chemistry feature—antimony’s relative enrichment. Therefore, the addition of lead from different sources resulted in CG7. A comprehensive ‘Copper Groups’ study including bronzes from major sites in the Chinese Bronze Age has been published [34]. On comparing the data of Chengdu Plains with other regions, we find that the combination of CG4 and CG7 is unique (Table 4). The four Han and three Tang Dynasty coins were all CG12(As + Sb + Ag) and CG7, respectively. These data are difficult to interpret. The ‘Copper Groups’ analysis of the Sanxingdui bronzes was based on published data [25]. The results showed that CG1(clean metal) and CG2(As) were the dominant groups, with no CG4 or CG7 examples (Table 4). The Sanxingdui bronze showed more connections with the Central Plains bronzes like Yinxu. Therefore, the copper used for the Sanxingdui bronzes was probably different from the possibly local copper sources used later. We also tried to observe a connection between LIA and ‘Copper Groups’ because the Jinsha and Warring States samples in the Baishoulu cemetery showed different lead isotope features but the same ‘Copper Groups’ pattern. We found no clear connection between LIA and trace element. It is reasonable to discuss lead sources and copper sources separately based on different datasets. No further explanation can currently be provided. Based on the interpretation of LIA and trace element data, we highlight two interesting points. First, from the Western Zhou Dynasty to the Warring States period, different lead sources were used, while the copper source probably remained the same (CG4). The people in the Chengdu Plains likely acquired lead and copper sources separately. The situation in the Han and Tang Dynasties

Table 4 Percentage results of ‘Copper Groups’ analysis and comparison with data from other Bronze Age sites. The grey areas represent the proportionally largest groups. ‘Copper Groups’ results of Sanxingdui, Yinxu, Metropolitan Western Zhou, and Dajing are cited from Pollard et al. [34].

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is unclear because coins circulate. However, the LIA data showed the possibility that the same lead sources were used in much later times. Second, both LIA and trace element data of the Sanxingdui bronzes showed marked differences with the Western Zhou Dynasty to Warring States period data. Both the lead and copper sources of Sanxingdui were different from later period metal sources, providing some evidence to pursue the Sanxingdui bronzes’ provenance. The Sanxingdui bronzes seem to appear suddenly with no technique base. Although many Sanxingdui bronze types are unique, some basic elements like motifs are still copies or variations of Shang bronzes. Since the lead and copper differed from later locally made objects, Sanxingdui bronzes were possibly not locally made, or the metals came from other regions. Addressing the different style of the Sanxingdui bronzes is complicated. Systematic observations on typology, motif, casting, and metal sources are needed to solve this question. We plan to explore this in future studies. Another project is the exact location of the lead and copper sources. The Chengdu Plains are adjacent to mountain areas in the west, such as the Longmen mountain belt, rich in many ore sources, especially copper. Ancient mining pits and slag have often been found in this region, and some have presumed it to be the major copper source of local bronze production in the Bronze Age and historical period [35,36]. However, the mining and smelting remains’ date is unclear, and proper chemistry and lead isotope data are lacking; therefore, the comparison study cannot be conducted. We plan to carry out a systematic survey near the Chengdu Plains to discover more archaeological evidence and scientific data from the ore sources and conduct a comprehensive study.

5. Conclusions We analysed 37 bronze samples from different periods to explore the development of bronze production in the Chengdu Plains. The results indicated that both tin and lead were widely used in the region. The alloying techniques were similar from the Western Zhou period to the Tang Dynasty, excepting for the Warring States bronzes uncovered in the Baishoulu cemetery. Whether the type or date differences contributed to the different alloy techniques remains undetermined. Casting was widely used across object types and periods. The cold-working and annealing technique was seen only in an exotic vessel. The LIA data suggested that the lead source used from the Western Zhou to Spring and Autumn periods was different from the Warring States period. However, the ‘Copper Groups’ study using trace elements indicated the same copper source was continually used from the Western Zhou to Warring States period. These lead and copper sources provided a material standard of local production. This standard could be used to solve many remaining puzzles. For instance, a substantial amount of bronzes with exotic features, such as the Chu- and Qin-state-style bronzes, were discovered in the Chengdu Plains. It is difficult to visually identify whether these items were directly circulated there or imitated locally. It was possible to address this question based on the material standard built in this study. After the Warring states period, LIA data showed the possibility that some coins might have been made from the same lead sources. However, this is an extremely preliminary assumption considering the circulation of coins. Comparing the later bronzes with the Sanxingdui bronzes, there is no connection, suggesting that the Sanxingdui bronzes might not have been locally made or the metal sources came from outside. The results from this study provide support for diachronic metallurgical studies in a closed region, as such research could

provide new clues for provenance studies and topics other than the development of bronze production. For instance, the coldworked and annealed Chu-style vessel showed similar lead isotope and ‘Copper Groups’ pattern with other local-style objects. Outside style and technique were combined with local materials. We can assume that it might have been made by Chu craftsman in the Shu state. Such hypotheses could be proved with more data in the future. Diachronic studies should also be conducted in other cultural regions. This is only preliminary research, and bronze production after the Warring States remains unclear. The Chengdu Plains played an increasingly important role in China after the Warring States period. Thus, it is necessary to analyse more bronze objects and combine evidences of ore sources to improve this research.

Funding The authors are thankful to the team led by Prof. Mark Pollard (University of Oxford) for their contribution on the Oxford System, which enabled the interpretation in this research. This work was supported by the China Postdoctoral Science Foundation [grant number 19YJC780001] and the Humanity and Social Science Youth Foundation of the Ministry of Education of China [grant number 17XJC780001].

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