Abstract Casting had symbolic significance and was strictly controlled in the Shang dynasty of ancient China. Vessel casting was mainly distributed around the Shang capital, Yin Ruins, which indicates a rigorous centralization of authority. Thus, for a casting mold to be excavated far from the capital region is rare. In addition to some bronze vessel molds excavated at the Buyao Village site, another key discovery of a bronze vessel mold occurred at Daxinzhuang. The Daxinzhuang site was a core area in the east of Shang state and is an important site to study the eastward expansion of the Shang. Here, combining synchrotron X-rays and other physicochemical analysis methods, nondestructive three-dimensional structure imaging and different elemental analyses were conducted on this mold sherd. Through high penetration X-ray tomography, we obtained insights on the internal structure and discovered some pores. We infer that the generation of pores inside the casting mold sherd was used to enhance air permeability during casting. Furthermore, we suppose that the decorative patterns on the surface were carved and not pasted onto it. Considering the previous compositional studies of bronze vessels, the copper and iron elements were analyzed by different methods. Unexpectedly, a larger amount of iron than of copper was detected on the surface. According to the data analysis and archaeological context, the source of iron on the casting mold sherd could be attributed to local soil contamination. A refined compositional analysis confirms that this casting mold was fabricated locally and used for bronze casting.

Citation: Zong Y, Yao S, Lang J, Chen X, Fan J, Sun Z, et al. (2017) Structural and compositional analysis of a casting mold sherd from ancient China. PLoS ONE 12(3): e0174057. https://doi.org/10.1371/journal.pone.0174057 Editor: Gayle E. Woloschak, Northwestern University Feinberg School of Medicine, UNITED STATES Received: July 20, 2016; Accepted: March 2, 2017; Published: March 15, 2017 Copyright: © 2017 Zong et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by the National Natural Science Foundation of China (31430031, U1332118), the National Social Science Foundation of China (14BKG009), Humanities and Social Science Youth Team Project, SDU (IFYT1505) and the Fundamental Research Funds of Shandong University (2014JC007). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction The origin of the Bronze Age in China may date back to the Majiayao culture [1,2], whereas the fabrication and use of bronze vessels prevailed in the Shang dynasty [3]. In northern China, many bronze vessels have been discovered around Yin Ruins, the capital of the later Shang dynasty [4,5]. Bronze vessel production seems to have been concentrated in the capital region and that these reported in the paper are the first vessel casting mold remains discovered outside of Anyang for the Anyang period [6,7]. In 2007, several bronze molds and vessels were excavated at the Buyao Village site [8,9]. Bronze vessels were highly significant markers of status and crucial media for ritual communication with the ancestral powers, and so the discovery of mold fragments outside of the Great Settlement Shang is highly significant. Daxinzhuang is east of Yin Ruins and existed in the middle or late Shang dynasty (Fig 1) [10–13]. It is located downstream of the Yellow River and is famous for the excavation of the Oracle Bone inscriptions of the Shang dynasty that have been found outside of Yin Ruins [14]. Daxinzhuang is a typical central territorial settlement and is one of the largest sites in the Haidai area [15,16]. The Daxinzhuang site is considered an important core area in the eastern region of Shang state. Moreover, these casting mold sherds may give clues of eastward expansion in the Shang regime [17,18]. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 1. Map of the Shang Dynasty and location of Daxinzhuang. Both Yin Ruins and Daxinzhuang sites are marked in the map. Reprinted from [11] under a CC BY license, with permission from [Lamassu Design], original copyright [2009]. https://doi.org/10.1371/journal.pone.0174057.g001 A key point in the use of casting molds is that bronze vessels could not be produced without molds in ancient China [4,19,20]. The manufacturing process of casting molds has been previously discussed [21–23]. As the process of casting is using a mold which contains a hollow cavity of the desired shape, in which the liquid metal is poured into to get a bronze ware after solidification. By referring to the decoration on the surface of the casting molds, archaeologists usually adopt the morphology method [5,19,20]. To determine the decorative structure and manufacturing process of the mold, X-ray computed tomography (XCT), a widely used nondestructive method in archaeology [24–27], is used to investigate a casting mold sherd. X-ray fluorescence mapping (XFM) is a two-dimensional elemental characterization method that obtains elemental mapping on the surface of materials [21,28–31] and is used to confirm a casting mold’s origin and whether it has been used. Inductively coupled plasma atomic emission spectrometry (ICP-AES) traces elements at a higher precision [32–34]. X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS) are other methods that are generally used for compositional analysis [35–38]. In this work, different methods compensated for one another to complete a final structural and compositional analysis.

