Tin Isotopy

Tin isotope ratios may be a useful tool for tracing back the tin in archaeological metal artefacts (tin metal, bronze) to the geological source and could provide information on ancient smelting processes. This study presents the results of laboratory experiments, which reduced (smelted) synthetic stannic oxide, natural cassiterite and corroded archaeological tin and bronze objects. The overall aim of the study is to find a reliable method for the decomposition of tin ores and corrosion products in order to determine their tin isotopic composition, and to explore possible effects on the tin isotope ratios during pyrometallurgy. We focused on five methods of reduction at high temperatures (900–1100 °C): reduction with CO (plain smelting), reduction with KCN/CO (cyanide reduction), reduction with Na2CO3/CO, reduction with Cu/CO (‘cementation technique’) and reduction with CuO/CO (‘co-smelting’). The smelting products are analysed by means of optical and scanning electron microscopy as well as X-ray diffraction, while their isotope composition is determined with a high-resolution multi-collector mass spectrometer with inductively coupled plasma ionisation. The results show that all five methods decompose synthetic stannic oxide, cassiterite and corrosion products. Ultimately, reduction with KCN is the best solution for analysing tin ores and tin corrosion because the chemical processing is straightforward and it provides the most reproducible results. Reduction with Na2CO3 and copper is an alternative, especially for bronze corrosion, but it requires laborious chemical purification of the sample solutions. In contrast, evaporation of tin and incomplete alloying during plain smelting and co-smelting can cause considerable fractionation among smelting products (Δ124Sn = 0.10 ‰ (0.03 ‰ u−1)). A less precise and even inaccurate determination of the tin isotopic compositions of the tin ores would be the consequence. However, the results of this study help to evaluate the possible influence of the pyrometallurgical processes on the tin isotope composition of tin and bronze artefacts.

BRONZE AGE TIN is a multidisciplinary project funded by the European Research Council comprising archaeology, history, geochemistry, and geology, conducted by scientists from the University of Heidelberg and the Curt-Engelhorn-Zentrum Archäometrie in Mannheim. The objective is to decipher the enigma of the origin of tin in the early bronzes by combining new archeological data and tin isotope ratios. These bronzes appear in a wide area stretching from the Aegean to the Persian Gulf, but this region is geologically devoid of any tin deposits.

The results of this research will be presented and discussed during four half-day sessions on geology and ore deposits, metallurgy and archaeology. In addition, there will be a welcome meeting on Wednesday evening, 14 March, a symposium dinner on Friday evening, 16 March, and the option for a tour through the Old City of Heidelberg on Saturday, 17 March 2018.

There will be only invited talks, but anyone interested is invited to participate the conference. Use the provided link for downloading the workshop programme and the registration form:

http://www.cez-archaeometrie.de/?p=1528

The sources and origin of tin, and the dispersion of bronze technology in the 3rd and 2nd millennium BC, are the central research topics of our multi-disciplinary research project, funded by an Advanced Grant of the European Research Council (ERC). It has the general goal to establish the tin isotopic composition of tin ores and tin-bearing artefacts, and considers the infl uence of anthropogenic processes on the isotope ratios. We discuss the tin isotopic composition of cassiterite from two major tin provinces in Europe: from Cornwall and Devon (Southern England), and from the Erzgebirge (Germany and Czech Republic). The samples from both tin provinces show a very large variation of isotopic compositions with δ124/120Sn-values ranging overall from -0.28 to 0.85‰. Although there is large overlap, on average, cassiterite from the Erzgebirge (δ124/120Sn = 0.09‰) is isotopically lighter than that of southwest England (δ124/120Sn = 0.18‰). This is due to a higher proportion of heavy isotope compositions in the samples from Cornwall and Devon. In addition, we compare the ore data with preliminary tin isotopic systematics in Early Bronze Age metal artefacts from the Únětice Culture in Central Germany and from several ancient settlements in Mesopotamia belonging to the Early Dynastic III and the Akkadian Periods. Bronze artefacts of the Únětice Culture containing more than 3 wt.% tin have rather constant isotopic compositions (δ124/120Sn = 0.2 to 0.31 ‰), despite having highly variable trace element concentrations and tin contents. This suggests the intentional addition of an isotopically homogeneous tin raw material (metal or cassiterite) to the copper ore or melt. In contrast, the tin isotopic composition of artefacts from Mesopotamia (>3 wt. % Sn) show a much larger δ124/120Sn variation from -0.2 to +0.4‰. This is even observed in single settlements such as Ur. Since there is no sizeable tin mineralization in the vicinity, this implies that the tin demand of the ancient metallurgist was covered by trading tin from different ore sources.

