THE ORIGIN AND ARCHAEOMETALLURGY OF A MIXEDSULPHIDE ORE FOR COPPER PRODUCTION ON THE ISLAND
OF KEA, AEGEAN SEA, GREECE*
A. PELTON†
Centre de Recherche en Calcul Thermodynamique, Département de genie chimique, École Polytechnique, C.P. 6079,succ. ‘Centre-ville’, Montréal, Québec, H3C 3A7, Canada
M. G. STAMATAKIS and E. KELEPERTZIS
National & Kapodistrian University of Athens, Department of Geology & Geoenvironment, Panepistimiopolis, Ano Ilissia,157 84 Athens, Greece
and T. PANAGOU
Hellenic Ministry of Education and Religious Affairs, Culture and Sports: Kea Project, Sp. Trikoupi 16, Athens, Greece
At the hill of Agios Symeon, on the island of Kea, Aegean Sea, Greece, ancient metallurgicalslags with a high Pb–Zn–Cu content have been found. Thermodynamic simulations have beencarried out, using the FactSage™ thermodynamic database computing system, with a view tounderstanding the ancient metallurgical processes that produced the observed slag composi-tions and morphologies. The simulations demonstrate that the slag samples resulted fromCu-making processes. It would thus appear that mixed ores were used, containing Cu2S–FeS–PbS with significant amounts of sphalerite (ZnS) as impurity. The roasted ores were reducedat relatively high oxygen potentials at ∼1125°C to form Cu containing low levels of Pb, Fe andZn.
The Lavrion Peninsula, in the southern part of the Attica Peninsula, and the adjacent southern partof the island of Evia to the east, as well as most of the islands of the Cyclades, in the Aegean Sea,have been well known for the development, exploitation and processing of metals such as silver,lead and copper from the Final Neolithic period to Late Antiquity (Coleman 1977; Conophagos1980; Economopoulos 1992; Georgakopoulou 2004, 2005; Kakavogiannis 2005; Bassiakos andPhilaniotou 2007; Kakavogianni et al. 2008; Tzachili 2008; Georgakopoulou et al. 2011). Ironmetallurgy has been demonstrated to have taken place in several other localities in the samegeographical area (Dimou et al. 2001). The broad Lavrion (or Laurium) area hosts the mainmining and metallurgical activities among those areas, which were mainly focused on theextraction of silver and lead periodically from Antiquity until the 20th century. The result was theproduction of slags that remained in situ, were re-melted over a long period of time for recoveryof Pb and/or Ag, or were partially reused as fluxing agents for further metallurgical processes.
*Received 18 April 2013; accepted 17 December 2013†Corresponding author: email [email protected]
The chemical composition of these slags was strongly affected by the initial compositions of theore, the fluxes used and the specific extractive techniques. Pernicka et al. (1990), Lutz andPernicka (1996) and Mangou and Ioannou (1998) have shown that copper produced in theEuropean Bronze Age is expected to contain a relatively high concentration of impurities. Ingeneral, ancient slags deriving from lead/silver metallurgy are rich in Pb–Ag and poor in Cu,whereas slags originating from copper metallurgy are rich in Cu and poor in Pb (Manasse andMellini 2002; Shugar 2003; Rothenberg et al. 2004; Gale et al. 2008; Tsakiridis et al. 2008). Ahigh zinc content is sometimes observed in either Pb–Ag or Cu slags (Manasse and Mellini 2002;Costagliola et al. 2008).
On the island of Kea (ancient Keos), the existence of mineral wealth was exploited inAntiquity, at least since Classical times, as can be easily proven based on written sourcesreferring to miltos (ruddle) (Cherry et al. 1991; Mendoni and Beloyannis 1993). For earlierperiods, there has been some dispute about whether metals were extracted from local ores or fromores that were imported, most probably from Lavrion (Gale et al. 1984; Wilson 1987;Papastamataki 1998). In any case, metallurgical activity had taken place on the island sinceprehistoric times (Final Neolithic to end of Early Bronze Age II, also in Late Classical I and III).Slag pieces and crucibles found in Kephala, Paoura, Ayia Irini and Troullos bear witness to thisfact (Caskey 1971; Coleman 1977).
Apart from these prehistoric sites, which are all notably confined to the northern part of theisland, metallurgical slags have been reported from Yaliskari, Sidhero Bay, Agios Symeonand Hellenika, while mining galleries are located in various sites on the eastern side of theisland: Kalamos/Trypospelies, Spathi, Orkos, Choste and Spasmata (Georgiou and Faraklas1985; Cherry et al. 1991; Mendoni 1991; Mendoni and Beloyannis 1993). Hand specimenscontaining gangue minerals and pyrite–chalcopyrite, as well as scattered metallurgical slags,were found by the authors in a north–south oriented stream, west of the ancient city of Karthaia(Vathypotamos). At Cape Faros, located south of Agios Symeon, Müller (2009) reports galena ina vein that also contains pyrite, minor zincblende and cerrusite, accompanied by the gangueminerals fluorite, ankerite, limonite and calcite. In dating all these activities, it is impossible to beeither certain or precise, because the dating is based on surface finds. The finds range fromprehistoric to Hellenistic, but one can note a particular density of Late Classical/Early Hellenisticmaterial.
Particularly at the hill of Agios Symeon, significant amounts of scattered slags are exposedover the site. Earlier studies of slags from Agios Symeon characterized them as copper slags,due to the existence of copper prills in the analysed samples (Caskey et al. 1988; Mendoni andBeloyannis 1993; Papastamataki 1998).
Analyses performed in the present study on a larger number of samples from the specific areareveal that these slags are mostly Cu–Pb–Zn rich. The aim of the present paper is to examine thechemistry, mineralogy and texture of the Agios Symeon slags in order to identify the metallur-gical processes that were used by the ancient metallurgists and possibly the composition of theoriginal ores.
GEOLOGICAL SETTING AND MINERALIZATION
The island of Kea belongs to the Attico–Cycladic metamorphic belt, lying within the WesternCycladic Detachment System (Müller 2009; Rice et al. 2012). The petrology of the islandis dominated by metamorphic rocks, mainly schists that locally host metamorphic basicrocks, cipollines and marbles (Davis 1972; Davis 1982; Avdis 1996; Higgins and Higgins
Copper production on the island of Kea, Aegean Sea, Greece 319
1996; Rice et al. 2012). Along the south-east coast, these rocks are overlaid by marbles. In thenorthern part of the island, Triassic carbonate rocks occur, thrusted above the metamorphicrocks.
