Tin was an extremely important but rare commodity in Europe during the Bronze Age and after. The provenance of prehistoric tin, which was used for the production of bronze, is still an unsolved problem in archaeological research. The first bronze finds in central Europe date to 2200 bc, while tin-bronze appears some c. 400–500 years earlier in Greece. The Aunjetitz Culture (2100–1600 bc), a very common prehistoric culture group in central Europe, is very rich in bronze finds. The fact that the Erzgebirge is the only region in this area with a notable tin mineralization has interested and inspired archaeologists and others for some time. However, evidence for prehistoric tin mining in the Erzgebirge region has not yet been found (Niederschlag et al. 2003). In Greece, for example, there are no tin ores, yet tin-bronze becomes common in the late Early Bronze Age: the tin was obviously imported. While several sources ranging from Turkey, Egypt, the former Yugoslavia, Italy and as far away as Afghanistan, Kirgistan and Uzbekistan, as well as the Erzgebirge region and Cornwall, have been suggested, there is no proof that these sources were used in Greece in the Greek Bronze Age (see, e.g., Muhly 1973; Penhallurick 1986; Pernicka 1987; Gillis 1991; Gillis and Clayton 2008). If the sources of the tin were known, this knowledge would provide much information about the mechanisms of ancient trade, mining, contacts and so on. Questions such as how many tin sources can be found in a single site, whether there are diachronic changes and how they relate to a greater contextual understanding of the site, what the range of the trading sphere was at any one time or place; and so on, could be answered. Thus if a method could be developed for sourcing tin, it would be invaluable not only in itself but also in a much wider, social context.
For Cornwall, the well-known old mining region in western Europe, tin mining over the past 3500 years is assumed by several authors (see, e.g., Penhallurick 1986). This assumption is founded on a small number of tin-containing slag finds and prehistoric remains in tin streamers. The oldest of the slags, excavated near St Austell, is dated to the 16th to 14th century bc. Otherwise the old tin ingots, which have been found mostly along the coast of Cornwall, can be dated to the Middle Ages rather than to prehistoric times (Rhoden 1985). In fact, there is no evidence that one prehistoric bronze find was made from Cornish tin.
The reason for the relatively scanty knowledge concerning ancient tin mining, smelting and trade is obvious: there is no scientific method to trace the tin to its source. Due to the fact that the lead contents, which could be found in ancient bronze artefacts, come mostly from the copper ore, lead isotope analyses cannot be applied for determining the provenance of the tin (Begemann et al. 1999). Hence, this present work deals with the use of tin isotopes for this purpose.
Tin has 10 stable isotopes, more than any other element, that cover a mass range of 12 amu (112Sn–124Sn). In the past it was assumed that there is no, or only a very small, variation of tin isotopes in nature (see, e.g., De Laeter et al. 1965, 1967; Rosman et al. 1984). McNaughton and Rosman (1991) analysed nine cassiterite samples from different locations and types of ore deposits. By using a multicollector TIMS and a double-spike procedure, they found that only one cassiterite sample (#1) with +0.15 ± 0.02‰ (2 s.d.) per mass unit differed significantly from a laboratory standard. Apart from this, the external reproducibility (2 s.d.) from seven of the nine samples was much poorer (e.g. #3, #7 and #9, 0.32‰ per mass unit). Thus it was not possible to detect any other significant variations. In 1998, the IUPAC Commission on Atomic Weights and Isotopic Abundances reported an atomic weight of 118.710 ± 0.007 for tin. Isotopic variations were considered insignificant.
The first published suggestion for using tin isotope ratios for solving an archaeological problem was made by Gale in 1997. This suggestion dealt with the idea that re-melted bronze must have a different tin-isotopic composition than primary metal. Thus it should be possible to detect recycling rates of bronze by measuring the tin isotopes. Unfortunately, Gale could not find significant isotopic variations.
Further investigations into tin isotopes for the purpose of creating a method for tracing archaeological tin were carried out between 1997 and 2002. By using a TIMS (VG Isolab 54) equipped with seven Faraday detectors, Gale (1997) achieved a reproducibility of 0.28‰ (2 s.d.) for 122Sn/116Sn, while Clayton et al. (2002) achieved 0.23‰ (n = 14) by using a MC–ICP–MS (Micromass IsoProbe) for the same isotope ratio. The measurements were carried out on an internal standard, Johnson Matthey Puratronic tin metal (Batch W14222). By measuring a cassiterite sample (Pen388/2) from Malaysia in comparison with the standard, Clayton et al. (2002) detected isotopic differences that were significantly larger than the analytical uncertainty. Furthermore, Clayton et al. (2002) presented tin isotope data of single cassiterite samples from Cornwall, the Erzgebirge, Egypt and Kirgizstan. Also, some archaeological material from the Aegean Bronze Age was analysed. The investigations showed that significant isotopic differences occur both in the ores and in the artefacts and between object types, ingots versus foils. Due to the fact that only a small number of samples were analysed, further conclusions could not be drawn. Further investigations in the field of tin isotopes with an archaeological background were also performed by Gillis and Clayton (2008), Nowell et al. (2002), Gillis et al. (2001) and Yi et al. (1999). For the TIMS or MC–ICP–MS measurements, the same standard material (Johnson Matthey Puratronic tin, as mentioned before) was always used, making the investigations comparable. An exception is Begemann et al. (1999), where a SnCl2 solution was applied. A more detailed description of this research is given by Clayton et al. (2002).
However, a clear idea of the scale of the isotope variations of tin in nature could not be gained from the existing publications. The question of whether tin isotope ratios can be used as a tool for provenance analyses of ancient tin still awaits an answer for two main reasons: first, as already mentioned, the number of analysed samples so far is much too small, so that one cannot consider them as systematic investigations. Secondly, not enough attention was paid to the purification of the measurement solutions—for example, by ion chromatography—especially when the MC–ICP–MS technique was applied, which in principle provides the possibility to generate a large number of isotope measurements within a short time. With the present work, this gap may be closed. Systematic investigations of ores from several deposits in the Erzgebirge region and Cornwall, as reported here, demonstrate that the tin isotope ratio of a source is widely homogeneous. On the other hand, we found significant differences between ores from different sources. This provides the foundation for a useful procedure to be used for tracing the ancient tin via tin isotopes.