Isotopic Ag–Cu–Pb record of silver circulation through16th–18th century SpainAnne-Marie Desaultya,b,c,1, Philippe Telouka,b,c, Emmanuelle Albalata,b,c, and Francis Albarèdea,b,c

aEcole Normale Supérieure de Lyon, F-69342 Lyon, France; bUniversité de Lyon, 69622 Villeurbanne, France; and cCentre National de la Recherche Scientifique,UMR 5276, 69364 Lyon Cedex 07, France

Edited* by Donald J. DePaolo, University of California, Berkeley, CA, and approved April 13, 2011 (received for review December 6, 2010)

Estimating global fluxes of precious metals is key to understandingearly monetary systems. This work adds silver (Ag) to the metals (Pband Cu) used so far to trace the provenance of coinage throughvariations in isotopic abundances. Silver, copper, and lead isotopeswere measured in 91 coins from the East Mediterranean Antiquityand Roman world, medieval western Europe, 16th–18th centurySpain, Mexico, and the Andes and show a great potential for prove-nance studies. Pre-1492 European silver can be distinguished fromMexicanandAndeanmetal. EuropeansilverdominatedSpanish coin-age until Philip III, but had, 80 y later after the reign of Philip V, beenflushed from the monetary mass and replaced by Mexican silver.

silver coinage | Spanish Americas | Price Revolution | MC-ICPMS

Aparticularly momentous time during the early history ofmodern European economy was the attempt by Hamilton

(1) to demonstrate that the great Price Revolution (1520–1650)was largely fueled by the influx of American silver rather thanby widespread coinage debasement and minting of the low-denomination copper “vellón”. The idea connecting silver influxto European inflation was actually proposed as far back as the16thcentury by the French philosopher Jean Bodin (2) and is com-monplace in classical economics. Huge amounts of silver, ∼300 tannually (3–5), weremined in the Spanish Americas from the 16thto the 18th centuries. That much silver could not be absorbedlocally by the American economy and therefore headed for theEuropean market through major Spanish harbors (6), notablySeville (7), and to the Far East either directly through the Phil-ippines or indirectly through Europe (8). The thesis that the PriceRevolution in Spain was fueled by the influx of American silverhas, however, become controversial in recent literature (9–11).More specifically, some authors emphasized that the arrival ofAmerican metals (ca. 1550 to ca. 1809) does not coincide with theperiod of inflation (ca. 1520 to ca. 1650) (9–11). Understandingsilver monetary mass and circulation relies on three types of pri-mary data: (i) the register of taxes collected when the silver barsreceived the royal stamp (the Quinto in Peru and the Diezmo inMexico) (12, 13), (ii) the register of the European harbors used toimport the silver shippings (1), and (iii) the compilation of con-temporaneous gazettes (9). These data are imprecise or even in-complete, especially for trade registers between 1660 and 1809(9), and do not take contraband and piracy silver into account(8, 14–17). In addition, any memory of the origin of the metal islost by recoinage, whenever silver is exported or a new king comesto power, or upon debasement. Reliable tracers of the monetarymass and exchange that can see through the destructive alter-ations of coinage silver therefore are needed. Over the last 30years, lead isotope compositions of metallic ores have been col-lected and gathered into large databases and broadly used as atool for provenance studies of archaeological artifacts (18–20).Themain factors of provenance analysis are (i) the contrast betweenores produced bymantle-derivedmagmas with low 207Pb/204Pb, suchas in Cyprus, southern Spain, and the Andes, and those producedin geologically ancient crust with high 207Pb/204Pb (such as mag-matism from the Altiplano) (21) and (ii) the age of the crustalprovinces from which the Pb ores were extracted. Unfortunately,the Pb isotope ratios of ores are strongly correlated with each