Materials and methods The casting mold sherd in this study comes from the Daxinzhuang site, which is located in the eastern part of Shang state. Although large amounts of bronze wares have been excavated here, few casting molds have been found. Importantly, a piece of casting mold with fine decoration was obtained. The casting mold sherd (sample number: 2010DXZJ9T2024H900②:18) was excavated in pit H900 at Daxinzhuang site and other remains including pottery sherds and animal bones were also excavated (S1 File). We infer that it is a vessel casting mold sherd from the arc-shaped front and back (Fig 2E and 2F), because the molds used to create bronzes with a shape of cambered surface. To confirm the age of the casting mold sherd, a pig bone found in the same excavation unit was used for standard AMS-14C dating in a Beta Analytic laboratory (S1 Fig). We also collected and studied the excavated pottery pieces that were made in local styles of the Daxinzhuang site to confirm the elements that are present in the casting mold sherd (S2 Fig). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 2. Different views of the casting mold sherd. (A-F) Six faces of the casting mold sherd. https://doi.org/10.1371/journal.pone.0174057.g002 Synchrotron X-ray tomographic microscopy was used to study the three-dimensional structure of the casting mold sherd, particularly to understand the relation between the decoration and the casting mold sherd matrix. Because of its short wavelength, X-rays have strong penetration ability and allow high-resolution imaging. Compared with laboratory X-ray sources, synchrotron X-rays have a higher flux, brightness and signal-to-noise ratio. To confirm the composition of the casting mold sherd, XPS, ICP-AES, EDS and XFM were conducted. Before these tests, the outermost layer of the entire mold was cleaned using a soft hairbrush with ethyl alcohol to eliminate external contamination. The sampling position was located in the back of the casting mold sherd to avoid the possible influence of the casting elements that are present in the decorative side. To further avoid some other surface contamination, the near-surface region whose depth was about 1mm was sampled. Then a knife was used to scrape the powder from the casting mold sherd. 0.2g, 0.1g and 0.01g of powder were respectively prepared for XPS, EDS and ICP-AES experiment. XFM was performed to directly detect the casting-related elemental information on the decorative side.

Conclusions In this work, we investigated one casting mold sherd that was excavated from Daxinzhuang. The standard AMS-14C dating confirms that it is a mold from the Shang dynasty. The nondestructive X-ray structural and technological analysis shows that this casting mold was carefully designed for casting. The pores that were discovered inside indicate that the people at the time were aware of the air permeability issue for casting molds. The three-dimensional structure analysis also shows that the decorated patterns on the surface were carved into the mold. Comprehensive elemental analysis methods combined with other excavations in Daxinzhuang prove that this mold was made locally and used for bronze vessel casting. By referring to previous excavations of objects connected with the royal capital, such as bronze articles, jadeware and oracular bones, the casting mold sherd could verify that the Daxinzhuang site was an important regional center in the late Shang state. This clue helps archaeologists to understand the cultural prosperity that was downstream of the Yellow River 3,000 years ago. However, more excavations will be helpful and necessary to reveal more details about the expansion of the late Shang state.

Acknowledgments We thank Drs. Hongliang Zheng, XinXin Yuan, Xiangfeng Guan, Xiao Long and Yu Liu for their technical assistance and beneficial discussion. We also thank Drs. Xiaoqiong Liu and Bo Liu for their constructive advice on figure revisions. This work was supported by the National Natural Science Foundation of China (31430031, U1332118), the National Social Science Foundation of China (14BKG009), Humanities and Social Science Youth Team Project, SDU (IFYT1505) and the Fundamental Research Funds of Shandong University (2014JC007).

Author Contributions Conceptualization: HDJ TQX HF. Formal analysis: YBZ XLD NNL. Funding acquisition: HDJ XXC. Investigation: YBZ SKY JDF GZZ AGL ZBS. Methodology: YBZ HDJ JFL. Project administration: HDJ. Resources: JFL XXC. Software: YBZ. Writing – original draft: YBZ HDJ. Writing – review & editing: YBZ HDJ.