Archaeological and Anthropological Sciences (2019) 11:6603–6613 https://doi.org/10.1007/s12520-019-00929-x ORIGINAL PAPER Copper granulation: scientific analysis on the ornaments from the coronet of Lady Pei of the early Tang Dynasty (618–712 A.D.) in Xi’an, Shaanxi, China Panpan Tan 1 & Junchang Yang 2 & Yaozheng Zheng 1 & Junkai Yang 3 Received: 31 January 2019 / Accepted: 27 August 2019 / Published online: 7 September 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Granulation is an ancient decorative technique for metalwork. Granulated finds are most commonly made of precious metals such as gold and silver, while copper granulation is very rare, and it has even been claimed that it was not employed in antiquity. This paper presents a scientific analysis of granulated copper ornaments from a gilded coronet belonging to Lady Pei (d. 691 A.D.) that was uncovered from an early Tang Dynasty tomb in Xi’an, China. In addition to common methods such as X-ray radiography, metallographic investigation, and scanning electron microscope observation including energy dispersive spectroscopy to examine the microstructure and elemental composition, X-ray micro-computed tomography was used to investigate the internal microstructure of granulation for the first time. The results affirm that copper granulation technique was used to decorate the coronet. Notably, further study shows that the copper granules and wires were joined to the substrate with silver–copper alloy hard solder and arrives at an estimate of the alloy mixing ratio. These observations are used to infer the manufacturing process: copper granules of two sizes were first brazed to the wires and substrate, and then mercury was used to gild the whole ornament. This work provides new insights for the study of ancient granulation techniques, especially through the scientific study of the very rare examples of copper work. Keywords Granulation . Copper . Silver–copper alloy solder . Mercury gilding . Tang Dynasty . μ-CT Introduction Granulation is an ancient decorative technique used in jewelry and metalwork in which tiny solid metal grains are applied to a metal surface or to each other. The materials used for ancient granulation are usually precious metals such as silver and, especially, gold. Both the earliest evidence of granulation (ca. 2500 B.C.) (Maxwell-Hyslop 1977) and the finest such work (7th–6th centuries B.C.) (Higgins 1962) were made of * Junchang Yang yangjunchang@nwpu.edu.cn 1 State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China 2 Institute of Culture and Heritage, Northwestern Polytechnical University, Xi’an 710072, China 3 Xi’an Institute of Conservation and Archaeology on Cultural Heritage, Xi’an 710068, China gold. The granule size is one of the criteria used to evaluate the refinement of the technique. The Etruscans were skilled at creating gold dust granulation with invisible joints, reaching a granule diameter as small as 0.07 mm (Nestler and Fomigli 2010). In China, granulation produced during the Tang Dynasty (618–907 A.D.) is comparable with Etruscan dust granulation, with granules of as little as 0.11 mm in diameter appearing on its gold artifacts (Yang et al. 2014a). This represents the highest level of technological expertise with gold and silver employed in the Tang Dynasty. In 2002, a group of exquisite ornaments was unearthed in a joint burial in the east of Xi’an (Shaanxi province) belonging to Yan Shiwei (d. 699 A.D.), a middle-rank Tang Dynasty official, and his wife Lady Pei (d. 691 A.D.) (Yang et al. 2014b). The metal items were scattered around the head of the female body, suggesting they were the ornamental components of a coronet (Fig. 1). After basic cleaning, a preliminary hypothetical reconstruction of the object shows that the coronet consist of about 300 ornaments and fragments made of a variety of mediums, including glass, pearl, semi-precious stone, feather, and textile (Fig. 2). Among them, metal 6604 Archaeol Anthropol Sci (2019) 11:6603–6613 Fig. 1 The coronet before cleaning. a General view of the metal ornaments. b Details of small fragmentary ornaments with corrosion ornaments in the shapes of wings, apricot leaves, and commas belong to the hierarchical system in the use of personal adornment for high-ranking females during the Tang Dynasty (Liu 1975). The numbers of these kinds of ornaments were ranked by the sumptuary laws and indicated the status of the owner. Of the ornaments, twenty-one (two wing-shaped, four apricot leaf-shaped, two comma-shaped, three rectangular-shaped, and