other, which often makes provenance assignment insufficientlydiscriminating. More recently, the high precision of the multiple-collector–inductively coupled plasma mass spectrometry (MC-ICPMS) technique (22) allowed Cu isotopes to be added to thecoinage tracers and a number of successful applications to theidentification of the sources of metals used for coinage have beensuggested (23). Although copper is primarily alloyed with coinagesilver to improvemetal hardness and resistance, it was also used formonetary debasement (17). Copper has two stable isotopes ofmass63 and 65, and, in contrast to the large variations in radiogenic Pbisotope abundances, which are due to the radioactive decay of Uand Th, the abundance variability of Cu isotopes is due exclusivelyto the physico-chemical conditions of ore-forming processes (pri-mary hydrothermal sulfides vs. low-temperature sulfides andhydrocarbonates) (23, 24) and remains within a few parts per 1,000.The other multi-isotopic coinage metal is silver (Au is mono-isotopic), but beyond some preliminary data on silver ores (25–27)no archeological application has been attempted. Silver has twonaturally occurring isotopes, 107Ag (51.4%) and 109Ag (48.6%).Evidence of 0.5‰Ag isotopic variability among silver ores (25–27)provides a strong incentive to use the 109Ag/107Ag ratio as a prov-enance tracer despite the need for time-consuming high-precisionisotopic analysis.With the incentive that the input of American silver into the

European monetary mass may be visible in the isotopic abun-dances of metals used for coinage, this work presents Pb, Cu, andAg isotope data on silver and billon coins from Europe and theSpanish Americas. We first analyzed reference material from theAntique world (Greek, Hellenistic, Roman, Near Eastern) andmedieval times, notably pre-Columbian Spain. We then analyzedthe isotope compositions of Pb, Cu, and Ag in 16th–18th centuryAmerican coinage from Mexico and South America and com-pared them to the isotope compositions of European Spanishcoins of the same age. We discuss the problems associated withthe allocation of the coinage metals to potential sources, notablyisotopic modification during the metallurgical processes. We alsodiscuss how these data can elucidate the history of the monetarymass and circulation in the world.

ResultsThe analytical techniques aredescribed inSIMaterials andMethodsand thedata are listed inTableS1. In the following,wefirst examinethe isotopic results one element at a time and then describehow thedata for different elements correlate with each other.

Lead. Lead has four isotopes, the stable 204Pb and the radiogenic206Pb, 207Pb, and 208Pb produced by radioactive decay of 238U,235U, and 232Th, respectively. The choice of plots used to rep-

Author contributions: A.-M.D. and F.A. designed research; A.-M.D. and P.T. performedresearch; E.A. contributed new reagents/analytic tools; A.-M.D. and E.A. analyzed data;and A.-M.D. and F.A. wrote the paper.

The authors declare no conflict of interest.

*This Direct Submission article had a prearranged editor.1To whom correspondence should be addressed. E-mail: annemarie.desaulty@gmail.com.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1018210108/-/DCSupplemental.

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resent the data is particularly important: The traditional207Pb/204Pb vs. 206Pb/204Pb, and 208Pb/204Pb vs. 206Pb/204Pb dia-grams have a long history in geochemistry and are based on thewell-understood control by the age of the ore and the U/Pb andTh/Pb ratios of its source (crust vs. mantle). In contrast, arche-ologists (28) favor different plots, notably 208Pb/206Pb vs.207Pb/206Pb, which reduce the analytical noise by removing thecorrelations induced by relatively higher counting errors associatedwith the low abundances of 204Pb. However familiar these plotsmay be, those used for archeological purposes are more difficult torelate to the geological history of the ore source, a particularlyimportant parameter because the Aegean, the Betic (southernSpain), and theAmericanCordilleras formed<120Maand are stillgeologically active, whereasmost of Central Europe is underlain byHercynian basement (250–400 Ma) (Fig. 1). The “model ages” Tlisted in Table S1 were calculated with the common Pb isotopecomposition and the 238U/204Pb of the crust (29, 30) using theformula given in SI Materials and Methods. As shown in207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb space (Fig. 2),207Pb/204Pb and 208Pb/204Pb ratios are higher in Antique andPotosi coins than inMexican and European medieval coins, whichsurprisingly overlap. The 206Pb/204Pb ratios of most Antique silvercoins are derived from isotopically young provinces (<120 Ma)and fit sources within the Aegean, Asia Minor, and southeasternSpain (Betic Cordillera) (20, 28, 31, 32) (Figs. 2 and 3). TheBasque–Cantabrian basin is also a suitable source but its Ag pro-duction was relatively minor (33). Exceptions are Gr2, a 500 BCIona Miletos diobol; R118, a 118 BC Licinus Crassus denier fromtheNarbomint inGaul; andR121, a 121 BC denier of Caius Plutiusfrom the Rome mint, which probably all derived their Pb fromHercynian (∼300 Ma) ores of the European basement. In contrast,Pb fromall of theEuropeanmedieval coins derives from thewesternEuropean Hercynian basement. Spanish medieval coins form twodistinct groups with coinage predating the Catholic Kings (1454–1474), having 206Pb/204Pb <18.5 and therefore bearing a EuropeanHercynian signature contrasting with the largely >18.6 values ofthe Catholic Kings samples (1479–1504), which are more similar tothe values found in the Betic Cordillera district. As expected, the206Pb/204Pb ratios of coins from Spanish America are stronglyimprinted by the rather recent magmatic activity of the Cordilleras(<130Ma) and overlap with theAntique and precolonial Europeancoins derived from the Aegean and Betic districts. 206Pb/204Pb in16th–18th century Spanish coinage varies between precolonialand Mexican values. The Pb model ages of two 17th century coins,one French and one English, are conspicuously young (<70 Ma).