At several places on the island, iron oxide ores occur, which have been exploited sinceAntiquity for various purposes (Mendoni and Beloyannis 1993; Argyriou and Korasidi 2007).Additionally, the presence and exploitation of massive iron ores, galena and argentiferouspolymetallic ores has been reported or assumed by various authors (Davis 1982; Gale et al. 1984;Mendoni and Beloyannis 1993; Argyriou and Korasidi 2007). According to published literaturedata and unpublished reports of IGME, Greece (Anonymous 1973), metallurgical slags, depositsand ore-grade deposits of mixed sulphides and/or galena have been identified on almost all of theislands of the Cyclades. Furthermore, arguments have been put forward for the existence ofcopper on the island (Cherry et al. 1991; Mendoni 1991; Mendoni and Beloyannis 1993;Papastamataki 1998).
The area studied is on the south-east slope of the hill at Agios Symeon, located in thesouth-eastern part of the island, which has a 25° inclination. The top of the hill is composed ofmarble that overlies schists. Catapotis (2007) has also noted that the Agios Symeon slags arefound close to a Cu–Fe sulphide deposit with surface oxidized outcrops.
THE SITE
The Agios Symeon slag site provides all the necessary conditions for a pyrometallurgicaloperation. It is located on a slope, at a distance of 3 km from the ancient city of Karthaia, whichhas a south-facing bay that is well protected from the north winds that commonly blow in theAegean Sea, with a harbour for ships delivering wood and ore. Karthaia was linked to the AgiosSymeon site by a well-preserved paved road made of tabular blocks of schists, as were manysimilar roads on the island (Mendoni 2004). In addition, fresh water is available from threesprings emanating close to the slag deposits.
The south slopes of the hill at Agios Symeon are steep, providing a natural air-channel, acharacteristic that has also been reported in other metallurgical sites around the Cyclades. InAntiquity, it was common to operate smelting furnaces high on ridges so that the wind would helpraise the furnace temperature by natural draught (Betancourt 2006).
The top of the hill is crowned by the church of Agios Symeon, which probably replaced anancient sanctuary. Two inscriptions dated to the Classical period inform us about the worshipof Aphrodite on this site. There is no surface evidence for other structures or remains of asettlement there. However, pottery sherds and ores scattered over the site attest to some activityother than worship. It is interesting to note that the worship of Aphrodite is connected withmetallurgy in Cyprus. Representative sherds collected from Agios Symeon have been datedto Classical, Hellenistic and Roman times. At a distance of 200 m north-west of the church,some prehistoric sherds have also been found (Caskey et al. 1988; Mendoni and Beloyannis1993).
Based on these data, it is impossible to date the ores of Agios Symeon more precisely.However, Catapotis (2007) includes Agios Symeon in the areas with Early Bronze Age coppersmelting sites of the southern Aegean.
In historical times, the region of Agios Symeon belonged to the city–state of Karthaia, whichflourished from the Archaic period down to Late Antiquity, but was also inhabited at least sincethe Middle Bronze Age. Karthaia is the closest well-known and established settlement to AgiosSymeon.
Slag pieces 2–10 cm long are scattered on or near the surface, a few metres from the church ofAgios Symeon, at an elevation of 415 m. A total of 50 slag pieces of various sizes wereexamined from this location. All are angular, having mostly a fluid ropy texture and a uniformgrey–black colour. A number of samples have light greenish–yellowish patches that partiallycover the greyish-black main mass. Several pieces host greenish, malachite-bearing nodules<1 mm in size. There are differences in their apparent density, presumably due to their differentcontents of base metals and voids. Most samples are massive and microporous, while a fewhave a spongy texture with large voids. The voids are either spherical or asymmetric. Theypresumably represent ghosts of metal prills removed during the recovery of entrapped metals inthe slag, and/or gas bubbles.
The total weight of the slags in this specific site is estimated to be several hundred kilogramsscattered on the slopes around the homonymous church. Eight slag pieces exposed on the surface,having different specific gravities, were selected for a series of analyses. A part of each samplewas cut and the remainder was pulverized in an agate mill. The powdered samples were analysedmineralogically by X-ray diffraction (XRD, Siemens 5005, NKUA) (Table 4 below) and chemi-cally (ALS Chemex Labs, Canada: by the ME-ICP06 method for trace elements, ME-ICP61afor major elements, OA-GRA05 for LOI and Pb-OG62 for lead) (Tables 1 and 2 below). Thecut samples were polished and examined microchemically and texturally by scanning electronmicroscopy (SEM–ADS, JEOL JSM-5600, LINK ISIS, NKUA) (Table 5, and Figs 1 and 2,below).
RESULTS
Bulk chemistry
The bulk chemical analyses of the samples are shown in Table 1. The analytical technique doesnot distinguish between different oxidation states. The Fe, Pb, Zn and Cu contents in Table 1 aretotal Fe, Pb, Zn and Cu contents expressed in terms of the elements. If these values are expressedinstead in terms of the oxides Fe2O3, PbO, ZnO and Cu2O, then the percentages in Table 1 foreach sample sum to 100%. The correlation of the concentrations of the major and trace elementsis shown in Table 2.
The bulk chemistry of the slags varies considerably. The total (Cu + Pb + Zn) content variesfrom 1.77 wt% to 16.45 wt%. The lead content varies from 0.16% to 10.80%. Based ondifferences in the copper and lead content of various Early Bronze Age slags and/or the compo-sition of their copper prills, Thornton et al. (2009) suggested that they were derived from differentmetallurgical processes.
The strong correlation of Mg with SiO2, TiO2, Al, K, Co, Cr and V (Table 2) indicates that thiselement originated from gangue aluminosilicates and Mg silicates. Cobalt is associated with Mg,Cr, TiO2 and V, and is also derived from the metamorphic rocks as a gangue constituent. Itshighest concentration is in sample SLA8, which is the poorest in Cu, Pb and Zn, indicating thatits origin was also from the parent rock.
