The Origin and Archaeometallurgy of a Mixed Sulphide Ore for Copper Production on the Island of Kea, Aegean Sea, Greece*

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Archaeometry 57, 2 (2015) 318–343 doi: 10.1111/arcm.12080

THE ORIGIN AND ARCHAEOMETALLURGY OF A MIXED SULPHIDE 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 metallurgical slags with a high Pb–Zn–Cu content have been found. Thermodynamic simulations have been carried out, using the FactSage™ thermodynamic database computing system, with a view to understanding the ancient metallurgical processes that produced the observed slag compositions and morphologies. The simulations demonstrate that the slag samples resulted from

Cu-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 reduced at relatively high oxygen potentials at ∼1125°C to form Cu containing low levels of Pb, Fe and Zn.

KEYWORDS: METALLURGICAL SLAG, KEA, THERMODYNAMIC SIMULATION, COPPER, LEAD, ROASTED ORES

INTRODUCTION The Lavrion Peninsula, in the southern part of the Attica Peninsula, and the adjacent southern part of 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; Conophagos 1980; Economopoulos 1992; Georgakopoulou 2004, 2005; Kakavogiannis 2005; Bassiakos and Philaniotou 2007; Kakavogianni et al. 2008; Tzachili 2008; Georgakopoulou et al. 2011). Iron metallurgy has been demonstrated to have taken place in several other localities in the same geographical area (Dimou et al. 2001). The broad Lavrion (or Laurium) area hosts the main mining and metallurgical activities among those areas, which were mainly focused on the extraction of silver and lead periodically from Antiquity until the 20th century. The result was the production of slags that remained in situ, were re-melted over a long period of time for recovery of Pb and/or Ag, or were partially reused as ﬂuxing agents for further metallurgical processes.

The chemical composition of these slags was strongly affected by the initial compositions of the ore, the fluxes used and the specific extractive techniques. Pernicka et al. (1990), Lutz and Pernicka (1996) and Mangou and Ioannou (1998) have shown that copper produced in the European Bronze Age is expected to contain a relatively high concentration of impurities. In general, 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 and Mellini 2002; Shugar 2003; Rothenberg et al. 2004; Gale et al. 2008; Tsakiridis et al. 2008). A high 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 in Antiquity, at least since Classical times, as can be easily proven based on written sources referring to miltos (ruddle) (Cherry et al. 1991; Mendoni and Beloyannis 1993). For earlier periods, there has been some dispute about whether metals were extracted from local ores or from ores 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 since prehistoric 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 this fact (Caskey 1971; Coleman 1977). Apart from these prehistoric sites, which are all notably confined to the northern part of the island, metallurgical slags have been reported from Yaliskari, Sidhero Bay, Agios Symeon and Hellenika, while mining galleries are located in various sites on the eastern side of the island: Kalamos/Trypospelies, Spathi, Orkos, Choste and Spasmata (Georgiou and Faraklas 1985; Cherry et al. 1991; Mendoni 1991; Mendoni and Beloyannis 1993). Hand specimens containing 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 in a vein that also contains pyrite, minor zincblende and cerrusite, accompanied by the gangue minerals fluorite, ankerite, limonite and calcite. In dating all these activities, it is impossible to be either certain or precise, because the dating is based on surface finds. The finds range from prehistoric to Hellenistic, but one can note a particular density of Late Classical/Early Hellenistic material. Particularly at the hill of Agios Symeon, significant amounts of scattered slags are exposed over 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 and Beloyannis 1993; Papastamataki 1998). Analyses performed in the present study on a larger number of samples from the specific area reveal that these slags are mostly Cu–Pb–Zn rich. The aim of the present paper is to examine the chemistry, mineralogy and texture of the Agios Symeon slags in order to identify the metallurgical processes that were used by the ancient metallurgists and possibly the composition of the original ores.

GEOLOGICAL SETTING AND MINERALIZATION The island of Kea belongs to the Attico–Cycladic metamorphic belt, lying within the Western Cycladic Detachment System (Müller 2009; Rice et al. 2012). The petrology of the island is dominated by metamorphic rocks, mainly schists that locally host metamorphic basic rocks, cipollines and marbles (Davis 1972; Davis 1982; Avdis 1996; Higgins and Higgins

1996; Rice et al. 2012). Along the south-east coast, these rocks are overlaid by marbles. In the northern part of the island, Triassic carbonate rocks occur, thrusted above the metamorphic rocks. At several places on the island, iron oxide ores occur, which have been exploited since Antiquity for various purposes (Mendoni and Beloyannis 1993; Argyriou and Korasidi 2007). Additionally, the presence and exploitation of massive iron ores, galena and argentiferous polymetallic 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 literature data and unpublished reports of IGME, Greece (Anonymous 1973), metallurgical slags, deposits and ore-grade deposits of mixed sulphides and/or galena have been identiﬁed on almost all of the islands of the Cyclades. Furthermore, arguments have been put forward for the existence of copper 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 the south-eastern part of the island, which has a 25° inclination. The top of the hill is composed of marble that overlies schists. Catapotis (2007) has also noted that the Agios Symeon slags are found 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 pyrometallurgical operation. It is located on a slope, at a distance of 3 km from the ancient city of Karthaia, which has a south-facing bay that is well protected from the north winds that commonly blow in the Aegean Sea, with a harbour for ships delivering wood and ore. Karthaia was linked to the Agios Symeon site by a well-preserved paved road made of tabular blocks of schists, as were many similar roads on the island (Mendoni 2004). In addition, fresh water is available from three springs emanating close to the slag deposits. The south slopes of the hill at Agios Symeon are steep, providing a natural air-channel, a characteristic that has also been reported in other metallurgical sites around the Cyclades. In Antiquity, it was common to operate smelting furnaces high on ridges so that the wind would help raise the furnace temperature by natural draught (Betancourt 2006). The top of the hill is crowned by the church of Agios Symeon, which probably replaced an ancient sanctuary. Two inscriptions dated to the Classical period inform us about the worship of Aphrodite on this site. There is no surface evidence for other structures or remains of a settlement there. However, pottery sherds and ores scattered over the site attest to some activity other than worship. It is interesting to note that the worship of Aphrodite is connected with metallurgy in Cyprus. Representative sherds collected from Agios Symeon have been dated to 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 Beloyannis 1993). 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 copper smelting sites of the southern Aegean. In historical times, the region of Agios Symeon belonged to the city–state of Karthaia, which ﬂourished from the Archaic period down to Late Antiquity, but was also inhabited at least since the Middle Bronze Age. Karthaia is the closest well-known and established settlement to Agios Symeon.

