Tracing the Resources of Iron Working at Ancient Sagalassos (South-West Turkey): A Combined Lead and Strontium Isotope Study on Iron Artefacts and Ores*

Archaeometry

49

, 1 (2007) 75–86. Printed in Singapore

*Received 29 March 2005; accepted 27 April 2006†Corresponding author: tel. +32 16326460; fax +32 16322980; email [email protected]© University of Oxford, 2007

Blackwell Publishing LtdOxford, UKARCHArchaeometry0003-813X© University of Oxford, 2007XXX 2007491

ORIGINAL ARTICLE

Iron working at ancient Sagalassos (south-west Turkey)

P. Degryse

et al.

TRACING THE RESOURCES OF IRON WORKING AT ANCIENT SAGALASSOS (SOUTH-WEST TURKEY):

A COMBINED LEAD AND STRONTIUM ISOTOPE STUDY ON IRON ARTEFACTS AND ORES*

P. DEGRYSE,

1

† J. SCHNEIDER,

1,2

N. KELLENS,

3

M. WAELKENS

3

and PH. MUCHEZ

1

1

Centre for Archaeological Sciences, Geology Section, Katholieke Universiteit Leuven, Celestijnenlaan 200E, 3001 Leuven, Belgium

2

Isotope Geochemistry and Geochronology Group, University of Geneva, Rue des Maraichers 13, CH-1205 Geneva, Switzerland

3

Department of Archaeology, Katholieke Universiteit Leuven, Blijde Inkomststraat 21, 3000 Leuven, Belgium

Lead and strontium isotope analyses were performed by thermal ionization massspectrometry (TIMS) on Roman to Byzantine iron artefacts and iron ores from the territoryof ancient Sagalassos (south-west Turkey), to evaluate Pb and Sr isotopes for provenancedetermination of ores for local iron production. It can be demonstrated that for early Romanartefacts and hematite iron ore processed in early Roman times from Sagalassos proper,as well as for magnetite placer sands and early Byzantine raw iron from the territory ofthe city, Sr isotopes are much less ambiguous than Pb isotopes in providing clearly coherentsignatures for ore and related iron objects. Late Roman iron objects were producedfrom iron ores that as yet remain unidentified. Early Byzantine iron artefacts display morescatter in both their Pb and Sr isotope signatures, indicating that many different ore sourcesmay have been used. Our study demonstrates that iron objects can be precisely analysedfor their Sr isotopic composition, which, compared to Pb isotopes, appears to be a muchmore powerful tool for distinguishing between chronological groups and determining theprovenance of raw materials.

KEYWORDS:

BYZANTINE, IRON ARTEFACTS, LEAD ISOTOPES, ORE PROVENANCE, ROMAN, SAGALASSOS, STRONTIUM ISOTOPES, TIMS, TURKEY

*Received 29 March 2005; accepted 27 April 2006© University of Oxford, 2007†Corresponding author: tel. +32 16326460; fax +32 16322980; email [email protected]

INTRODUCTION

In the study of ancient iron production and processing, it remains notoriously difficult toestablish the origin of the raw materials of iron artefacts on the basis of main and trace elementcontents (Gale and Stos-Gale 1982; Pernicka 1987; Heimann

et al.

2001). In the directreduction method for preparing raw iron from its ores, the ore is reduced in a bloomery furnacein one step (Cleere 1976; Pleiner 2000). With the aid of carbon monoxide as a reducing agent,the ore reacts to produce a bloom or ‘spongy’ iron at 1150–1200

°

C (Wertime 1980), fromwhich the slag drains away as a melt (White 1986). Temperatures as high as 1540

°

C, themelting point of iron, could normally not be achieved in antiquity (Tylecote 1980) and it isthought that the use of limestone as a flux was generally unknown (Cleere 1976), althoughsome archaeological evidence exists that this technique may have been used in ancient iron

76

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© University of Oxford, 2007,

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ore processing (e.g., Heimann

et al.

2001). Nevertheless, for the production of metallic iron, aconsiderable amount of ferric oxide must be sacrificed to produce slag (Cleere 1976). Duringsuch reduction process, the gangue materials, which are incompatible with metallic iron, areenriched in the slag. Due to these extensive effects of chemical fractionation that occur duringiron production, it is difficult to correlate slag material, bloom, iron objects and potential oressolely on the basis of their main and trace element chemistry and mineralogy when ore, slagand bloom of one furnace charge are not found in the same archaeological context (e.g.,Buchwald and Wivel 1998; Heimann

et al.