The origin of the tin used for the production of bronze in the Eurasian Bronze Age is still one of the mysteries in prehistoric archaeology. In the past, numerous studies were carried out on archaeological bronze and tin objects with the aim of determining the sources of tin, but all failed to find suitable fingerprints. In this paper we investigate a set of 27 tin ingots from well-known sites in the eastern Mediterranean Sea (Mochlos, Uluburun, Hishuley Carmel, Kfar Samir south, Haifa) that had been the subject of previous archaeological and archaeometal-lurgical research. By using a combined approach of tin and lead isotopes together with trace elements it is possible to narrow down the potential sources of tin for the first time. The strongly radiogenic composition of lead in the tin ingots from Israel allows the calculation of a geological model age of the parental tin ores of 291 ± 17 Ma. This theoretical formation age excludes Anatolian, central Asian and Egyptian tin deposits as tin sources since they formed either much earlier or later. On the other hand, European tin deposits of the Variscan orogeny agree well with this time span so that an origin from European deposits is suggested. With the help of the tin isotope composition and the trace elements of the objects it is further possible to exclude many tin resources from the European continent and, considering the current state of knowledge and the available data, to conclude that Cornish tin mines are the most likely suppliers for the 13 th-12 th centuries tin ingots from Israel. Even though a different prov-enance seems to be suggested for the tin from Mochlos and Uluburun by the actual data, these findings are of great importance for the archaeological interpretation of the trade routes and the circulation of tin during the Late Bronze Age. They demonstrate that the trade networks between the eastern Mediterranean and some place in the east that are assumed for the first half of the 2 nd millennium BCE (as indicated by textual evidence from Kü ltepe/Kaneš and Mari) did not exist in the same way towards the last quarter of the millennium.

Provenance studies of metal artefacts are well-established in the interdisciplinary field of science-based archaeology primarily using the chemical and isotopic composition. In the last decades, tin isotopes became gradually more important as a fingerprinting tool for the provenance of tin, but many questions especially regarding the behaviour of tin isotopes during pyrometallurgical processes are still not satisfactorily answered. This paper is a contribution to the understanding of tin isotope fractionation on tin ore smelting under prehistoric conditions and discusses the consequences for tin provenance studies. It presents the results of smelting experiments that were carried out with cassiterite in the laboratory and in the field, respectively. Besides chemical characterisation with XRF, SEM-EDX and Q-ICP-MS, tin isotope composition of tin ores and smelting products (tin metal, tin vapour, slag) were determined using solution MC-ICP-MS. Although tin recovery on smelting in the field was low (20-30%) due to tin losses to fuming and slag formation, the results indicate that the tin isotope composition is less affected than anticipated from theoretical considerations (Rayleigh fractionation). If cassiterite is completely reduced during the smelting reaction the tin metal becomes enriched in heavy tin isotopes with a fractionation of D124 Sn = 0.09-0.18‰ (0.02-0.05‰ u-1) relative to the original cassiterite. An estimate of the provenance of the original cassiterite and the potential ore source would still be possible because the variability of tin isotope ratios in tin ore provinces is much larger. If the cassiterite becomes incompletely reduced, however, then fractionation increases significantly up to D 124 Sn = 0.88‰ (0.22‰ u-1) and conclusions on tin sources are limited. Similarly, condensed tin vapours (D 124 Sn = 1.13‰ (0.28‰ u-1)) and slags (D 124 Sn = 0.42-1.32‰ (0.11-0.33‰ u-1)) that are by-products of the smelting process show large fractionation with respect to the original tin ore as well, which makes them unsuitable for provenance studies.

