Micro-chemical investigation of corrosion products naturally grown on archaeological Cu-based artefacts retrieved from sea-water

The mineral species forming the patinas naturally grown on copper-based artefacts retrieved from sea-water have been studied by means of the combined use of scanning electron microscopy coupled with energy dispersive spectrometry (SEM+EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and optical microscopy (OM). The selected artefacts include fragments of bronze statues and commonly used objects that were recently found in the Sicilian channel and along the Italian coasts. Our findings show that the composition and structure of the corrosion products is different depending on several parameters and the local environment. Furthermore, the study of the metallurgical features of the artefacts revealed also some interesting manufacturing details. From a conservation point of view, the SEM-EDS, XPS, FTIR, XRD and OM results show, as a whole, that the alloy matrix is often heavily corroded also in the bulk where the ubiquitous and dangerous presence of chlorine was always detected as the main dangerous corroding agent. Chlorine forms nantokite (CuCl) which is the species inducing the cyclic and inexorable copper corrosion commonly called "bronze disease". Other elements from the marine environment that contribute to form the patina are sulphur, phosphorous and seawater cations like Ca and Mg. On the basis of our results, an attempt is made to find a correlation between the nature of the patina, the chemical composition of the objects and the marine retrieving environment where the objects lived for many hundreds, or thousands, years. From a conservation point of view, our findings may be useful to suggest, or select, tailored conservation methods and materials with the purpose to turn the dangerous copper chlorides, or oxy-chlorides (atacamite and its polymorphs), into less reactive and more stable phases. This should hamper the copper cyclic reaction thus ensuring a long-term chemical-physical stability to the bronze artefacts.