Silver lining: evidence for Inka silver refining in northern Chile Colleen Zori a, * , Peter Tropper b a Cotsen Institute of Archaeology, University of California, Los Angeles, 308 Charles E. Young Drive North, A201 Fowler Building, Box 951510, Los Angeles, CA 90095, USA b Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52f, Innsbruck A-6020, Austria article info Article history: Received 7 February 2013 Received in revised form 18 March 2013 Accepted 21 March 2013 Keywords: Inka Archaeometallurgy Silver Cupellation Tarapacá Valley abstract Prehistoric silver purification using lead cupellation has been documented in multiple places throughout the Andes, but direct evidence of the Inka use of this technology has remained elusive. In this study, we use X-ray fluorescence, scanning electron microscopy, and electron-microprobe analysis to document direct evidence of Inka period (AD 1400e1532) silver purification using lead cupellation in the Tarapacá Valley of northern Chile. Local metalworkers used wind-driven huayra furnaces to produce pure lead metal, sustaining temperatures of ca. 900e1100 C to smelt lead-bearing ores that may have included galena. The lead metal was then used in open-vessel cupellation of silver-bearing ores, some of which may have been cupriferous and derived from the nearby Inka mines at Huantajaya. Phase analyses of the slagged interiors of bowl-shaped ceramic vessels used for cupellation indicate that the metalworkers maintained the oxidizing environment and temperatures between 800 and 1100 C requisite for cupellation. We argue that the Inka introduced this technique to Tarapacá metalworkers. The absence of finished silver artifacts in local valley contexts suggests that the refined silver was removed from the valley for use elsewhere in the empire. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction For the Inka (AD 1400e1532) of Andean South America (Fig. 1), silver represented the moon and was associated with women of the imperial lineage (Cieza de León, 1959 [1553]; Cobo, 1979 [1653]). Sumptuary laws regulated the use of silver, limiting it to individuals upon whom the Inka had conferred the privilege (Betanzos, 1996 [1557]). As noted by Sallnow (1989: 223), “all personal gifts and ornaments [of gold and silver] circulating in the empire could then be construed as fragments of this celestial world, derivingdlike the prestige and rank which they signaleddfrom the Inka king him- self”. In the non-market economy of the prehistoric Andes, the primary value of silverdand of metals in generaldwould not have been in their accumulation per se, but derived instead from sym- bolic associations with the Inka state and their worth in creating vertical and horizontal social relationships through their bestowal, use, and exchange (Lechtman, 1993; Owen, 2001). Despite the presence of native silver deposits, most silver in the Andes exists in polymetallic ore bodies, combined with other metals such as lead, gold and copper (Lechtman, 1976). There is a growing body of data suggesting that techniques employing leaddsuch as cupellationdwere used to refine silver in Andean prehistory, but direct evidence of Inka use of these technologies has not been heretofore documented. We first present a brief discussion of the techniques used in pre- Columbian silver refining, and the evidence supporting the use of such technologies in various places in the ancient Andes (Fig. 1). We then discuss new data from the Tarapacá Valley of northern Chile (Fig. 2), where metalworkers at the site of Tarapacá Viejo refined local silver-bearing ores for the Inka state. We provide direct evi- dence of lead smelting and the cupellation of high-silver ores in open ceramic vessels. There is no indication that these silver refining techniques were used in the Tarapacá Valley prior to Inka incorporation, suggesting that they were introduced through con- tact with the empire. In contrast with copper and bronze also produced by Tarapacá metalworkers (Zori et al., 2013), silver ap- pears to have been appropriated for state use elsewhere in the empire rather than circulating in the local political economy. This case study expands our understanding of silver production tech- nology in the Inka empire and how imperial incorporation altered metal production economies in the provinces. 2. Pre-industrial silver refining Throughout the pre-modern world, lead played a critical role in refining silver. At the smelting stage, leaddadded to a furnace as * Corresponding author. Tel.: þ354 612 5180 (Iceland). E-mail addresses: [email protected](C. Zori), [email protected](P. Tropper). Contents lists available at SciVerse ScienceDirect Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas 0305-4403/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jas.2013.03.020 Journal of Archaeological Science 40 (2013) 3282e3292
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Journal of Archaeological Science 40 (2013) 3282e3292
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Journal of Archaeological Science
journal homepage: http: / /www.elsevier .com/locate/ jas
Silver lining: evidence for Inka silver refining in northern Chile
Colleen Zori a,*, Peter Tropper b
aCotsen Institute of Archaeology, University of California, Los Angeles, 308 Charles E. Young Drive North, A201 Fowler Building, Box 951510,Los Angeles, CA 90095, USAb Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52f, Innsbruck A-6020, Austria
a r t i c l e i n f o
Article history:Received 7 February 2013Received in revised form18 March 2013Accepted 21 March 2013
