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Late Bronze and Early Iron Age copper smelting technologies in the South Caucasus: the view from ancient Colchis c. 1500-600 BC

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"Many of the arguments for how and why people began to use iron in Southwest Asia rely on assumptions about the technology and relative organization of copper and iron smelting. However, research on the technological transformations of the Late
  Late Bronze and Early Iron Age copper smelting technologies in theSouth Caucasus: the view from ancient Colchis c. 1500 e 600 BC Nathaniel L. Erb-Satullo a , * , Brian J.J. Gilmour b , Nana Khakhutaishvili c a Department of Anthropology, Harvard University, 11 Divinity Ave, Cambridge, MA 02138, USA b Research Lab for Archaeology and the History of Art, Oxford University, Dyson Perrins Building, South Parks Rd, Oxford OX1 3QY, United Kingdom c Department of History, Archaeology, and Ethnology, Shota Rustaveli State University, 10 Rustaveli Ave, Batumi 6010, Georgia a r t i c l e i n f o  Article history: Received 10 November 2013Received in revised form13 March 2014Accepted 29 March 2014Available online xxx Keywords: Bronze AgeIron AgeNear EastMetalSlagTechnologyProduction a b s t r a c t Many of the arguments for howand why people began to use iron in Southwest Asia rely on assumptionsabout the technology and relative organization of copper and iron smelting. However, research on thetechnological transformations of the Late Bronze Age and Early Iron Age suffers from a lack of investi-gation of primary metal production contexts, especially in regions outside the Levant. The currentresearch examines metal production debris from a large number of smelting sites in western Georgia,and addresses questions of technology and resource utilization through detailed examination of fewselect sites. Through the chemical and mineralogical analysis of slag samples, we demonstrate the ex-istence of an extensive copper-production industry and reconstruct several key aspects of the smeltingtechnology during the Late Bronze Age and Early Iron Age. Combining a statistical analysis of slagmineralogy with other lines of evidence, we argue that copper was extracted from sul fi de ores through aprocess of roasting and smelting in deep pit furnaces. The data also suggest that metalworkers atdifferent sites exploited different ore sources within the same ore body. These results form a funda-mental basis for further examination of spatial and chronological patterns of technological variation,with implications for models of Near Eastern copper production in this crucial period. Intriguing evi-dence of bloomery iron smelting, though currently undated, reinforces the region ’ s potential to providedata on a key technological transformation.   2014 Elsevier Ltd. All rights reserved. 1. Introduction The organization of metal production and the processes of technological change are key areas of interest for archaeologistsstudying the Eastern Mediterranean and the Southwest Asia. Yetdespite a number of theories about the reasons for the rise of ironproduction, many signi fi cant questions remain about technologicaland social changes occurring during this period.Perhaps the single most signi fi cant reason for the lack of reso-lution on many of these issues is the absence of data on the tech-nology and organization of metal production activities in the LateBronze Age (LBA) and Early Iron Age (EIA). Investigations of LBA e EIA copper smelting are rare in Southwest Asia outside of a fewwell-studied regions such as Cyprus (Kassianidou, 2012; Knapp,2012) and the Southern Levant (Barker et al., 2007; Hauptmann,2007; Levy et al., 2012). For iron, the picture is even more sparse.There is only one well-studied example of primary iron smelting(Tell Hammeh, Jordan), and only a few secondary iron smithingworkshops have been found dating to before about 500 BC(Eliyahu-Behar et al., 2008; Eliyahu-Behar et al., 2012; Veldhuijzen,2012; Veldhuijzen and Rehren, 2007).Without evidence from primary production contexts for bothiron and copper alloys, it is very dif  fi cult totest theories about howiron production emerged. Many argue that the organization anddistribution of copper/bronze and iron production differed in sig-ni fi cant ways, making iron more attractive than bronze for