ten flower-shaped) are decorated with tiny granules and embellished with pearls and semi-precious stones, indicative of sophisticated craftsmanship. More strikingly, the small granules of some of the ornamental parts appeared to be not gold but gilt copper (Fig. 3), which is rarely found in ancient Asia. However, many of the coronet ornaments were very corroded, hindering their study, analysis, and interpretation. Previous studies of granulation were often concentrated on gold and silver. Prior to the twentieth century, scholars and goldsmiths attempted to restore the ancient craft of granulation by means of experimental practice to imitate historical gold and silver jewelry (Hackens and Winkes 1983). A few successes revealed that the technique for manufacturing granulation consisted of three basic processes, namely, granule production, positioning with an organic adhesive, and joining (Maryon 1949; Higgins 1962; Wolters 1981; Hackens and Winkes 1983; Nestler and Fomigli 2010). Granules can be produced by various methods such as heating small metal plates or wire cuttings laid separately on a charcoal block and pouring the molten metal into water or a bowl filled with powdered charcoal from a certain height. The most efficient way to create large numbers of spherical granules is the crucible method, that is, to place alternately metal strips into different layers of powdered charcoal in a crucible until it is full, then to heat the crucible until the metals melt to form global grains. Organic adhesive (animal or vegetable glue) Fig. 2 The coronet after basic cleaning. a The coronet comprises a variety of ornamental elements. b Glass bird-shaped ornament. c Flower-shaped ornament decorated with pearl. d Metal rosette with precious stones and pearl inlay. e Traces of feather on the back of a rosette-shaped ornament. f Traces of textile on the back of a flower-shaped metal ornament. g Gilding on the back of a corroded metal ornament Archaeol Anthropol Sci (2019) 11:6603–6613 6605 Fig. 3 Typical photographs of metal ornaments decorated with granules. (a) Photograph of HDP1. (b) Micrograph of the granulated area of HDP-1. (c) Photograph of HDP-10. (d) Micrograph of the granulated area of HDP-10 is used to fix the granule before heating, then carbon monoxide, the reaction product, prevents oxidation of the materials being joined. Joining encompasses three main methods, which were brazing with metal alloys, surface welding, and copper-salt diffusion. With the popularization and development of analytical technology, several scientific methods, particularly non-destructive methods, have been applied to the study of ancient granulated artifacts (Huang et al. 2009; Schlosser et al. 2012; Yang et al. 2014a; Ro and Yu 2016; Tan et al. 2016; Ashkenazi et al. 2017; Scrivano et al. 2017) and have enabled a deeper understanding of this technique. However, these nondestructive analyses mainly focus on the acquisition of information about the surface or the near-surface. Furthermore, the use of other metals, such as granulated copper artifacts, has received little attention. No historical text recorded the specific manufacturing technique used for copper granulation. It has even been claimed that there was no copper granulation in antiquity (NatuniewiczSekuła 2017). However, a report regarding copper granule decoration on hairpins belonging to Empress Xiao (Dang et al. 2018) indicates that copper granulation was already in existence in the seventh century in ancient Asia. The coronet of Pei is very likely another example of copper granulation. In this paper, multi-analytical methods including X-ray radiography, metallographic investigation, and scanning electron microscopy including energy dispersive spectroscopy (SEM-EDS) were used to study the structure and chemical compositions of granulated ornaments from the Pei coronet, based on which the manufacturing techniques were disclosed. Particularly, X-ray micro-computed tomography (μ-CT) was used for the first time to analyze the internal microstructure of the granulation, as this technique is ideally suited to the noninvasive investigation of small items. Samples and methods Samples Three representative samples were selected from different kinds of granulated ornamental elements from the coronet and labeled GCG001, GCG002, and GCG003 (Fig. 4). GCG001 is a complete wing-shaped ornament. A row of pearls decorates the frame, and semi-precious stone, pearl, and metal wire comprising flowers decorate inside the frame. It measures 16.4 cm in length, a maximum