Copper. Isotopic data are reported in δ notation, for which theisotope composition is cast as the deviation of a particular iso-topic ratio with respect to the same ratio in a standard material

[here National Institute of Standards and Technology (NIST)976]: δ65Cu = [((65Cu/63Cu)sample/(

65Cu/63Cu)standard) – 1] × 1,000.The δ65Cu unit is the part per 1,000 (per mil or‰). Fig. 3A showsthe Cu isotope compositions of analyzed samples. Except forsample (Gr2: δ65Cu = −4.06‰), which was also anomalous for Pbisotopes, the δ65Cu values of Antique coins range between −1.00and +0.15‰, whereas the medieval, Mexican, Andean, and16th–18th century European coins have δ65Cu ranging from −0.5to +1‰. δ65Cu values for medieval samples fall between −0.5 and0.5‰. The range of δ65Cu values for both Mexico (0.0 ± 0.2‰)and Potosi (+0.7 ± 0.2‰) is particularly narrow. The δ65Cu ofmost European coins from the 16th–18th century, regardless ofcountry, is similar to Mexican values (Fig. 3A).

Silver. Isotopic data are reported in parts per 10,000, (ε), withε109Ag= [((109Ag/107Ag)sample/(

109Ag/107Ag)standard) – 1]× 10,000with respect to the standard SRM 978a.The observed range of isotopic variations (from −1 to +2

ε-units) is ∼30 times the analytical uncertainty (∼0.1ε). The Agisotope variability of Antique coins defines two groups (Fig. 3),the oldest having ε109Ag ∼ 0.0 and the younger, mainly composedof Roman and Gallic coins, having ε109Ag ∼ −0.5. The two groupsare statistically different at the 99% confidence level. The ε109Agvalues of the medieval, European, and Spanish coins, with theexception of the Catholic Kings coins, overlap with those of thesecond Antique group (Fig. 3). As in the case for Pb, the Agisotope composition of the Catholic Kings coins also is distinctfrom that of the rest of the medieval coins (ε109Ag = from 0.0to +1.5). The eight Mexican samples show very little ε109Agvariability (0.7 ± 0.2). This range overlaps with the values of twoMexican native silver ores that we measured (ε109Ag = 0.3 and1.2), Pachuca and Guanajuato, two large mines actively exploitedin colonial times. The ε109Ag value of −5.3 reported by Hauriet al. (26) for the silver ore form Zacualpan (Mexico) is much

Fig. 1. Map of Europe with the <90-Ma-old Alpine (in particular Aegean,Basque–Cantabrian, and Betic) domains indicated in gray overprinting theolder western European Hercynian (250- to 400-Ma-old) basement.

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Fig. 2. Lead isotope compositions for Antique (n = 24), medieval (n = 23),Andean (n = 11), Mexican (n = 8), and 16th–18th century European (n = 25)coins. Analytical errors are smaller than the symbols.

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more negative. In contrast, the Ag isotope compositions of Potosicoins are quite variable. In the Spanish samples from the 16th–18th century, the ε109Ag varies from −1.0 to +1.0, a range thatincludes both the French (0.35) and English coins (0.32).

Ag–Cu–Pb correlations. Fig. 3A shows that each group of coins fallsin a specific part of the ε109Ag–δ65Cu diagram with a few outliers.The field of Antique and medieval coins before the CatholicKings plots toward the negative δ65Cu and ε109Ag quadrant.Seven Potosi coins stand out for their high δ65Cu, whereas threeof them have values close to 0.0‰: δ65Cu and ε109Ag are notcorrelated. Most European 16th–18th century and Mexican coinsplot in a narrow field at δ65Cu ∼ 0.0‰ and ε109Ag ∼ 0.7. The Agand Cu isotope compositions of Philip III Spanish coins aresurprisingly similar to those of the pre-Catholic Kings medievalsamples. The only significant correlation between Ag and Pbisotopes is observed for the Antiques coins with Pb model agesT < 200 Ma (r = −0.8) (Fig. 3B).