Die Herkunft des Zinns der Bronzezeit ist seit langem eines der größten Rätsel der archäologischen Forschung. Obwohl das Metall in Form von Bronze bereits im späten 4. und dem 3. Jt. in Anatolien, der Ägäis und dem Nahen Osten in Erscheinung tritt, liegen die Bezugsquellen dafür praktisch noch immer im Dunkeln. Das liegt vor allem daran, dass im Gebiet der ersten Zinnbronzen keine ergiebigen Zinnvorkommen existieren. Außerdem gab es bis vor kurzem keine analytischen Mittel, der Herkunft des Zinns gezielt nachzuspüren. Archäologische Evidenzen für die Ausbeutung von Zinnressourcen sowie Textdokumente aus dem 2. Jt. waren daher die einzigen greifbaren Anhaltspunkte, auf Basis derer die Forschung davon ausgeht, dass das bronzezeitliche Zinn des östlichen Mittelmeerraums und des Vorderen Orients zuvorderst aus Afghanistan stammt. Aber auch die Bergwerke nahe der türki-schen Ortschaften Kestel und Hisarcık sowie des iranischen Deh Hosein werden immer wieder als mögliche Quellen genannt. Teilweise werden sogar die fernen Zinnressourcen in Usbekistan, Tadschi-kistan oder Kasachstan angeführt (zusammenfassender Forschungsstand von Pigott in Druck). Untersuchungen im Rahmen des vom ERC finanzierten Projektes "BronzeAgeTin" liefern nun erste analytische Hinweise darauf, dass das Zinn der Spätbronzezeit (ca. 1300-1200 v. Chr.) im östlichen Mittelmeerraum-zumindest teilweise-nicht aus Asien, sondern aus europäischen Lagerstätten stammt (Berger et al. 2019). Dazu wurden insgesamt 38 Zinnbarren von unterschiedlichen Fundstellen in Israel (Hishuley Carmel, Kfar Samir, Haifa), Griechenland (Mochlos, Kreta; 16. Jh. v. Chr.), der Türkei (Schiffswrack von Uluburun) und zum Vergleich in Großbritannien (Salcombe) auf ihre Zinn-und Bleiiso-topenverhältnisse (MC-ICP-MS) sowie Spurenelementmuster ((LA-)Q-ICP-MS) hin untersucht. Ein-schränkungen diesbezüglich ergaben sich vor allem bei den Barren von Uluburun und Mochlos, bei denen das Zinn fast vollständig korrodiert war. Hier war von Vornherein mit einer geringeren Aussagkraft der Analyseergebnisse zu rechnen. Die übrigen Artefakte waren jedoch durchweg metallisch, und so ist davon auszugehen, dass die in Abbildung 1a erkennbare lineare Korrelation in den 206 Pb/204 Pb-und 207 Pb/204 Pb-Verhältnissen nicht von einer Kontamination durch die Lagerung der Barren im Meer herrührt...

This study uses MC-ICP-MS for the precise analysis of the stable tin isotopic composition in ore minerals of tin (cassiterite, stannite), tin metal and tin bronze. The ultimate goal is to determine the provenance of tin in ancient metal objects. We document the isotope compositions of reference materials and compare the precision of different isotope ratios and the accuracy of different procedures of mass fractionation correction. These data represent a base with which isotopic data of future studies can be directly compared. The isotopic composition of cassiterite and stannite can be determined after reduction to tin metal and bronze, respectively. Both metals readily dissolve in HCl, but while the solutions of tin metal can be directly measured, the bronze solutions must be purified with an anion exchanger. The correction of the mass bias is best performed with an internal Sb standard and an empirical regression method. A series of Sn isotope determinations on commercially available mono-element Sn solutions as well as reference bronze materials and tin minerals show fractionations ranging from about-0.09‰ to 0.05‰/amu. The combined analytical uncertainty (2s) was determined by replicate dissolutions of reference materials of bronze (BAM 211, IARM-91D) and averages at about 0.005‰/amu.