of 5.5 cm and a minimum of 2.6 cm in width, and a maximum of 0.4 cm in thickness. GCG002 is the triangular tail-end of another wing-shaped ornament. Three pearls are inlaid at its center, and tiny granules decorate along the two edges. It measures 1.2 cm in width and 1.3 cm in height. GCG003 is a small piece taken from the tail of the edge of a flower-shaped fragment with semi-precious stone and pearl inlay. Granules run along the edge of the ornament, which measures 1.4 cm in length and 0.8 cm in maximum width. All of the three samples are severely corroded. Nondestructive techniques were applied to GCG001 and GCG002 to determine the basic structure of the ornaments, the distribution of granules, and the internal microstructure of the granulation. GCG003 was subjected to metallurgical investigation and chemical compositional analysis, to reveal the nature of the materials and the metallurgical manufacturing processes involved in their production. Archaeol Anthropol Sci (2019) 11:6603–6613 6606 Fig. 4 The three granulated experimental samples from the coronet of Pei Methods Results X-ray radiography Granule distribution GCG001 was tested with a GILARDONI ART—GIL350/6 X-ray detector with energy settings of 160 kV and electric current settings of 4.5 mA; the working distance was 80 cm. The digital imaging system is a DUERR CRNet/HD-CR 35 NDT Plus, and the imaging plate (IP) is HD-IP Plus 10 cm × 14 cm, with a pixel size of 15 μm. The X-ray image in Fig. 5 reveals the basic structure and granule distributions of GCG001. The framework and patterns are created out of wire, and it features types of granule ornamentation. The first type is linear granulation: a single row of granules running along the border wire. The second is field granulation, in which countless granules fill in the background. The granules in both of these areas are regular and uniform. A total of 320 granules were counted from the linear and field granulations in the X-ray image. According to the imaging principle of X-ray radiography, the relationship between the image size and the item size follows the formula (Qiang 2007): μ-CT GCG002 was scanned with a CD-130BX μ-CT (Chongqing Zhence Science and Technology Co. Ltd.) set to an energy of 150 kV and an electric current of 66 μA. The charge-coupled device (CCD) detector has a spatial resolution of 3.0 μm, an exposure time of 39 ms, and a 360-degree scanning angle. Metallographic investigation and SEM-EDS GCG003 was cold-mounted, ground, and polished following standard metallographic procedure. The polished section was then etched with alcoholic ferric chloride solution (FeCl3+ HCl+C2H6O) to reveal its metallographic structure. The sample was examined and photographed with both a ZEISS optical microscope and a ZEISS EVO MA 25 SEM. Chemical compositional analysis was carried out in the SEM with an Oxford X-max 20 EDS. The operating conditions were an accelerating voltage of 20 kV and a working distance of approximately 8–9 mm. W 0 ðL1 þ L2 Þ ¼ W L1 ð1Þ where W′ is the image size, W is the item size, L1 is the working distance, and L2 is the distance from the item to the IP. As GCG001 was placed directly on the IP, L2 can be approximated to the thickness of GCG001, making L2 ≪ L1. Thus, W ′ ≈ W: the granule size in the X-ray image is approximately that in the sample. The distributions of the approximate diameters of the linear and field granules are shown in Fig. 6. The measured diameters of the linear granulation are distributed around 0.85 mm, while those of field granulation range around 0.65 mm, indicating that these two kinds of granules were pre-designed to decorate different parts. In addition, fillers can be seen in the joining areas of the wires, indicating that the components were bonded by soldering or brazing (Fig. 5 inset). Archaeol Anthropol Sci (2019) 11:6603–6613 6607 Fig. 5 The X-ray image of GCG001 Granulation materials and techniques CT provides both a 2D cross-sectional image and a 3D numerical model reconstruction and can thus reveal more detailed information about the inner microstructure of the sample than can X-ray radiography (Bernardini et al. 2018). However, the principles underpinning CT mean that larger and denser metal objects require higher energy, which comes at a cost in terms of resolution (Bettuzzi et al. 2015; Ding et al. 2017). Compared with CT in general, μ-CT has a higher spatial resolution (lower energy and electric current) and is specifically suitable for investigating items within a small volume (in the order of millimeters) (Rossi et