2001).Conversely, the isotopes of lead are not fractionated during such a process and the lead

isotopic composition of a metal or glass artefact is identical to that of the raw materials fromwhich it was produced (e.g., Brill and Wampler 1965; Gale and Stos-Gale 1982). Therefore,lead isotopes have been extensively used in archaeometry to trace the provenance of metalores, especially in the study of bronze (e.g., Gale and Stos-Gale 1982; Yener

et al.

1991).However, due to analytical difficulties arising with the generally low Pb contents of ironartefacts, lead isotopes have been rarely employed for the provenance determination of ironores (Gale

et al.

1990; Schwab

et al.

2003). Furthermore, the potentially large chemical andPb isotopic variation of metal ores is a general problem in provenance studies based on leadisotopes. Lead isotope signatures can be ‘broad’ and isotopic populations of distinct oredistricts may overlap considerably (e.g., Gale and Stos-Gale 1981, 1982; Yener

et al.

1991;Sayre

et al.

1992; Pernicka 1992, 1993; Budd

et al.

1993; Stos-Gale

et al.

1995; Sayre

et al.

2001), making it impossible to identify specific ore types used for metal production.As with the isotopes of lead, the

87

Sr/

86

Sr ratio of natural strontium in the Earth varies withage and rock type due to differential decay of

87

Rb to stable

87

Sr, thereby providing the basisfor the well-established Rb–Sr dating technique in geochronology (see, e.g., Faure and Powell1972; Faure 1986). Over the past 25 years, geologists have also employed Sr isotopes for rockprovenance studies and chronostratigraphy (Faure 1986, 2001; Banner 2004), whereas eco-logists and archaeologists have analysed Sr isotopes in the food chain for tracing animal andhuman migration (e.g., Grupe

et al.

1997). Isotopes of strontium are also not fractionatedduring natural exchange reactions or by kinetic effects due to their relatively high masses atlow internal mass differences (Faure 1986, 2001), leaving the opportunity to use the

87

Sr/

86

Srratio for provenancing the raw materials of artefacts. The potential of this approach has yetbeen largely unexplored, although some studies have been undertaken to trace the origin ofmarbles (Brilli

et al.

2005) and Roman glass (Wedepohl and Baumann 2000; Freestone

et al.

2003; Degryse

et al.

2006).Here, we evaluate both lead and strontium isotopes of ores and iron objects in combination

to study the origin of raw materials used for iron production at ancient Sagalassos (south-westTurkey). The application of strontium isotopes to trace the origin of ores used for iron produc-tion (and metal in general) in antiquity is unprecedented. As with lead isotopes, only negativeconclusions can be drawn on the raw materials of artefacts using

87

Sr/

86

Sr ratios. However, inthis study we will show that the strontium isotopic composition of iron artefacts can provideadditional and decisive information on the origin of their raw materials that could not bederived from lead isotope data alone.

THE ARCHAEOLOGICAL CONTEXT

The archaeology of Roman to early Byzantine Sagalassos is the subject of an interdisciplinaryresearch project coordinated by the Katholieke Universiteit Leuven since 1985 (Waelkens 1999;

Iron working at ancient Sagalassos (south-west Turkey)

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Degryse

et al.

2003d). The site is located in the Taurus Mountains of southwestern Turkey, 120km north of the modern beach resort of Antalya, within the ancient region of Pisidia. Apartfrom exploiting the obvious resources such as agriculture and forestry, the extensive territoryof Sagalassos offered a rich variety in mineral resources, among which are also iron ores(Degryse

et al.

2003a,d). The Lycean limestone nappes, just north of the ancient town, containoccurrences of hematite. This mineralization is related to fluid expulsion from an ophioliticmélange into the overlying allochtonous limestone (Degryse

et al.

2003a). The olistostromedeposits of the Bey Da

g

ları limestone platform show placer deposits of magnetite and titanite(Degryse

et al.

2003b).The frequent occurrence of iron slag in several excavation layers, dating from the first to the

seventh centuries

ad

, proves that iron metal was processed at Sagalassos (Kellens

et al.