Die Schwerter vom Typ „Apa“ werden als die ältesten frühbronzezeitlichen Schwerter Europas angesehen. Sie haben eine weite Verbreitung vom nördlichen Karpatenbecken (Rumänien, Ungarn), über Mitteleuropa bis nach Skandinavien, was auf ein weitreichendes Austauschnetz in der frühen und der beginnenden mittleren Bronzezeit hindeutet. Ihr Ursprungsgebiet wird im Karpatenbecken vermutet, von wo aus sie als Exportstücke in die anderen Regionen gelangten. Es gibt jedoch auch lokale Imitationen außerhalb des Karpatenbeckens und es ist strittig, in welcher Beziehung ähnliche Schwertypen, wie der Typ „Valsømågle“, zu ihnen stehen. Die Schwerter von Typ „Apa“ haben gemeinsame typologische Eigenschaften; dennoch repräsentiert jedes Bronzeobjekt ein Unikat, da die Verzierung von Stück zu Stück unterschiedlich ist und auch die Formen der Klingen und Griffe variieren. Das absolute Alter dieser und nahestehender Artefakte liegt zwischen 1700 und 1500 v. Chr. und fällt damit in eine Zeit, in der in einer Reihe von europäischen Regionen bedeutende Veränderungen in der materiellen Kultur stattfanden. Daher ist es interessant zu untersuchen, ob die Zusammensetzung der Zinnbronze-Legierungen den typologischen Zusammenhang bestätigt oder ob unterschiedliche Werkstoffe zur Herstellung der einzelnen Objekte verwendet wurden.

We report the largest published dataset to date of Sn-isotopic compositions of Bronze Age artifacts (338) along with 150 cassiterite samples (75 new) from six potential tin ore sources from which the tin in these artifacts were thought to have likely originated. The artifacts are from a broad area, Central Europe through the Central Balkans, and the six tin sources are Cornwall, three sites in the Erzgebirge, and two sites in Serbia. A clustering analysis on mean site-level isotopic values of δ 124 Sn identifies regional variation that can be attributed to the use of different tin ore sources in different regions. Therefore, geographically meaningful regions were identified to group the Bronze Age artifact assemblages and a probabilistic, Bayesian analysis was performed to determine the proportional contribution of each tin source to each regional assemblage. Artifacts enriched in heavy isotopes (δ 124 Sn > 0.7‰) that cluster in west-central Serbia are likely associated with the ores from Mt. Cer in west Serbia. Mixed artifact assemblages (high and low δ 124 Sn) in this region are attributed to the use of cassiterite from the two Serbian sites (Mt. Cer and Mt. Bukulja). Moderate composition artifacts that occur north of the Middle Danube in Vojvodina, Transylvania, and Central Europe are likely associated primarily with ores from the West Pluton of the Erzgebirge. Compositionally light bronzes (δ 124 Sn < 0.2‰) in southern Serbia and the lower Danube river valley cannot be linked to a documented ore source. There is no indication of the use of ores from Cornwall or the East Pluton of the Erzgebirge in Central Europe and the Balkans during the Late Bronze Age.

"BronzeAgeTin" - das vom European Research Council mit einen Advanced Grant geförderte Projekt zur Erforschung der Verwendbarkeit der Zinnisotopenanalyse für Herkunftsbestimmungen in der Archäologie neigt sich nach viereinhalb Jahren seinem Ende entgegen - Zeit, eine kurze Bilanz der experimentellen Arbeiten zu ziehen. Abgesehen von neuen Erkenntnissen zum Ursprung und zur Verbreitung der Bronzetechnologie im 3. und 2. Jahrtausend im Mittleren Osten und Europa, konnten innerhalb der Projektlaufzeit knapp 500 Kassiterit- und Stannitproben aus Europa bis Mittelasien zinnisotopisch charakterisiert werden. Damit steht am Ende eine umfangreiche Zinnerzdatenbasis für zukünftige Projekte zur Verfügung. Dem gingen zahlreiche Experimente mit dem Ziel voraus, eine geeignete Methode zum Aufschluss von Kassiterit (Zinnstein) zu finden, der aufgrund seiner Widerstandsfähigkeit weder durch Säuren noch durch Laugen in Lösung gebracht werden kann. Da dies jedoch zwingende Voraussetzung für Messungen mittels Multikollektor-Massenspektrometrie mit induktiv-gekoppelter Plasmaanregung (MC-ICP-MS) ist, wurden Experimente zu verschiedenen Verhüttungsmethoden durchgeführt, um den Zinnstein zunächst in gut lösliches Zinnmetall zu überführen. Die Wahl fiel schließlich auf die Verhüttung bzw. Reduktion mit Kaliumcyanid, weil hierdurch im Gegensatz zu anderen Methoden (Reduktion mit CO, Zementation mit Kupfer, co-smelting mit Kupfermineral) keinerlei Zinn in Form leichtflüchtiger Zinnverbindungen (z. B. SnO) verlorengeht, das zu einer ungewollten Isotopenfraktionierung des Zinns führt (Brügmann u. a. 2017; Berger u. a. 2017)...