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A Flux That Binds: The Southeast Asian Lead Isotope Project

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The continued lack of a widely agreed upon Southeast Asian prehistoric ceramic sequence means that archaeologists working in the region sometimes struggle to reliably identify and interpret medium- to long-range social interactions, including
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  113 A Flux that Binds? The Southeast AsianLead Isotope Project Thomas Oliver Pryce abstract The continued lack of a widely agreed upon Southeast Asian prehistoric ceramic sequence means thatarchaeologists working in the region sometimes struggle to reliably identify and interpret medium- to long-range socialinteractions, including neighboring populations in East and South Asia. The Southeast Asian Lead Isotope Project(SEALIP) has been designed to contribute to our comprehension of regional connectedness for the late prehistoricMetal Age using copper-base and lead metal exchange networks as a social interaction proxy—a proxy that will of course omit social groups that did not for whatever reason develop or adopt metallurgical production or consumption behaviors. This paper outlines why archaeometallurgy is more than just the “history of metallurgy” and how suchlead isotope research has the potential to furnish critical data for all scholars of Southeast Asian late prehistory. Thecurrent SEALIP database permits the two long-known prehistoric copper production centers, the Khao Wong Prachan valley in central Thailand and Phu Lon in northeast Thailand, to be reliably distinguished isotopically, fullling the Provenance Hypothesis and justifying the ongoing regional metal exchange program. The data also indicate that thesetwo major copper producers were simultaneously importing copper-base metal, supporting previous suggestions thatearly Southeast Asian metal exchange was motivated more by gifting than commerce. Archaeometallurgical Underperformance in theFar East Lead is a metal that has been signicantly overlooked by archaeologists working in Southeast Asia. Sanskrit nameslike Suvarnabhumi (Land of Gold) and Suvarnadvipa  (Islands of Gold) (Wheatley 1961) have long assured aninterest in the region’s precious metal output (e.g., Miksic1990; Richter 2000), just as the famous Southeast AsianTin Belt (e.g., Schwartz   et al . 1995) has garnered extensivespeculation over this metal’s long-term role in trans-Asiatic exchange (e.g., Bellwood 2007; Bronson 1977,1992; Jacq-Hergoualc’h 2002)—a topic slowly approach-ing the realms of empiricism (e.g., Bennett and Glover 1992; Pryce, Murillo-Barroso,   et al . , in press). On copper- base metallurgy, debate has been raging on its “srcins”in Southeast Asia since the late 1960s (e.g., Bayard 1972,1979, 1981; Gorman and Charoenwongsa 1976; Higham1975, 1996–97; Higham   et al . 2011; Muhly 1981; Solheim1968; Spriggs 1996–97; White 1986, 1988, 2008), thoughthe current balance of evidence has resulted in a consensusof the late second millennium bce for the inception of the Bronze Age, alongside the accord establishing the mid-rst millennium bce for the Iron Age (e.g. Ciarla et al . , in press;Higham and Higham 2009; Higham   et al. 2011; Pigott andCiarla 2007; cf. White and Hamilton 2009). Arguably morefruitful in terms of density of reliable data, the ThailandArchaeometallurgy Project’s holistic approach to primaryand secondary copper production in Thailand has beencontributing invaluable information on prehistoric metal-lurgical behavior in the broadest sense since 1984 (Pigottand Natapintu 1988; e.g., Pigott   et al . 1997; see also Smith1973; Wheeler and Maddin 1976, for earlier work) andhas recently been complemented by comparable investi-gations at Sepon in central Laos (Sayavongkhamdy   et al .  2009). Southeast Asian ferrous archaeometallurgy, despitean early start and enormous potential (e.g., Bennett 1982;Dizon 1988; Pigott and Marder 1984; Suchitta 1983), isin need of substantial development, but continued assaults(Pryce and Natapintu 2009) on the notion of ubiquitous lat-eritic iron sources will hopefully pay dividends in the formof regional studies on material and technological exchange(e.g., Biggs 2009).In comparison, lead has scarcely been mentioned inthe Southeast Asian literature, with only a few possible premodern mining sites noted during a 1984 survey bythe Thailand Archaeometallurgy Project in Loei province,northern Thailand (Pigott 1986; see Sitthithaworn 1990).Lead is a major component of regional copper-base alloys,certainly from the Iron Age onward (e.g., Higham 1989,240), but the only comments we generally hear are broaddiscussions of lead ore availability (e.g. Higham 1989, 136, 185) and the use of lead as a “ux” to “improve a cop - per-alloy’s casting characteristics by reducing the melting point and viscosity, and increasing the workable castingrange and rendering greater detail in the mould” (Murillo-Barroso et al . 