2003).In early Roman excavation contexts in the city of Sagalassos proper, rejected hematite ore(associated with chert, less than 20% Fe

2

O

3

) has been found in association with metallurgicalwaste (mainly smithing slag), trade iron (‘hooked billets’) and many shaped iron artefacts. Inlate Roman and early Byzantine excavation contexts, large amounts of metallurgical waste(mainly smithing slag) and shaped iron artefacts have again been identified. However, noindication of the ore has been found. Remarkably, an early Byzantine iron workshop is alsoknown at Sagalassos, as iron slag (smithing hearths), hammer scale (debris of the activity onthe anvil) and iron flakes are indicative of the activity there (Kellens

et al.

2003). It has been datedto the sixth to seventh centuries

ad

. In early Byzantine times, iron-smelting sites were activeacross the entire territory, apart from the obvious iron smithing in and around the city itself.Large concentrations of iron tap slag (from smelting) and furnace materials (furnace lining,tuyères) have been found in the hills between the villages of Dereköy and Çanaklı, 5–7 km tothe south-east of Sagalassos (Degryse

et al.

2003a,d). Remains of furnace hearths with bloomand tap slag attached have been recovered from the surface (Degryse

et al.

2003b). Theactivities there have been dated to the sixth to seventh centuries

ad

on the basis of associatedceramics. The material from the two iron-producing localities, Sagalassos and Dereköy, isclearly distinct, as is the ore used (Degryse

et al.

2003a,b). Around Sagalassos, the iron wasteshows a normal chemical composition for Roman slag, with high FeO and SiO

2

contents andmoderate to low contents of other constituents (cf., e.g., Serneels 1993). It is mineralogicallydominated by wuestite and fayalite. Around the Bey Da

g

ları massif, the metallurgical wasteshows an exceptional chemical composition, with remarkably elevated TiO

2

, V

2

O

5

, CaO,MgO, Sr, REE, Zr, U and Th contents. Mineralogically, the slag has a magnetite-dominatedcomposition. The magnetite–titanite placer sands at this location, which also contain somemonazite, pyroxene and amphibole, show an excellent chemical correspondence to this slag.From all these data, it is clear that two entirely different settings for iron production wereactive in the territory of Sagalassos (Degryse

et al.

2003b).

METHODS

In this study, we have characterized relevant types of iron ores from the territory of Sagalassosand related iron-working products (bloom and trade iron), as well as iron artefacts from well-defined excavation contexts, by means of their lead and strontium isotopic compositions,which have been measured by thermal ionization mass spectrometry (TIMS). We have alsodetermined the Sr concentrations for most of these samples and the Pb contents of selectediron objects by the isotope dilution technique (ID–TIMS), using enriched

84

Sr–

206

Pb tracersolutions.

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Sampling

A suite of 21 samples has been selected for isotopic analysis. The chronology of the sampleswas determined by stratigraphic and contextual analysis. Material from three distinct periods wassampled: early Roman (imperial),

ad

1–150; late Roman,

ad

300–450; and early Byzantine,

ad

450–650/75. Samples were taken from the recovered hematite ore, a hooked billet and ironartefacts from early Roman excavation contexts. Also, samples of the magnetite–titaniteplacer sands at Dereköy and associated early Byzantine raw iron from this location in the territorywere sampled. In the case of Dereköy, ore and bloom were found in a single productionlocation, alongside smelting waste, undiagnostic slag and furnace fragments (Degryse

et al.

2003b). Their ore–iron relation is therefore undoubted. Geochemical, mineralogical andpetrographic investigation of the furnace fragments at Dereköy (Degryse

et al.

2003b) indicatedthat these were identical in raw material composition to the early Byzantine common waresproduced at Sagalassos, using local ophiolitic clay (Degryse

et al.

2003d). Supplementalsamples were taken from bloom, an iron billet and shaped iron artefacts from both late Romanand early Byzantine excavation contexts, where there was no obvious indication of the oresused. Also, iron flakes from the early Byzantine workshop (Kellens

et al.

2003) at Sagalassos,likely to be hammer scale from the smithing process, were included in our analyses.