certaintypes of objects. Iron ’ s geological ubiquity, contrasted with copperand tin ’ s geological rarity, remains a signi fi cant feature of manyexplanations (Mirau,1997:110 e 111), even if the hypothesis of a tinshortage driving the spread of iron has lost popularity (Muhly,2003:180, Waldbaum, 1999:39). The assumption that the distri- bution of early ironproduction matched the geological distribution *  Corresponding author. Tel.:  þ 1 413 441 2331. E-mail addresses:  nsatullo@fas.harvard.edu (N.L. Erb-Satullo), brian.gilmour@rlaha.ox.ac.uk (B.J.J. Gilmour), nana_khakhutaishvili@yahoo.com(N. Khakhutaishvili). Contents lists available at ScienceDirect  Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas http://dx.doi.org/10.1016/j.jas.2014.03.0340305-4403/   2014 Elsevier Ltd. All rights reserved.  Journal of Archaeological Science 49 (2014) 147 e 159  of ore deposits ignores the human element of production. Thelandscapeoftechnicalknowledgeandsocio-technicpracticeslikelyhadahugein fl uenceonwhereandhowironproductiondeveloped.The western part of the Republic of Georgia, known in ancienttimes as Colchis, is an ideal place to investigate the relationshipbetween copper and iron production (Fig. 1). Archaeological exca-vations and chance  fi nds have yielded huge numbers of bothcopper-base and iron artifacts, and the region is extremely rich incopper and iron ores (Gambashidze et al., 2001; Mikeladze andBaramidze, 1977; Nazarov, 1966; Papuashvili, 2011; Tvalchrelidze,2001). Evidence of metal production occurs both in settlementsites (casting molds and tuyères) (Mikeladze, 1990:26), and atdedicated smelting sites. Previous analyses of slags from Colchianmetal production sites offer contradictory interpretations aboutwhether iron or copper was produced (Inanishvili, 2007; Nieling,2009:257 e 259, Tavadze et al., 1984).InordertoexaminetheorganizationandtechnologyofLBA e EIAmetal production, we have started a new  fi eld project to locate,map, and investigate new and previously-identi fi ed smelting sites.This paper focuses on the identi fi cation of metal smelting debrisand the technologies of metal smelting. The clari fi cation of theseproduction processes is fundamental to further research for tworeasons. First, by establishing the various activities which occurredateachsite,wecangainaclearerpictureoftheorganizationofcraftproduction. Spatial variations in resource acquisition and produc-tion practices may suggest varying practices across differentcommunities, and may hint at the participation of distinct socialgroups in the production process.Second, through a reconstruction of smelting technologies, wecan begin to understand how metalworkers adapted or preservedtraditions of copper production during the emergence of ironsmelting technology. A number of scholars have argued that ironwasinvented bytheaccidentalproductionofironintheprocessof copper or lead smelting (Charles,1980:165 e 166, Gale et al.,1990;Pigott, 1982:21, Wertime, 1964:1262), though direct evidence for this is lacking (Merkel and Barrett, 2000). Regardless of whetherthe invention of iron occurred in this way, it is reasonable to hy-pothesize that experience in manipulating ores at high tempera-tures, gained through the smelting of copper, would haveimpacted the adoption and spread of iron technology. Oneperspective might argue that early iron technologies would fl ourish in regions that also had long-standing traditions of cop-per production. A contrary view might propose that an elaborateand conservative tradition of bronze production would slow thesocial acceptance and adoption of iron and iron-making technol-ogies ( Japaridze,1999:65). Testing these models requires accuratereconstructions of technical practices and clear evidence for thecontexts of different production activities. The goal of the presentstudy is to determine what kinds of metal were produced atsmelting sites in western Georgia, identify the types of ores used,and reconstruct the practices used in the smelting process. This isa fundamental  fi rst step in the investigation of questions of  Fig. 1.  Map of Colchis showing key metal producing areas. Smelting sites outside of the Supsa e Gubazeuli production area are marked on the map. Elevation data: SRTM. N.L. Erb-Satullo et al. / Journal of Archaeological Science 49 (2014) 147  e 159 148  economic organization, resource acquisition, and the socialcontext of technological change. 