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 object s all over Europe. Their chemical composition and crystal symmetry have bee n determined by means of scanning electron microscopy (SEM/EDS) and X-ray p owder diffraction. Data on location, sampling, object characteristics, gene ral environment and air pollution level; type, colour and chemical composit ion of the corrosion layers have been obtained and evaluated by multivariat e statistical analysis. The results verify that the highest air pollution l evels are usually associated with the occurrence of thick, black or dark gr ey corrosion layers on copper or bronze objects, preferentially containing soot, iron oxide hydroxides, and antlerite, Cu-3(SO4)(OH)(4). Pale green co rrosion usually contains brochantite, Cu-4(SO4)(OH)(6), and is rather assoc iated with less polluted areas. Atacamite, a copper hydroxide chloride with the chemical formula Cu2Cl(OH)(3), is preferentially observed in coastal r egions. In addition, sulphur isotope analyses have been performed on eleven corrosion samples from city centers. The delta S-34 values are typically i n the region from +4 to +6 parts per thousand relative to the sulphur isoto pe 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 th e form of brochantite or antlerite, mainly originates from a similar source despite geographic variation, most likely sulphur contained in air polluta nts.