DiscussionFirst, we assess the extent to which isotope compositions can beused to identify the source of metal ores. We review the variousprocesses that cause isotopic variability within a given ore de-posit, including the mass-dependent thermodynamic fraction-ation of Ag and Cu isotopes, and the variability over regionaldistances, which is important for Ag as well as for Pb isotopes.Processing of coins may alter the original isotopic signatures, inparticular isotopic fractionation during metallurgical processes,and willful additions or accidental contamination can overprintthe original initial isotopic signature. Second, we use the Pb, Ag,and Cu tracers to infer the provenance of the metals containedwithin the samples analyzed in this study. Third, we examine howthese provenance inferences relate to known historical eventsand what they teach us about silver circulation through the 16th–18th century Spanish economy.

Thermodynamic Isotope Variability Among Coexisting Ores. The ex-tent of Ag isotope fractionation among native metal and other Agminerals (sulfides, sulfo-antimonides, and chlorides) from a singledeposit or from nearby localities is not well studied, but someinferences can be made from available data. A single observationshows that the Ag isotope compositions of coexisting pyrite andnative silver ore from Pribram (Czech Republic) are within errorof each other (27). The conspicuous isotopic homogeneity ofMexican samples, however, makes a strong case against sub-stantial thermodynamic fractionation among minerals. For ex-ample, themining of both oxidized [colorados (red ores), inclusiveof native silver, Ag chlorides and bromides, and Fe oxides] andsulfidic [negros (black ores), inclusive of pyrargyrite, pyrite, ga-lena, and sphalerite] silver at the major camp of Zacatecas (34,35) does not translate into measurable Ag isotopic variabilityamong the coins struck in Mexico.The range of Cu isotope variation among coexisting ores is

clearly larger than that for Ag (23, 24). Both Cu isotopic homo-geneity and values close to zero argue for the incorporation ofmetal from high-temperature ores into Mexican coins, possiblybornite or chalcopyrite. Likewise, the 10 samples fromPotosi havesimilar δ65Cu of +0.7 ± 0.2‰, whereas the 3 samples dated be-tween 1620 and 1670 have values between −0.3 and +0.1‰. Thenarrow range of δ65Cu within each group signals the use of high-temperature ores.In contrast to the cases of Ag and Cu, lead isotope variability

results from the radioactive decay of U and Th rather than ther-modynamic fractionation. Consequently, Pb isotopes do not re-flect on the predominant sulfide type, but mainly on the regionalgeologic context of the ore.

Origin of Regional Isotopic Variability. Silver. Natural silver isotopevariability is very narrow (25–27) and a meaningful isotopic signalcan be detected only with extremely precise measurements(±0.1ε). Except for a handful of data on native silver, the isotopiccomposition of silver ores is largely unknown. Native silver oresfrom Italy, Mexico, Canada, Russia, and Norway show ε109Agvariations of ∼5 parts per 10,000 (25–27). A major finding of ourwork is that a substantial proportion of American Ag has isotopecompositions that can be distinguished from those of the metalused in pre-1492 European mints. This is the case for the 8Mexican coins analyzed here (ε109Ag = 0.7 ± 0.2), which aredistinct from the 24 Antique and 18 medieval European coins(ε109Ag = −0.2 ± 0.6) (the Catholic Kings, 1479–1504, stand outas an exception) (Fig. 3). In contrast to the Viceroyalty of Peru,Mexican silver production was divided among several majorcamps (4). The metals analyzed here most probably come fromdifferent deposits, yet the Mexican Ag isotope signature is dis-tinct. The spread of ε109Ag in Potosi and Antique coinage reflectsa different situation. The geological setting of the Cerro Ricomine at Potosi, which is hosted by a young (13.8 Ma) volcanicdome intrusive into a much older (Ordovician or ∼450 Ma) series

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Fig. 3. (A) δ65Cu (in parts per 1,000) vs. ε109Ag (in parts per 10,000). (B) Pbmodel ages T (in millions of years) vs. ε109Ag for Antique (n = 20), medieval (n= 23), Andean (n = 11), Mexican (n = 8), and 16th–18th century European (n =24) coins. Sample GR2 is not reported on this graph due to its very differentCu isotope composition (δ65Cu = −4.06‰, see text). Errors on δ65Cu (2 SD)and ε109Ag (2 SEM) are shown for each sample. Probability ellipses arerepresented for three groups of samples: Antique and medieval coins,Mexican coins, and Andean coins.