Analytical techniques

The iron artefacts studied were sawn through and uncorroded samples were cut from the coresof the objects involved. All samples were weighed into clean Teflon

®

screw-top beakers. Ironartefacts (sample weights of several hundred mg) were completely dissolved in aqua regia,while iron ores were dissolved in a 3:1 mixture of 22 N HF/14 N HNO

3

on a hotplate. Thelatter solutions were dried and dissolved in aqua regia. All but two solutions were spiked witha highly enriched

84

Sr tracer, which allows for determination of the Sr isotopic compositionand the Sr elemental concentration from one mass spectrometry run. Some of the solutionscontaining dissolved iron artefacts were subsequently split into two aliquots for separate deter-mination of Pb isotope ratios and Pb concentrations by isotopic dilution, using a

206

Pb tracer.Strontium and lead were chemically separated with 3 N HNO

3

, using EICHROM Sr resin on250

µ

l Teflon columns, following the methods of Horwitz

et al.

(1991a,b). Sr was strippedfrom the columns with 5 ml of H

2

O. Subsequently, Pb was eluted from the same column with5 ml of HCl 6 N. The Pb cut was further processed through a 250

µ

l column containingEICHROM Pre Filter Resin. After evaporation to dryness, the entire procedure was repeatedusing 50

µ

l columns to further purify both the Sr and Pb elutes. For mass spectrometry, Sr wasloaded with TaCl

5

–HF–H

3

PO

4

solution (Birck 1986) on to W single filaments, and Pb was loadedon to single Re filaments using silica gel – H

3

PO

4

bedding. All Sr and Pb isotopic measurementswere performed on a six-collector FINNIGAN MAT 261 solid-source mass spectrometer, runningin static multi-collection mode. Sr isotopic ratios were normalized to

88

Sr/

86

Sr = 0.1194. Repeatedstatic measurements of the NBS 987 standard over the duration of this study yielded an average

87

Sr/

86

Sr ratio of 0.71025

±

4 (2

σ

mean,

n

= 22). Pb isotopic ratios were corrected for mass frac-tionation using a mean discrimination factor of 0.123

± 0.029 (2σ) %/[amu], based on replicatemeasurements of the NBS SRM 981 common lead (n = 84) standard. Errors and error corre-lation were calculated after Ludwig (1980), the 2σ uncertainties for the corrected 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios being 0.06%, 0.09% and 0.12%, respectively. Individual a prioriuncertainties (2σ) are given for 87Sr/86Sr ratios and Sr–Pb elemental concentrations (Table 1).

Iron working at ancient Sagalassos (south-w

est Turkey)79

© U

niversity of Oxford, 2007, A

rchaeometry 49, 1 (2007) 75

–86

Table 1 Lead and strontium isotope and concentration data for ore and iron samples from Sagalassos

Sample Context Location Material 206Pb/204Pb 207Pb/204Pb 208Pb/204PbConc.

Pb (ppm) 2σ 87Sr/86Sr 2σConc.

Sr (ppm) 2σ

Fe ores02PDC11 Early Roman Sagalassos Hematite 18.056 15.588 37.859 n.d. 0.71149 0.00003 34.61 0.3603PDS7 (Outcrop) Dereköy Magnetite placer sand 19.363 15.709 39.257 n.d. 0.70379 0.00004 762.37 7.3703PDS5 (Outcrop) Dereköy Magnetite placer sand 19.365 15.717 39.269 n.d. 0.70373 0.00001 1.70% 0.01%03PDS8 (Outcrop) Dereköy Magnetite placer sand 19.375 15.730 39.327 n.d. 0.70372 0.00004 2.47% 0.03%03PDS4 (Outcrop) Dereköy Magnetite placer sand 19.375 15.714 39.281 n.d. 0.70375 0.00004 451.93 4.63

Trade iron and bloom91N352 Early Roman Sagalassos Billet 18.169 15.656 38.176 0.493 0.005 0.71097 0.00001 0.62 0.01CS 70 Early Byzantine Sagalassos Billet 18.814 15.707 38.818 n.d. 0.70534 0.00001 10.51 0.1403PDC33 Early Byzantine Sagalassos Iron flake 18.637 15.704 38.720 n.d. 0.70535 0.00001 11.59 0.1502PDS1 Early Byzantine Dereköy Billet 19.011 15.674 38.971 n.d. 0.70375 0.00001 n.d.02PDS25 Early Byzantine Dereköy Billet 19.454 15.685 39.254 n.d. 0.70375 0.00001 n.d.

