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Journal of Archaeological Science 82 (2017) 31e39

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Journal of Archaeological Science

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Evidence of arsenical copper smelting in Bronze Age China: A study ofmetallurgical slag from the Laoniupo site, central Shaanxi

Kunlong Chen a, Siran Liu a, *, Yanxiang Li a, **, Jianjun Mei a, b, Anding Shao c,Lianjian Yue c

a Institute of Historical Metallurgy and Materials, University of Science and Technology Beijing, Beijing 100083, Chinab Needham Research Institute, Cambridge CB3 9AF, UKc Shaanxi Provincial Institute of Archaeology, Xi'an 710043, China

a r t i c l e i n f o

Article history:Received 21 October 2015Received in revised form20 April 2017Accepted 25 April 2017

Keywords:Bronze ageChinaArsenical copperSlagPolymetallic ore

* Corresponding author.** Corresponding author.

E-mail address: driverliu1987@gmail.com (S. Liu).

http://dx.doi.org/10.1016/j.jas.2017.04.0060305-4403/© 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t

Previous archaeometallurgical studies on Bronze Age China mainly focused on finished artefacts,whereas our understanding of copper smelting technology of this period is still limited. This paper, forthe first time, presents analytical results of metal production remains from the site of Laoniupo inGuanzhong Plain, central Shaanxi. It reveals that arsenical copper was produced at this site by smeltingarsenic-rich polymetallic ores with raw copper or high purity copper ores. The identification of metalproduction in the Guanzhong Plain is significant for the investigation of regional development and inter-regional interaction of Bronze Age cultures in China. The possible exploitation of ores from deposits inthe Qinling Mountain region during this period is also discussed in this article.

© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Research into the metalwork of the Early-Middle Bronze AgeChina (ca. 2000 BCE e 1000 BCE) has made significant progress inthe past decades. Issues such as the beginning of metallurgy inChina, and compositional characteristics and casting technology ofShang-Zhou bronze ritual vessels have kept attracting scholars andresulted in a large number of crucial publications in both Chineseand English (e.g. Zhao, 2004; Mei, 2009; Chen et al., 2009; Bagley,2009; Mei et al., 2012, 2015). However, most of these studies areartefacts-based, only reflecting information of the final few steps ofancient metallurgical chaine op�eratoire (Ottaway, 2001;Hauptmann, 2007) and, unavoidably, leaving many research la-cunas. For example, we still have little knowledge about the earlysmelters' choice of ores and details of their smelting and alloyingtechnologies. In recent years, a series of archaeometallurgical fieldinvestigations and analytical work have been carried out by aresearch group at the Institute of Historical Metallurgy and

Materials, University of Science and Technology Beijing (IHMM,USTB). Much attention has been paid to the production debrisfound at ancient metallurgical sites, aiming at a direct under-standing of ancient metal production activities. Investigation on themetal production remains unearthed from the site of Laoniupo, aspresented in this paper, is one of the initial results of these efforts.

2. Archaeological context and samples

The site of Laoniupo is located at the eastern suburb of Xi'an City,Shaanxi Province. This area in the lower valley of the Wei River iscalled “Guanzhong Plain”meaning “Inside the Passes”. Neighbouredby the Loess Plateau in the north and the Qinling Mountains in theSouth, this area connects the Central Plain of China and NorthwestChina, and has long been occupied by many archaeological cultures(Fig. 1). The site of Laoniupo was first inhabited during the Yang-shao period (6th to 4thmillennia BC), andwas continuously used asa settlement by the following Late Neolithic cultures such asKeshengzhuang II culture (24th to 21st centuries BC) and Don-glongshan culture (ca. 20th to 18th centuries BC). Pottery typologysuggests that during the Donglongshan period the site receivedinfluences from both the Erlitou culture (19th to 16th centuries BC)in the Central Plain and the Qijia culture (23rd to 18th centuries BC)

Fig. 1. Map of the Guanzhong Plain and adjacent regions, showing the locations of Laoniupo and other Shang period sites.

K. Chen et al. / Journal of Archaeological Science 82 (2017) 31e3932

in Hexi corridor, Gansu province, demonstrating its important rolein the cultural exchanges of this period (Liu, 2002; Zhang, 2000;Han, 2009).