SAMPLING AND ANALYSIS Slag pieces 2–10 cm long are scattered on or near the surface, a few metres from the church of Agios Symeon, at an elevation of 415 m. A total of 50 slag pieces of various sizes were examined from this location. All are angular, having mostly a fluid ropy texture and a uniform grey–black colour. A number of samples have light greenish–yellowish patches that partially cover 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 different contents of base metals and voids. Most samples are massive and microporous, while a few have a spongy texture with large voids. The voids are either spherical or asymmetric. They presumably represent ghosts of metal prills removed during the recovery of entrapped metals in the slag, and/or gas bubbles. The total weight of the slags in this specific site is estimated to be several hundred kilograms scattered 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 sample was cut and the remainder was pulverized in an agate mill. The powdered samples were analysed mineralogically 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-ICP61a for major elements, OA-GRA05 for LOI and Pb-OG62 for lead) (Tables 1 and 2 below). The cut samples were polished and examined microchemically and texturally by scanning electron microscopy (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 does not distinguish between different oxidation states. The Fe, Pb, Zn and Cu contents in Table 1 are total Fe, Pb, Zn and Cu contents expressed in terms of the elements. If these values are expressed instead in terms of the oxides Fe2O3, PbO, ZnO and Cu2O, then the percentages in Table 1 for each sample sum to 100%. The correlation of the concentrations of the major and trace elements is shown in Table 2. The bulk chemistry of the slags varies considerably. The total (Cu + Pb + Zn) content varies from 1.77 wt% to 16.45 wt%. The lead content varies from 0.16% to 10.80%. Based on differences in the copper and lead content of various Early Bronze Age slags and/or the composition of their copper prills, Thornton et al. (2009) suggested that they were derived from different metallurgical processes.

The strong correlation of Mg with SiO2,TiO2, Al, K, Co, Cr and V (Table 2) indicates that this element 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. Its highest concentration is in sample SLA8, which is the poorest in Cu, Pb and Zn, indicating that its 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 primary sulphide ores are also rich in Ni (Kamariza area) (Marinos and Petrascheck 1956; Voudouris et al. 2008a,b). It is correlated with Cu and Mn and is probably derived from the primary ore.

Table 1 ‘Whole rock’ bulk chemical analysis of Agios Symeon, Kea island slags

SLA1 SLA2 SLA3 SLA4 SLA5 SLA6 SLA7 SLA8

SiO2 (%) 36.8 25.6 30.3 39.8 30.7 38.3 42.4 49.5 Al2O3 (%) 1.7 1.3 3.34 6.74 3.73 6.91 4.7 6.2 CaO (%) 5.81 3.7 2.32 6.47 3.06 2.37 10.3 9.84 MgO (%) 0.22 0.44 0.82 1.49 0.68 1.5 1.37 2.94

Na2O (%) 0.11 0.1 0.12 0.15 0.09 0.11 0.16 0.16 K2O (%) 0.3 0.4 0.56 1.05 0.59 0.93 0.81 0.96 TiO2 (%) 0.06 0.05 0.15 0.23 0.11 0.22 0.19 0.24 MnO (%) 2.05 0.35 5.56 3.28 0.33 2.73 1.89 1.27

P2O5 (%) 0.13 0.14 0.12 0.17 0.12 0.14 0.16 0.13 Fe (%) 29.9 34.2 31.2 21.9 33.9 25.8 26.2 20 Pb (%) 3.97 10.8 1.92 0.6 2.79 0.5 0.16 0.53 Zn (%) 1.62 2.91 3.65 3.35 3.62 3.11 1.02 0.51 Cu (%) 2.78 2.74 3.51 3.94 4.21 2.98 2.48 0.73 Ag (ppm) 26 18 42 29433 As (ppm) 480 180 360 190 250 290 150 270 Ba (ppm) 350 240 700 290 360 190 180 210 Be (ppm) <10 <10 <10 <10 <10 <10 <10 <10 Bi (ppm) <20 <20 20 20 <20 <20 <20 <20 Cd (ppm) 10 <10 10 20 10 10 <10 <10 Co (ppm) 60 80 100 100 50 90 120 170 Cr (ppm) 50 50 80 220 60 190 80 290 Ga (ppm) <50 <50 <50 <50 <50 <50 <50 <50 La (ppm) <50 <50 <50 <50 <50 <50 <50 <50 Mo (ppm) <50 <50 <50 <50 <50 <50 <50 <50 Ni (ppm) 280 190 330 370 240 230 300 110 S (%) <0.1 <0.1 0.1 <0.1 <0.1 <0.1 <0.1 0.1 Sb (ppm) 570 70 120 60 70 90 <50 <50 Sc (ppm) <10 <10 10 20 10 10 10 10 Sr (ppm) 130 270 100 150 120 270 120 60 Th (ppm) <50 <50 <50 <50 <50 <50 <50 <50 Tl (ppm) <50 <50 <50 <50 <50 <50 <50 <50 U (ppm) <50 <50 <50 <50 <50 <50 <50 <50 V (ppm) 10 10 30 50 20 50 40 50 W (ppm) 310 460 480 1080 500 710 360 240 Pb + Cu + Zn (%) 8.37 16.45 9.08 7.89 10.62 6.59 3.66 1.77

