Final Report Summary - BRONZEAGETIN (Tin Isotopes and the Sources of Bronze Age Tin in the Old World)
A new method has been developed that offers the chance to trace ancient tin via tin isotope signatures. It was found that the isotope ratios of tin exhibit small but measurable variations in nature making different tin deposits identifiable so that in principle it should be possible to relate archaeological bronze objects to specific ore deposits. For this purpos the analytical uncertainty of the measurements was substantially improved compared with previous studies. Also, an in-house isotopic standard was prepared to facilitate international comparability and which may serve as international reference material for tin isotope studies.
A new protocol for the isotope analysis of tin ores was developed. Tin isotope ratios are measured in solution but the major tin mineral in ores, cassiterite (SnO2), is insoluble in mineral acids. A newly developed method with a reduction of SnO2 at around 1100°C to tin metal in a closed graphite vessel by cementation with copper powder proved to be most reliable approach by avoiding evaporation of tin monoxide (SnO), which causes significant fractionation of the tin isotopes in the metal.
To assess possible tin fractionation in the ancient pyrometallurgical processes two approaches were tested, namely model experiments in the laboratory and life-size experiments in the field. It was found that assuming a 30% recovery of tin or more a fractionation of around Δ124Sn = 0.1‰ must taken into account when trying to trace back the tin in metal artefacts to its deposit. On the other hand, it was established that small tin losses as result of repeated melting (‘re-cycling’) and casting of bronze induced no substantial fractionation. This is an important conclusion for studies on the provenance of tin in the Bronze Age.
However, it turned out that the tin isotope ratios of some 500 tin ore samples from Europe and western Asia show substantial overlap so that it is difficult to distinguish e.g. between the two major tin mineralised regions in Europe, namely Cornwall and the Saxo-Bohemian Erzgebirge. On a smaller scale it may be possible to differentiate single tin deposits, because some exhibit only small variations in tin isotope ratios. On the other hand, the major sulfide mineral of tin, stannite, has distinctly different tin isotope ratios which suggests that redox reactions are the main driving force for the fractionantion of tin isotopes in nature. Indeed, fractionation of the Sn isotope system may provide a better monitor of high-temperature redox reactions than those in whichsuch signatures may be masked by isotopic partitioning between coexisting phases (e.g. Fe).
From the archaeological work it is evident that in the Near East tin bronze technology is particularly linked to early cities and their administrations. An elite-controlled long-distance trade network guaranteed the procurement of raw materials, which simultaneously formed the basis for specialised metal workers and other craftsmen. In contrast, Bronze Age Europe had numerous local centres without highly developed administration, which only influenced small geographic areas. Here certain ‘hot zones’ in southern England, central Germany, northern Italy as well as northeastern and southwestern Spain appeared, where the tin bronze technology might have been introduced earlier than in other regions. Each of these ‘hot zones’ has nearby tin deposits, whereas every centre in the Near East was dependent on long-distance trade of raw materials. The large variation of tin isotope ratios in Mesopotamian bronzes of the 3rd millennium BC objects suggests that several sources may have supplied the region. In Europe it seems that tin metallurgy was first introduced to the coastal regions of Atlantic Europe and southern England. In central Europe bronze is obviously a prestige material found mainly in burials. Tin may have been traded together with gold from southwest England as the material analysis of the Sky Disc of Nebra indicate.
A new protocol for the isotope analysis of tin ores was developed. Tin isotope ratios are measured in solution but the major tin mineral in ores, cassiterite (SnO2), is insoluble in mineral acids. A newly developed method with a reduction of SnO2 at around 1100°C to tin metal in a closed graphite vessel by cementation with copper powder proved to be most reliable approach by avoiding evaporation of tin monoxide (SnO), which causes significant fractionation of the tin isotopes in the metal.
To assess possible tin fractionation in the ancient pyrometallurgical processes two approaches were tested, namely model experiments in the laboratory and life-size experiments in the field. It was found that assuming a 30% recovery of tin or more a fractionation of around Δ124Sn = 0.1‰ must taken into account when trying to trace back the tin in metal artefacts to its deposit. On the other hand, it was established that small tin losses as result of repeated melting (‘re-cycling’) and casting of bronze induced no substantial fractionation. This is an important conclusion for studies on the provenance of tin in the Bronze Age.
However, it turned out that the tin isotope ratios of some 500 tin ore samples from Europe and western Asia show substantial overlap so that it is difficult to distinguish e.g. between the two major tin mineralised regions in Europe, namely Cornwall and the Saxo-Bohemian Erzgebirge. On a smaller scale it may be possible to differentiate single tin deposits, because some exhibit only small variations in tin isotope ratios. On the other hand, the major sulfide mineral of tin, stannite, has distinctly different tin isotope ratios which suggests that redox reactions are the main driving force for the fractionantion of tin isotopes in nature. Indeed, fractionation of the Sn isotope system may provide a better monitor of high-temperature redox reactions than those in whichsuch signatures may be masked by isotopic partitioning between coexisting phases (e.g. Fe).
From the archaeological work it is evident that in the Near East tin bronze technology is particularly linked to early cities and their administrations. An elite-controlled long-distance trade network guaranteed the procurement of raw materials, which simultaneously formed the basis for specialised metal workers and other craftsmen. In contrast, Bronze Age Europe had numerous local centres without highly developed administration, which only influenced small geographic areas. Here certain ‘hot zones’ in southern England, central Germany, northern Italy as well as northeastern and southwestern Spain appeared, where the tin bronze technology might have been introduced earlier than in other regions. Each of these ‘hot zones’ has nearby tin deposits, whereas every centre in the Near East was dependent on long-distance trade of raw materials. The large variation of tin isotope ratios in Mesopotamian bronzes of the 3rd millennium BC objects suggests that several sources may have supplied the region. In Europe it seems that tin metallurgy was first introduced to the coastal regions of Atlantic Europe and southern England. In central Europe bronze is obviously a prestige material found mainly in burials. Tin may have been traded together with gold from southwest England as the material analysis of the Sky Disc of Nebra indicate.