The nickel content is low compared with the Pb/Cu-rich slags of the island of Keros(Georgakopoulou 2007a,b) and those of the Kamariza area in the broad Lavrion, where primarysulphide ores are also rich in Ni (Kamariza area) (Marinos and Petrascheck 1956; Voudouriset al. 2008a,b). It is correlated with Cu and Mn and is probably derived from the primary ore.
Copper production on the island of Kea, Aegean Sea, Greece 321
The bismuth content is low, compared with the Pb–Cu–Ni–Bi-rich slags of the island ofKeros and also with the Cu–Pb–Ni–Bi-rich ores from several locations in the broad Kamarizaarea of Lavrion (Marinos and Petrascheck 1956; Georgakopoulou 2004, 2005; Voudouris et al.2008a,b).
Silver is correlated with barium and arsenic and it may have been derived from both lead andcopper minerals contained in the polymetallic ores processed at Agios Symeon, such as the oresfrom Lavrion described by Voudouris et al. (2008a,b).
The arsenic content is low, compared with other slags of the Aegean (Georgakopoulou 2007a).It is strongly correlated with Sb; this is indicative of the presence of tetrahedrite–tennantiteassemblages in the ore (Table 3). The zinc content is high in all samples. It is strongly correlated
Table 1 ‘Whole rock’ bulk chemical analysis of Agios Symeon, Kea island slags
with copper, suggesting that part of the zinc entered the system from Cu minerals, as well as fromzinc-blende. Iron is correlated with copper, zinc and lead, suggesting that this element wasoriginally included in the polymetallic ores and acted as a self-fluxing agent.
XRD mineralogy
Although mineral names properly refer only to natural materials, the same names are used in Table4 to designate the phases in the slags (Manasse and Mellini 2002). As shown in Table 4,the main crystalline phase identified by XRD is fayalite, followed by magnetite (spinel). Otherphases detected include hedenbergite, augite, quartz and feldspars, while calcite and cristobalitewere each found in one sample (SLA4 and SLA8, respectively). Augite, K-feldspars (sanidine or
Figure 1 (Continued)
Copper production on the island of Kea, Aegean Sea, Greece 325
Figure 2 The mapping of three elements (Pb, Cu and Fe) in selected slag samples. The Zn pattern was not satisfactory,as this element has a uniform distribution in almost all crystalline and amorphous phases.
microcline) and quartz or cristobalite were detected together in three samples. The calcite in SLA4probably originated from pieces of furnace refractory entrapped in the slag. Also, a glassy phase,evidenced by a hump in the XRD pattern, is a major constituent of all the samples analysed, aswitnessed by the relatively low peak heights of the crystalline phases in the XRD patterns.
Textural and microprobe analysis
Under the SEM, the glassy groundmass varies in brightness and chemical composition. Twogeneral groups can be defined: a Pb-silicate-rich (SLA1, SLA2, SLA5 and SLA4) and an
Figure 2 (Continued)
Table 3 Ag minerals or argentiferous copper sulphides reported from Cu-rich polymetallic ores in the Lavrionmetallogenetic district (Marinos and Petrascheck 1956; Voudouris et al. 2008a,b)
Mineral Composition Colour
Galena PbS Light grey, dark lead greyTetrahedrite (Cu,Fe,Ag,Zn)12Sb4S13 Iron grey, steel greyTennantite (Cu,Fe,Ag,Zn)12As4S13 Steel grey, blackEnargite (Cu,Fe,Zn,Ag)AsSbS Steel grey, blackish grey, violet blackChalcosite Cu2S Blue black, grey, blackChalcopyrite CuFeS2 Brass yellowPrustite Ag3AsS3 Reddish greyPolybasite (Ag, Cu)16Sb2S11 Black, dark ruby red
Copper production on the island of Kea, Aegean Sea, Greece 327
Fe-silicate-rich groundmass (the rest of the samples). The Pb-rich siliceous groundmass has abright off-white colour, whereas the Pb-free/poor siliceous glass has a dark grey–black colour(Fig. 1).
The main crystalline phases observed can be divided into three major groups: spinel-type,fayalite-type and hedenbergite/augite (pyroxene)-type. The spinels are mainly in the form ofZn-rich magnetite. It is noteworthy that dendritic fayalite crystals are observed in all slag samplesstudied, indicating rapid cooling conditions (Fig. 1).
Metallic and semi-metallic inclusions were observed in all samples (Table 5 and Figs 1 and 2).The inclusions occur as symmetric nodules, mostly metallic copper and secondarily leadoxide prills of various sizes up to 2 mm, and as asymmetric sub-microscopic exsolutions in thegroundmass. Other elements detected in the inclusions are As, Ag, Sb, S and Fe. Silver wasdetected in SLA1 as curved wire-like or symmetric inclusions alone or with antimony, hosted ina copper-rich prill, suggesting an origin of a silver-bearing tetrahedrite–tennantite ore. In thesame Cu prill, small irregular inclusions of variable composition also occur (Table 5 and Fig. 1).
In all samples analysed, copper-containing inclusions occur in the form of Cu prills with minorFe, or Fe–As–Sb, asymmetric masses. Some copper prills have a discontinuous thin rim of Cu–Scomposition, with minor Fe. High amounts of Cu–S, Cu–Fe–O and Cu–Fe–S–O occur in nodularform in slags SLA7 and SLA8, which are less rich in Cu + Pb + Zn. In the samples containingPbO or Pb-alloy inclusions (SLA1, SLA2 and SLA3), lead occurs as PbO nodules with minor Feand Cu contents, as Pb–As alloy with minor Cu and S contents, or as Cu–Pb alloy with minor Asor Fe contents. Interestingly, even the relatively Pb-poor sample SLA8 (Pb 0.53%) containssubmicroscopic Cu prills that are discontinuously rimmed with PbO.
The coexistence of copper with arsenic and antimony is indicative of at least partial tennantite–tetrahedrite content in the ore (Table 3). On the other hand, the presence of both copper oxidesand sulphides in SLA7 and SLA8 is indicative of the existence of chalcopyrite and bornite,cuprite and/or chalcocite ores.
In some inclusions of SLA1 and SLA3, both lead and sulphur were detected, indicating thepresence of relics of the original galena content of the ore. The lead-rich slag sample SLA2contains lead sulphide relics and also PbO prills with minor Cu and/or Fe contents, or metallicPb–Cu–Fe alloy with variable percentages of each component in each prill.