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of sediments, is complex (36). This complexity is apparent in theregional Pb model ages (Fig. S1). Different sources of silver maytherefore be involved in the same ore deposit. In addition, dif-ferent mines contribute to the same mint. In contrast to Mexico,where no particular silver mine dominated, Potosi was moreproductive than the other camps in Peru, which had difficultieswith labor and capital (4). Ambiguities about the origin of silvercoins arise from both the metal registration and minting. Regis-tered Potosi production accounts for 80% of the silver mintedover the history of colonial Peru, but other neighboring mines,such as Sicasica (1600), Tatasi (1612), and Padua (1652), also hadtheir production registered in Potosi during the last half of the17th century (4). The Potosi mint processed Potosi silver but alsoindependently registered metal from nearby mines (Porco andOruro) and occasionally dealt with silver from themajor andmoreremote camp of Cerro de Pasco whenever the Lima mint closeddown (Fig. S1). The spread of Ag isotope compositions of theAntique coins also calls for multiple sources of metal: ε109Agcorrelates with Pbmodel agesT (r=−0.8 if the three samples withT > 200 Ma are disregarded). The ore deposits therefore tap avery young geological source (T ∼ 0) of metal with ε109Ag ∼ 0.0,mostly represented by the coins from the eastern MediterraneanBasin on the one hand andmetal from the older basement (T> 200Ma)mostly representedbyRomanandGaelic coins (ε109Ag∼−0.5)on the other hand.Copper. As noted before, the δ65Cu values separate most Potosicoins from the rest of the corpus. Because copper isotope com-positions are controlled by ore genesis, it is unlikely that δ65Cu isa true regional variable. Despite this, our observations indicatethat the Potosi mint was using copper that came from an iso-topically well-defined, but unidentified ore deposit. We thereforeconsider δ65Cu of ∼+0.7‰ as a geographically meaningful tracerof Potosi copper.Lead. The Pb model ages T of the samples (Table S1 and Fig. 3B),and to a large extent the 207Pb/206Pb and 208Pb/206Pb ratios, arereliable indicators of the geological age at which the crustal vol-ume that gave rise to a particular ore body was isolated by geo-dynamic processes. For example, the 206Pb/204Pb ratios of theEuropean Betic, Mexican, and Andean districts, which all derivefrom young provinces (<130 Ma), are similar and distinct fromthose derived from Hercynian northern Spain and westernEurope, where crust mostly formed 250–450 Ma. Thus, it is dif-ficult to use Pb model ages T (or, equivalently, Pb isotope ratios)to distinguish coins from southern Spain from those of the NewWorld. Other ratios, such as 207Pb/204Pb, may fingerprint oldPrecambrian crustal segments, such as the AndeanAltiplano (21),whereas 208Pb/204Pb variation relative to 206Pb/204Pb reflects thepoorly understood variability of Th/U ratios among source rocks.One possible interpretation of the spread of Pb model ages T and206Pb/204Pb ratios observed for Potosi coinage, and that contrastswith the homogeneity of Pb in Mexican coins, is the mixing ofmodern Pb from the Cerro Rico volcanics with Pb from the oldsedimentary basement.

Effects of Extractive Metallurgy, Recycling, and Coinage. Differentprocesses were involved in the extractive metallurgy of silver:

i) Smelting consists of heating the metal ore, possibly aftera first stage of roasting, with a reducing agent, commonlycharcoal. During the medieval period, some metallic Pb wasadded at the beginning of the smelting process to facilitatethe recovery of Ag directly from galena (37).

ii) After the 1550s in Mexico and the 1570s in Peru, the kind ofamalgamation known as the patio process of silver extractionallowed silver recovery from very low-grade ore. The orewas first finely crushed. Added to this ore were large quan-tities of common table salt (NaCl), vitriol (CuSO4 andFeSO4), known as magistral, and mercury, with a typicalratio of lost mercury to silver produced of 1.5 (38). The

resulting amalgam was boiled off and both silver and mer-cury were retrieved. Mercury used in Spanish America camefrom three sources: the Huancavelica mines (1,500 kmnorth of Potosi) (Fig. S1), which provided mercury to theviceroyalty of Peru; the Almaden mines in southern Spain,which supplied mainly Mexico and less frequently Potosi;and the Idria mines in modern Slovenia, which were tappedoccasionally to make up for any shortfalls from the twoprincipal sources (12, 39).

iii) Cupellation is a purification stage that separates metals eas-ily oxidized, typically Pb and Cu, from Ag, which remainsmetallic. It often involves litharge (PbO) addition. Thisstage was used after smelting and for recycling preexistingmetals (coins and silverware) with large Cu contents.