The bismuth content is low, compared with the Pb–Cu–Ni–Bi-rich slags of the island of Keros and also with the Cu–Pb–Ni–Bi-rich ores from several locations in the broad Kamariza area 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 and copper minerals contained in the polymetallic ores processed at Agios Symeon, such as the ores from 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–tennantite assemblages in the ore (Table 3). The zinc content is high in all samples. It is strongly correlated

© 2014 University of Oxford, Archaeometry 57, 2 (2015) 318–343 oprpouto nteiln fKa eenSa Greece Sea, Aegean Kea, of island the on production Copper Table 2 Correlation of the major and trace elements of the slags studied

Cu Cr Mn Ni V W Zn Pb As Ba Ag Sb Sr Co Al Ca K Mg Fe SiO2

Cu 1 Cr 0.47 1 Mn 0.25 0.08 1 Ni 0.72 −0.3 0.62 1 V −0.25 0.85 0.3 0.03 1 W 0.62 0.29 0.31 0.58 0.41 1 Zn 0.87 −0.29 0.33 0.46 −0.18 0.63 1 Pb 0.07 −0.56 −0.41 −0.26 −0.8 −0.2 0.21 1 04Uiest fOxford, of University 2014 © As −0.02 −0.18 0.35 0.05 −0.35 −0.31 −0.02 −0.01 1 Ba 0.43 −0.35 0.7 0.44 −0.29 −0.02 0.5 0.03 0.49 1 Ag 0.22 −0.56 0.55 0.26 −0.58 −0.28 0.32 0.36 0.63 0.49 1 Sb 0 −0.38 0.06 0.16 −0.54 −0.3 −0.19 0.18 0.84 0.63 0.45 1 Sr 0.23 −0.18 −0.18 −0.07 −0.15 0.36 0.41 0.51 −0.22 0.84 −0.11 −0.09 1 Co −0.77 0.74 0.12 −0.37 0.69 −0.2 −0.63 −0.45 −0.29 −0.22 −0.32 −0.04 −0.4 1 Al −0.05 0.82 0.23 0.04 0.94 0.55 0.03 −0.78 −0.28 −0.29 −0.63 −0.49 −0.02 0.46 1 Ca −0.61 0.36 −0.29 −0.14 0.33 −0.29 −0.86 −0.34 −0.28 −0.28 −0.49 −0.02 −0.53 0.64 0.12 1 K 0.01 0.79 0.25 0.14 0.95 0.64 0.08 −0.72 −0.46 −0.28 −0.63 −0.63 −0.03 0.49 0.96 0.17

Archaeometry Mg −0.6 0.93 0.02 −0.41 0.87 0.05 −0.44 −0.63 −0.3 −0.37 −0.58 −0.53 −0.31 0.87 0.77 0.48 0.75 1 Fe 0.5 −0.9 −0.15 0.08 −0.87 −0.23 0.5 0.7 0.17 0.4 0.58 0.2 0.31 −0.8 −0.8 −0.6 −0.8 −0.9 1 − − − − − − − − − − − − SiO2 0.64 0.76 0.02 0.22 0.73 0.1 0.74 0.73 0.06 0.47 0.58 0.07 0.48 0.76 0.61 0.77 0.55 0.81 0.92 1 57 21)318–343 (2015) 2 , 323 324 A. Pelton et al.

Figure 1 SEM images and microanalysis of the studied slag samples.

Figure 1 (Continued) with copper, suggesting that part of the zinc entered the system from Cu minerals, as well as from zinc-blende. Iron is correlated with copper, zinc and lead, suggesting that this element was originally 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 Table 4 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). Other phases detected include hedenbergite, augite, quartz and feldspars, while calcite and cristobalite were each found in one sample (SLA4 and SLA8, respectively). Augite, K-feldspars (sanidine or

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.

Figure 2 (Continued)

Table 3 Ag minerals or argentiferous copper sulphides reported from Cu-rich polymetallic ores in the Lavrion metallogenetic district (Marinos and Petrascheck 1956; Voudouris et al. 2008a,b)

Mineral Composition Colour

Galena PbS Light grey, dark lead grey

Tetrahedrite (Cu,Fe,Ag,Zn)12Sb4S13 Iron grey, steel grey Tennantite (Cu,Fe,Ag,Zn)12As4S13 Steel grey, black Enargite (Cu,Fe,Zn,Ag)AsSbS Steel grey, blackish grey, violet black

Chalcosite Cu2S Blue black, grey, black Chalcopyrite CuFeS2 Brass yellow Prustite Ag3AsS3 Reddish grey Polybasite (Ag, Cu)16Sb2S11 Black, dark ruby red microcline) and quartz or cristobalite were detected together in three samples. The calcite in SLA4 probably 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, as witnessed 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. Two general groups can be deﬁned: a Pb-silicate-rich (SLA1, SLA2, SLA5 and SLA4) and an