Even though most of the arsenic and antimony contained in the original ore was released asgases during roasting and smelting; a small portion of them was fixed in the metallic inclusions,in both Cu and Pb prills.
Table 4 XRD bulk mineralogical analysis of crystalline phases of Kea slag samples
Fayalite(olivine)
Magnetite(spinel)
Hedenbergite(pyroxene)
Quartz/cristobalite
Augite(pyroxene)
Sanidine, microcline(K-feldspars)
Calcite
SLA1 Major Major MajorSLA2 Major Major MajorSLA3 Major Medium MajorSLA4 Major Trace Major Medium MediumSLA5 Major Major MediumSLA6 Major MediumSLA7 Trace Trace Major MediumSLA8 Trace Medium Major Trace
The mapping of the three major metallic components, Fe, Cu and Pb (Fig. 2) shows thatthe iron is mostly in the form of well-formed polygonal Fe spinels and prismatic fayalite/hedenbergite, whereas copper is confined only to copper prills. Lead forms bright areas where itoccurs as Pb silicates and snow-white areas where there are Pb prills. In the sample SLA4 (centre)where Pb is present in a copper-rich prill, it is discontinuously developed on the margin ofthe prill. Note the differences on the grey shadows in the asymmetric central copper prill. Thebrighter area is almost exclusively copper, whereas the darker area is the Cu-oxide area. Themapping of zinc failed to present a good pattern, as it is uniformly distributed in almost all phases,both crystalline and glassy.
In conclusion, of the three metals Cu, Zn and Pb, copper was detected only in nodules ofvarious shapes; zinc is present in the groundmass in neoformed silicates and spinels, while leadwas detected in nodules, mostly associated with copper, and in a glassy Pb-silicate phase(Fig. 1).
DISCUSSION
The origin of the raw materials
On the island of Kea, the presence of galena and base metal sulphide deposits that wereaccompanied by silver has already been demonstrated by Tournefort (2003 [1770]) and morerecent researchers. However, the deposits are not large enough for us to assume ancient metal-lurgy based on the local sources only. In the area of Agios Symeon, the archaeological findingsrange in age from prehistoric and Classical through Roman (Caskey et al. 1988, Mendoni andBeloyannis 1993), raising questions about the exact period of the metallurgical activity at the site.In the present study, it has been shown that the slags of Agios Symeon are rich in Pb–Zn–Cu (plusminor Sb–As) with silver contents up to 42 ppm. Silver micro-inclusions were detected by SEMin sample SLA1, associated with Cu-rich prills.
The assumption that there was a long ongoing metallurgical activity in Agios Symeon,reflecting differences in the metallurgical techniques or different origins of the processed ores, isnot supported by the presence of large amounts of accumulated slags, as is the case on the islandsof Lavrion or Kythnos (personal observation; see also Bassiakos and Philaniotou 2007). Marinosand Petrascheck (1956) have pointed out that the older Pb/Ag slags on Lavrion contained as muchas 12% Pb (before the Persian Wars, c. 490 bc), changing to Pb-poor slags by Roman times. Thehigh percentage of lead (>10%) that one slag sample (SLA8) contains could suggest that itpredates 490 bc. However, the same authors state that during the rule of Athens by the oratorDemetrius Phalereus (345–280 bc), and throughout the Roman period, there was a re-melting ofslags on Lavrion to recover retained silver.
The microanalysis, as well as the ‘whole rock’ chemical analysis, of the Agios Symeon slagsdiffers significantly from slags characterized as copper slags found in Crete and other locationsin Greece (Betancourt 2006). The main differences with all Aegean copper metallurgy slagsstudied, with the exception of the Group 2 slags of the island of Keros (Georgakopoulou 2004),are the high contents of Zn and Pb, as well as the presence of silver in amounts up to 42 ppm.Similar slags with high Cu/Zn/Pb contents have been located in eastern Bulgaria in Eneolithicmetallurgy for copper production (Ryndina et al. 1999).
Even though the analysed slags contain arsenic and antimony, the original content of theseelements in the ores cannot be estimated, as most of the Sb and As, as well as almost all thesulphur, were released as gases during the roasting/smelting of the ore.
The average Cu/Pb ratio of the slags studied is 1.1. The same Cu/Pb ratio was found byGeorgakopoulou (2004, 2007a,b) in the Cu/Pb-rich slags of the island of Keros, which liessouth-east of the island of Kea.
The metallurgical slags of Agios Symeon have as a major distinguishing feature their highPb–Zn–Cu content, which was not common in the ancient slags of the Aegean at any time. Theonly similar Cu/Pb-rich slags (Zn was not measured) have been described by Georgakopoulou(2005, 2007a) from the island of Keros, which is also part of the Cyclades. However, the Kerosisland slags also contain significant amounts of bismuth, arsenic and nickel, a chemistry thatresembles ores that have been studied from Kamariza Lavrion ore deposits (Marinos andPetrascheck 1956; Voudouris et al. 2008a,b). Given the uncommon base metal chemistry of theKeros island slags and their high Pb–Cu content (the ‘2nd Group’ of slags), the author proposedthe possibility of two types of copper-based metallurgy on the island of Keros, namely arsenicalcopper rich in lead, and pure copper (Georgakopoulou 2007b).
Therefore, the island of Kea is the second location in the Aegean region that has been foundto host metallurgical slags of uncommon composition. The coexistence in slags of Pb, Zn and Cu,in high amounts, may reflect a mixing of Cu-bearing and Cu-rich ores of diverse origin, or apolymetallic ore in bulk. Taking as the basis of assumption the metallogenetic district of theLavrion area and the Cyclades, where a series of common and rare metallic minerals has beendescribed, the following minerals can be counted as sources of the various elements identified inthe slags studied: galena/cerrusite (Pb source), zinc-blende (Zn source) and primary or secondaryCu-minerals such as chalcopyrite, cuprite, azurite and malachite (Cu source). In addition,rarer minerals such as tetrahedrite (Cu–Ag–Zn–Sb source), boulangerite (Pb–Sb source) andbournotite (Pb–Cu–Sb source) also occur (Marinos and Petrascheck 1956; Alfieris 2006;Voudouris et al. 2008a,b). The existence of Ag-rich minerals such as prustite and/or polybasitecannot be excluded, as polybasite and prustite are found in the polymetallic ores of Lavrion andthe southern part of the island of Evia, which is part of the same metallogenetic zone of theAttico-Cycladic massif (Voudouris et al. 2008a,b).