By the late secondmillennium BC, contemporarywith the ShangCulture in the Central Plain, the Laoniupo site reached its mostflourishing stage. Having a maximum extension of 20 km2, the sitecontrolled a great strategic landscape on the north bank of the BaRiver, a tributary of the Wei River. In the late 1980s, NorthwestUniversity in Xi'an carried out six seasons of excavations at Lao-niupo and revealed a total area of 5000m2. The site is divided into 4sub-zones by the excavators according to their different landscapesand topographies (Fig. 2). These excavations have yielded abundantarchaeological features including rammed-earth foundations forlarge buildings, pottery kilns, house foundations, tombs, sacrificialpits of chariots and horses, and ash pits. A large number of artefactsmade with various materials were unearthed while the mostinteresting ones are fragments of casting moulds and slags, directlysuggesting a copper processing workshop at this site. (Liu, 2002).

From 2008 to 2011, the IHMM carried out four field surveys atLaoniupo and its neighbouring regions. Metal working remains,mainly slag fragments, were found in the sub-zone I and II. Ac-cording to the published excavation reports and recent fieldwork, itis clear that the metalworking remains are concentrated in thecentral-south part of the site facing southward to the Ba River(Fig. 2). In April 2010, a small landslide on top of a local cave-houseat the southern edge of the site exposed the profile of a culturallayer. This layermainly consist of grey podzolic soil and ash, bearingdomestic pottery sherds, bone fragments, and metallurgical

remains such as slags and technical ceramics (Fig. 3). All samplesanalysed in this research are collected from this layer. The typologyof pottery sherds suggest this layer can be dated to the Shangperiod. A charcoal inclusion found in one slag piece (No. LN112) wassent for radiocarbon dating at the School of Archaeology andMuseology and the School of Physics, Peking University (lab code:BA090234). The calibrated age with 2s confidential level is be-tween 1415 BCE and 1295 BCE. This result places the specimen tothe Middle-Late Shang period, and corresponds well with thechronology suggested by the associated pottery sherd (Fig. 4).

Twelve slag and four technical ceramic fragments shown inFig. 3 (No. 1e12) were selected for more detailed analyses due totheir relatively large size. The specimens are irregular lumps withdark grey, dark brown and black colours. Vacuoles in different sizesas well as green and/or some red-brown corrosion products areregularly identified. Flowing patterns were not observed on any ofthem, indicating they were not tapped from the furnace. Technicalceramic samples (Fig. 3: No.13e15) are also small fragments,showing reddish ceramic body on one side and dark brown to blackslag linings on the other side. The relatively small sizes and fairlyfine texture of these ceramics suggest they are fragments of cru-cibles rather than furnace walls, which is likely to be thicker andcoarser (Martin�on-Torres and Rehren, 2014).

3. Analytical methods

Bulk chemical composition of slag samples were analysed withShimadzu Lab Center XRF-1800 Wavelength Dispersive X-ray

Fig. 3. Photographs of metallurgical remains discovered at the Laoniupo site; No. 1 to 12 are slag pieces while No. 13 to 15 are technological ceramics.

Fig. 2. Map of the Laoniupo site, showing the general location, sub-zones and excavated areas.

Fig. 4. Calibrated date of charcoal inclusion found in slag LN112.

K. Chen et al. / Journal of Archaeological Science 82 (2017) 31e39 33

Fluorescence Spectrometer (XRF) at the School of Metallurgical andEcological Engineering, USTB. Samples were first powdered andthen prepared into pressed pellets for analysis. Microscopic ana-lyses of slag and technical ceramic were conducted with an opticalmicroscopy (OM) at the IHMM, USTB and a Hitachi SN-3200Scanning Electron Microscope equipped with an EDAX EnergyDispersive Spectrum analyzer (SEM-EDS) at the Chinese Academyof Cultural Heritage (CACH). Samples were mounted with epoxyresin and polished with diamond paste down to 0.25 mm. Theanalytical conditions of SEM-EDS were set as an accelerationvoltage of 20 kV, a working distance of 10e15 mm and an acqui-sition time of 60s.