al. 2004). A small part of GCG002 was scanned by μ-CT to obtain higher resolution and more detailed information about its internal characteristics (Fig. 7a). The μ-CT slice in Fig. 7b shows the internal structure and materials of the scanned part of GCG002. The dark gray and gray areas in the granules and wire reveal that they are severely corroded and not made of gold. Fillers were found at the joints between the granules and wire, which is consistent with the X-ray image of GCG001 and indicates that soldering or brazing was used to attach the granules. A bright layer covering the surface of the sample can be interpreted as gilt, demonstrating that the ornament was gilded after the joining process. A cross-section of GCG003 was examined to investigate its metallographic structure and the nature of its materials. The chemical composition of GCG003 is presented in Table 1. The metallographic images in Fig. 8 show that the granule is fully corroded and the wire (which is around 0.3 mm thick) and substrate are partially corroded. Fillers are visible in the joining areas, and a gilt layer covers the surface of the sample (Fig. 8 a and b). Although the granule is fully corroded, the high ratio of copper indicates that the granule was made of pure copper. The copper phase in the residual metal cores of the metal wire and substrate is α-Cu (Fig. 8 a and b), affirming the chemical compositional data indicating that they, too, were made of pure copper. The recrystallized grain structure, featuring twinned grains, indicates that the wire and substrate were made by hot forging (Fig. 8 c and d). The fillers have a silver and copper (α+β) eutectic structure, indicating that Fig. 6 Granule diameter distributions of a linear granulation and b field granulation Archaeol Anthropol Sci (2019) 11:6603–6613 6608 Fig. 7 (a) The area for μ-CT scanning is highlighted in the frame. (b) Reconstructed μ-CT slice image GCG003 was brazed with silver–copper alloy, a type of hard solder (Fig. 9). Moreover, the Ag–Cu eutectic structure (gray layer in the SEM image) is seen on the surface of the whole sample, reflecting that the solder flooded the sample during the heating process (Fig. 10). A 1.2– 2.6-μm-thick gilt layer (bright layer in the SEM image) coats the sample and overlies the solder layer, confirming that gilding was the final procedure carried out on the ornament (Fig. 10a). The existence of elemental gold and mercury reveals that the sample was mercury gilded. The analytical results thus confirm the findings obtained from GCG001 and GCG002. On the basis of the results above, ornaments decorated with gilded copper granulation were manufactured by way of the following sequence: producing and sorting the copper granules, positioning the granules on the designated areas, brazing the ornament with Ag–Cu alloy, and mercury gilding the entire ornament. Table 1 Discussion Copper granulation The most interesting finding of this study is that copper, rather than precious metals, was used for granulation on Lady Pei’s coronet. Copper granulation is a very rare archeological discovery (Wolters 1981). One reason for this may be that copper corrodes easily in the burial environment. The only comparable examples are the hairpins of Empress Xiao, in which gilded copper field granulation was utilized to decorate the heads (Dang et al. 2018). The hairpins were likewise severely corroded, and the granules were fully corroded, with only a mercury gilt layer covering the surface. Little has been reported regarding the bonding method used for the hairpins. Therefore, the current study of the Pei coronet is the most comprehensive scientific analysis so far of copper granulation from ancient Asia and its brazing technique. Chemical composition of GCG003 Zone of analysis Granule Metal wire Solder (granule + substrate) Solder (metal wire + substrate) Solder layer (granule surface) Gilt layer (surface of granule) Gilt layer (surface of metal wire) Composition (wt%) O Cl Cu Ag Micro-area Micro-area Micro-area Micro-area Micro-area Micro-area Micro-area 13.8 15.6 1.3 1.2 1.3 2.5 2.1 83.8 82.4 98.7 98.8 98.7 7.9 10.7 91.6 88.8 Micro-area Micro-area Micro-area Micro-area Micro-area Micro-area Micro-area Micro-area 2.5 2.4 1.8 13.4 9.7 7.5 7.7 6.4 6.9 11.3 23.3 83.0 87.9 89.2 91.4 7.5 3.1 5.0 3.2 0.5 0.5 1.1 1.5 0.9 1.1 Au Hg 71.7 12.8 11.0 14.1 14.4 77.2 72.6 58.3 Archaeol Anthropol Sci (2019) 11:6603–6613 6609 Fig. 8 Metallographic images of GCG003 after etching. a, b Basic structure of CGC003. c Twinned α-Cu grains in copper wire. d Joints