Keywords:InkaArchaeometallurgySilverCupellationTarapacá Valley
0305-4403/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jas.2013.03.020
a b s t r a c t
Prehistoric silver purification using lead cupellation has been documented in multiple places throughoutthe Andes, but direct evidence of the Inka use of this technology has remained elusive. In this study, weuse X-ray fluorescence, scanning electron microscopy, and electron-microprobe analysis to documentdirect evidence of Inka period (AD 1400e1532) silver purification using lead cupellation in the TarapacáValley of northern Chile. Local metalworkers used wind-driven huayra furnaces to produce pure leadmetal, sustaining temperatures of ca. 900e1100 �C to smelt lead-bearing ores that may have includedgalena. The lead metal was then used in open-vessel cupellation of silver-bearing ores, some of whichmay have been cupriferous and derived from the nearby Inka mines at Huantajaya. Phase analyses of theslagged interiors of bowl-shaped ceramic vessels used for cupellation indicate that the metalworkersmaintained the oxidizing environment and temperatures between 800 and 1100 �C requisite forcupellation. We argue that the Inka introduced this technique to Tarapacá metalworkers. The absence offinished silver artifacts in local valley contexts suggests that the refined silver was removed from thevalley for use elsewhere in the empire.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
For the Inka (AD 1400e1532) of Andean South America (Fig. 1),silver represented themoon andwas associated with women of theimperial lineage (Cieza de León, 1959 [1553]; Cobo, 1979 [1653]).Sumptuary laws regulated the use of silver, limiting it to individualsupon whom the Inka had conferred the privilege (Betanzos, 1996[1557]). As noted by Sallnow (1989: 223), “all personal gifts andornaments [of gold and silver] circulating in the empire could thenbe construed as fragments of this celestial world, derivingdlike theprestige and rank which they signaleddfrom the Inka king him-self”. In the non-market economy of the prehistoric Andes, theprimary value of silverdand of metals in generaldwould not havebeen in their accumulation per se, but derived instead from sym-bolic associations with the Inka state and their worth in creatingvertical and horizontal social relationships through their bestowal,use, and exchange (Lechtman, 1993; Owen, 2001).
Despite the presence of native silver deposits, most silver in theAndes exists in polymetallic ore bodies, combined with othermetals such as lead, gold and copper (Lechtman, 1976). There is agrowing body of data suggesting that techniques employing
leaddsuch as cupellationdwere used to refine silver in Andeanprehistory, but direct evidence of Inka use of these technologies hasnot been heretofore documented.
We first present a brief discussion of the techniques used in pre-Columbian silver refining, and the evidence supporting the use ofsuch technologies in various places in the ancient Andes (Fig. 1). Wethen discuss new data from the Tarapacá Valley of northern Chile(Fig. 2), where metalworkers at the site of Tarapacá Viejo refinedlocal silver-bearing ores for the Inka state. We provide direct evi-dence of lead smelting and the cupellation of high-silver ores inopen ceramic vessels. There is no indication that these silverrefining techniques were used in the Tarapacá Valley prior to Inkaincorporation, suggesting that they were introduced through con-tact with the empire. In contrast with copper and bronze alsoproduced by Tarapacá metalworkers (Zori et al., 2013), silver ap-pears to have been appropriated for state use elsewhere in theempire rather than circulating in the local political economy. Thiscase study expands our understanding of silver production tech-nology in the Inka empire and how imperial incorporation alteredmetal production economies in the provinces.
2. Pre-industrial silver refining
Throughout the pre-modern world, lead played a critical role inrefining silver. At the smelting stage, leaddadded to a furnace as
Fig. 1. Map of Andean South America, showing the approximate borders of the Inka empire and the locations of sites mentioned in the text.
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lead-bearing ores, lead metal, litharge (PbO or lead oxide), or lead-containing slagdgives bulk to the metal fraction, thereby facili-tating separation of the metal from the slag and coalescence into asingle mass of metal. While a leadesilver bullion results fromsmelting argentiferous lead ores, such as galena, silver can also beobtained from ores that are cupriferous (Barba, 1923 [1639];Tylecote, 1964). Lead is then added in an additional step to separatethe silver from the silver-bearing copper metal. Known as leadsoaking, this process involves first melting the silverecopper metalwith up to three to four times its weight in lead (Bayley, 1992). Thesilver dissolves in the lead, but since lead is almost completelyinsoluable in copper, the cooledmetal comprises two interdigitatedphasesdone of copper and the other of silver mixed with lead. Themetal mixture is then heated to between 500 and 700 �C, above themelting point of lead (327 �C) but significantly below that of copper(1083 �C). The silverelead phase melts and trickles out, leavingbehind a porous mass of copper. Whether produced from silver-
bearing ores of copper or lead, the resulting leadesilver metal iscontained in matte cakes, comprising a mixture of metal sulfides,silicates, and traces of other base metals from the original ores.