4. Analytical results

Bulk composition of 12 slag pieces are shown in Table 1.Remarkably, all samples have high copper and arsenic contentswith an average of 11.2 wt% CuO and 3.6 wt% As2O3. The main

Table 1WD-XRF bulk chemical analytical results of the Laoniupo slag (wt%).

Sample Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO Fe2O3 NiO ZnO BaO CuO As2O3

LN101 e 0.45 3.25 56.11 0.30 0.18 1.49 1.88 e 0.25 10.13 0.11 1.12 0.47 21.29 2.95LN102 0.08 0.33 2.43 77.43 0.31 0.31 1.19 1.47 0.10 0.10 5.76 0.01 0.23 1.10 6.95 2.20LN103 0.11 0.28 2.02 65.09 0.23 0.15 0.97 2.03 0.17 0.15 11.58 0.03 0.62 0.32 13.62 2.63LN104 e 0.61 1.55 72.60 0.34 0.14 0.63 3.68 0.12 0.21 8.97 0.04 0.63 1.34 7.43 1.70LN105 0.45 0.96 3.68 61.99 0.77 0.21 1.43 4.41 0.24 0.10 8.70 0.01 0.22 0.54 12.09 4.19LN106 e 0.39 2.49 76.14 0.62 0.06 1.29 2.58 0.37 0.38 7.83 0.08 1.21 0.42 4.96 1.18LN107 0.30 2.08 2.95 35.13 0.53 0.18 1.25 15.59 0.20 1.01 22.68 0.09 1.81 2.72 8.86 4.61LN108 0.86 1.38 8.10 52.29 0.60 0.08 3.27 6.82 0.54 0.30 11.98 0.06 0.46 1.03 9.87 2.34LN109 0.22 2.16 2.57 45.71 0.41 0.28 1.10 11.05 0.16 0.64 18.53 0.07 1.11 1.82 10.32 3.84LN110 0.25 1.50 3.50 34.53 0.64 0.10 1.70 14.23 0.26 0.72 21.37 0.07 1.86 3.57 10.10 5.62LN111 0.22 1.41 4.63 38.31 0.45 0.31 2.06 10.13 e 0.56 17.33 0.08 1.52 2.64 13.57 6.79LN112 0.40 1.43 4.48 36.11 0.64 0.21 1.79 10.74 0.27 0.59 17.10 0.06 1.68 2.58 16.41 5.50

Note: Normalized data; “–” means the concentration is below the detection limit. It should be note that copper (Cu) and arsenic (As) are presented here as oxides because anoxide standard was employed during the XFR analysis, but a big portion of these elements are existed in metallic form as numerousmetal prills were observed frequently in allthe samples. The calculation of iron (Fe) as Fe2O3 rather than FeO is based on the fact that hercynitic magnetite (Fe(Fe, Al)2O4) was identified in some samples, however, smallparticles/dendrites of wüstite are detectable under the microscope.

K. Chen et al. / Journal of Archaeological Science 82 (2017) 31e3934

components of slag matrix, namely SiO2, Fe2O3 and CaO havenotably varied concentrations. Silica content of these samplesranges between 35 wt% and 78 wt% while their iron oxide contentsare between 5 wt% and 23 wt%. The variation of lime content iseven more significant, with a difference in one order of magnitudebetween the richest and the poorest samples. Barium oxide andzinc oxide are consistently present in these samples.

Microscopic analysis shows that most samples are heteroge-neous with frequent semi-reacted quartz grains (Fig. 5). Thisobservation explains the highly varied basic oxides concentrationin slags' bulk composition. SEM-EDS analysis of the fully molten/reacted areas of slags shows that they are mainly composed of SiO2and FeO, together with notable amounts of CaO, Al2O3, BaO andMgO. Melilite and pyroxene group crystals were found in the glassymatrix (Fig. 6). Cuprite and magnetite-rich spinel clusters with asignificant amount of iron oxide and alumina were also frequentlyidentified. Occasionally, delafossite crystals (CuFeO2) and smallwüstite particles/dendrites (FeO) were spotted, indicating theheterogeneous redox conditions inside the smelting container.