For the metallurgical process to induce isotope fractionation,it must cause partial vaporization of the metals or involve a solidor a liquid phase that coexists with the metal, notably silicate-richslag or Pb-rich oxides. Because little isotope fractionation ofstable metal isotopes is expected at the high temperatures ofmetallurgy, the yield has to be poor for isotope fractionation tobe observed. Baron et al. (37) concluded from smelting experi-ments that the Pb isotope signature of ores is preserved duringmetallurgy. Likewise, Cu extraction and refining processes do notalter the copper isotope signature of copper ores (40). For Ag,no effort was spared to keep the yield as high as possible, andhence any potential isotope fractionation of Ag was minimized.The homogeneity of Ag isotopic compositions in Mexican coinsand their similarity with those of local ore suggest that frac-tionation related to metallurgy is weak. The patio process spreadin Mexico starting in the 1550s while smelting and silver recoveryby lead cupellation still persisted. Smelting was largely used inthe late 17th century due to a shortage of mercury in Mexico (4)and was in general preferred for the treatment of Ag-rich ores,which was the case of Pb ores (34). Poor miners and Indianlaborers, who received part of their wages in ore (39), also fa-vored it. Until the 18th century, it has been estimated thatroughly half of the silver produced in Mexico came from thesmelting process (4). Extractive metallurgy, whether smelting oramalgamation, is therefore not a significant source of Ag isotopevariability. This assessment may not be valid for Peru, where it isknown that yields were poor (4).Use of additives during metallurgical processes or recoinage

(37, 41) is expected to distort fingerprinting if the additives andthe ore have a different origin. Likewise, litharge (PbO) additionduring postsmelting purification by cupellation may overwhelmthe original Pb isotope signature. Such problems are particularlyserious if mining, ore treatment, and metal purification take placeat different locations. In the patio process, the many additives area concern, notably mercury. The Pb content of mercury andcinnabar is not recorded, but given the solubility of lead, both inmercury and in silver (∼1%) (42, 43), amalgamation may havealtered the Pb isotope composition with respect to the originalore. In addition, lead contamination by cupellation is expectedduring the refinement frequently required before recoinage (44).Accession of new monarchs, design of new coins, silver import,and coinage debasement were all opportunities to introduceforeign Pb into the metal. New World silver reminted in Spain isexpected to involve European lead and to obscure the AmericanPb isotope signature of the metal. Likewise, Andean and Mexicansilver went through multiple recoinage in the Americas (1728,1772, and 1786) (17), but local reprocessing is less likely to cor-rupt the isotopic signature.The purpose of copper alloying is either improved coin

hardness and resistance (∼5–10%) or debasement (17). In gen-eral, Pb and Cu ores form in different environments (13). Pbcontents in chalcopyrite are rather limited and, for fine silvercoins, Pb contamination associated with Cu addition should beless important than during purification. The Ordinance of Me-

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dina de Campo (1497) defines the weight of the Real (3.434 g)and stipulates that it contains 3.195 g of silver (93% Ag and 7%Cu) (45). The use of Cu isotopes to trace Andean and Mexicansilver in Spanish colonial coins is relevant only for exports con-sisting of silver coins. Otherwise the copper signature is inheritedfrom wherever copper was added, possibly in Europe. Accordingto Garner (14), if there is little doubt that silver was exported indifferent forms, coins were the favorite one. Although up toa few percent metallic Cu can be alloyed with Ag (46), silver oflower fineness is prone to unmixing and, during oxidative high-temperature reprocessing, to oxidation (47): Removal of Cu bycupellation is then necessary before realloying. Coinage recyclingpreexisting coins can therefore be suspected to contain copperfreshly added at the site where recycling took place.

