Atmospheric bronze and copper corrosion as an environmental indicator. A study based on chemical and sulphur isotope data

Corrosion products have been taken from 130 copper or bronze outdoor objects all over Europe. Their chemical composition and crystal symmetry have been determined by means of scanning electron microscopy (SEM/EDS) and X-ray powder diffraction. Data on location, sampling, object characteristics, general environment and air pollution level; type, colour and chemical composition of the corrosion layers have been obtained and evaluated by multivariate statistical analysis. The results verify that the highest air pollution levels are usually associated with the occurrence of thick, black or dark grey corrosion layers on copper or bronze objects, preferentially containing soot, iron oxide hydroxides, and antlerite, Cu-3(SO4)(OH)(4). Pale green corrosion usually contains brochantite, Cu-4(SO4)(OH)(6), and is rather associated with less polluted areas. Atacamite, a copper hydroxide chloride with the chemical formula Cu2Cl(OH)(3), is preferentially observed in coastal regions. In addition, sulphur isotope analyses have been performed on eleven corrosion samples from city centers. The delta S-34 values are typically in the region from +4 to +6 parts per thousand relative to the sulphur isotope standard CDT (Canyon Diablo Troilite) with a mean value of 4.7 +/-1.2 (1 sigma), thereby indicating that the sulphur in the corrosion layers, in the form of brochantite or antlerite, mainly originates from a similar source despite geographic variation, most likely sulphur contained in air pollutants.