Besides Lavrion, other islands of the Cyclades or the southern part of the island of Evia mightbe the source of the polymetallic ores. On the neighbouring island of Seriphos, the ancient Cuslags contain 0.8–1.04% copper in the form of copper prills sunk in an iron-silicate groundmass(Papadimitriou and Fragiskos 2008; Georgakopoulou et al. 2011). However, the copper contentof Seriphos slags is lower than in most of the Kean samples analysed. Mining and metallurgicalworks for Pb/Ag have been located on the island of Siphnos, dated from the third millennium bc(Wagner et al. 1979).
Several authors have performed isotopic analyses of Aegean slags in order to provide evi-dence for the origin of the primary ores by correlating isotopes of slags and a few ore depositslocated on Lavrion or on a few islands in the Cyclades. Our opinion is that it is not possibleto prove that the ancient consumers, traders and metallurgists asked for ores from a singlespecific place. Common practice through the ages was to use all available resources of accept-able quality, independently of their location. Similar to present-day practice, the ancient tradersprobably supplied the local furnaces with raw materials from several sources around theEastern Mediterranean, at least. Trading has been well established throughout the Mediterra-nean region and Europe since prehistoric times (Knapp 2000; Broodbank 2006; Watson 2006;Jones 2007).
Based on pottery and metal findings on the island of Kea that were imported from the islandof Melos, Wilson (1987) and Broodbank (2002) pointed out the significance of the interconnec-tion and shipments between the metalliferous Cyclades and neighbouring Lavrion. Stos-Gale
Copper production on the island of Kea, Aegean Sea, Greece 331
(2000) also suggested transportation of ores for smelting between adjacent Cyclades islands,probably due to the availability of fuels in sufficient quantities. It is plausible to assume ship-ments from Lavrion, most probably from the Ano Sounion area, where Pb–Cu–Zn ores wereexploited in vast quantities in ancient times, or from another locality in the vicinity of Kea. TheKamariza area, the other Lavrion area that hosts Cu-rich ores, can be excluded since these oresare rich in Ni and Bi, elements that are present in the Keros island slags, but not in those fromKea. The possibility of transportation of ores from other locations of the Aegean to the island ofKea can be explained by the scarcity of any ‘fuels’ on Lavrion, the presence of high-quality basicfluxes in sufficient quantities close to Agios Symeon and in other localities of Kea, and thepresence of skilled copper metallurgists in the Cyclades. In general, high-quality ore is preferablefor shipments, a practice followed to the present day.
METALLURGY
In this section, we present a thermodynamic analysis, based on the measured slag compositions,in order to shed light on the metallurgical processes used by the ancient metallurgists on theisland of Kea.
These calculations require accurate databases of the thermodynamic properties of multicompo-nent solutions (most importantly, of solid and liquid oxide and metallic solutions) as functionsof temperature and composition. All thermodynamic calculations were performed using theFactSage™ thermodynamic computer system, of which one of the present authors is a principaldeveloper, coupled with the large evaluated FACT databases, which contain data for over 6000pure substances and hundreds of multicomponent solutions (Bale et al. 2009; Pelton et al. 2012).FactSage consists of a suite of programs that use these databases to perform chemical equilibriumcalculations by means of a general Gibbs energy minimization algorithm. The FACT solutiondatabases give the thermodynamic properties (chemical potentials) as functions of temperature andcomposition for liquid and solid multicomponent solutions of oxides, metals, sulphides and so on.These solution databases are prepared by first developing an appropriate mathematical model,based upon the structure of the solution, giving the thermodynamic properties as functions ofcomposition and temperature. Next, all available thermodynamic and phase diagram data from theliterature are simultaneously ‘optimized’ to obtain one set of critically evaluated self-consistentparameters of the model for all phases in two-component, three-component and, if available,higher-order subsystems that reproduce all experimental data simultaneously within the experi-mental error limits. Finally, the models are used to estimate the thermodynamic properties ofN-component solutions from the database of parameters of lower-order subsystems. All data of alltypes (phase diagrams as functions of temperature, composition and oxygen potential; activitymeasurements, calorimetric data; etc.) from thousands of original references have simultaneouslybeen taken into account in developing the optimized FACT databases over the past 35 years. Forcomplete details and lists of references, see Pelton et al. (2012).
The thermodynamic analysis must necessarily be based on many simplifying assumptions,such as the assumptions of chemical equilibrium and the absence of temperature, compositionand oxygen potential gradients during smelting. It is also assumed that the slags did notbecome highly segregated upon cooling, so that the compositions of the analysed samples arerepresentative of the composition of the entire slags during smelting. Clearly, these assump-tions are not minor. Nevertheless, we believe that the thermodynamic analysis provides at leasta semi-quantitative insight into the nature of the metallurgical processes that produced theseslags.
The aforementioned ropy texture of the slags indicates that they were most probably tappedand cooled quickly. That the cooling was relatively rapid is also suggested by the presence ofdendritic fayalite crystals, as has already been mentioned. Fast cooling would minimize segre-gation, thereby supporting the assumption that the compositions of the samples are representativeof the overall slag compositions.
As will be shown, a general conclusion of the thermodynamic analysis is that, despite the highPb/Cu ratios observed in some samples, all the slags examined resulted from the production ofCu, with lead as an impurity.
The first step in the treatment of sulphide ores is roasting (heating in air) in order to remove thesulphur as SO2, thereby converting the sulphides to oxides, which can be subsequently reducedwith carbon in the smelting step. Arsenic and antimony are also removed in this step as volatiles.Roasting was carried out by heating the ores over open fires, but also occurred in situ as thecharge was heated in the smelting kiln. The roasting of the Kea island slags was quite complete,as witnessed by the low residual sulphur content of the analysed slags (Table 2).