Provenance Assessment. Antique and pre-1492 European coins. MostAntique coins are characterized by young Pb model ages T andlow (<0.1) ε109Ag values. The very young Pb model ages T of thesilver coins from the eastern Mediterranean area (with ε109Ag ∼0.0, see above) are consistent with the prevalence of either localmetal sources or sources in the Betic district in Southern Spain(33). The intermediate (∼100 Ma) model Pb ages T of the Romanand Gaelic coins are geologically unusual and suggest that mixedsources of Pb were used in silver metallurgy. In contrast, mostEuropean medieval and a handful of Roman coins are charac-terized by Pb model ages T older than 200 Ma and negativeε109Ag isotope compositions. Clearly, Antique silver either isa minor component of the medieval silver monetary mass or hasbeen largely reprocessed using old Hercynian Pb. A puzzling caseis that of the coins of the Catholic Kings (1479–1504), in whichthe young Pb model ages T (28–120 Ma) reflect the prevalence ofa Betic metal (20, 31, 32). This period corresponds to the captureof the kingdom of Granada and of its rich Ag mines by theSpanish kings. The scatter of ε109Ag for the Catholic Kings coinsis uncharacteristic. The only coin of the Catholic Kings (ES32)with a Hercynian signature was struck in a northern mint atBurgos, whereas the other coins were struck in Seville or Granadain southernmost Spain.Mexico. The ε109Ag of Mexican coins is very similar to native silverfrom Pachuca and Guanajuato, two large mines exploited by theSpaniards, analyzed in this study (+0.3 and +1.1, Table S1), butdiffers from the value of −5.3 ± 0.5 found by Hauri et al. (26) forZacualpan. This difference suggests that Zacualpan silver was notused for coins analyzed so far. The Pb isotope compositions areconsistent with what is known of the Mexican sulfide ores in theregion of colonial Spanish mining in the center and the north ofthe country (48, 49).Andes. The only available ε109Ag value from a local mine (Porco,Table S1) falls within the range of Potosi coins. Likewise, theisotope composition of Pb present in the coins struck in Potosi isconsistent with literature values for the neighboring Ag oredeposits of Cerro Rico, Oruro, and Porco (ref. 50 and referencestherein) (Fig. S2). In contrast, the Pb isotope composition of Cerrode Pasco next to Lima is clearly different (51, 52). The spread of Pbisotope compositions requires the contribution of a rather old endmember, which can be European Pb introduced by amalgamation,local Pb from the Paleozoic basement, or possibly an unknownsource (Fig. S2). The two samples PotoD and PotoF with theoldest Pb model ages T (329 and 252 Ma, respectively) and alsodistinctly high ε109Ag (+1.6 and +3.3) suggest that they representa local variation rather than contamination or isotope fraction-ation during the amalgamation process. For the rest of the Potosicoins, compelling evidence for what created the Ag isotope vari-ability is missing. Whether a low yield of the extractive metallurgyor regional isotopic variations, the Ag isotopic signature is het-erogeneous and not particularly distinctive.European 16th–18th century coins. The Spanish vellón coin ES4 ofCharles V (1516–1555), the half-real ES40 of Philipp II (1556–1598), and four coins of Philipp III (1598–1621) (ES19, ES21,

ES48, and ES45) all have δ65Cu at ∼0.0‰. With the exception ofthe intermediate ES40, this group of coins also has negativeε109Ag. These characteristics do not fit American metal supplyand can be explained by the alloying of pre-1492 silver with Pb oflocal origin. Both Charles V and Philipp II coins have HercynianPb model ages T (287 and 247 Ma, respectively). The Pb modelages T of the Philipp III coins are consistent with the locality ofthe mint: ES19 (39 Ma) and ES21 (36 Ma) were struck in Seville,whereas ES45 (268 Ma) was struck in Valladolid. The Toledosample (ES48, 147 Ma) is intermediate. In contrast, from PhilippV (1700–1746) onward, silver isotopes (ε109Ag ∼ +0.7) un-ambiguously demonstrate the prevalence of Mexican silver in theSpanish monetary mass of the 18th century. The observation thatfew Spanish colonial coins have Pb model ages T as young asthose of Potosi and Mexico does not imply that Spanish silver wasnot imported from the Americas. Rather, it emphasizes wide-spread recoinage and refining, which mostly used local Europeanlead. The δ65Cu and ε109Ag of the 17th century French and En-glish coins and their young model ages point to a strong contri-bution of Mexican metals (Fig. 3).