Iron objects00LL4 Early Roman Sagalassos Nail 18.139 15.641 38.191 n.d. 0.71124 0.00001 n.d.03NK145 Late Roman Sagalassos Arrowhead 18.777 15.667 38.846 15.188 0.157 0.70631 0.00002 8.96 0.1300LL5 Late Roman Sagalassos Nail 18.648 15.702 38.835 n.d. 0.70637 0.00002 2.27 0.0303NK78 Early Byzantine Sagalassos Scissors 18.749 15.687 38.959 6.691 0.067 0.70379 0.00001 73.50 0.9603NK72 Early Byzantine Sagalassos Spatula 18.873 15.687 38.883 0.745 0.007 0.70535 0.00001 1.95 0.0303NK86 Early Byzantine Sagalassos Catapult bolt/pilum (?) 18.751 15.689 38.851 0.175 0.002 0.70503 0.00001 13.29 0.1803NK95 Early Byzantine Sagalassos Hammer 18.423 15.625 38.432 0.352 0.004 0.70587 0.00002 1.23 0.0103NK147 Early Byzantine Sagalassos Pliers 18.773 15.692 38.892 n.d. 0.70514 0.00001 42.16 0.6000LL15 Early Byzantine Sagalassos Clamp 18.670 15.668 38.717 n.d. 0.70411 0.00001 6.58 0.0903NK79 Early Byzantine Sagalassos Sickle 18.733 15.675 38.822 3.737 0.037 0.70484 0.00001 12.68 0.1703NK146 Early Byzantine Sagalassos Axe 18.756 15.684 38.871 n.d. 0.70469 0.00001 17.13 0.22

80 P. Degryse et al.

© University of Oxford, 2007, Archaeometry 49, 1 (2007) 75–86

Samples containing less than 10 ppm of Pb and Sr were corrected for maximum total proce-dure blanks (n = 6) of 30 pg for both Pb and Sr: for all other samples, these blank values werefound to be negligible (< 0.5 wt% of the analysed sample Pb and Sr amounts).

RESULTS

The results of the Pb–Sr isotope analyses are listed in Table 1, along with Sr concentrationsfor all but three samples and Pb contents of seven iron objects. With the exception of the hem-atite sample from Sagalassos (c. 35 ppm Sr), the analysed iron ores (magnetite–titanite placersands from Dereköy) have generally high Sr contents of 450 ppm to 2.5%. As expected, theiron objects show much lower concentrations, with most samples containing < 15 ppm Sr. Thesame applies to the Pb contents of the seven analysed iron objects, which range from 175 ppbto 15.2 ppm.

The Pb isotopic composition of the samples is depicted in Figures 1 (a) and 1 (b). The recoveredhematite ore from Sagalassos is clearly distinct in isotopic composition from the Dereköymagnetite-titanite placer sands, which have much higher radiogenic signatures. The workediron objects display considerable variation in their Pb isotopic composition, Early Roman ironfrom Sagalassos has the least radiogenic lead, similar to that of the hematite ore sample fromthe same site. Conversely, late Roman iron from Sagalassos displays higher Pb isotope ratios,which are indistinguishable from the majority of early Byzantine artefacts within analyticalerrors. One sample of early Byzantine iron from Sagalassos has a less radiogenic Pb isotopesignature. The two samples of early Byzantine iron from the territory at Dereköy differ signi-ficantly in their Pb isotopic composition, with one sample being similar to the Dereköy magnetiteplacer sands and the other one shifted towards the population defined by most of the earlyByzantine and the late Roman iron objects from Sagalassos.

Figure 2 shows a 87Sr/86Sr versus 206Pb/204Pb diagram for all samples. Figure 3 shows ahistogram of the 87Sr/86Sr ratios recorded in the ores, bloom and iron artefacts. The magnetite–titanite placer sands from Dereköy are identical in their Sr and Pb isotopic compositions.The hematite ore from Sagalassos has a significantly higher 87Sr/86Sr ratio. Based mainly onthe Sr isotope signature, distinct sample populations can be distinguished. The two earlyRoman iron artefacts and the haematite ore sample from the same context define a cluster atlow 206Pb/204Pb and high 87Sr/86Sr ratios. Late Roman iron from Sagalassos also has virtuallyidentical and relatively high 87Sr/86Sr ratios, clearly distinct from most of the early Byzantineartefacts. One early Byzantine iron object from Sagalassos (03NK78, scissors) has a lower87Sr/86Sr ratio that is virtually identical to that of early Byzantine iron and magnetite placersands from Dereköy.

