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