Cupellation separates the silver from the lead and other impu-rities. In cupellation, the leadesilver bullion is heated to a tem-perature of 900 �C or higher in an oxidizing environment, causingthe formation of lead oxide, or litharge (PbO). Cupellation some-times takes place in a hearth lined with bone ash or other materialthat absorbs the litharge as it forms, eventually leaving behind anunoxidized button of pure silver (Tylecote, 1964). Litharge alsocollects oxides of the base metals still present in the leadesilverbullion. High levels of copper can be taken as evidence that thesilver derived from a copper-rich matrix not subjected to liquationprior to cupellation (Bayley, 1992: 48). Cupellation in a bone ash-lined hearth is relatively efficient and results in little silver loss,although silver can sometimes be detected in the rim around thedepression where the button of silver formed.
Fig. 2. The Tarapacá Valley of northern Chile, showing the study area and the locations of Tarapacá Viejo and all smelting sites.
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Cupellation can also be carried out in an open ceramic vessel. Ifunlined with bone ash or other absorbent material, the silica in theclay of the ceramic vessel vitrifies when heated and the resultinglead silicate slag blocks the absorption of the liquid metal oxides(Bayley, 1992, 2008; Söderberg, 2004). Because litharge is immis-cible with silver metal and is lighter (specific gravity of silver: 10.9;specific gravity of litharge: 9.5), it floats on top and can be skimmedoff, eventually leaving behind pure silver (Tylecote, 1964;Lechtman, 1976). This process is less efficient than cupellation in alined hearth and results in the loss of some silver in the matrix oflead-rich slag coating the interior of the ceramic cupellation vessel(Bayley, 2008: 138).
3. Archaeological evidence for silver production in theprehistoric Andes
Archaeological evidence dates the use of lead in Andean silverrefining to as early as the Late Formative Period (250 BCeAD 475) inthe Titicaca Basin. Calibrated radiocarbon dates associated withcupellation debris and a possible cupellation hearth at the site ofHuajje range between AD 60e120 (Schultze, 2013; Schultze et al.,2009). Lacustrine sediments from Laguna Taypi Chaka, located inthe southern Titicaca Basin near the site of Tiwanaku, document asignificant increase in volatilization and subsequent deposition oflead beginning around AD 400 and peaking at approximately AD1040 (Cooke et al., 2008). These layers track the smelting of leadores and the use of lead in silver purification in close correlationwith the development and collapse of the Tiwanaku polity.
Lead deposition in several highland lakesd Laguna Pirhuacocha,located in the Morococha mining district of central Peru; LagunaTaypi Chaka, found approximately 25 km east of Tiwanaku; andLaguna Lobato, adjacent to the rich silver deposits of Potosíd then
rose slowly throughout the Late Intermediate Period (AD 1000e1450; Cooke et al., 2008). Near the silver mines of Potosí in Bolivia,evidence for the use of wind-driven huayra furnaces to producelead-silver bullion, as well as scorification and/or cupellation inceramic bowls, has been recovered at the Late Intermediate Periodand Late Horizon (AD 1450e1532) site of Juku Huachana (Téreygeoland Castro, 2008). Huayras are columnar furnaces with walls madeeither of clay or stones and mud mortar, with holes or gaps left forthe wind to stoke the charge of fuel and unsmelted ores. Evidencefor silver production has also been found on the Peruvian coast.Litharge cakes representing the final stages of the cupellation ofargentiferous galena have been found at the central coast site ofAncón (Lechtman, 1976). The site was in use from the Middle Ho-rizon through the Inka period, although the precise chronologicalaffiliation of the metallurgical materials is unknown.
There is abundant ethnohistoric documentation that the Inkastate supported silverworkers in Cuzco, outlying Inka administra-tive settlements (including Huánuco Pampa, Hatun Xauxa, Caja-marca, and Tumbes), royal estates like Machu Picchu, and inconsolidated provincial production enclaves (Cieza de León, 1986[1553]; Cobo, 1979 [1653]; Gordon and Knopf, 2007; Ramírez,2007; Rowe, 1948). Until now, however, there has only been indi-rect evidence suggesting that lead played a role in silver refiningunder the Inka. Late Horizon deposits at the Chincha Valley site ofTambo de Mora, a likely example of a state-sponsored silverworkshop, comprise numerous furnaces used for heating andrefining, as well as slag, tuyere fragments, and artifact molds(Alcalde et al., 2002). Testing of these molds indicated that they hadbeen used to cast silver with remnant traces of lead and copper,suggesting that the metal had been produced through lead cupel-lation (Alcalde et al., 2002). The investigators did not document anyproduction debris, such as crucibles, matte cakes, litharge cakes, or
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lead-rich slags, directly indicative of silver refining activities. Thissuggests that refining took place elsewhere, with purified silvermetal brought to the enclave for the production of finished artifacts.