DISCUSSION

Significance and precision of data: Pb versus Sr isotopes

It is well known that isotope ratios measured by thermal ionization mass spectrometry aresusceptible to instrumental mass fractionation occurring in the course of evaporation andthermal ionization of the sample (e.g., Dickin 1997). For elements with more than onenon-radiogenic isotope, such as Sr and Nd, isotopic fractionation can be readily corrected bynormalization to an internal standard; that is, an invariable isotope ratio of the respective element.In the case of strontium, the 86Sr/88Sr ratio is monitored during the mass-spectrometric run,

Iron working at ancient Sagalassos (south-west Turkey) 81


compared to its natural, constant value of 0.1194 (Steiger and Jäger 1977), and then used forcorrection of the raw 87Sr/86Sr ratios. In this way, 86Sr/88Sr measurements with an analyticalprecision of better than 100 ppm (1 in 104) are routinely achievable using TIMS. Naturallead, however, has only one invariable, non-radiogenic isotope, 204Pb, whereas the other threePb isotopes vary in their abundance due to decay of 238U, 235U and 232Th. As a consequence, Pbisotope ratios produced by TIMS can only be externally corrected for mass fractionation,using a mean fractionation factor δ determined from repeated measurements of internationallyused lead isotope standards and subsequent comparison to the certified Pb isotope ratios. Evenunder rigorous control of all experimental conditions, these standard measurements usuallyvary significantly along a fractionation line, and the reproducibility of δ may be as poor as

Figure 1 Pb–Pb diagrams for all analysed ore and iron samples.



several tens of per cent. Propagation of this error σδ (e.g., Ludwig 1980) will then result inenhanced analytical errors on fractionation-corrected sample Pb isotope ratios, in the range of500–1000 ppm, even though the internal precision of a Pb run on the TIMS may be better than100 ppm. Consequently, even the best-quality Pb data produced on a TIMS have precision5–10 times worse than Sr isotope measurements (see the error bar in Fig. 2). For archaeologicalstudies, 86Sr/88Sr ratios, with their better precision, are therefore a priori more suitable toresolve even small variations among, for example, chronological groups of metal artefacts andore types than Pb isotopes, which may be ambiguous because of both a possible chemical and

Figure 2 A 87Sr/ 86Sr versus 206Pb/204Pb diagram for all analysed ore and iron samples.

Figure 3 A histogram of the 87Sr/86Sr ratios recorded in the ores, bloom and iron artefacts.



isotopic variability of the samples and their comparatively poor precision. In the present study,this can be seen nicely in Figures 1 and 2, where distinct chronological groups of iron artefactsoverlap in their Pb isotope ratios, but resolve into clearly separated clusters when their Srisotope signature is considered.

Combined Pb and Sr isotope systematic of iron artefacts and provenance of iron ores

The 87Sr/86Sr and 206Pb/204Pb ratios of the early Roman (rejected) hematite ore, the hookedbillet and artefact 00LL4 (a nail) correspond very well, providing a similar signature for earlyRoman ore and iron samples (Fig. 2). While the 87Sr/86Sr signatures define an extremely tightcluster, the lead isotopes of these samples have a larger spread and do not unambiguouslyidentify them as belonging to one group (Fig. 1). As with the early Roman samples, the magnetiteplacer sands and the two early Byzantine iron billets from Dereköy show an excellent corre-spondence in their 87Sr/86Sr signatures, whereas the lead isotopes considered alone are muchmore ambiguous. However, as both ore and iron billets were found in a single productionlocation at Dereköy, their relation in the smelting activities on the territory is undoubted. Thisis readily supported and confirmed by the strontium isotopic data, but not by the Pb isotopesignatures, which give the impression that one of the iron billets may have been producedfrom the same raw materials as the early Byzantine iron objects from Sagalassos. This isdefinitely not the case, as the Sr signature of the Dereköy billets clearly points to the magnetiteplacer sands of the area. Remarkably, a contemporary iron object from the early Byzantineexcavation contexts at Sagalassos proper (sample 03NK78; scissors) shows a good corre-spondence with the Sr isotope ratios defined by the Dereköy ore and bloom (Fig. 2). It is likelythat this object was also manufactured from the Dereköy placer sands.