2010, 1767). Statements like this are entirelycorrect but rather underplay the importance of lead’s rolein Southeast Asian metallurgical traditions. Although leadis relatively easy to smelt, its ores, like those of copper,tin, and high-quality iron, have a nonuniform distribution,and consumption of leaded copper or leaded bronze in one 11 SRAAM Pryce v3.indd 11323/11/2011 13:46  Scientific ReSeaRch on ancient aSian MetalluRgy 114 part of the region very probably involved the exchange of substantial quantities of lead from elsewhere. For exam- ple, the Iron Age leaded bronze Dong Son drums mayeach have a mass in the dozens of kilograms, which, withlead contents in excess of 10 wt.% (Murillo-Barroso   et al .  2010, table 2), means that each artifact required severalkilograms of lead metal. If we assume that alloys this richin lead are due to the intentional addition of lead metal asopposed to the smelting of naturally lead-bearing copper ores, then Southeast Asia’s leaded Dong Son bronzes alonerequire a technological and economic explanation for thathigh quantity of lead (see Pryce 2010). Where was it beingmade? When? By whom? And in what social context of  production? Lead’s Potential Contribution to SoutheastAsian Archaeology Of the broad research questions presented, the latter threerequire the surveying of Southeast Asia’s known lead depos-its (see, e.g., Pryce and Abrams 2010; Pryce, Brauns, etal., in press, for copper), followed up with excavation of those mineralizations associated with premodern industrialactivity (see, e.g., Pigott and Weisgerber 1998; Pigott   et al .  1997, for copper), and technological analysis of the result-ing assemblages (see, e.g., Pryce   et al . 2010, for copper). This approach is promising but will take time. For the rst question, however, lead offers some particular analyticalaffordances that have been dramatically underexploited inthe Southeast Asian arena and that, as argued here, couldresult in rapid advances in our understanding of the region’searly metal exchange networks.It has been some decades since large international ele-mental analysis programs were instituted for Europeanmetal exchange research (e.g., Junghans   et al . 1960, 1968,1974), but this approach is now widely recognized to be problematic due to the differential partitioning of traceelements during metal production processes and the data conation that all additive technologies suffer during alloy -ing and recycling (e.g., Chippindale 2000; Pollard 2009;Pollard and Heron 2008)—though recent research suggeststhat these enormous databases can be exploited for informa-tion about past production processes, if not provenance aswas srcinally intended (Bray 2009).With extreme care, and taking into account elemen-tal datasets, the provenance baton can be seen as having passed to lead isotope analysis (LIA). Lead, within widelyaccepted caveats, can be geochemically linked to its sourcemineralization via the analysis of its stable isotopes. Thistechnique does not apply only to pure lead artifacts or toheavily leaded copper-base alloys; instead the detectionlimit of present-day instrumentation permits the determi-nation of lead isotope ratios at trace element concentrations(parts per million or even parts per billion; see Niederschlag   et al . 2003). As most copper ores contain trace amounts of lead, we are in a position to assess the exchange networksof both lead-base and copper-base alloys, although this dual usage does present some difculties. As per many parts of the world, Southeast Asia stands to benet considerably from an anthropologically and geochemically well-founded diachronic reconstruction of its metals sector economy. However, our need is greater than that. With an area of ca. 5,000,000 sq. km and a present-day population of ca. six hundred million, thescope of the regional archaeological challenge could becompared against that for the European Union. A verywell-attended Southeast Asian archaeological meeting nets in the region of ve hundred scholars and students. 1  I am uncertain what the average attendance for a major European archaeology conference is, but I suspect that one of the signicant differences will be that of “research density.” Despite decades of commendable advances inSoutheast Asian archaeology, we still lack a broadlyagreed upon regional prehistoric ceramic sequence (Whiteand Hamilton 2009, 358), which, combined with linguis-tic and publication barriers, presents a real problem for synthetic accounts of shared history between neighboringterritories whose former populations we know throughcommon sense were most probably in contact. I hope todemonstrate that archaeometallurgical methodologiestried and tested elsewhere can serve to ameliorate thissituation, from the advent of the Bronze Age onward, by paying obeisance to geological rather than national  boundaries. Lead, is thus, a ux that binds. Lead Isotope Characterization and Provenance Archaeological LIA is not new to Southeast Asia, having been carried out by Japanese researchers since 1993 (seeKakukawa   et al . 