Large numbers of metallic prills (from several micrometres tomillimetres in diameter) were found trapped in the slag matrix(Fig. 7, Table 2). Most of them are rich in arsenic (up to 30 wt%),showing a dendrites and sometimes independent g phase. In only a

Fig. 5. BSE image of sample LN101, showing large vacuoles and unreacted remnant in aheterogeneous substrate.

few samples, pure copper prills were identified. Iron is alsofrequently detected in these prills as impurity and inmany cases, itsconcentration is over 1 wt%. Antimony was found in some arsenicalcopper prills, and occasionally its concentration can be more than2 wt%. Copper sulphide with a minor amount of iron was occa-sionally found as halos surrounding metallic prills.

Mineral remains containing a significant amount of copper and/or arsenic were identified in some samples. A triangular inclusionwas found embedded in a partially reacted matrix of sample LN107(Fig. 8). Microanalysis of the un-corroded strip (indicated by thewhite cross in Fig. 8) gave a Cu:S atomic ratio around 1.06,approximating the formula of covellite (CuS). In sample LN102 andLN109, bright clusters were found in BSE images (Fig. 9, Table 3).They mainly contain oxygen, iron, copper and arsenic. Chemicalcomposition of the whole cluster and its sub-phases are shown inTable 4. Both the “skeleton-frame” microstructure and chemicalcomposition indicate these clusters are very likely to be unreactedore remains, probably secondary minerals of copper/iron sul-pharsenide ores such as enargite and arsenopyrite (H€oppner et al.,2005).

Cross-section of technical ceramics show that they have avitrified inner part with dark-grey/black colour and a thin layer ofglassy slag lining on the interior surface (Fig. 10). The chemicalcomposition of ceramic body, vitrified ceramic and slag lining are

Fig. 6. Optical photomicrograph of sample LN107, showing grey pyroxene crystalstogether with clusters of iron oxides (light grey) and metal prills (bright) in a silicatematrix (dark) (Optical photomicrograph, as polished).

Fig. 7. Examples of metal prills observed in the Laoniupo slags. Upper left: LN106, pure copper prill with cuprite inclusion formed (Cu þ Cu2O) eutectic structure; upper right:LN107, noting dendrites already visible due to a relatively high As content; lower left: LN111, high As content prill with dendrites and grey sulphide inclusions; lower right: LN112, apure copper prill with copper sulphide on the rim. (Optical photomicrograph).

K. Chen et al. / Journal of Archaeological Science 82 (2017) 31e39 35

determined by SEM-EDS area analysis (Table 4). It is noticed that incomparison to the ceramic body, the levels of iron oxide, lime,barium oxide, arsenic and copper considerably increased in thevitrified part of ceramic and slag lining. The find of arsenic andcopper in the inner part of ceramics clearly indicates they wereused for processing copper alloy while the enrichment of alkali andalkali earth oxides in this part might reflect the influence of fuelash. The gradient in colour and degree of vitrification from theoutside to inside suggest these vessels were heated from above(Bayley and Rehren, 2007).

5The nature of the Laoniupo slags in comparative perspective:technological pathways to arsenical copper

The Laoniupo slag samples have relatively low iron oxide buthigh silica content and heterogeneous substrate. The dominance ofarsenic-bearing copper prills in all analysed samples indicates thatarsenical copper was produced at this site, however, a furtherinterpretation of its production mechanism is not straightforward.Zinc and barium are consistently detected in the Laoniupo slags in aconsiderable level but generally absence in the body of technicalceramics. Additionally, the copper and arsenic rich minerals such ascovellite and complex sulphurarsenide minerals are found in theslag matrix. These finds suggest that the charge inside the Laoniupocrucibles included some complex ore minerals with barium andzinc rich gangue. The existence of cuprite and delafossite in slagmatrix indicates a relatively oxidising reaction condition. This typeof “dross” is usually considered to be relatedwith copper melting orrefining process (Bachmann, 1982; Craddock, 1995; Liu et al., 2015).However, Burger et al. (2010)'s experimental simulation has indi-cated, in a crucible smelting process, when the charge has a rela-tively high O:S weight ratio (>2e3), magnetite, delafossite andeven cuprite can become the dominant phases in the smelting slag.

