Tin isotope analysis

Description

The isotopic composition of tin in archaeological tin and bronze objects can be used to answer questions regarding origin, production techniques (e.g. recycling) or the connection between artefacts. The method can also be helpful for questions concerning geological and modern material.

Application

The isotope ratios of chemical elements in natural rocks and cultural-historical objects can vary. One reason for this is the radioactive decay of unstable isotopes, for example of rubidium, uranium or rhenium, which leads to variable isotope compositions of strontium, lead or osmium. On the other hand, stable isotopes are also separated from each other due to their mass differences in the course of physical (diffusion, evaporation), chemical (smelting, oxidation, reduction) and biological processes. Such processes are of great importance both during the production of cultural-historical objects (metals, alloys, slags, ceramics, glass) and during the formation of their starting materials (ores, minerals). They provide the reaction products with a specific isotopic fingerprint. This fingerprint offers a twofold information: one on the nature of the source of the element in question and the other on the transfer process from the source to the reaction product. With the help of the isotopic fingerprint of tin in bronze and tin metal, questions regarding origin, production technique or the connection of artifacts with each other can be answered. The measurement of the ratios of the stable isotopes of tin to answer archaeological questions as well as their specific informative value are still in the early stages of research, and especially details of the isotope systematics still need to be investigated. For the determination of origin, however, it can already be estimated that a single isotope system can only be used to a limited extent due to large variations in the isotope compositions of tin deposits. Strictly speaking, we are not talking about determining the origin either, but – as with the lead isotope method – about narrowing down the possible origin. This is primarily due to the isotopic overlapping of the deposits, which means that only the principle of exclusion can be applied. This means that if the isotope ratios are not matching, deposits can be excluded, but a positive allocation is not possible. In this case, the integration of additional data from other isotope systems (e.g. copper, lead), geochemistry (chemical composition) or even from the historical sciences is useful to test hypotheses on provenance. However, a so-called multi-parameter approach also holds great potential with regard to the specific interrelationships of artefacts with each other. By combining tin, copper and lead isotopes with the chemical composition, for example, questions regarding material mixtures or recycling can be pursued. CEZA has „state-of-the-art“ analytical equipment, extensive databases and natural scientists at its disposal to advise and interpret the analytical data.

Basics

Tin has ten stable isotopes and thus the largest number of all elements of the periodic table. This results in a large atomic mass range (112 to 124), over which the mass fractionation can be determined with high precision. The element has siderophile and chalcophile properties and is therefore particularly predestined to characterize processes involving metallic and sulfide phases. For archaeometry, tin metal and bronze – an alloy consisting of tin and copper – are such phases. The most important tin mineral, both today and in prehistory, is tin oxide or cassiterite (SnO2). To determine the isotope ratios of tin, samples must be dissolved. For metal this is achieved with hydrochloric acid, whereas cassiterite must first be thermally reduced to tin metal. The reason for this is the high resistance of tin oxide to acids. The sample solutions are then subjected to chromatographic steps in order to isolate the tin from the sample matrix and thus to avoid interferences during the measurement. In addition, an antimony standard is added to the tin solution, which serves to correct mass discrimination in the spectrometer. For the measurement, CEZA has a Neptune Plus multi-collector mass spectrometer at its disposal with which the isotope ratios can be determined with high precision. The ratios are given relative to a tin isotope standard as delta values (δ notation). These can be compared with the tin isotope database of the CEZA, which meanwhile comprises far more than 1,000 data sets of tin ores (Europe, Egypt, Asia) and metal objects. Extensive knowledge about the behaviour of tin in pyrometallurgical processes is also available from many experiments. In this way it is possible to systematically investigate a wide range of questions.

Sample properties

The sample size depends on the tin concentrations in the sample, which can be quite variable! For bronze samples we recommend a sample size of 50 mg, for metallic tin or tin alloys it should be at least 20 mg. From a purely technical point of view, 1 mg or less may be sufficient. However, with this small quantity, the question arises as to how representative it is for the entire object and to what extent the analytical results can be interpreted. This aspect is even more critical for ore samples, especially since ore deposits are heterogeneous. Therefore, the larger the amount of ore, the more representative and meaningful the result is. For tin isotope analysis of cassiterite, a sample powder is produced at CEZA from about 50–100 individual crystals, which is then thermally reduced to tin metal. To acquire this number of crystals, the amount of tin ore should not be less than 50 g; however, this depends largely on the content of tin minerals in the sample.