Other indirect evidence for the use of lead in Inka silver refiningis the presence of low levels of lead in finished silver artifacts fromthroughout the empire, remnants of lead used in purification. Silverartifacts from Machu Picchu (Gordon and Knopf, 2007) and fromLate Horizon contexts in the Mantaro Valley of Peru (Howe andPeterson, 1994) contain an average of between 0.2 and 0.9% lead,typical of silvers produced through cupellation elsewhere in theancient world (see e.g. Gale and Stos-Gale, 1981). A final indirectindicator of Inka silver refining is provided by the lacustrine sedi-ment profiles from the three Andean lakes, which document sig-nificant increases in lead deposition after the respective regionswere incorporated into the Inka empire (Cooke et al., 2008). This isconsistent with the expansion of local lead-based silver refining tomeet state demands (Cooke et al., 2008).
Table 1Materials related to silver production from excavation and survey.
TR4016 TR 05 1051.00** Furnace fragmentTR4034 L2-FF01 Furnace fragment
L2-A01-02 Slagged ceramicTR4119 FF01 Furnace fragment
FF05 Furnace fragment
4. Materials and methods
Augmenting the indirect indications of lead cupellation underthe Inka, we now present direct evidence of Inka period silverrefining in the Tarapacá Valley of northern Chile. The materialsanalyzed derive from excavations at Tarapacá Viejo and from 26smelting sites identified in a full-coverage pedestrian survey of18 km2 of the lower Tarapacá Valley (Fig. 2; Zori, 2011). TarapacáViejo was the principal administrative site in the valley, and wasoccupied continuously from the Late Formative Period until itsabandonment in AD 1717 (Núñez, 1984; Zori, 2011). Seven 1 � 2 mtest units and one 1 � 4.5 m trench were excavated at the site,sampling approximately 10% of the rooms. These excavationsyielded hundreds of pieces of unsmelted copper ore, as well ashuayra furnace fragments, slag, slagged ceramics, crucible frag-ments, casting molds, drips of metal and other production debris,and finished copper and copper-alloy artifacts (Zori, 2011; Zoriet al., 2013).
Metallurgical materials were also recovered from 26 smeltingsites. The smelting sites comprise clay-walled huayra fragmentswith slagged interiors, along with unsmelted ore, loose slag, slag-ged or vitrified ceramics, and unconsumed charcoal fuel. Thesesites were dated using surface ceramic collections and 13 calibrated14C dates obtained from charcoal collected at nine of the sites(Damiata, unpublished results).1
All of the slagged ceramics, crucibles, mold fragments, metalproduction debris, and metal objects and a sub-sample of thefurnace fragments and slag from both excavation and survey weresubjected to X-ray fluorescence (XRF) using a Bruker KeymasterPortable XRF unit with rhodium anodes. All samples were me-chanically cleaned using a brush and then subjected to 200 s of1.35e2.50 mA of radiation using a 40 kV X-ray tube. This portableXRF unit does not yield quantitative data, and so the amounts of thevarious metals can only be assessed in a relative fashion. Tables ofX-ray emission lines were used to determine the major, minor, andtrace elements in each sample.
Materials related to silver production included furnace frag-ments, loose slag, and sherds of slagged ceramic vessels with highlevels of lead and variable quantities of silver and auxiliary base
1 The smelting sites, located on wind-swept hilltops, are quite deflated and donot have a sub-surface component. Nonetheless, the hyper-aridity of the Atacamaclimate has preserved quantities of charcoal on the surface. The charcoal tested wasin clear spatial association with the furnace fragments, and the internal consistencyof the dates obtained provides assurance that they pertain to the smelting activitiesin question.
metals (Table 1). A sample of these artifacts was analyzed usingpolarized light microscopy (PLM), scanning electron microscopy(SEM), and electron microprobe analysis (EMPA). The elementalanalyses were carried out at the Institute of Mineralogy andPetrography of the University of Innsbruck, Austria, using a JEOLJXA 8100 SUPERPROBE with five WDS detectors and a ThermoNoran EDS system. To cover the chemical composition of sulfides,sulfosalts, and metals, a first analytical routine was designed toexamine the elements S, Cu, Fe, Zn, Hg, Mn, Mo, Cd, Ni, Pb, Co, Au,Ag, Ge, In, As, Sb, Bi, Se, Sn, and Te, with 50-s peak and 40-s back-ground counting times. A second routine focused on the silicateminerals, analyzing the elements O, S, Si, Mg, Fe, Mn, Cr, Ca, K, Na,Ba, Sr, Al, Ti, Ba, P, Zn, Cl, F, Sb, Cd, As, Pb, Ag, Cu, and Ni. Forstandardization, natural as well as synthetic standards (metals,oxides, silicates, glasses and sulfides) were used. Counting timeswere 20 s for the peak and 10 s for the background. Both analyticalroutines employed an acceleration voltage of 15 KV and a beamcurrent of 10 nA. Backscattered electron (BSE) images of the sampletextures were also obtained using the electron microprobe. These
TR 05 1024.002** SlagTR 05 1024.003** SlagA1-L2-M01 Lead metal
TR1065 TR05 1065.001 Slag
*Analyzed using SEM/EMPA by Dr. Peter Tropper.**Analyzed using XRF by Dr. David Scott (unpublished results).