Radivojevi�c et al. (2010) analysed slag pieces from the site ofBelovode in eastern Serbia dated back to the 5th millennium BC.These samples are rich in copper (10.1e27.8% Cu2O) and containdelafossite as well. However, according to their consistently highiron, manganese, zinc and cobalt contents, these slags were inter-preted to be remains of a smelting process. Similar slags were foundin a Portuguese site and dated to the Chalcolithic period. Analyticalresult and experimental smelting also suggested them as by-products of a smelting process (Hanning et al., 2010). In this light,the Laoniupo slags were likely to be remains from processing nat-ural ores due to their high zinc and barium content as well as thepresence of unreacted oreminerals in the slagmatrix. However, it isnot clear whether the crucible charge was one complex poly-metallic ore yielding both copper and arsenic or an arsenic rich oretogether with pre-smelted metallic copper and/or high puritycopper minerals (e.g. malachite and covellite).

Previous researchers have suggested several different routes toproduce arsenical copper: the direct smelting of polymetallic oresbearing both copper and arsenic (Müller et al., 2004), the co-smelting of copper oxide minerals with copper/iron sulpharse-nide ores, cementation of metallic copper with arsenic minerals(Lechtman and Klein, 1999) or alloying of metallic copper witharsenic-rich speiss (Thornton et al., 2009; Rehren et al., 2012). Thedifferentiation of these different processes, especially betweenintentional alloying and unconsciously smelting naturally poly-metallic ores has troubled many researchers.

Müller et al. (2004) analysed a coherent assemblage of metal-lurgical remains including slags, crucible slag lining, ores, cruciblefragments and metal prills unearthed from Chalcolithic layer at thesite of Almizaraque in southeast Spain. Chemically, most of theslags are rich in copper (CuO up to 49%) and arsenic (As2O3 up to7.2%), and their magnesia and iron oxide contents are much higherthan that of the ceramic body of crucibles. The paragenesis of

Table 2Composition and dimensions of metal prills in Laoniupo slag.

No. Number of prills Size of prills (um) Composition (wt%)

Cu As Fe Sb

LN101 5 10e40 Max 89.9 24.2 2.7 3.7Mean 77.1 19.2 1.4 2.4Min 70.4 8.6 e e

SD. 7.9 6.3 0.9 1.5LN102 10 5e20 Max 91.6 30.6 2.7 4.7

Mean 81.9 15.2 1.6 1.2Min 63.2 8.4 e e

SD. 12.7 10.1 1.2 2.0LN103 7 15e1500 Max 100.0 5.6 3.1 e

Mean 96.6 2.2 1.1 e

Min 94.4 e e e

SD. 2.4 2.3 1.4 /LN104 11 5e150 Max 96.5 24.7 e e

Mean 85.9 10.9 e e

Min 75.3 e e e

SD. 8.5 9.0 / /LN105 3 10e250 Max 67.0 31.0 25.5 15.9

Mean 49.1 21.9 12.8 6.6Min 27.7 5.5 e e

SD. 19.9 14.2 12.8 8.3LN106 10 5e400 Max 98.7 29.5 2.8 e

Mean 83.8 14.3 1.9 e

Min 68.2 e 1.3 e

SD. 11.4 11.1 0.7 /LN107 4 5e700 Max 88.4 22.8 e e

Mean 83.2 16.8 e e

Min 77.2 11.6 e e

SD. 5.6 5.6 / /LN108 10 5e50 Max 100.0 4.9 1.8 e

Mean 96.8 3.0 0.3 e

Min 94.9 e e e

SD. 2.3 2.1 0.7 /LN109 7 5e50 Max 100.0 32.4 3.8 e

Mean 81.3 17.3 1.5 e

Min 67.5 e e e

SD. 13.2 13.6 1.5 /LN110 5 10e150 Max 89.7 15.1 2.7 e

Mean 87.4 11.1 1.5 e

Min 84.9 7.8 e e

SD. 1.8 3.1 1.4 /LN111 8 10e200 Max 96.8 31.5 2.2 e

Mean 90.7 8.4 0.9 e

Min 68.5 1.4 e e

SD. 10.0 10.6 1.1 /LN112 11 5e300 Max 98.4 11.0 2.6 3.3

Mean 91.1 7.7 0.6 0.5Min 86.9 e e e

SD. 3.7 3.7 1.0 1.2

Note: “–” means the concentration is below the detection limit; “/” means the SD isnot applicable.