2. Metal smelting in western Georgia Hundredsofsmeltingfurnaceshavebeenreportedintheregion,some of which have been excavated (Gzelishvili, 1964;Khakhutaishvili, 1976, Khakhutaishvili, 2009 [1987],Khakhutaishvili, 2006; Khakhutaishvili, 2008). Most of the previ-ous radiocarbon and paleomagnetic dates for these sites fall be-tween c. 1500 e 600 BC, with one or two sites attributed to theperiod of Greek colonization and in fl uence (beginning in the mid1st millennium BC), and a few to the  fi rst half of the 2nd millen-nium BC (Khakhutaishvili, 2009 [1987]:105 e 106). Limited ceramicevidence also suggests that most sites belong to the Late Bronzeand Early Iron Age.In two seasons of   fi eldwork, we have mapped over 50 smeltingsites in the region. The main focus in these two seasons was theregion of the Supsa and Gubazeuli rivers (Fig. 2), whichyielded theoldest dates in earlier  fi eldwork (Khakhutaishvili, 2009[1987]:105 e 106). From a regional perspective, metallurgical activ-ity seems to have been clustered in several production areas,probably due to the location of the necessary resources: ore, fuel, Fig. 2.  Map of smelting sites discovered in the Supsa e Gubazeuli production area. Elevation data: ASTER GDEM (a product of METI and NASA). Fig. 3.  Copper smelting furnace excavated at site 46, showing a ring of reddish burnedclay surrounding the pit. The length of the scale bar is 1 m. (For interpretation of thereferences to color in this fi gure legend, the reader is referred tothe webversion of thisarticle.) N.L. Erb-Satullo et al. / Journal of Archaeological Science 49 (2014) 147  e 159  149  and clay (Fig. 1). However, within these areas, metal productionsites are quite dispersed. Previous excavations have shown thatthey generally consist of the one or two furnaces, a scatter of slag,and sometimes a small platform with signs of burning. Furnacestakethe formofpits dugintothe natural claysubstrate,usuallytoadepth of a little over 1 m (Khakhutaishvili, 2009 [1987]) (Fig. 3). Large amounts of partially vitri fi ed and slagged ceramic materialstrongly suggest that these pits were lined with a layer of clay. Thepresence of tuyèrefragments, some which are slagged, indicate themethod of delivering air to the furnace, but the exact geometry of tuyère positioning is not clear. A large proportion of tuyère frag-ments are not slagged, and larger fragments show unusual curva-ture,  fl ared interlocking sections, and occasional side holes,suggesting a rather complex air deliverysystem that probably useda forced draft (see Khakhutaishvili, 2009 [1987]:43, 67, 73, 88,100).Buildings or other habitation evidence have not been found inclose proximity to these sites, and non-metallurgical pottery isoften present in only small amounts, even in fully excavated sites(e.g. Khakhutaishvili, 2009 [1987]:55, 58). While the size of eachindividual slag heap does not approach the massive scale seenelsewhere in the Near East (Levyet al., 2004), in aggregate the sitesrepresent an extensive landscape of production. Considering theextremelydensevegetationandthesmallsizeoftheslagheaps,itislikely that the rough count of 400 sites mentioned by DavidKhakhutaishvili (2009 [1987]:17), represents a lower limit for theactual number of sites in the LBA e EIA.As the primary form of material culture found at smelting sitesin Colchis, slags are crucial for reconstructing the kinds of activitiesthat occurred there. Detailed analysis of these materials can revealthe type of metal produced, the kinds of ores used, and candifferentiate between secondary shaping (casting and forging) andprimarysmelting.Moreover,theexaminationofthemineralphasespresent in the slag can reveal the atmospheric conditions in thesmelt, and help to reconstruct the smelting process.The discussion about whether sites in western Georgia repre-sent the remains of copper or iron production has centered arounda limited number of chemical and microstructural analyses. Wüs-tite (FeO) and metallic iron are characteristic features of bloomeryiron slags. Although copper smelting slags contain various ironoxides andoccasionally metallic iron, theyare clearly distinguishedby the presence of copper-bearing phases, visible in polished sec-tions under the microscope.Previous analyses of slags from smelting sites in western Geor-gia offer