Fig. 4. BSE image of the rim of the sample TR4000-L3-FF01, showing Pb contaminationevident by the bright Pb-bearing area. This area contains Pb drops, glass and quenchcrystals. Mineralogical remnants of the temper from an amphibole-bearing host rocksuch as amphibole (Amp), plagioclase (Pl), K-feldspar (Kfs), ilmenite (Ilm) and quartz(Qtz) can be seen.
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images allow a qualitative distinction between chemical composi-tions of different minerals since minerals with high concentrationsof heavy elements appear bright in these images.
5. Results: Archaeological evidence for Inka silver productionin the Tarapacá Valley
Evidence indicative of several stages in the production of silverwas found in the excavations at Tarapacá Viejo and at 10 of the 26smelting sites in the Tarapacá Valley (Fig. 3). These stages includethe smelting of lead in huayra furnaces and cupellation in openceramic vessels.
5.1. Lead smelting
Although XRF analysis indicates that copper was the primaryobjective of most smelts in the Tarapacá Valley, a fraction of thefurnace fragments (N ¼ 9 out of 117 fragments tested) displayedsignificantly elevated levels of lead relative to copper or othermetals. We argue that these represent fragments of furnaces usedto smelt lead for refining silver.
One high-lead furnace fragment (sample TR4000-L3-FF01) fromthe surface of site TR4000was selected for testing using SEM-EPMAanalysis. Analysis confirmed the presence of a thin (ca. 100e200 mm) rim of lead-bearing glass, containing larger lead droplets(Fig. 4). SEM-EPMA yielded as newly formed phases Pb-bearingglass, Pb prills and extremely fine-grained Pb-bearing oxides(possibly spinels) and did not indicate the presence of silver metal.This suggests that the furnacewas used to produce lead, rather thanleadesilver bullion. This practice has been observed ethnographi-cally in Bolivia, where metalworkers use the lead metal in subse-quent cupellation (Van Buren and Mills, 2005).
Fig. 3. Smelting sites in the Tarapacá Valley where lead was produced. Inset: se
Three pieces of loose slag with high levels of lead were alsorecovered from the surface of site TR4000. SEM-EPMA documentedcomplex textures in slag sample TR4000-L2-SL01, comprising alead-bearing silicate glass matrix with a multitude of silicatequench crystals and droplets of metal in which copper (with up to2 wt.% Fe) and iron (with up to 8 wt.% Cu) occur intergrown withlead (with 3 wt.% Cu; Table 2 and Fig. 5). The coexistence of copper
ven radiocarbon dates obtained from four of the ten lead-production sites.
Table 2Percentage weight of elements in prills of metal as well as oxides (EMPA analysis).
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and iron indicates temperatures of 900e1000 �C in the system CueFe (Schwarzendruber, 1993). Although not subjected to SEM-EMPA,PLM of the second slag sample from TR4000 (TR4000-L3-SL01)documented numerous prills in which copper and lead are inter-grown with iron, indicating a temperature range similar to sampleTR4000-L2-SL01. Returning to sample TR4000-L2-SL01, Pb alsooccurs as droplets in the slag. The newly formed silicate quenchcrystals are melilite (åkermanite Ca2MgSi2O7-gehlenite Ca2Al2SiO7solid solution) þ kalsilite (KAlSiO4) þ clinopyroxene (diopside,CaMgSi2O6) þ wollastonite (CaSiO3) þ alamosite (PbSiO3). Thequench crystals contain high levels of PbO, ranging from 3% PbO inkalsilite to 8% PbO in melilite to 8e37% PbO in the glass. One of twoPb-rich areas analyzed with the microprobe (sample 4000-L2-SLO1Pb-17) had a relatively high percentage weight of sulfur (almost10 wt.%), which could be a remnant of the original PbS (lead sulfide,or galena) composition.
Fig. 5. BSE image of a metal prill from sample TR4000-L2-SL01. Cu (light gray, with2 wt.% Fe) and Fe (dark gray, with up to 8 wt.% Cu) occur intergrown with Pb(white).