Table 4 XRD bulk mineralogical analysis of crystalline phases of Kea slag samples

Fayalite Magnetite Hedenbergite Quartz/ Augite Sanidine, microcline Calcite (olivine) (spinel) (pyroxene) cristobalite (pyroxene) (K-feldspars)

SLA1 Major Major Major SLA2 Major Major Major SLA3 Major Medium Major SLA4 Major Trace Major Medium Medium SLA5 Major Major Medium SLA6 Major Medium SLA7 Trace Trace Major Medium SLA8 Trace Medium Major Trace

Fe-silicate-rich groundmass (the rest of the samples). The Pb-rich siliceous groundmass has a bright 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 of Zn-rich magnetite. It is noteworthy that dendritic fayalite crystals are observed in all slag samples studied, 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 lead oxide prills of various sizes up to 2 mm, and as asymmetric sub-microscopic exsolutions in the groundmass. Other elements detected in the inclusions are As, Ag, Sb, S and Fe. Silver was detected in SLA1 as curved wire-like or symmetric inclusions alone or with antimony, hosted in a copper-rich prill, suggesting an origin of a silver-bearing tetrahedrite–tennantite ore. In the same 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 minor Fe, or Fe–As–Sb, asymmetric masses. Some copper prills have a discontinuous thin rim of Cu–S composition, with minor Fe. High amounts of Cu–S, Cu–Fe–O and Cu–Fe–S–O occur in nodular form in slags SLA7 and SLA8, which are less rich in Cu + Pb + Zn. In the samples containing PbO or Pb-alloy inclusions (SLA1, SLA2 and SLA3), lead occurs as PbO nodules with minor Fe and Cu contents, as Pb–As alloy with minor Cu and S contents, or as Cu–Pb alloy with minor As or Fe contents. Interestingly, even the relatively Pb-poor sample SLA8 (Pb 0.53%) contains submicroscopic 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 oxides and 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 the presence of relics of the original galena content of the ore. The lead-rich slag sample SLA2 contains lead sulphide relics and also PbO prills with minor Cu and/or Fe contents, or metallic Pb–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 as gases during roasting and smelting; a small portion of them was ﬁxed in the metallic inclusions, in both Cu and Pb prills.

© 2014 University of Oxford, Archaeometry 57, 2 (2015) 318–343 Table 5 The chemical composition and ranges of elements (%) for the metallic inclusions oprpouto nteiln fKa eenSa Greece Sea, Aegean Kea, of island the on production Copper Cu Fe Pb As Sb S PbO Ag2OAs2O3 Sb2O3 Fe2O3 CuO

SLA1 Cu–Fe–As–Sb 86.21–92.88 2.15–6.36 0.6–1.7 1.18–2.27 SLA1 Cu–As–Sb 92.86–92.77 2.12–2.55 1.16–1.78 SLA1 Cu–Fe–S 62.53–75.42 1.41–2.19 14.31–35.37

SLA1 Ag2O–Sb2O3 65% 33% SLA1 Cu–Fe 92.03–98.16 1.61–1.96 SLA1 Cu–S 78.41 19.71 29.66

SLA1 PbO–As2O3–Sb2O3 55.21 20.75 2.89 SLA1 Cu–Pb–S 48.61 37.27 15.01 SLA1 Cu–Fe–As 93.67 0.27 2.63 SLA1 Cu–Pb–As 56.44 42.63 2.05 2.66–3.34

SLA2 PbO–Fe2O3 73.97–88.11 0.88–6.26 1.4–5.8

04Uiest fOxford, of University 2014 © SLA2 PbO–Fe2O3–CuO 65.11–91.94 SLA2 Cu–Fe 92.02–94.94 0.67–3.75 SLA2 Cu–Fe–Pb 3.94–83.56 0.7–1.12 10.82–91.55 SLA3 Cu–Fe 83.62–98.17 0.62–3.54 SLA3 Cu–Fe–S 80.16 12.45 1.44 SLA3 Cu–S–As–Pb 1.79 25.82 10.79 1.77 SLA4 Cu–Pb–S 50.50 35.40 14.10 SLA4 Cu–Fe 95.8–97.33 2.67–3.87 SLA5 Cu–Fe 93.03 4.42

Archaeometry SLA5 Cu–Fe–S 91.22–95.15 1.72–2.44 0.44–1.41 SLA6 Cu–Fe 88.96–95.26 2.07–2.54 SLA7 Cu–Fe–S 72.72 2.7 22.06 SLA7 Cu–Fe–O 80.97–82.08 2.67–3.29 SLA8 Cu–Fe–S–O 65.95–66.2 1.53–2.45 20.15–21.38 57

21)318–343 (2015) 2 , SLA8 Pb–Cu–Fe–S 33.31 9.33 17.06 29.11 SLA8 Cu–Fe–S 66.74–69.18 2.06–6.01 22.2–23.59 329 330 A. Pelton et al.