Fig. 8. Covellite particle embedded in a partly reacted matrix (LN107), cross shows theposition of EDS microanalysis.

Fig. 9. BSE image of complex ore remnant, compositions of different phases are shownin Table 3.

Table 3Chemical composition of different phases showing in Fig. 9.

Position Composition (wt%)

O Cu As Fe Si Al Ca Mn Zn P

1 9.2 87.7 e 2.4 0.7 e e e e e

2 16.3 61.7 e 15.7 4.3 2.0 e e e e

3 22.3 19.5 36.0 9.9 0.8 1.1 5.1 1.7 2.8 0.9

Note: Normalized data; “–” means the concentration is below the detection limit.

K. Chen et al. / Journal of Archaeological Science 82 (2017) 31e3936

magnetite, cuprite and delafossite in the slags indicates a fairlyoxidising condition during the smelting process, although tempo-rally and regionally, conditions could have been reducing enough tosmelt copper indicated by the notable iron content in the trappedarsenical copper prills. The authors argued the Almizaraque slagsindicated a crucible smelting process using secondary altered fah-lore ores, but they also pointed out that the possibility of alloyingcopper metal with arsenic-rich ores at this site.

Murillo-Barroso et al. (2017) presented a detailed analysis ofores and copper production remains from the 3rd millennium BCsite Las Pilas in Spain not far from Almizaraque. The ore analysisshows that most samples contain oxidic copper minerals such asazurite and malachite while arsenic and zinc bearing minerals suchas zincolivenite was frequently identified. Most slags were not fullyliquefied and highly heterogeneous, bearing a large quantity ofpartially reacted arsenic and copper rich minerals. The authorssuggested a low efficiency smelting technology with a limited

degree of specializationwas used to reduce complex ores directly incrucibles and produced copper with a range of arsenic contents.

Rehren et al. (2012) suggested that smelted speiss had beenused as the source of arsenic to produce arsenical copper at theEarly Bronze Age site of Arisman in northwest Iran. The speissmight have been added to an oxidic copper ore during the smeltingprocess, or later mixed with metallic copper in an alloying process.Either operation would result in an iron silicate slag with notableamounts of CaO and Al2O3. Bulk chemical composition of slagsamples shows they are rich in copper (2.0 wt% CuO in average) butrelatively poor in arsenic (0.1 wt% As2O3 in average). However,metal prills trapped in the slag are mainly arsenical copper with

Table 4Analytical result of technical ceramic fragments from the Laoniupo site (wt%).

Sample No. Defined areas Number of analysis Na2O MgO Al2O3 SiO2 K2O CaO FeO BaO CuO As2O3

LN113 Ceramic body 1 1.9 2.8 12.8 65.5 4.3 7.7 5.2 e e e

Vitrified ceramic 3 1.8 2.8 11.8 60.1 3.1 12.5 5.0 0.8 2.2 e

Slag lining 3 0.6 1.3 8.7 46.8 2.4 18.6 8.7 2.3 5.9 4.8LN114 Ceramic body 2 1.8 2.5 13.6 66.6 3.8 6.1 5.6 e e e

Vitrified ceramic 1 1.3 e 11.4 58.8 3.2 12.3 6.1 1.6 2.1 3.3Slag lining 1 1.2 e 8.2 46.0 2.1 20.2 7.9 2.2 5.9 6.3

LN115 Ceramic body 1 1.8 2.4 13.7 68.2 4.0 4.7 5.2 e e e

Vitrified ceramic 1 1.7 2.5 12.3 65.6 3.9 9.0 5.0 e e e

Slag lining 3 1.2 e 10.0 51.6 2.7 14.6 8.0 2.0 3.7 6.0

Note: Normalized data; “–” means the concentration is below the detection limit.

Fig. 10. Binocular photomicrograph of cross-section of a technical ceramic fragment(LN114, width of image is 15 mm).