The third slag sample from TR4000 (sample TR4000-L2-SL02)comprises lead-bearing silicate glass containing a number ofsmall lead prills (Figs. 6 and 7). Within the Pb-bearing glass (20e55 wt.% PbO) occur a large number of silicate quench crystals ofmelilite þ clinopyroxene þ wollastonite þ alamosite. Testing ofone prill revealed that it consists of almost pure lead metal with asmall quantity of lead dioxide (Table 2). There was little inter-growth between lead and copper or other metals in this particularcase, possibly because of the high temperatures reached bythe sample: complete melting of the protolith assemblage K-feldspar þ plagioclase þ quartz documents overstepping of thedry granite melting curve, indicating temperatures in the range of950e1050 �C (Huang and Wyllie, 1975; Wyllie, 1971).
Taken together, the high levels of lead in the slagged interior ofthe furnace fragment and the slag pieces and the absence of re-sidual silver indicate that these are the byproducts of smelting lead.
Fig. 6. BSE image of the rim of sample TR4000-L2-SL02. There is strong variation in thePb content of the glass, evident by bright and dark areas in the Pb-bearing glass, whichalso contains wollastonite quench crystals. The relict black rounded grains are quartz.
Fig. 8. BSE image of the rim of sample TR4005-L1-INL4 showing strong Pb contami-nation, evident by the bright area, which contains Pb-rich glass, bright Pb quenchphases and gray silicate quench crystals.
Fig. 9. BSE image of a prill of Cu (gray, 8 wt.% Ag) and Ag (white, 17 wt.% Cu) in PbeSiglass (light gray), which recrystallized to alamosite (Al) and black silicate quenchcrystals (Ks, kalsilite and Me, melilite). Cuprite, Cu2O, also occurs. Sample TR4005-L1-INL4.
Fig. 7. Sample TR4000-L2-SL02. BSE image of high temperature quench crystals ofwollastonite (Wo) and melilite (Me) in Pb-bearing glass (L). Pb occurs as small droplets.
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Examples of pure lead metal, including two of amorphous shapeand one piece of folded sheet metal, were found in the upperexcavation levels at Tarapacá Viejo and likely served as metalliclead stock for cupellation.
5.2. Open-vessel cupellation
Recovered in the upper excavation contexts at Tarapacá Viejoand from the surface at nearby smelting site TR4005, fragments ofopen ceramic vessels with dark gray, lead-rich slag on their in-teriors testify to silver refining through cupellation in the TarapacáValley (Table 1). Rather than being produced specifically formetallurgical purposes, these open vessels are repurposed ceramicbowls or fragments of larger vessels. The majority are in the LocalInka style, while a small number date to the Colonial Period. Sincemetalworkers in the valley employed vessels not specificallydesigned for metal production, it is likely that they chose vesselsreadily available at the time, supporting the assertion that theadoption of silver refining dates to the Late Horizon and continuedinto the historic period (Zori and Tropper, 2010).
Of the 14 fragments of slagged ceramics recovered at TarapacáViejo and subjected to XRF testing, six were shown to containelevated levels of lead and traces of silver and base metals,including copper. This is indicative of the removal of lead and othermetal impurities through the formation of litharge and lead silicateslag. It is telling that all of the high-lead vessel fragments and two ofthe three pieces of stock lead were found in the area of the site withthe highest concentration of Inka artifacts, supporting the LateHorizon date for the adoption of silver refining techniques.
Three of the 14 high-lead slagged ceramic fragments recoveredfrom TR4005 were subjected to SEM-EMPA. All three derive fromLocal Inka style bowls, characterized by extremely fine (<300 mmdiameter) and uniform temper, typical of Inka-influenced LateHorizon ceramics.
Sample TR4005-L1-INL4 displays a dark gray lead silicate slagcontaining up to 28 wt.% PbO coating the interior to a thickness of300e500 mm (Fig. 8). This lead-rich rim contains at least one prill inwhich copper (with 4 wt.% Ag) occurs intergrown with silver (with7.5e17wt.% Cu; Table 2 and Fig. 9). The composition of the coexistingmetals in this prill indicates temperatures of ca.1000 �C in the systemAgeCu (Bale et al., 2002). This is in accordance with the observed
melting of the temper assemblage amphiboleþ plagioclaseþ quartz(Table 3), which takes place at temperatures of ca. 1050 �C (Grapes,2011). Within the Pb-rich glass, alamosite, melilite and kalsilitealso occur (see Fig. 8). The quench crystals contain high levels of PbO,ranging from 2 to 16% PbO in kalsilite and 5e8% PbO in melilite. Thepresence of Cu in the silver droplet and additional prills of Cu metalcould suggest that the silverwas obtained fromcopper-richminerals(Bayley,1992: 48). Theoccurrenceof the copperoxide (cuprite, Cu2O;Fig. 9, Table 2) supports the assertion that the sample reached tem-peratures of ca. 1000 �C, according to the system CueO (Bale et al.,2002) and is consistent with the interpretation that the reactionstook place in the oxidizing environment needed for cupellation.