The mapping of the three major metallic components, Fe, Cu and Pb (Fig. 2) shows that the 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 it occurs 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 of the prill. Note the differences on the grey shadows in the asymmetric central copper prill. The brighter area is almost exclusively copper, whereas the darker area is the Cu-oxide area. The mapping 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 of various shapes; zinc is present in the groundmass in neoformed silicates and spinels, while lead was 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 were accompanied by silver has already been demonstrated by Tournefort (2003 [1770]) and more recent researchers. However, the deposits are not large enough for us to assume ancient metallurgy based on the local sources only. In the area of Agios Symeon, the archaeological findings range in age from prehistoric and Classical through Roman (Caskey et al. 1988, Mendoni and Beloyannis 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 (plus minor Sb–As) with silver contents up to 42 ppm. Silver micro-inclusions were detected by SEM in 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, is not supported by the presence of large amounts of accumulated slags, as is the case on the islands of Lavrion or Kythnos (personal observation; see also Bassiakos and Philaniotou 2007). Marinos and Petrascheck (1956) have pointed out that the older Pb/Ag slags on Lavrion contained as much as 12% Pb (before the Persian Wars, c. 490 bc), changing to Pb-poor slags by Roman times. The high percentage of lead (>10%) that one slag sample (SLA8) contains could suggest that it predates 490 bc. However, the same authors state that during the rule of Athens by the orator Demetrius Phalereus (345–280 bc), and throughout the Roman period, there was a re-melting of slags on Lavrion to recover retained silver. The microanalysis, as well as the ‘whole rock’ chemical analysis, of the Agios Symeon slags differs significantly from slags characterized as copper slags found in Crete and other locations in Greece (Betancourt 2006). The main differences with all Aegean copper metallurgy slags studied, 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 Eneolithic metallurgy for copper production (Ryndina et al. 1999). Even though the analysed slags contain arsenic and antimony, the original content of these elements in the ores cannot be estimated, as most of the Sb and As, as well as almost all the sulphur, 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 by Georgakopoulou (2004, 2007a,b) in the Cu/Pb-rich slags of the island of Keros, which lies south-east of the island of Kea. The metallurgical slags of Agios Symeon have as a major distinguishing feature their high Pb–Zn–Cu content, which was not common in the ancient slags of the Aegean at any time. The only 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 Keros island slags also contain significant amounts of bismuth, arsenic and nickel, a chemistry that resembles ores that have been studied from Kamariza Lavrion ore deposits (Marinos and Petrascheck 1956; Voudouris et al. 2008a,b). Given the uncommon base metal chemistry of the Keros island slags and their high Pb–Cu content (the ‘2nd Group’ of slags), the author proposed the possibility of two types of copper-based metallurgy on the island of Keros, namely arsenical copper rich in lead, and pure copper (Georgakopoulou 2007b). Therefore, the island of Kea is the second location in the Aegean region that has been found to 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 a polymetallic ore in bulk. Taking as the basis of assumption the metallogenetic district of the Lavrion area and the Cyclades, where a series of common and rare metallic minerals has been described, the following minerals can be counted as sources of the various elements identified in the slags studied: galena/cerrusite (Pb source), zinc-blende (Zn source) and primary or secondary Cu-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) and bournotite (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 polybasite cannot be excluded, as polybasite and prustite are found in the polymetallic ores of Lavrion and the southern part of the island of Evia, which is part of the same metallogenetic zone of the Attico-Cycladic massif (Voudouris et al. 2008a,b). Besides Lavrion, other islands of the Cyclades or the southern part of the island of Evia might be the source of the polymetallic ores. On the neighbouring island of Seriphos, the ancient Cu slags 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 content of Seriphos slags is lower than in most of the Kean samples analysed. Mining and metallurgical works 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 evidence for the origin of the primary ores by correlating isotopes of slags and a few ore deposits located on Lavrion or on a few islands in the Cyclades. Our opinion is that it is not possible to prove that the ancient consumers, traders and metallurgists asked for ores from a single specific 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 traders probably supplied the local furnaces with raw materials from several sources around the Eastern 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 island of 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

(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 shipments from Lavrion, most probably from the Ano Sounion area, where Pb–Cu–Zn ores were exploited in vast quantities in ancient times, or from another locality in the vicinity of Kea. The Kamariza area, the other Lavrion area that hosts Cu-rich ores, can be excluded since these ores are rich in Ni and Bi, elements that are present in the Keros island slags, but not in those from Kea. The possibility of transportation of ores from other locations of the Aegean to the island of Kea can be explained by the scarcity of any ‘fuels’ on Lavrion, the presence of high-quality basic fluxes in sufficient quantities close to Agios Symeon and in other localities of Kea, and the presence of skilled copper metallurgists in the Cyclades. In general, high-quality ore is preferable for 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 the island of Kea. These calculations require accurate databases of the thermodynamic properties of multicomponent solutions (most importantly, of solid and liquid oxide and metallic solutions) as functions of temperature and composition. All thermodynamic calculations were performed using the FactSage™ thermodynamic computer system, of which one of the present authors is a principal developer, coupled with the large evaluated FACT databases, which contain data for over 6000 pure 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 equilibrium calculations by means of a general Gibbs energy minimization algorithm. The FACT solution databases give the thermodynamic properties (chemical potentials) as functions of temperature and composition 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 of composition and temperature. Next, all available thermodynamic and phase diagram data from the literature are simultaneously ‘optimized’ to obtain one set of critically evaluated self-consistent parameters 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 experimental error limits. Finally, the models are used to estimate the thermodynamic properties of N-component solutions from the database of parameters of lower-order subsystems. All data of all types (phase diagrams as functions of temperature, composition and oxygen potential; activity measurements, calorimetric data; etc.) from thousands of original references have simultaneously been taken into account in developing the optimized FACT databases over the past 35 years. For complete 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, composition and oxygen potential gradients during smelting. It is also assumed that the slags did not become highly segregated upon cooling, so that the compositions of the analysed samples are representative of the composition of the entire slags during smelting. Clearly, these assumptions are not minor. Nevertheless, we believe that the thermodynamic analysis provides at least a semi-quantitative insight into the nature of the metallurgical processes that produced these slags.