The artefacts from the late Roman contexts at Sagalassos, a period for which no bloom ortrade iron has been identified, have comparable Pb isotopic compositions (Fig. 1). Theirstrontium isotopic composition, however, shows a much better clustering than their leadsignature (Fig. 2). No ore material for this chronological group of iron artefacts was identifiedfrom the archaeological context. The signature of these artefacts is clearly different fromboth that of the hematite ore used in early Roman times at Sagalassos and that of themagnetite–titanite placer sand from Dereköy used in early Byzantine times. It is assumedthat a completely different ore source was used for manufacturing of iron objects in lateRoman times at Sagalassos proper, or that these objects were imported from one and the sameproduction site.

Viewing the data scatter and limited correspondence in both the strontium and lead isotopiccomposition of the early Byzantine iron at Sagalassos (Fig. 2), including trade iron, ironartefacts and iron flakes from the workshop, it is suggested that different ores from variouslocations were used for the production of these objects. It is very unlikely that all these wereimported and smelted locally. It can be inferred that some iron was only worked locally,besides the import of finished objects and alongside the primary production centre active onthe territory. However, original signatures of local ore sources also may have been alteredduring recycling or repairing iron (e.g., Schwab et al. 2003). Again, the strontium isotopes canhelp here to discriminate possible ore sources. For instance, object 03NK95, an early Byzantinehammer, has a lead signature shifted towards early Roman ore and iron (Fig. 1; cf., Table 1).However, its strontium isotopic composition is entirely different and rather points to the use ofsimilar raw material(s) used for the production of the other early Byzantine iron artefacts fromSagalassos (Fig. 2).



CONCLUSIONS

Thermal ionization mass spectrometry (TIMS) is still the most widely used analytical techniquefor producing high-quality Sr and Pb isotope data in geochemistry and archaeometry, eventhough other, more sophisticated, methods are now available that allow us to measure Pbisotopes with much better precision due to an improved control and correction of instrumentalmass fractionation, such as double/triple spiking (e.g., Galer 1999) and analysis of Tl-dopedPb samples by multiple-collector ICP mass spectrometry (e.g., Niederschlag et al. 2003).However, Sr isotope ratios can be routinely determined with great precision using TIMS.Our study demonstrates that even iron artefacts can be reliably analysed for their Sr isotopiccomposition, despite their generally very low Sr concentrations at the ppb to lower ppm level.Moreover, we have shown here for the first time that the provenance determination of ironartefacts through Sr isotopes is possible, and that it decisively overcomes the limitations of Pbisotope data. While lead isotopic signatures of artefacts and ores may be ambiguous due to theoften highly variable chemistry of the raw materials and their enhanced analytical errors whenmeasured by TIMS, 87Sr/86Sr ratios, with their better precision, may represent a more straight-forward and powerful tool to exclude or corroborate specific point sources of ores used formetal production.

The material from ancient Sagalassos (south-west Turkey) analysed in this study provides acritical test of the usefulness of Sr isotopes in provenance determination and sample correla-tion. We have shown that samples found in the same archaeological context have virtuallyidentical Sr isotope signatures, but may differ in their Pb isotopic composition. Consideringthe Pb isotopic correspondence of the late Roman and most of the early Byzantine iron objectsfrom Sagalassos, 87Sr/86Sr ratios can also help to discriminate between overlapping lead isotopepopulations, which will warrant further application of this approach.

ACKNOWLEDGEMENTS

Professor Udo Haack is sincerely thanked for comments and discussions. This research wassupported through a fellowship of the Alexander von Humboldt Foundation (P. Degryse).This research is also supported by the Interuniversity Attraction Poles Programme – BelgianScience Policy (IUAP V/9 and VI) and presents results of a Concerted Action of the FlemishGovernment (GOA 02/2 and 07).

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Documents

Tracing the Resources of Iron Working at Ancient Sagalassos (South-West Turkey): A Combined Lead and Strontium Isotope Study on Iron Artefacts and Ores*