XRFanalyses of the dark gray slag lining sample TR4005-L1-A01-02 showed high levels of silver and lead, with lower percentages ofcopper. SEM-EMPA confirmed that it was comprised of a silicateglass (<300 mm in thickness) with strong lead contamination and
Table 3Protolith rocks used as temper for metallurgical ceramics and their possible geological origin.a
Sample Mineral assemblage of the temper Protolith rock/possible geologicalorigin
a The geological origin of the temper materials was reconstructed using the detailed map of the area (1:50,000) and local formation names according to SERNAGEOMIN(2003).
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containing a prill in which copper (with 8 wt.% Ag) is intergrownwith silver (with up to 7 wt.% Cu; Figs. 10 and 11; Table 2). Thecomposition of these coexistingmetals indicates temperatures of ca.800e900 �C in the systemAgeCu (Bale et al., 2002). These should beconsidered as minimum temperatures, as melting of the granitic/
Fig. 10. BSE image of the rim of the sample showing strong Pb contamination, evidentby the bright Pb-rich glass, Pb-bearing quench phases and gray silicate quench crystals.Sample TR4005-L1-A01-02.
Fig. 11. BSE image of a drop of Cu (gray, 8 wt.% Ag) and Ag (white, 7 wt.% Cu) coexistingwith alamosite (Al, light gray) and black quench crystals (Kls, kalsilite). Cuprite (Cu2O)also occurs. Sample TR4005-L1-A01-02.
granodioritic temper assemblage (Table 3) indicates overstepping ofthe 950e1050 �C temperature range at which dry granite becomesmolten. Within the Pb-bearing glass, quench crystals ofmelilite þ kalsilite þ alamosite occur. The quench crystals containhigh percentages of PbO: 5% PbO in kalsilite and 3e7% PbO in
Fig. 12. BSE image of quench crystals of kalsilite (Ks, black), Pb3O4 (minium), alamosite(Al) and melilite (Me) in Pb-Si glass (L) of sample TR4005-L3-A1.
Fig. 13. High-Pb slagged ceramic (artifact TR4005-T1L) with detachment scar fromremoval of purified silver.
Fig. 14. Geological map of the study area in northern Chile (1:50,000, redrawn from SERNAGEOMIN, 2003).
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2 A small number of silver artifacts have been recovered from Middle Horizon(AD 500e1000) cemeteries surrounding the oasis of San Pedro de Atacama. Evi-dence for local silver production has not been found, however, suggesting that theywere probably brought to the site from elsewhere (B. Maldonado, personalcommunication, 2013).
3 There is at least some evidence that the Inka did not restrict access to finishedsilver goods in places where the elite had had them previously. In his work in theCentral Andean sierra, Owen (2001) shows that Xauxa elites saw only a minordecline in access to silver during the Late Horizon compared to the pre-Inka period.Silver was not local to this area and had to be obtained through long-distance trade.Owen argues that “since the amount of silver in circulation did not decline, the Inkaevidently either failed to control traditional Xauxa channels for acquiring silver ormore characteristically, took over the redistribution role without reducing the totalflow of silver” (Owen, 2001: 287).
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melilite. Fig. 11 depicts a particularly large droplet of silver metal,containing 4e6 wt.% Cu. As with sample TR4005-L1-INL4, thepresence of cuprite, (Cu2O; Fig. 11, Table 2) is indicative of relativelyhigh temperatures in the system CueO, as well as an oxidizingenvironment. The Cu prills and low but ubiquitous presence ofcopper in the silver droplets also indicate that the silver was mostlikely present as metal coexisting with OH-bearing copper mineralssuch as atacamite [Cu2(OH)3Cl] and azurite [Cu3(CO3)2(OH)2]. Theseminerals have been reported from the Huantajaya silver mines,located some 60 km west of the Tarapacá Valley (Maksaev et al.,2007). Huantajaya is known to have been a locus of Inka silverextraction (Cobo, 1979 [1653]: Ch. 33; Pizarro, 1986 [1571]: 189).
The dark gray slag of sample TR4005-L3-A1 contained up to52 wt.% lead oxide (PbO), or litharge. Quench crystals of kalsiliteand melilite were found together with Pb-bearing minerals ala-mosite and minium (Pb3O4) in the PbeSi glass of the slag (Fig. 12),indicating that the slag reached temperatures necessary to becomefully molten. The melilite quench crystals contain 11e16 wt.% andkalsilite contains 2e11 wt.% PbO. Although none of the prills werelarge enough to be tested using the electron microprobe, XRFanalysis did not indicate the presence of silver in the slag.
One final piece of evidence testifies to the cupellation processescarried out at TR4005. Artifact TR4005-T1L is a fragment of a LocalInka style vessel bearing a dark gray slag that XRF testing revealed isvery high in lead. This fragment displays a circular depression that islikely a detachment scar, resulting from the removal of the button ofsilvermetal produced through cupellation (Fig.13).While silverwasnot detected in the slag on the vessel walls, XRF of the rim of thedetachment scar identified small quantities of residual silver.Several other examples of slagged ceramics with possible detach-ment scars were encountered in excavations at Tarapacá Viejo.