The aforementioned ropy texture of the slags indicates that they were most probably tapped and cooled quickly. That the cooling was relatively rapid is also suggested by the presence of dendritic fayalite crystals, as has already been mentioned. Fast cooling would minimize segre- gation, thereby supporting the assumption that the compositions of the samples are representative of the overall slag compositions. As will be shown, a general conclusion of the thermodynamic analysis is that, despite the high Pb/Cu ratios observed in some samples, all the slags examined resulted from the production of Cu, with lead as an impurity. The first step in the treatment of sulphide ores is roasting (heating in air) in order to remove the sulphur as SO2, thereby converting the sulphides to oxides, which can be subsequently reduced with 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 the charge 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 and contributed to forming a low melting point slag during smelting. Additional fluxes might have been added to further decrease the melting point. A slag with the approximate composition of fayalite, 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 gangue was high in silica, iron oxide flux would be added, whereas a gangue high in iron oxides would require a silica flux. Other fluxing agents such as CaO or MgO might also have been added; these have approximately the same function as FeO. As will be shown below, the melting points and viscosities of the Kea island slags were quite low. That is, they were either fortuitously (through the gangue material), or intentionally, properly fluxed. The oxide mixture was then reduced with charcoal in a kiln. Temperatures of the order of 1150–1250°C are necessary in order to produce a liquid slag. The heat was supplied by the combustion 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 oxides according to the reaction:

MOx +=+ COx M CO2, (1) where M = Cu, Pb, Zn and so on. The equilibrium (1) is controlled by the ratio of partial pressures, PCO/PCO2; the higher this ratio, the more reducing is the gaseous mixture. Rather than specifying the actual CO/CO2 partial pressure ratio, it is common metallurgical practice to give the equivalent equilibrium oxygen pressure (or oxygen potential) of the gaseous mixture. At complete thermodynamic equilibrium with carbon at 1200°C, the theoretical oxygen potential is 7.65 × 10–18 bar. However, such a highly reducing potential could never have been achieved in practice. 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 the furnace 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 the components SiO2–Al2O3–CaO–MgO–MnO–Fe–Pb–Zn–Cu. All other minor components were ignored.

Smelting simulation In the ﬁrst simulation, a mixture consisting of slag of the composition of SLA4 was equilibrated at 1200°C, at ﬁxed oxygen potentials varying from 10–11 to 10–7 bar. The FactSage software calculates the equilibrium state of the system by minimizing the total Gibbs energy while adding or removing oxygen from the system iteratively until the calculated equilibrium oxygen potential becomes 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 molten oxide phase and 3.05 g of a liquid copper phase of the compositions shown in Table 6. It can be seen that most of the Fe in the oxide phase is present as FeO, while the metallic phase contains 99.59% Cu. After smelting, the liquid copper phase and the slag phase would be separately tapped and cooled. The equilibrium composition of the molten metal phase in the kiln during smelting was the same as the composition (Table 6) of these Cu prills, which are droplets of the molten metal that became entrained in the liquid slag. Let us suppose that the total masses of molten slag and metal in the kiln during smelting were approximately equal—say, 100 kg each. Then, as can be seen from Table 6, the kiln during smelting contained 0.70 kg of PbO in the slag phase and

0.29 kg of Pb in the metal phase, along with 1.29 kg of Cu2O in the slag phase and 99.59 kg of Cu in the metal, giving an overall Pb/Cu weight ratio in the initial ore of only 0.01 (molar ratio of 0.03). That is, an ore with a Pb/Cu ratio of only 0.01 can produce a molten slag with a

PbO/Cu2O ratio of 0.54. The distribution of Pb and Cu between the molten slag and metal phases is determined by the following equilibrium between Pb and Cu in the metal phase and PbO and

Cu2O in the slag:

0 Pb+=+ Cu2 O2 Cu PbO,.ΔG =− 23 1 kJ at 1200 ° C . (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, the standard Gibbs energy change must be corrected to take into account the chemical activities of the components. This is done by the FactSage software. The Gibbs energy change is still very

= −9 Table 6 Calculated equilibrium composition (wt%) of slag SLA4 at 1200°C and PO2 10 bar

96.95 g Liquid oxide phase 3.05 g Liquid metal phase (prills)