Other oxides such as aluminosilicates, iron oxides and so on were present in the gangue andcontributed to forming a low melting point slag during smelting. Additional fluxes might havebeen added to further decrease the melting point. A slag with the approximate composition offayalite, Fe2SiO4, has a conveniently low melting point and also a low viscosity, which contrib-utes to a better separation of the metal and slag during and after smelting. Hence, if the ganguewas high in silica, iron oxide flux would be added, whereas a gangue high in iron oxides wouldrequire a silica flux. Other fluxing agents such as CaO or MgO might also have been added; thesehave approximately the same function as FeO. As will be shown below, the melting points andviscosities of the Kea island slags were quite low. That is, they were either fortuitously (throughthe gangue material), or intentionally, properly fluxed.
The oxide mixture was then reduced with charcoal in a kiln. Temperatures of the order of1150–1250°C are necessary in order to produce a liquid slag. The heat was supplied by thecombustion of the charcoal in a forced current of air, supplied by a bellows and/or by the wind.The combustion produces a gaseous mixture of CO and CO2, which reduces the metal oxidesaccording to the reaction:
M O CO M COx x+ = + 2, (1)
where M = Cu, Pb, Zn and so on. The equilibrium (1) is controlled by the ratio of partialpressures, PCO/PCO2; the higher this ratio, the more reducing is the gaseous mixture. Rather thanspecifying the actual CO/CO2 partial pressure ratio, it is common metallurgical practice to givethe equivalent equilibrium oxygen pressure (or oxygen potential) of the gaseous mixture. Atcomplete thermodynamic equilibrium with carbon at 1200°C, the theoretical oxygen potential is7.65 × 10–18 bar. However, such a highly reducing potential could never have been achieved inpractice. Georgakopoulou et al. (2011) propose that oxygen potentials in the range of 10–6 to 10–10
bar were actually achieved in ancient kilns.During the smelting process, the liquid slag and metal phases are separated because of their
different densities, with the less dense slag on top. In some furnaces, a tap hole was built into thefurnace so that the slag could be drained out and separated from the metal after smelting(Tylecote 1987).
In the following simulations, the overall slag compositions from Table 1 were used for thecomponents SiO2–Al2O3–CaO–MgO–MnO–Fe–Pb–Zn–Cu. All other minor components wereignored.
Copper production on the island of Kea, Aegean Sea, Greece 333
In the first simulation, a mixture consisting of slag of the composition of SLA4 was equilibratedat 1200°C, at fixed oxygen potentials varying from 10–11 to 10–7 bar. The FactSage softwarecalculates the equilibrium state of the system by minimizing the total Gibbs energy while addingor removing oxygen from the system iteratively until the calculated equilibrium oxygen potentialbecomes equal to the targeted value. At an oxygen pressure of 10–9 bar at 1200°C for example,the calculations show that in a total mass of 100 g at equilibrium, there are 96.95 g of a moltenoxide phase and 3.05 g of a liquid copper phase of the compositions shown in Table 6. It can beseen that most of the Fe in the oxide phase is present as FeO, while the metallic phase contains99.59% Cu.
After smelting, the liquid copper phase and the slag phase would be separately tapped andcooled. The equilibrium composition of the molten metal phase in the kiln during smelting wasthe same as the composition (Table 6) of these Cu prills, which are droplets of the molten metalthat became entrained in the liquid slag. Let us suppose that the total masses of molten slag andmetal in the kiln during smelting were approximately equal—say, 100 kg each. Then, as can beseen from Table 6, the kiln during smelting contained 0.70 kg of PbO in the slag phase and0.29 kg of Pb in the metal phase, along with 1.29 kg of Cu2O in the slag phase and 99.59 kg ofCu in the metal, giving an overall Pb/Cu weight ratio in the initial ore of only 0.01 (molar ratioof 0.03). That is, an ore with a Pb/Cu ratio of only 0.01 can produce a molten slag with aPbO/Cu2O ratio of 0.54. The distribution of Pb and Cu between the molten slag and metal phasesis determined by the following equilibrium between Pb and Cu in the metal phase and PbO andCu2O in the slag:
Pb Cu O Cu PbO kJ at C+ = + = − °202 23 1 1200, . .ΔG (2)
The standard (that is, for the pure metals and oxides) Gibbs energy change for this reaction,ΔG0, is negative. Of course, the metals and oxides are not pure but in solution. Hence, thestandard Gibbs energy change must be corrected to take into account the chemical activities ofthe components. This is done by the FactSage software. The Gibbs energy change is still very
Table 6 Calculated equilibrium composition (wt%) of slag SLA4 at 1200°C and PO2 = 10−9 bar
96.95 g Liquid oxide phase 3.05 g Liquid metal phase (prills)
negative. That is, Cu2O is more easily reduced than PbO. As a result, the equilibrium (2) is shiftedto the right, with Cu being favoured in the metal phase and lead, as PbO, being favoured in theslag phase. Only at very low oxygen pressures, when most of the Cu2O in the ore has beenreduced and the concentration of Cu2O, and hence its chemical activity, in the slag becomes verylow, will the Pb content of the metal phase become significant. In Figure 3, the calculatedequilibrium PbO and Cu2O contents of the liquid slag and the Pb content of the liquid metal areplotted over the range of oxygen potentials expected for the ancient kilns. It may be noted that theCu2O will not be reduced to Cu at 1200°C unless the oxygen potential is less than 10–7.2 bar. Thatis, in the kiln in which SLA4 was produced, the oxygen potential must have been lower than this;otherwise, no metal would have been formed.
The FactSage software was also used to calculate the liquidus temperature of SLA4; that is, thetemperature above which the slag is fully liquid. This temperature depends somewhat uponthe oxygen potential. At potentials of 10–10 and 10–9 bar, the calculated liquidus temperaturesare 1137°C and 1124°C, respectively, with the primary solid being olivine of the approximatecomposition (Fe0.34Mg0.11Zn0.02Mn0.03)2SiO4, while at PO2 = 10–8 bar, the liquidus occurs at 1141°C,with spinel (containing 93 mol% Fe3O4) as the primary solid. As discussed previously, theappearance of the slags indicates that they were nearly fully liquid. Hence the kilns were mostprobably operating at temperatures above 1125°C. The thermodynamic simulations were carriedout arbitrarily at 1200°C. However, the results are reasonably independent of temperature overthe range from 1100°C to 1300°C.
PbO in liquid slag
Cu2O in liquid slag
Pb in metal
log10PO2 (bar)
weig
ht
%
-11.0 -10.0 -9.0 -8.0 -7.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Figure 3 The calculated equilibrium PbO and Cu2O contents of the liquid slag phase and the Pb content of the liquidmetal phase at 1200°C as a function of the equilibrium oxygen potential for sample SLA4.