between the filigree and substrate show a eutectic structure in the fillers and twinned α-Cu grains in the substrate The copper work is of similar dimensions to contemporary examples of granulated gold ornaments. Linear granulation and field granulation were used to decorate the coronet of Pei. The observed distinction between larger linear granules decorating the edges and smaller field granules filling the background is the most common decorative form in the granulation works of the Tang Dynasty (Yang et al. 2014a). In Fig. 9 Back-scattered electron (BSE) images of the microstructure of the joining area. a Eutectic structure in joints between the wire and substrate. b Eutectic structure in joints between the granule and substrate terms of the granule diameter, the linear granules of the Pei coronet range 0.59–1.15 mm, and the field granules range 0.44–0.90 mm. By comparison, the diameters of the field granules on the hairpins of Empress Xiao are in the range of 0.20 to 0.50 mm (Dang et al. 2018). In gold granulation work, granules from 0.30 mm to 0.35 mm in diameter were used as linear granulation, and 0.11 mm to 0.28 mm in diameter were Archaeol Anthropol Sci (2019) 11:6603–6613 6610 Fig. 10 Elemental distribution maps of GCG003. a Distribution of copper, silver, gold, and mercury over the granule surface. b Distribution of copper and silver in GCG003 used as field granulation (Yang et al. 2014a). Although the above relates only to granule size, it demonstrates a lack of a difference between producing gold and copper granules. Granulation in different metal types appears to have been a well-developed technique in the Tang Dynasty, with metalsmiths being skilled at using granules of varies sizes for metal decoration. Moreover, though there are various methods of producing granules, they are mostly based on the principle of metal solidification. Hence, the most efficient method must have been chosen to produce such large numbers of spherical granules. The most likely option is the crucible method, which is able to produce uniform granules of a pre-determined size. In addition, copper and gold artifacts share similar decorative techniques during the Tang period, such as the joint use of wire work, granulation, and semi-precious stone and pearl inlay. The copper hairpins of Empress Xiao also employ wire to create the flowers into which semi-precious stones are set, and copper granules fill in the background (Dang et al. 2018). A set of gold coronet ornaments unearthed from the western suburbs of Xi’an (Xi’an Institute of Conservation and Archaeology on Cultural Heritage 2011) resembles the copper ornaments of the Pei coronet. Filigree creates the framework and patterns, bigger granules run along the edges, innumerable smaller granules fill in the background, and gemstones or semi-precious stones are inset into the patterns. Religious relics also have the same decorative elements as these ornaments, for example, gold caskets that contain the Buddha’s relics from the Qingshan Temple (Zhao 1985) and Famen Temple (Han et al. 1988); small flower-shaped ornaments decorate the caskets that gold wire forms the patterns, semiprecious stones and pearl are inlaid into this framework, and granules are bonded along the edge. Copper has similar physical properties to gold, enabling it to be hammered and shaped. Meanwhile, gilding allows copper objects to be made to look like gold. Qi (2005) pointed out that many sophisticated copper items produced during the Tang period imitated gold and silver artifacts. The gilded coronet of Pei detailed herein could therefore be an imitation of a gold coronet of the Tang Dynasty. Joining technique This study revealed that silver–copper alloy was used to braze the copper granules and wire to the substrate. Silver-based brazing filler is a kind of hard solder that has a lower melting point than the materials to be joined but makes strong joints (Maryon 1949; Maryon and Plenderleith 1954). Silver has a melting point of 961.93 °C, that of copper is 1084.5 °C, and silver and copper form a eutectic structure at 780 °C with 71.9 wt% Ag (Scott 1991; Mortazavi et al. 2017). In GCG003, the average content of the eutectic structure is approximately 88.7 wt% Ag and 9.3 wt% Cu, but this does not reflect their original proportions due to corrosion so serious that the α phase in the joining area was fully corroded. To Archaeol Anthropol Sci (2019) 11:6603–6613 6611 Fig. 11 Copper–silver binary diagram (Scott 1991) understand the original mixing ratio used for the solder, we calculated