SiO2 43.24 Cu 99.59 FeO 30.02 Pb 0.29

Fe2O3 0.68 Zn 0.09 Al2O3 7.32 Fe 0.03 CaO 7.03 Mn 0.00001 MgO 1.62 MnO 3.56

Mn2O3 0.004 Cu2O 1.29 PbO 0.70 ZnO 4.53

© 2014 University of Oxford, Archaeometry 57, 2 (2015) 318–343 Copper production on the island of Kea, Aegean Sea, Greece 335 negative. That is, Cu2O is more easily reduced than PbO. As a result, the equilibrium (2) is shifted to the right, with Cu being favoured in the metal phase and lead, as PbO, being favoured in the slag phase. Only at very low oxygen pressures, when most of the Cu2O in the ore has been reduced and the concentration of Cu2O, and hence its chemical activity, in the slag becomes very low, will the Pb content of the metal phase become signiﬁcant. In Figure 3, the calculated equilibrium PbO and Cu2O contents of the liquid slag and the Pb content of the liquid metal are plotted over the range of oxygen potentials expected for the ancient kilns. It may be noted that the –7.2 Cu2O will not be reduced to Cu at 1200°C unless the oxygen potential is less than 10 bar. That is, 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, the temperature above which the slag is fully liquid. This temperature depends somewhat upon the oxygen potential. At potentials of 10–10 and 10–9 bar, the calculated liquidus temperatures are 1137°C and 1124°C, respectively, with the primary solid being olivine of the approximate = –8 composition (Fe0.34Mg0.11Zn0.02Mn0.03)2SiO4, while at PO2 10 bar, the liquidus occurs at 1141°C, with spinel (containing 93 mol% Fe3O4) as the primary solid. As discussed previously, the appearance of the slags indicates that they were nearly fully liquid. Hence the kilns were most probably operating at temperatures above 1125°C. The thermodynamic simulations were carried out arbitrarily at 1200°C. However, the results are reasonably independent of temperature over the range from 1100°C to 1300°C.

6.0

5.0

4.0

Cu O in liquid slag 3.0 2 weight %

2.0 Pb in metal

1.0 PbO in liquid slag

0.0 -11.0 -10.0 -9.0 -8.0 -7.0

log10PO2 (bar)

Figure 3 The calculated equilibrium PbO and Cu2O contents of the liquid slag phase and the Pb content of the liquid metal phase at 1200°C as a function of the equilibrium oxygen potential for sample SLA4.

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 reactions are even more negative than that of reaction (2):

0 (3) Zn+=+ Cu2 O2 Cu PbO,.ΔG =− 120 7 kJ at 1200 ° C ,

0 Fe+=+ Cu2 O2 Cu PbO,.ΔG =− 110 5 kJ at 1200 ° C . (4)

Hence, reactions (3) and (4) are displaced strongly to the right and the tendency of Fe and Zn to 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%. This is much lower than the measured Fe content of 2.67–3.87% for the Cu-rich prills in SLA4 shown in Table 5. Calculations show that such high-equilibrium Fe contents would require oxygen potentials in the range 10–12.6 to 10–12.9. It is very unlikely that such low potentials could have been achieved. Furthermore, such low oxygen potentials would have resulted in a very low PbO content of the slag, of the order of 0.25%, which is inconsistent with the observation that the PbO content of the oxide groundmass of the cooled slags is relatively high. The high apparent Fe content of the Cu-rich prills is most probably due to an error of analysis in which some of the oxide phases surrounding the prills were inadvertently included in the analysis. Georgakopoulou et al. (2011) came to the same conclusion regarding the analysis of the Fe content of the Cu prills in their slag samples. Their conclusion was supported by the analysis of a very large prill in which the Fe content was below the detection limit of the analysis. Samples SLA1 and SLA2 have the largest observed Pb/Cu ratios. Accordingly, thermodynamic analyses, similar to that just discussed for SLA4, were carried out on these slags to see if they could have 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 of the liquid metal phase at 1200°C. In SLA1, operation at 10–9 bar still yields a metal phase containing only 4.42 wt% (1.3 mol%) Pb.While it is just conceivable that SLA2 could have resulted from a Pb-making process operating at low oxygen partial pressure, this seems very unlikely in view of the facts that all the other slag samples are clearly the result of Cu-making, that a large amount 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 a liquid Cu-rich phase. It seems much more likely that SLA2 resulted from a Cu-making process –8 operating at a high PO2 of the order of 10 bar, where the equilibrium slag at 1200°C is calculated to 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 molten metal, 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 is calculated as 1128°C, 1111°C and 1156°C, respectively, while for SLA2, the corresponding temperatures 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. The viscosity of molten slags depends strongly upon the silica content and upon the temperature. The dependence upon the oxygen potential is less important. The calculated viscosities for seven of the eight analysed slags at 1200°C at an oxygen partial pressure of 10–9 varied from 1.6 poise for

SLA2, with the lowest SiO2 content, to 15 poise for SLA4. Only SLA8, with the highest SiO2

© 2014 University of Oxford, Archaeometry 57, 2 (2015) 318–343 Copper production on the island of Kea, Aegean Sea, Greece 337 content, had a higher viscosity (38 poise). At a temperature of 1125°C, the viscosities are approximately double their values at 1200°C. Modern copper slags have viscosities at operating temperatures of the order of 1–50 poise. Hence, the ancient slags were quite ﬂuid. This would favour good separation of slag and metal during tapping.

Formation of prills The thermodynamic calculations can also help elucidate the events that occurred as the liquid metal 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 the = –9 liquid metal phase that was at equilibrium with SLA1 at 1200°C and PO2 10 bar. As mentioned above (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 a binary Cu–Pb alloy with 4.42% Pb, as shown on Figure 6. Solid Cu begins to precipitate at a liquidus 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 and 4.4 g of liquid containing 48.3% Pb (point B) remains. In reality, the cooling would have been too rapid for equilibrium conditions to prevail. As a result, the precipitated solid Cu phase would have

10.0

9.0

8.0

Pb in metal 7.0

6.0

5.0

weight % PbO in liquid slag 4.0

3.0

2.0

Cu2O in liquid slag 1.0

0.0 -11.0 -10.0 -9.0 -8.0

log10PO2 (bar)

Figure 4 The calculated equilibrium PbO and Cu2O contents of the liquid slag phase and the Pb content of the liquid metal phase at 1200°C as a function of the equilibrium oxygen potential for sample SLA1.