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high iron content (3.1 wt% in average).Arsenical copper production in the Bronze Age China did not

receive much attention until a copper smelting site at Xichengyinear Zhangye, in the middle of the Hexi Corridor, Gansu provincewas identified (see Fig. 1). The site provides important referencematerial for the study of early arsenical copper production in China.Dated to the early second millennium BC, Xichengyi is one of theearliest copper smelting sites excavated in China so far (Chen et al.,2014). Li et al. (2015) reported analytical results of metallurgicalremains, including 32 slags, 29 ores and 5 furnace lining from thissite. All slag samples have similar iron silicate matrix with severalto tens percent of copper. They can be clearly divided into twogroups on the basis of their different metallic inclusions. In mostslag samples (27 of 32), pure copper prills are consistently foundand occasionally matte prills are also identified. The remaining 5samples are, however, dominated by arsenical copper prills (up to30 wt% of arsenic) with some of them also containing notableamounts of iron and antimony. The analysis of furnace lining showsthe similar pattern that only one sample has arsenical copper prillswhile the other 4 are dominated by pure copper prills. The oresamples from this site are mainly oxidic minerals of copper (26 of29) with a small proportion of them (3 samples) rich in arsenic andlead bearing minerals such as fahlore, cerrusite, galena and mem-tite. Chen (2015) then divided these materials into two differentcategories referring to two separated metallurgical processes ortwo different stages of the same process. Copper metal was verylikely to be smelted first from relatively clean copper ores, and thenalloyed with arsenic-rich ores in a separated cementation/co-smelting process.

Another evidence of arsenical copper production dated to the2nd millennium BC was from a Shang city site at Yuanqu in Shanxi,

located to east of the Guanzhong Plain just on the other side of theYellow River (see Fig. 1). The excavation in 1990s revealed smallpieces of metallurgical remains at this site. One slag sample shows asignificant arsenic content in the matrix and numerous trappedarsenical copper prills (Arsenic up to 36 wt%). The similarcementation/co-smelting process was proposed to explain theformation of this slag (Liang et al., 2005). This find providesimportant evidence that can potentially fills the chronological gapbetween Laoniupo and Xichengyi, and indicate the existence of thistechnology in the further east areas.

Considering the relatively high and stable arsenic content in theLaoniupo slag and a potentially long history of producing arsenicalcopper via alloying process in north China exemplified by the site ofXichengyi and Yuanqu Shang city, it is argued that the Laoniuposlag were also derived from a cementation/co-smelting processinvolving arsenic-rich mineral and metallic copper/high puritycopper minerals. During this process, copper metal/mineral andarsenic-rich ores (potentially copper-bearing as well) were chargedtogether in the reaction vessel, probably a crucible, to producearsenical copper. The ores might contain sulphide minerals whichaccounts for the sulphide rim surrounding the metal prills andunreacted sulphide minerals observed in the slag. Since no evi-dence of pure copper production has been found at the Laoniuposite so far, the metallic copper, if used, was more likely to besmelted at other sites and then imported.

7. Discussion

The study of the Laoniupo slags for the first time provides evi-dence of local arsenical copper production in the Guanzhong Plain.The technical features of the Laoniupo slags including their rela-tively low quantity and small size, heterogeneous slag matrix withabundant unreactedminerals, high copper content, and presence ofmagnetite, delafossite and metal prills with sulphidic rim areconsistent with early smelting slag derived from a relatively lowtemperature and oxidising process.

Though the ore source of this site has still not been confirmed, itis suggested that the Qinling Mountain at the southern boundary ofthe Guanzhong Plain is currently the best candidate (see Fig. 1).There are a number of important metallogenetic belts in this regionbearing copperminerals such as bornite, chalcopyrite often co-existwith fahlores in polymetallic deposits (Qi and Hou, 2005; Ren et al.,2007). Geographically, some of the deposits are accessible throughthe valleys of small rivers originated from themountain, such as theBa River passing by the site of Laoniupo. Although we do not haveany direct evidence for the copper mining in this area so far, therecent discovery of ancient turquoise mining site dated back to thelate Neolithic period in the Luonan County (see Fig. 1) providedsolid evidence for the early exploration of mineral resources in theQinling region (Xian et al., 2016). Metalliferous deposits in theQinlingMountain have been proposed as the potential source of the

K. Chen et al. / Journal of Archaeological Science 82 (2017) 31e3938

Shang period metalwork found in the Hanzhong Basin, south of themountain range (Chen et al., 2009) and might also be explored bythe Bronze Age people living to the north in the Guanzhong plain.