Silver Circulation. The Spanish quest for silver in the New Worldstarted as early as 1498 (53), but it was only in the early to mid-16th century that the major mines were opened (4). EuropeanSpanish coins from the 16th and early 17th centuries (Charles Vto Philip III) show no input of American metal, which suggeststhat coins and cobs struck in Mexico and Peru were not realloyedinto Spanish coinage. Of the silver mined in the Americas, 20%stayed on the continent (12). Another 10% were used to buyAsian silk, porcelain, and spices (14), and yet another 15% fell inthe hands of pirates or was smuggled (15), leaving ∼200 tons toreach Seville every year of the late 1500s and early 1600s (4, 5, 9).Philip II defaulted on the Spanish debt in 1557, 1560, 1575, and1596 (54, 55) and reopened the Rio Tinto and other Spanishsilver mines (56), which confirms that American silver did not stayin Spain very long. As stated by Braudel (ref. 57, p. 205) “everyconsignment of American silver was quickly dispersed in alldirections, almost like an explosion.” Silver was mostly exportedfrom Spain to repay the huge loan obtained from the Germanbankers to secure Charles V’s election and to repay Genoesebankers for other major loans. It was also lost to subsidize thewars in The Netherlands; to buy grains, cloth, and paper thatSpain did not produce itself in sufficient quantities; or simplybecause petty money was driving out good silver (16). However,the absence of the New World treasures from Spain is not suffi-cient to reject their role in the Price Revolution. According toFlynn (10), the influx of American silver may have had a globalinfluence on international markets and may explain Spain’s in-flation as a reflection of the overall European Price Revolution.Our isotopic data on the coins of Philip V (1700–1746) indicatethat Mexican silver found its way to the Spanish silver monetarymass only in the aftermath of the Utrecht treaty (1713), whichmarks a major break in Spanish involvement in foreign wars,which in turn coincides with the onset of a major phase of silvermining expansion in America (4, 5, 58). These data also show thatthe isotopic signature of European silver until Philip III (1598–1621) is not detectable in Philip V coins. If the corpus analyzed inthe present work is a representative sample of the contempora-neous metal, the implication is that by the time of Philip V,Mexican silver had actually replaced the silver monetary masscirculating under Philip III, which was partly exported andpartly diluted.

ConclusionsThis work demonstrates that silver isotopes can be used suc-cessfully to trace the origin of coinage. The usefulness of Pbisotope compositions as tracers can be strengthened by usingmodel ages T, which represent a geologic characteristic of oredeposits, but is made ambiguous by silver purification and

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reprocessing, which often involve local sources of Pb distinctfrom the ore sources. The combined use of Ag, Cu, and Pb candistinguish pre-1492 European silver from Mexican and Andeanmetal sources. European silver dominated Spanish coinage untilPhilip III but, 80 years later under the reign of Philip V, had beenflushed from the monetary mass and replaced by Mexican silver.

Materials and MethodsThe corpus is composed of 94 samples described in Table S1. After cleaning,a piece of coin was cut off with pliers and dissolved in nitric acid. Ag wasprecipitated by addition of ascorbic acid. Cu was separated in HCl mediumand Pb in HBr medium on anion exchange columns and then analyzed byMC-ICPMS (22), using Pd (Ag), Zn (Cu), and Tl (Pb) to correct for instrumental

mass bias. Details relative to sample preparation and MC-ICPMS measure-ment protocols are provided in SI Materials and Methods.

ACKNOWLEDGMENTS. We thank Jacques Samarut, Chantal Rabourdin-Combes, Dominique Le Quéau, and Mireille Perrin. We thank all the profes-sional numismatic dealers around the world who provided the coins and, inparticular, Daniel Sedwick from Orlando for expert advice. Florian Tereygeolkindly gave twoAndean samples andGeorgeRossmanMexicanores.We thankMerlin Méheut and Ghylaine Quitté for informal discussions and Maia Kuga,Chantal Douchet, and Aline Lamboux for their friendly help in the laboratory.Janne Blichert-Toft provided expert, quick, and generous help by editing thetext. An anonymous reviewer is particularly thanked for detailed commentsand useful suggestions. This work was supported by the Institut National desSciences de l’Univers and EcoleNormale Supérieure. Late but critical support bythe program CIBLE (Créativité-Innovation-Projets blancs) funded by the RégionRhône-Alpe helped us acquire high-quality material.

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