2008 and references therein). However, thegeographical and temporal extent of regional LIA sampling would benet from a sharp expansion as well as an increased density of coverage to enable a systematic unveiling of  premodern nonferrous regional metal exchange networks.This is the aim of the Southeast Asian Lead Isotope Project(SEALIP; Pryce   et al . 2011). To appreciate SEALIP’s potential in regards to Southeast Asian archaeology, it isnecessary to recognize LIA’s own historic trajectory. Thetechnique was srcinally developed in the 1930s as a geo-logical dating method (Russell and Farquhar 1960, 14) but was co-opted for archaeological purposes in the late1960s (Brill and Wampler 1967), its methodology being further modied in subsequent decades into a reliable and mature methodology for studying ancient metal exchange and provenance, though not without a signicant period of  negotiation in the late 1980s and early 1990s (reviewed inPollard 2009).The premise is that lead has four stable isotopes—  204 Pb, 206 Pb, 207 Pb, and 208 Pb—of which 204 Pb is primeval lead andthe others the end members of radioactive decay sequences.The ratios between the lead isotopes in rocks and mineralsare related to the geological age of their formation and, assuch, can be used to characterize those rocks and miner- als from specic locations. Ore minerals also contain trace amounts of lead, or predominantly lead in the case of galenaand cerussite, of which, crucially, the isotope ratios remain 11 SRAAM Pryce v3.indd 11423/11/2011 13:46  A Flux thAt Binds? the southeAst AsiAn leAd isotope project 115constant throughout metal production processes. 2 This isunlike trace element patterns, which, as already mentioned, shift signicantly. Thus, in theory, the lead isotope ratios of  archaeological copper-base and lead-base artifacts can belinked to the mineralization from which they were made.The reality of archaeological LIA requires the broaching of  the following major difculties. 1. Geographically separate mineralizations may havesimilar lead isotope signatures if they are of a compa-rable geological age.2. Single mines may have a dispersed signature if com- prised of multiple sequential mineralizations of different geological ages.3. Most critically, metallurgy is an additive technol-ogy, meaning the same type of metal (e.g., copper)from multiple sources with different signatures can be blended, different types of metal (e.g., copper and tin)can be alloyed, and the resulting metal can be recy-cled many times, thoroughly mixing the srcinal leadisotope (and trace element) signatures.4. One must pay great attention to whether the alloy has been deliberately leaded. That is, if lead metal has beenadded to the alloy, it will totally overwhelm the leadisotope signature of the trace lead. The result is thatwe have two metal exchange systems, copper and lead, plotted on the same graphs with an uncertain boundary between them. Artifacts with 5–10 wt.% Pb are clearlyintentionally leaded, but at 1–2 wt.% Pb interpretationis not so simple and context and discretion must beapplied. Nevertheless, archaeological LIA is much thestronger for its disadvantages being well understood.Another issue is a necessarily slavish obedience to theProvenance Hypothesis, which, among other measures,states that, “Inter-source variation must be greater thanintra-source variation for successful source discrimina-tion” (Wilson and Pollard 2001, 508). Variation in sourcehere refers to isotopic variation in production signatures based upon analyses of smelting slag (Pryce,   Brauns, etal . , in press; Pryce et al . 2011). Unless one can distin-guish between regional production signatures (e.g., centraland northern Thailand), then one should simply give up,as the resultant lead isotope analyses of artifacts would be demonstrably meaningless. Even when sources can bedistinguished, an artifact can only be considered “prov-enanced” when its lead isotope signature is “consistent”with a known source, and even then it could come fromanother source with an identical signature. If an artifactis not consistent with any of the known sources, then itis not provenanced but characterized, potentially await-ing provenance as more production systems are studied.Strict attention to context and alternate datasets (typologi-cal, technological, and elemental) is imperative to avoiddisastrous misinterpretations. Primary Copper Source Differentiation inThailand Throughout the 1980s, 1990s, and most of the 2000suntil Sepon arrived on the scene (Sayavongkhamdy   etal . 2009), all that was known about prehistoric extrac-tive metallurgy in Southeast Asia was derived from theefforts of the Thailand Archaeometallurgy Project team,working at either end of the Loei-Petchabun Volcanic Belt (see, e.g., Natapintu 1988) (g. 1). In the northern ThaiLoei province, the rst-millennium- bce copper mine andore processing facility of Phu Lon on the banks of theMekong River was investigated in two seasons in 1984and 1985 (Pigott et al. 1992; Pigott and Weisgerber 1998).During 1986, 1990, and 1992, the team’s focus shiftedsouth to the Khao Wong Prachan valley in the central ThaiLopburi province, where several copper mineralizationswere recorded in close association with two major copper  production and settlement sites, Non Pa Wai and Nil KhamHaeng, along with other locations of interest in the vicin-ity (e.g., Ciarla 2005, 2007, 2008; Pigott   et al . 