contradictory interpretations. Early studies of these slags(Tavadze et al.,1984) argue that they represent the remains of ironproduction, pointing tothe presence of wüstite and metallic iron inthe slags. Published photomicrographs show abundant dendriticminerals, and metallic ironwas reported. Neither copper metal norcopper-bearing mineral phases are mentioned (Inanishvili, 2007;Tavadze et al., 1984). On the other hand, the discovery of coppersul fi des in a more recent analysis of a few slags led Nieling tointerpret them as the result of a matte smelting operation, whichproduces a consolidated mass of copper sul fi des, known as matte,as an intermediate stage of production (2009:257 e 259). However,not all slags with sul fi des in them are the result of a true mattesmelting process, inwhich the matte is produced in an initial stagebeforebeingcrushed, roastedinan oxidizingatmosphere,andthensmelted again.Unfortunately,onlyasmallnumberofphotomicrographs,whichare crucial for determining the type of slag, have been published.Bulk chemical analysis is reported for a larger number of samples(Inanishvili, 2007:12 e 13), but copper and iron smelting slags canbe very close in bulk chemical composition, with the only distinc-tion being the presence in the former of roughly 0.5 e 3.0 wt.%copper (Pleiner, 2000:254). Iron smelting slags typically havecopper values under 200 ppm (0.02 wt.%) (Humphris et al.,2009:364, Veldhuijzen and Rehren, 2007:194). However, pub- lished bulk chemical analyses of slags from Colchis do not reportthe copper content. 3. Analytical methods of slag analysis Slag was collected from surface scatters, previously excavatedmaterial remaining at the sites, and new excavated contexts. 134samples of slag and 1 sample of matte from 34 sites were mountedand analyzed by the author (NES) via optical microscopy, while asubset of these (102 slag samples from 24 sites) has also beenanalyzedusingscanningelectronmicroscopy inordertodeterminethe mineralogy and chemistry of the sample. The samples comefrom sites in the Supsa e Gubazeuli, Choloki e Ochkhamuri, and theSkhalta e Adjaristskali production areas (Fig. 1). For each sample,mineral phases and inclusions (e.g. charcoal, partially reactedgangue fragments, pieces of partially melted technical ceramic)were coded as being present in a signi fi cant number of instances(coded as  “ 2 ” ), present in rare isolated instances (coded as  “ 1 ” ), ornot identi fi ed in the sample (coded as  “ 0 ” ). Mineralogical identi fi -cations were carried out by re fl ected-light optical microscopycross-checked with energy dispersive X-ray microanalysis (EDS).Morphology, optical properties, elemental composition, and para-genetic associations were used to make identi fi cations.In order to obtain major element chemical compositions, EDSarea analyses were carried out on fully melted regions of thesample, avoiding unmelted inclusions, corroded areas, and largevoids. Analyses of at least four different areas were averagedtogether. In nearly all cases, intra-sample variation was minor. AllSEManalyseswerecarriedoutbytheauthor(NES),usinganOxfordInstrumentsINCAX-SightEDSsystemattheMuseumofFineArtsinBoston. 4. Analytical results of slag analysis 4.1. Macroscopic analysis of copper smelting slags The surfaces of most slags are covered with buff to reddish-orange corrosion. Tell-tale copper-green corrosion was only rarelyobserved. Slags from the Supsa e Gubazeuli and Choloki e Ochkha-muri production areas can be categorized into several macroscopicgroupings.The most distinct andeasily recognizable type ofslag consists of fragments of dense cakes with few voids. The slag matrix is veryhomogeneous, and there are usually very few partially reacted in-clusions.Larger, more complete examplesshowthatthese slags areparts of large cakes, with variable diameters typically about 20 e 30 cm, and thicknesses around 10 cm. The rarity of complete cakesis most likely a result of the ancient metalworkers breaking themapart to free material that pooled underneath, probably coppermatte (sul fi des) or copper metal. While the upper surface of theseslag cakes is more common, one example (Fig. 4, right) has aparticularly well preserved bottom surface, which shows the for-mation of a meniscus at the interface between the slag and themetal or matte below it. Slags of this type probably formed at thebottom of the