Copper production on the island of Kea, Aegean Sea, Greece 335
It can be seen from Table 6 that most of the Fe and Zn are found as oxides in the slag phase,with very low concentrations in the metal. The Gibbs energy changes of the following reactionsare even more negative than that of reaction (2):
Zn Cu O Cu PbO kJ at C+ = + = − °202 120 7 1200, . ,ΔG (3)
Fe Cu O Cu PbO kJ at C+ = + = − °202 110 5 1200, . .ΔG (4)
Hence, reactions (3) and (4) are displaced strongly to the right and the tendency of Fe and Znto report to the slag phase is high, contrary to the predictions of Cooke and Aschenbrenner(1975). The calculated equilibrium Fe content of the metal phase in Table 6 is only 0.03%. Thisis much lower than the measured Fe content of 2.67–3.87% for the Cu-rich prills in SLA4 shownin Table 5. Calculations show that such high-equilibrium Fe contents would require oxygenpotentials in the range 10–12.6 to 10–12.9. It is very unlikely that such low potentials could havebeen achieved. Furthermore, such low oxygen potentials would have resulted in a very lowPbO content of the slag, of the order of 0.25%, which is inconsistent with the observation that thePbO content of the oxide groundmass of the cooled slags is relatively high. The high apparent Fecontent of the Cu-rich prills is most probably due to an error of analysis in which some of theoxide phases surrounding the prills were inadvertently included in the analysis. Georgakopoulouet al. (2011) came to the same conclusion regarding the analysis of the Fe content of the Cu prillsin their slag samples. Their conclusion was supported by the analysis of a very large prill in whichthe Fe content was below the detection limit of the analysis.
Samples SLA1 and SLA2 have the largest observed Pb/Cu ratios. Accordingly, thermodynamicanalyses, similar to that just discussed for SLA4, were carried out on these slags to see if they couldhave resulted from the production of Pb rather than Cu. The results are shown in Figures 4 and 5,which show the equilibrium Cu2O and PbO contents of the liquid slag phase and the Pb content ofthe liquid metal phase at 1200°C. In SLA1, operation at 10–9 bar still yields a metal phasecontaining only 4.42 wt% (1.3 mol%) Pb.While it is just conceivable that SLA2 could have resultedfrom a Pb-making process operating at low oxygen partial pressure, this seems very unlikely inview of the facts that all the other slag samples are clearly the result of Cu-making, that a largeamount of Cu would have been produced simultaneously with consequent problems of separation,and that spherical Cu-rich prills are observed in the SLA2 sample, indicating the presence of aliquid Cu-rich phase. It seems much more likely that SLA2 resulted from a Cu-making processoperating at a high PO2 of the order of 10–8 bar, where the equilibrium slag at 1200°C is calculatedto contain 13.32 wt% PbO and 3.08% Cu2O, while the equilibrium metal contains 7.12 wt% Pb(Fig. 5). Assuming as before that the kiln contained 100 kg of molten slag and 100 kg of moltenmetal, the Pb/Cu weight ratio of the initial ore is calculated to be 0.20 (molar ratio 0.061).
The calculated liquidus temperature of SLA1 at oxygen potentials of 10–10, 10–9 and 10–8 bar iscalculated as 1128°C, 1111°C and 1156°C, respectively, while for SLA2, the correspondingtemperatures are 1152°C, 1172°C and 1213°C, respectively.
Similar thermodynamic analyses were carried out for the other five slag samples (SLA3,SLA5, SLA6, SLA7 and SLA8), with results very similar to those for SLA4, SLA1 and SLA2.
The FactSage software also permits the calculation of the viscosities of molten slags. Theviscosity of molten slags depends strongly upon the silica content and upon the temperature. Thedependence upon the oxygen potential is less important. The calculated viscosities for seven ofthe eight analysed slags at 1200°C at an oxygen partial pressure of 10–9 varied from 1.6 poise forSLA2, with the lowest SiO2 content, to 15 poise for SLA4. Only SLA8, with the highest SiO2
content, had a higher viscosity (38 poise). At a temperature of 1125°C, the viscosities areapproximately double their values at 1200°C. Modern copper slags have viscosities at operatingtemperatures of the order of 1–50 poise. Hence, the ancient slags were quite fluid. This wouldfavour good separation of slag and metal during tapping.
Formation of prills
The thermodynamic calculations can also help elucidate the events that occurred as the liquidmetal phase was cooled after being separated from the slag. The same events would, of course,have occurred as the entrained metallic prills were cooled inside the separated slag phase.
The phase diagram of the Cu–Pb system is shown in Figure 6. Let us take as an example theliquid metal phase that was at equilibrium with SLA1 at 1200°C and PO2 = 10–9 bar. As mentionedabove (Fig. 4), this phase was calculated to contain 4.42 wt% Pb at equilibrium (as well as 0.04%Fe and 0.04% Zn). Ignoring the small Fe and Zn contents, consider the cooling of 100 g of abinary Cu–Pb alloy with 4.42% Pb, as shown on Figure 6. Solid Cu begins to precipitate at aliquidus temperature of 1071°C. If we assume approximately equilibrium cooling conditions,then, when the temperature has decreased to just above the monotectic temperature of 956.7°C,from the lever rule 95.5 g of solid Cu alloy containing 2.41% Pb (point A) has precipitated and4.4 g of liquid containing 48.3% Pb (point B) remains. In reality, the cooling would have been toorapid for equilibrium conditions to prevail. As a result, the precipitated solid Cu phase would have
PbO in liquid slag
Cu2O in liquid slag
Pb in metal
log10PO2 (bar)
weig
ht
%
-11.0 -10.0 -9.0 -8.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Figure 4 The calculated equilibrium PbO and Cu2O contents of the liquid slag phase and the Pb content of the liquidmetal phase at 1200°C as a function of the equilibrium oxygen potential for sample SLA1.
Copper production on the island of Kea, Aegean Sea, Greece 337
contained less than 2.41% Pb. Assuming Scheil–Gulliver (very rapid) cooling conditions, it canbe calculated that the Cu phase contains approximately 1.2% Pb after being cooled to 956.7°C.
