The characterization of Sn-based corrosion products in ancient bronzes: a Raman approach Francesca Ospitali, a * Cristina Chiavari, b Carla Martini, b Elena Bernardi, c Fabrizio Passarini c and Luc Robbiola d The characterization of corrosion patinas on bronzes is preliminarily connected to the determination of proper conservation strategies. Moreover, it also provides an essential contribution to the comprehension of the mechanisms of formation of patinas. Concerning the last aspect, several studies highlighted that bronze corrosion behaviour cannot be assimilated to that of pure copper. In particular, decuprification and relative enrichment of tin in the corrosion layers were observed. Tin therefore, together with its insoluble salts, plays an important role in the mechanism of formation of bronze patinas. Thus, the characterization of Sn-based corrosion products becomes fundamental, although remaining quite problematic. This work reports several case studies where Sn-based corrosion products grown in bronzes have been identified, thanks to the hyphenated system scanning electron microscopy–energy dispersive spectrometry–Raman structural and chemical analyser. Combining the main characteristics of these techniques, different Sn-containing compounds, mainly crystalline and nano- sized tin dioxides, were detected in bronze patinas exposed to different environments (to the atmosphere, in both natural and accelerated ageing conditions, and to the soil). The main issues regarding the interpretation of Raman spectra of these compounds are presented and discussed. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: Raman microscopy; SEM–EDS; bronze corrosion; tin oxide; copper oxide Introduction The characterization of corrosion patinas on bronzes is mainly and preliminarily connected to the determination of proper con- servation strategies: for instance, the choice of protective coat- ings or corrosion inhibitors should follow the characterization of the patinas, that is the support on which they are applied. Characterization studies give also an important contribution to the comprehension of the mechanisms of formation of patinas, both for outdoor and archaeological bronzes. Several studies highlighted that the formation process of bronze patinas in natural environments is mainly related to a decuprification phenomenon linked to the oxidation of the alloy, i.e., the selective dissolution of copper connected with a relative enrichment of tin inside the corrosion layers. [1–3] Tin as an alloy- ing element has an important role in the formation and stability of patinas. Therefore, the identification of Sn compounds plays a key role in the study of bronze corrosion mechanisms. For outdoor bronzes, the characterization studies acquire more and more significance when correlated with the exposure condi- tions (the role of environmental parameters – multi-pollutants, geometries of exposure to the rain...) and the alloy composition (the role of alloying elements). For archaeological bronzes, a detailed characterization of complex multi-layered patinas can also lead to the identification of ‘dangerous’ compounds and structures such as cuprous chlo- ride under cuprous oxide, for the prevention and care of the bronze disease. For buried bronzes, the corrosion behaviour depends on the dominant mechanism of ion migration: either cationic (type I ‘even’ patina, linked to copper ions’ migration from the alloy) or anionic (type II ‘coarse’ patina, relative to mass transportation of negative ions from the soil). [4] Also in this case, the role of Sn and its insoluble salts appears fundamental for un- derstanding the mechanism of formation of patinas. For these reasons and also from a conservation point of view, a deep characterization of Sn-based corrosion products is of primary importance, although quite problematic because of the detection problems as discussed subsequently. Therefore, it becomes essential to refine a new approach concerning patinas in order to clarify both their structural nature and their real com- position in terms of Sn-based compounds. In fact, elemental analytical techniques, such as energy disper- sive spectrometry (EDS), only reveal the presence of Sn-containing compounds without information on their structure, whereas ‘molecular’ or ‘structural’ characterization techniques, such as X-ray diffraction (XRD), infrared (IR), or Raman spectroscopies, have some difficulties in detecting Sn-based species. The identification * Correspondence to: Francesca Ospitali, Department of Industrial Chemistry and Materials, University of Bologna, Viale Risorgimento 4, 40136 Bologna Italy. E-mail: francesca.ospitali@unibo.it a University of Bologna, Department of Physical and Inorganic Chemistry, Viale Risorgimento 4, 40136 Bologna, Italy b University of Bologna, Department SMETEC, Viale Risorgimento 4, 40136 Bologna, Italy c University of Bologna, Department of Industrial Chemistry and Materials, Viale Risorgimento 4, 40136 Bologna, Italy d Université de Toulouse, Laboratoire TRACES (UMR 5608 CNRS), UTM, Maison de la Recherche, 5 Allées Antonio Machado, 31058 Toulouse Cedex 9, France J. Raman Spectrosc. (2012) Copyright © 2012 John Wiley & Sons, Ltd. Research Article Received: 13 December 2011 Revised: 10 January 2012 Accepted: 26 January 2012 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/jrs.4037