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Micro-Raman study of copper hydroxychlorides and other corrosion products of bronze samples mimicking archaeological coins

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Abstract

Three bronze samples created by CNR-ISMN (National Research Council—Institute of Nanostructured Materials) to be similar to Punic and Roman coins found in Tharros (OR, Sardinia, Italy) were studied to identify the corrosion products on their surfaces and to evaluate the reliability of the reproduction process. Micro-Raman spectroscopy was chosen to investigate the corroded surfaces because it is a non-destructive technique, it has high spatial resolution, and it gives the opportunity to discriminate between polymorphs and to correlate colour and chemical composition. A significant amount of green copper hydroxychlorides (Cu2(OH)3Cl) was detected on all the coins. Their discrimination by Raman spectroscopy was challenging because the literature on the topic is currently confusing. Thus, it was necessary to determine the characteristic peaks of atacamite, clinoatacamite, and the recently discovered anatacamite by acquiring Raman spectra of comparable natural mineral samples. Clinoatacamite, with different degrees of order in its structure, was the major component identified on the three coins. The most widespread corrosion product, besides hydroxychlorides, was the red copper oxide cuprite (Cu2O). Other corrosion products of the elements of the alloy (laurionite, plumbonacrite, zinc carbonate) and those resulting from burial in the soil (anatase, calcite, hematite) were also found. This study shows that identification of corrosion products, including discrimination of copper hydroxychlorides, could be accomplished by micro-Raman on valuable objects, for example archaeological findings or works of art, avoiding any damage because of extraction of samples or the use of a destructive analytical technique.

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Acknowledgments

The authors thank Gabriel Maria Ingo (CNR-ISMN) and Valérie Hayez for kindly providing, respectively, the bronze coins and the clinoatacamite sample and allowing their study.

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Authors and Affiliations

  1. Dipartimento di Fisica, University of Parma, Viale G.P. Usberti 7/a, 43100, Parma, Italy

    Giulia Bertolotti, Danilo Bersani & Pier Paolo Lottici

  2. Dipartimento di Energetica, University of Rome “La Sapienza”, Via A. Scarpa 16, 00161, Rome, Italy

    Marcella Alesiani

  3. Mineralogisches Museum, Universität Hamburg, Grindelallee 48, 20146, Hamburg, Germany

    Thomas Malcherek & Jochen Schlüter

Authors
  1. Giulia Bertolotti
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  2. Danilo Bersani
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  3. Pier Paolo Lottici
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  4. Marcella Alesiani
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  5. Thomas Malcherek
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  6. Jochen Schlüter
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Corresponding author

Correspondence to Giulia Bertolotti.

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Published in the special issue Analytical Techniques in Art, Archaeology and Conservation Science with guest editor Oliver Hahn.

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Bertolotti, G., Bersani, D., Lottici, P.P. et al. Micro-Raman study of copper hydroxychlorides and other corrosion products of bronze samples mimicking archaeological coins. Anal Bioanal Chem 402, 1451–1457 (2012). https://doi.org/10.1007/s00216-011-5268-9

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  • DOI: https://doi.org/10.1007/s00216-011-5268-9

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