The following monotectic reaction then occurs isothermally at 956.7°C:
liquid I at point B Cu solid liquid II at point C( ) = + ( ). (5)
When the monotectic reaction is complete at 956.7°C, from the lever rule, there are 97.56 g ofsolid Cu and 2.44 g of liquid containing 84.8% Pb (point C). That is, the solidification of Cu isnearly complete at 956.7°C. The remaining Pb-rich liquid then cools to 327.41°C, precipitatingsome Cu as it cools. At 327.41°C the liquid is nearly pure Pb, which then solidifies. As a result,much of the Pb impurity would have ended up as a separate phase that could have beenmechanically separated after cooling, leaving a quite pure Cu final product.
This sequence of events can account for many of the observed features of the prills. Althoughthe entrained prills, while still liquid at 1200°C, would have contained 4.42% Pb, this Pb wouldhave soon separated out as a Pb-rich liquid phase as the prills cooled after the slag was removedfrom the furnace. The remaining Cu-rich prills would then contain very little Pb, as observed.The liquid Pb that separated out would collect around the outer edges of the Cu prills, account-ing for the observations of tiny Pb prills near the edges of some of the Cu prills. Some of thisPb-rich liquid would oxidize during cooling in air, accounting for the observation of smallPbO-rich prills or nodules. If the amount of the Pb-rich liquid was relatively large, as would bethe case with SLA2, it could have become separated from the Cu prill and be observed as an
PbO in liquid slag
Cu2O in liquid slag
Pb in liquid metal
log10PO2 (bar)
weig
ht
%
-10.0 -9.5 -9.0 -8.5 -8.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
Figure 5 The calculated equilibrium PbO and Cu2O contents of the liquid slag phase and the Pb content of the liquidmetal phase at 1200°C as a function of the equilibrium oxygen potential for sample SLA2.
independent Pb-rich or PbO-rich prill, as observed for SLA2. The Cu-rich prills, on the otherhand, being larger and solid, would not have time to oxidize significantly during the coolingin air.
Hence we believe that the observed PbO nodules are not relics from the ore. In any case, sincethe melting point of PbO is 886°C, PbO relics would not have survived smelting at temperaturesabove 1125°C.
Finally, the prills containing significant amounts of S, Sb and As are in all probability relics ofthe original ore.
Mineralogy
The mineralogy of the samples is not particularly useful for elucidating either the sources of theores or the metallurgical processes that were used since the phases are all synthetic, having beenformed during cooling in air of slags of the overall compositions shown in Table 2. As the slagswere cooled in air, the Fe2+ would have been partially oxidized to Fe3+. The extent of thisoxidation is unknown. Hence, in simulating the slag solidification with thermodynamic calcu-lations, arbitrary amounts of oxygen were added until the results agreed approximately with theobservations. For example, taking 100 g of SLA1 at equilibrium at 1200°C, adding 1.5 g ofoxygen and cooling showed that olivine (mainly fayalite), clinopyroxene and spinel crystallizeat relatively high temperatures. The remaining liquid at lower temperatures would be veryviscous and would not crystallize, but would instead form a glassy groundmass containing mostof the PbO. This is consistent with the observations in Table 1. Calculations for SLA2 indicate
Liquid
Two liquids
Liquid + Cu
Liquid + Cu
Cu + Pb
956.7o
327.41o
A (2.41) B (48.3)
C (84.8)
0.0442
1071o
Weight fraction Pb
T(o C
)
0 0.2 0.4 0.6 0.8 1
200
300
400
500
600
700
800
900
1000
1100
1200
Figure 6 The Cu–Pb phase diagram.
Copper production on the island of Kea, Aegean Sea, Greece 339
the high-temperature crystallization of olivine, clinopyroxene and two spinel phases, one rich inFe and one rich in Zn. Calculations for SLA4, SLA5 and SLA6 are in reasonable agreementwith the observations in Table 1. In particular, SLA6 is calculated to contain very littleclinopyroxene. Calculations for SLA7 and SLA8 are in qualitative agreement with the obser-vations in Table 1. High clinopyroxene contents were calculated, but also high olivine andspinel contents. Since K2O was not considered in the calculations, the precipitation ofK-feldspars was not reproduced. However, the ‘medium’ amounts of K-feldspars reported insome samples in Table 1 are difficult to understand, given the very low K2O content of all slagsshown in Table 2. As mentioned previously, the calcite observed in one sample probably camefrom dislodged furnace lining.
CONCLUSIONS
The source of the ores used in ancient times on the island of Kea for the production of copper isdebatable. The metallurgical slags of Agios Symeon have as a major distinguishing feature theirhigh Pb–Zn–Cu content, which was not common in the ancient slags of the nearby islandof Seriphos or in the Aegean at any time. Similar Pb/Cu-rich slags have been reported fromthe island of Keros, located east of Kea in the Aegean. On the island of Kea, argentiferouspolymetallic ores and galena have been reported. Lavrion Peninsula and the adjacent southernpart of the island of Evia, as well as most of the Cyclades islands in the Aegean Sea, have beenwell known for the development, exploitation and processing of metals such as silver, lead andcopper since late Neolithic times. Hence, the origin of the Cu ores might be one or more of theabove-mentioned sites.
The thermodynamic simulations demonstrate that the slag samples resulted from Cu-makingprocesses in which there was a large amount of Pb impurity. It would thus appear that mixed oreswere used, containing Cu2S–FeS–PbS with significant amounts of PbS as impurity. The roastingof the ores appears to have been quite thorough, since the sulphur content of the samples is low.The roasted ores were reduced at relatively high oxygen potentials, at temperatures in excessof about 1125°C, to form Cu metal containing 1–2 wt% or less of Pb and very low levels of Feand Zn. The melting points and viscosities of the slags were both low, indicating that they wereproperly fluxed, either fortuitously or by design. The slags were tapped and cooled relativelyrapidly in air.
ACKNOWLEDGEMENTS
Thanks are expressed to Dr Lina Mendoni for discussions on the archaeological importance ofthe ancient towns of the island of Kea and the possible importance of the local lead ores. Theauthors also wish to thank Dr Myrto Georgakopoulou for her valuable comments and suggestionson the revised version of the paper, and Professor Marcello Mellini for his valuable comments onthe manuscript. Figures 3–6 were prepared using FactSage™.
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