deep pit furnaces.A second category of slag consists of amorphous, sponge-likemasses, often with charcoal fragments encased within the slagmatrix. Slags of this type often contain numerous partially reactedminerals and rock fragments (Fig. 4, left). Small amorphous drips,splashes, and lumps of slag were also assigned to this category. Arare third type of slag, sometimes dif  fi cult to distinguish from theamorphous spongy slag, was designated tap slag. These glassierslags show evidence of rapid cooling and  fl ow patterns. However, N.L. Erb-Satullo et al. / Journal of Archaeological Science 49 (2014) 147  e 159 150  this type of slag is rare at sites in the Supsa e Gubazeuli and Chol-oki e Ochkhamuri areas.There are strong similarities between the slag cake fragmentsand the copper smelting slags found at the LBA site of PolitikoPhorades on Cyprus (Kassianidou, 1999; Knapp and Kassianidou,2008). Knapp and Kassianidou (2008:144) argue, on the basis of  experimental work (Bamberger and Wincierz,1990:133) that slagsof this type were formed by draining the whole contents of thefurnace into a pit. In our case, however, this tapping process ishighly unlikely, given the evenpattern of burning encircling the pitfurnaces we have excavated (Fig. 3). This pattern, along with thedepth of the pit, suggests that the pits are the reaction chambers of the furnaces, not forehearths into which the furnace contents weredrained. Moreover, there is no trace of any possible furnace struc-ture off to the side of the pits we excavated. Earlier excavation re-ports reveal similar patterns for nearly all other furnaces, with twopossibleexceptions.Inthesecases,theexcavatordescribedunusualshallow depressions  fi lled with burned clay and charcoal andadjoining the typical deeper stone-lined pit. However, theseshallowdepressionswereinterpretedasthelocationof thebellows(Khakhutaishvili, 2009 [1987]:69, 72). Because we have not foundanyoftheseshallowdepressionsinourcurrentproject,itisdif  fi cultto speculate further on their function. 4.2. Mineralogy and chemistry of copper smelting slags The vast majority of the slags analyzed for the current study,including all samples from the Choloki e Ochkhamuri and Supsa e Gubazeuli production areas, are the remains of copper smelting.Most slags are characterized by the presence of olivine((Fe,Mg) 2 SiO 4 ), as well as variable amounts of magnetite (Fe 3 O 4 )and less commonly wüstite (FeO) (Fig. 5). Hematite (Fe 2 O 3 ) wasobserved in a few samples, but may be either post-depositionalalteration or a partially reacted addition to the furnace. A widerange of copper and iron sul fi de phases were identi fi ed in theslags. Most common are chalcopyrite (CuFeS 2 ), bornite (Cu 5 FeS 4 ),iron sul fi de (probably srcinally pyrite (FeS 2 ), but transformedinto pyrrhotite (Fe 1   x S) at high temperatures), sphalerite ((Fe,Zn)S), and covellite (CuS). Chalcocite (Cu 2 S), and digenite (Cu 9 S 5 ) alsoappear frequently. These mineral phases were identi fi ed micro-scopically in matte prills solidi fi ed from the melt, and, in the caseof copper e iron sul fi des, the iron sul fi de, and sphalerite, in pri-mary, partially-reacted ore and gangue fragments. Bornite andchalcopyrite are often  fi nely interspersed in both matte prills andpartially reacted ore inclusions. Bycontrast, copper oxides such ascuprite (Cu 2 O) and malachite (Cu 2 CO 3 (OH) 2 ) were found onlyoccasionally, and their morphology suggests that most are post-depositional corrosion of copper sul fi des or copper metal. Frag-ments of partially reacted ore and gangue consist of variouscombinations of   fi nely interspersed quartz, iron oxide, sphalerite,copper e iron sul fi de, and iron sul fi de (Fig. 6). Chalcopyrite andpyrite were probably the srcinal copper and iron sul fi des in theore, but high temperatures has partially transformed chalcopyrite Fig. 4.  Examples of a spongy amorphous copper smelting slag (sample 301) and a copper smelting slag cake fragment (sample 4301). Fig. 5.  SEM backscatter image of a typical copper smelting slag (sample 2702) showingfayalite (1), magnetite (2), copper e iron sul fi de (ex-solution texture of chalcopyrite andbornite) (3) and iron sul fi de (4) in a glassy matrix (5). N.L. Erb-Satullo et al. / Journal of Archaeological Science 49 (2014) 147  e 159  151
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