On smelting cassiterite in geological and archaeological samples: preparation and implications for provenance studies on metal artefacts with tin isotopes

Abstract

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 Na 2 CO 3 /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 Na 2 CO 3 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 (Δ 124 Sn = 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.

Isotope systematics and chemical composition of tin ingots from Mochlos (Crete) and other Late Bronze Age sites in the eastern Mediterranean Sea: An ultimate key to tin provenance?

“…Since there is still no internationally certified tin reference material, an in-house standard was prepared from ultraclean tin metal (Puratronic, Batch W14222, Johnson Matthey, Royston, GB) by dissolving it in HCl. This metal had already been used in previous studies [31–33; 64; 70; 72; 75–81]. The isotopic ratios reported here are related to the in-house standard and are given in the delta notation in units of permil (‰) with 120 Sn as the common denominator.…”

“…In contrast to the metallic samples, the corroded specimens of the Mochlos and Uluburun ingots had to be converted to tin metal prior to TIA because common corrosion products of tin, such as stannic oxide (SnO 2 ) and hydrated stannic oxide (SnO 2 ·nH 2 O) [82–83], are almost insoluble in acids (although stannic oxide is identical with cassiterite we use the term for the corrosion product in order to distinguish it from the natural ore mineral). Conversion to tin metal was achieved by reduction of a small amount of pulverised material (~10 mg) in a muffle furnace at 950°C, according to the protocol established by Berger et al [70]. In order to prevent tin loss during heating due to the formation of volatile SnO [84], reduction was performed in presence of potassium cyanide (KCN) using graphite crucibles (Fig 5).…”

“…In order to prevent tin loss during heating due to the formation of volatile SnO [84], reduction was performed in presence of potassium cyanide (KCN) using graphite crucibles (Fig 5). This is the most reliable procedure for cassiterite/stannic oxide reduction, as no tin loss and isotopic fractionation due to evaporation has been observed so far [64; 70; 85]. After reduction, the tin metal was processed for TIA like the Israeli specimens.…”

Microstructure and phase composition of bronze Montefortino helmets discovered Mediterranean seabed to explain an unusual corrosion

“…In one case-study, MacLeod et al observed that during bronze corrosion under partially aerobic condition, tin reacts forming the oxide sulphate Sn 3 O 2 SO 4 40 . Other studies 41 found that tin oxidation in anaerobic environment produces cassiterite (SnO), a product always present together with cuprite (CuO) because of the similar redox potential (Eh) which drives their formation. Finally, a small amount of antlerite was found in samples 2B, 2C and 2D.…”

An old problem in a new light: Elemental and lead isotopic analysis of Luristan Bronzes

“…Based on previously published works, it has been suggested that the prehistoric tin bronze artefacts from the Bronze Age and Iron Age (Iron Age tin bronzes from Luristan) were produced using an uncontrolled (or uncontrollable) alloying process (Oudbashi et al, 2016a;Oudbashi and Hessari, 2017;Oudbashi and Hasanpour, 2018). In literature, different procedures to produce tin bronze alloy in the ancient time have been suggested (Pigott 2004;Rovira et al, 2009;Valério et al, 2013;Murillo-Barroso et al, 2010;Erb-Satullo et al, 2015;Nezafati 2006;Figueiredo et al, 2010;Berger et al, 2019;Farci et al, 2017, Rademakers andFarci 2018;:…”