5.3. The nature of the temper and its geological context
Table 3 lists the observed silicate temper assemblages in the claysof the furnace and slagged ceramic fragments. All of the investigatedsamples showextremelywell-ground tempermaterialsdalmost norock fragments and only single mineral grains were observed. Thetemper materials cluster into two groups: granodioritic (TR4000-L3-FF01, TR4005-L1-A01-02, TR4005-L3-A1) and basaltic (TR4005-L1-INL4). Textural observations show that the fragment size oftemper is <300 mm in the crucible fragments and <600 mm in theslag and furnace fragments. The geological map of the area (Fig. 14)indicates that all tempermaterials could have been obtained locally:the relevant rock types occur locally either in situ or as fragments inthe alluvial deposits (SERNAGEOMIN, 2003).
6. Discussion and conclusions
Evidence from the Tarapacá Valley of northern Chile providesdirect documentation that the Inka used lead cupellation in thepurification of silver. Tarapacá metalworkers employed wind-driven huayra furnaces to produce pure lead metal, sustainingtemperatures of ca. 900e1100 �C to smelt lead-bearing ores thatmay have included galena. Phase analysis demonstrates a highdegree of melting at the rims of the samples, producing Pb-bearingglass and high-temperature quench mineral such as melilite, kal-silite and wollastonite, indicative of temperatures of ca. 1000 �C inthe chemical system PbOeZnOeFe2O3e(CaO þ SiO2) (Jak andHayes, 2002). As the slag cooled to lower temperatures (600e700 �C), typical Pb-quench phases such as alamosite formed (Ettleret al., 2009).
The lead metal was then used in open-vessel cupellation ofsilver-bearing ores, some of which may have been cupriferous andderived from the Inka mines at Huantajaya. Phase analyses of the
metal prills (Table 2) and silicate temper assemblages (Table 3)indicate that the metalworkers maintained the oxidizing environ-ment and temperatures between 800 and 1100 �C requisite forcupellation. Some litharge accumulated on the vessel interiors andwas incorporated into the lead silicate slag, while the remainder ofthe litharge must have been removed manually. Several examplesof slagged ceramics with detachment scars demonstrate where thepurified silver metal was removed after cupellation.
Although copper production had been on-going in the TarapacáValley since at least AD 1250 (Zori et al., 2013), evidence suggeststhat the Inka probably introduced purification of silver using open-vessel cupellation to Tarapacá Valley metalworkers. Smelting oflead occurs only at sites where calibrated 14C dates and ceramicassemblages include or are limited to the Late Horizon (Fig. 3). Noproduction debris indicative of silver refining were found in pre-Inka contexts, neither in the valley nor in other pre-Inka metal-lurgy sites in northern Chile2 (see e.g. Figueroa et al., 2009; Graffamet al., 1996; Núñez,1987; Salazar et al., 2010), and all of the ceramicsused in lead cupellation in the Tarapacá Valley date to either LateHorizon or the Colonial Period. In addition, no silver artifacts havebeen recovered from pre-Inka contexts anywhere in the valley.
It is critical to note, however, that artifacts of silver are absentfrom Inka period contexts in the Tarapacá Valley as well, and thereis no evidence for the production of finished silver objects.3 Thisstands in stark contrast with the copper and bronze also worked byTarapacá metalworkers, for which there is abundant evidence ofboth the production (e.g. casting molds, drips, lamina) and circu-lation of finished metal objects (Zori et al., 2013). It is likely that theInka removed the refined silver from the Tarapacá Valley to betransformed into finished objects by state-supported specialists atInka centers or production enclaves, such as Tambo de Mora. Acomparable arrangement has been observed at the Inka silvermining site of Porco: subjects fulfilled their labor obligations bymining, smelting, and refining silver, but there is no indication ofthe production of silver artifacts (Van Buren and Presta, 2010: 185).By introducing silver refining techniques to the experienced met-alworkers of the Tarapacá Valley, the Inka both reoriented localproduction toward a metal desired by imperial elites and ensuredthat all of the silver would be directed upwards toward state con-sumption, albeit away from where it was originally refined.
Acknowledgments
These investigations were conducted under the Tarapacá ValleyArchaeological Project (TVAP) and Proyecto FONDECYT 1030923.Financial support was provided by the National Science Founda-tion, the Cotsen Institute of Archaeology, and UCLA Department ofAnthropology. Our sincerest gratitude to Mauricio Uribe, RanBoytner, David Scott, and Charles Stanish. Thanks also to DavideZori, Carol Schultze, and two anonymous reviewers for commentson earlier drafts of this paper.
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