1997). TheKhao Wong Prachan area seems to transition to the BronzeAge in the terminal second millennium bce , with copper- base founding and possibly copper smelting initiated at thesame time or soon after and persisting with a trajectoryof increasing intensity and technical competence until the early/mid-rst millennium ce (Ciarla   et al ., in press; Pryce   et al . 2011). The long academic hegemony, unsought andwell deserved, of the Thailand Archaeometallurgy Projecthas led to many assumptions about where Southeast Asia’scopper was being produced (e.g., White and Pigott 1996,158, for a well-reasoned example). The following data-sets may provide the framework to put those assumptions to the test, but we must rst establish that we can distin -guish Phu Lon and Khao Wong Prachan copper, lest wefail the Provenance Hypothesis (Wilson and Pollard 2001).Though there may be sequential hydrothermal depositions of metal-bearing uids at both these mineralizations, as they lie on the same fault system, the major formation of which is assumed to be a single event in geological time,this discrimination test is critical for the ongoing validityof SEALIP research. Figure 1. General map of study area showing sites and geographical  features mentioned in the text. KWPV = Khao Wong Prachan valley; LPVB = Loei-Petchabun Volcanic Belt; PL = Phu Lon; TFB =Truongson Fold Belt; XEP = Xepon. 11 SRAAM Pryce v3.indd 11523/11/2011 13:46  Scientific ReSeaRch on ancient aSian MetalluRgy 116 Methodology SEALIP’s pilot study involved the LIA of three Metal Age(ca. 1000 bce  –ca. 500 ce ) slag samples and one Bronze Age bronze ax from Non Pa Wai, and three Metal Age slags andthree copper cordiform implements thought to be ingotsfrom Nil Kham Haeng (Pryce   et al . 2011). This pilot study did indeed provide “Southeast Asia’s rst isotopically-dened prehistoric copper production system,” with ve of the six slag samples as well as the three ingots plottingclosely together, and only the Bronze Age unleaded bronzeax proving inconsistent with the Khao Wong Prachan valleycopper signature (Pryce   et al . 2011, g. 4). Nevertheless, the outlying slag lead isotope determination undermined theintegrity of that signature and, with more funding in hand, afurther fourteen Non Pa Wai and Nil Kham Haeng slag sam- ples from the Thailand Archaeometallurgy Project archiveat the University of Pennsylvania Museum of Archaeologyand Anthropology have been added to the SEALIP data- base. As per the pilot study, these samples were analyzed bymultiple collector–inductively coupled plasma–mass spec-trometry (MC-ICP-MS) at the Curt-Engelhorn-ZentrumArchäometrie gGmbH (CEZA) in Mannheim, Germany(see Niederschlag   et al . 2003 for a detailed methodology).This central Thai material was accompanied to Germany by ten Phu Lon samples from the Penn Museum archive:seven copper mineral fragments, one slag fragment, one  bronze casting “drip,” and one bronze ax (g. 2; table 1). The ax had previously been analyzed for major elementsat the Museum Applied Science Center for Archaeology(Vincent C. Pigott, pers. comm.), but trace elements wereincluded in the present study (table 2). Although slag is the preferred material for characterizing production systems,representing only those minerals actually used by ancientmetalworkers and pooling the lead isotope signatures of other potential minor contributors like degrading ceramicsand fuel ash (Pryce   et al . 2011), very little slag was recov-ered at Phu Lon. This was possibly due to recognition issues with eldworkers inexperienced in metallurgical materials,  but the result is that the Phu Lon production signature is primarily ore mineral-based at present. Results The lead isotope results of the thirteen additional slagsamples from Non Pa Wai and Nil Kham Haeng have sub- stantially rmed up the copper production signature of  the Khao Wong Prachan valley, leaving sample SEALIP/  NPW/1 as an apparent outlier (g. 2, table 2). The Phu Lon copper production signature, based upon seven mineral andone slag samples, is far more dispersed than that for theKhao Wong Prachan valley, but most importantly, there isno overlap between those two signatures. Of Phu Lon’s twocopper-base samples, SEALIP/PL/9 and SEALIP/PL/10, the rst appears consistent with the Phu Lon copper pro -duction signature whereas the second does not (see table 2). Figure 2. Three-dimensional plot of lead data for artifacts. BA = Bronze Age; IA = Iron Age; KWPV = Khao Wong Prachan valley; NKH = Nil Kham Haeng; NPW = Non Pa Wai; PL = Phu Lon. 11 SRAAM Pryce v3.indd 11623/11/2011 13:46
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