Considering its chronological context, the production of arsen-ical copper at the Laoniupo site is worthmore attention since it is inconflict with the traditional view that tin bronze metalwork (lea-ded or not) played a dominant role in most of the Bronze Age sitesin China. Kwang-chih Chang (1980) pointed out that in contrast tomany other regions of the world, the use of bronze in ancient Chinawas mainly linked to politics, in the form of ritual vessels andweapons, rather than to economic functions. However, in a regionallevel, significant diversity of bronzemetalwork in terms of function,style and alloy composition have been noticed. Chen et al. (2009,2016) revealed a remarkable regional compositional characteristicof Hanzhong bronzes in southern Shaanxi and challenged thesimplistic ‘core-periphery’ interpretation paradigm, whichemphasized the cultural predominance of the Central Plain butoversimplified the complex historical trajectories and interactionsamong many different geographical regions. The identification ofarsenical copper production at the site of Laoniupo providesanother important example of this regionalised characteristic ofmetal production during the Shang period.

Last but not least, as it has been widely discussed for decades,the cultural exchange with Northwest China could have played animportant role for the early development of metallurgical tech-nology in the Central Plain during the Bronze Age, and part of theCentral Plain's technological know-how might be originated fromthe northwest (e.g. An, 1981; Fitzgerald-Huber, 1995; Li, 2005; Meiet al., 2012, 2015). However, due to the lack of research on theproduction remains, this hypothesis remains untested. In consid-eration of the important geographic position of Laoniupo, the re-sults presented here may have the potential to provide empiricalevidence for technological connections between Northwest Chinaand the Central Plain. As mentioned before, in spite of its later date,the technology of Laoniupo are generally in the same line withthose revealed at Xichengyi and Yuanqu, showing a potentialtechnological continuity. It should also be noted that the site ofLaoniupo had been occupied continuously since the Neolithicperiod and its cultural connection with the Hexi corridor and theCentral Plain started at least in the first half of the second millen-nium BC. During this period, the copper artefacts have been used bythe people living in those two regions. Thus, it is worth payingattention in the future investigation to check whether there areearlier evidence of metallurgical production at the site of Laoniupo,testing whether the metallurgical technology had spread via theGuangzhong plain.

6. Conclusion

Scientific analyses of metallurgical remains from the Laoniuposite have, for the first time, revealed evidence of arsenical copperproduction in the Guanzhong plain during the late second millen-nium BC. Complex ores consisting of arsenic-bearing minerals mayhave been used as the source of “alloying” element and smeltedtogether with raw copper/high purity copper minerals to producearsenical copper. Similar cementation/co-smelting process wasidentified at the Xichengyi site in the Hexi Corridor and the ShangCity site at Yuanqu. This result throws new light on the under-standing of regional metalwork development in the Bronze AgeGuanzhong Plain and its potential role as a bridging zone con-necting Northwest China and the Central Plain during the spreadmetallurgical technology. Future research is expected to provideinformation about the general archaeological background andtechnical details of the smelting activity at the site of Laoniupo,especially the nature of the ores and other crucible charges used at

this site. Its potential relationship with the ore resource in theQinglingMountain should be examined as well. It is also interestingto conduct comparative studies of bronze production in theGuanzhong Plain and other regions of China to find out how variedsocial contexts and different production and consumption modelsof bronzes influenced the technological choices of ancient workers.

Acknowledgments

The authors are grateful to Professors Thilo Rehren, Ernst Per-nicka and Marcos Martin�on-Torres as well as colleagues from USTBfor their insightful comments, support and assistance. The com-ments made by four anonymous reviewers helped to improve thequality of the manuscript and are gratefully acknowledged. Thiswork was supported by research grants awarded by the NationalNatural Science Foundation of China (51304020, 51474029) and theNational Administration for Cultural Heritage (2014220). Therevision of this paper is finished at the UCL Institute of Archaeology,where Chen Kunlong is working as a Newton International Fellow(2017e2019, NF160456) supported by the Newton InternationalFellowship awarded by the British Academy.

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