the Cu and Ag contents using the “lever principle” in the Cu–Ag binary diagram. First, five 6 μm × 6 μm sample areas were randomly selected from the eutectic structure to calculate the average ratios of the α and β phases, then the contents of Cu and Ag were derived by applying the “lever principle” and α/β. The final solid solution of the α phase is considered to fall approximately at point M and that of the β phase at point N; the relative contents of the α phase and β phase are calculated by formulas (2) and (3) according to the diagram (Fig. 11): 91:2−Wt Ag  100% MN WtAg −8:0  100% β¼ MN α¼ ð2Þ ð3Þ where WtAg is the silver content in the solder recipe used for the copper granulation and MN is taken as MN = 91.2%– 8.0% = 83.2%. As α/β = 12/25, we can deduce that the weight percent of Ag is 64.2 wt%, Cu is 35.8 wt%, and the ratio Ag:Cu ≈ 2:1. These indicate that the melting point of the solder is around 820 °C. Silver–copper alloy solder has received little attention in the studies of ancient solder. However, a few examples of soldering of copper suggest that its use was not limited to copper granulation. Silver-based solder is an ancient solder and has been used for brazing copper since no later than 2500 B.C. (Maryon and Plenderleith 1954). Of particular interest is a recent publication finding that a copper loop from Sanmenxia, Henan, and belonging to the late Tang Dynasty (ca. 809 A.D.) was brazed with copper and silver alloy (62.24 wt% Cu and 36.46 wt% Ag on average) (Kang et al. 2017). The average Ag to Cu weight ratio was also calculated for this loop but was found to be approximately 1:2 rather than the value of 2:1 found here for the Pei coronet. Both of these ratios represent hypoeutectic structures, but 2:1 has a lower melting point. Few ancient documents record the solder recipe used for copper work. It is not until the Qing Dynasty (1636– 1912 A.D.) in China that literature named Jingjinglingchi (Zheng 2015) and Qingdaijiangzuozeli (Wang 2000) recorded a recipe deemed suitable for brazing copper, and this was different from that used in the Tang Dynasty: four parts of silver and six parts of brass (named Caihuatong, a kind of natural brass). Hence, though silver and copper alloy was utilized to braze copper in Tang China, the specific ratio is as yet uncertain. Conclusion The coronet of Lady Pei is a typical representative of the hierarchical system in the use of personal adornment for high-ranking females during the Tang Dynasty. This study analyzed the nature of the materials and mode of granulation application on three granulated samples from the coronet. These non-destructive analyses show that two types of granulation of different sizes, that is, linear granulation and field granulation, were jointly used in the same ornament. Observations of the microstructure of the granulation indicate that the granules were brazed with fillers and then gilded. Metallographic investigation and chemical compositional analysis further affirm that the granulated decoration on the 6612 Pei coronet was made of copper and that Ag–Cu alloy was employed to join the granules, wires, and substrate together. Moreover, mercury gilding was applied to the ornament as the final step to give it a gold-like appearance. The physical features and bonding method reveal that copper granulation was a well-developed technique in Tang China and shared many similarities with contemporary gold granulation. Analysis of these aspects also provides a deeper understanding of the ancient granulation technique. Furthermore, the application of the μ-CT technique, an effective and non-invasive method, reveals the state of the copper granules, fillers at joints, and the inner structure of the granulation. μ-CT provides an effective solution for conducting research on small metal artifacts. Acknowledgments We would like to thank the Xi’an Institute of Conservation and Archaeology on Cultural Heritage for the samples. The authors are grateful to Ms. Xiaojuan Dang, Ms. Juan Ji, and Mr. Jiankai Xiang from Shaanxi Institute for the Preservation of Cultural Heritage, for the support and help on the metallographic investigation, SEM-EDS, and X-ray radiography. Special thanks go to Dr. Yan Liu, Ms. Yingzi Zhangsun, and Ms. Yuanyuan Zhang for suggestions on the manuscript. Funding information The study was financially supported by the National Natural Science Foundation of China (51674206) and Science and Technology Project of Inner Mongolia Autonomous Region (JH20180250). 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