15.0

14.0

13.0

12.0

11.0 PbO in liquid slag

10.0

9.0

8.0 Pb in liquid metal 7.0 weight % 6.0

5.0

4.0

3.0

2.0 Cu2O in liquid slag

1.0

0.0 -10.0 -9.5 -9.0 -8.5 -8.0

log10PO2 (bar)

Figure 5 The calculated equilibrium PbO and Cu2O contents of the liquid slag phase and the Pb content of the liquid metal phase at 1200°C as a function of the equilibrium oxygen potential for sample SLA2. contained less than 2.41% Pb. Assuming Scheil–Gulliver (very rapid) cooling conditions, it can be 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 of solid Cu and 2.44 g of liquid containing 84.8% Pb (point C). That is, the solidification of Cu is nearly complete at 956.7°C. The remaining Pb-rich liquid then cools to 327.41°C, precipitating some 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 been mechanically 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. Although the entrained prills, while still liquid at 1200°C, would have contained 4.42% Pb, this Pb would have soon separated out as a Pb-rich liquid phase as the prills cooled after the slag was removed from 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, accounting for the observations of tiny Pb prills near the edges of some of the Cu prills. Some of this Pb-rich liquid would oxidize during cooling in air, accounting for the observation of small PbO-rich prills or nodules. If the amount of the Pb-rich liquid was relatively large, as would be the case with SLA2, it could have become separated from the Cu prill and be observed as an

1200

o 1100 1071 Liquid

1000 Liquid + Cu Two liquids C (84.8) o A (2.41) B (48.3) 956.7 900

800 C)

o 700 T( Liquid + Cu

600

500

400

o 327.41 300 Cu + Pb 0.0442 200 0 0.2 0.4 0.6 0.8 1 Weight fraction Pb

Figure 6 The Cu–Pb phase diagram. independent Pb-rich or PbO-rich prill, as observed for SLA2. The Cu-rich prills, on the other hand, being larger and solid, would not have time to oxidize signiﬁcantly during the cooling in air. Hence we believe that the observed PbO nodules are not relics from the ore. In any case, since the melting point of PbO is 886°C, PbO relics would not have survived smelting at temperatures above 1125°C. Finally, the prills containing signiﬁcant amounts of S, Sb and As are in all probability relics of the original ore.

Mineralogy The mineralogy of the samples is not particularly useful for elucidating either the sources of the ores or the metallurgical processes that were used since the phases are all synthetic, having been formed during cooling in air of slags of the overall compositions shown in Table 2. As the slags were cooled in air, the Fe2+ would have been partially oxidized to Fe3+. The extent of this oxidation is unknown. Hence, in simulating the slag solidiﬁcation with thermodynamic calculations, arbitrary amounts of oxygen were added until the results agreed approximately with the observations. For example, taking 100 g of SLA1 at equilibrium at 1200°C, adding 1.5 g of oxygen and cooling showed that olivine (mainly fayalite), clinopyroxene and spinel crystallize at relatively high temperatures. The remaining liquid at lower temperatures would be very viscous and would not crystallize, but would instead form a glassy groundmass containing most of the PbO. This is consistent with the observations in Table 1. Calculations for SLA2 indicate

© 2014 University of Oxford, Archaeometry 57, 2 (2015) 318–343 340 A. Pelton et al. the high-temperature crystallization of olivine, clinopyroxene and two spinel phases, one rich in Fe and one rich in Zn. Calculations for SLA4, SLA5 and SLA6 are in reasonable agreement with the observations in Table 1. In particular, SLA6 is calculated to contain very little clinopyroxene. Calculations for SLA7 and SLA8 are in qualitative agreement with the observations in Table 1. High clinopyroxene contents were calculated, but also high olivine and spinel contents. Since K2O was not considered in the calculations, the precipitation of K-feldspars was not reproduced. However, the ‘medium’ amounts of K-feldspars reported in some samples in Table 1 are difﬁcult to understand, given the very low K2O content of all slags shown in Table 2. As mentioned previously, the calcite observed in one sample probably came from dislodged furnace lining.

CONCLUSIONS The source of the ores used in ancient times on the island of Kea for the production of copper is debatable. The metallurgical slags of Agios Symeon have as a major distinguishing feature their high Pb–Zn–Cu content, which was not common in the ancient slags of the nearby island of Seriphos or in the Aegean at any time. Similar Pb/Cu-rich slags have been reported from the island of Keros, located east of Kea in the Aegean. On the island of Kea, argentiferous polymetallic ores and galena have been reported. Lavrion Peninsula and the adjacent southern part of the island of Evia, as well as most of the Cyclades islands in the Aegean Sea, have been well known for the development, exploitation and processing of metals such as silver, lead and copper since late Neolithic times. Hence, the origin of the Cu ores might be one or more of the above-mentioned sites. The thermodynamic simulations demonstrate that the slag samples resulted from Cu-making processes in which there was a large amount of Pb impurity. It would thus appear that mixed ores were used, containing Cu2S–FeS–PbS with signiﬁcant amounts of PbS as impurity. The roasting of 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 excess of about 1125°C, to form Cu metal containing 1–2 wt% or less of Pb and very low levels of Fe and Zn. The melting points and viscosities of the slags were both low, indicating that they were properly ﬂuxed, either fortuitously or by design. The slags were tapped and cooled relatively rapidly in air.

ACKNOWLEDGEMENTS Thanks are expressed to Dr Lina Mendoni for discussions on the archaeological importance of the ancient towns of the island of Kea and the possible importance of the local lead ores. The authors also wish to thank Dr Myrto Georgakopoulou for her valuable comments and suggestions on the revised version of the paper, and Professor Marcello Mellini for his valuable comments on the manuscript. Figures 3–6 were prepared using FactSage™.

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