Environmental threats to archaeological artefacts

Archaeological artefacts which have remained in soil for thousands of years are often seriously deteriorated. A preliminary study of 600 bronze objects from Swedish museum collections have shown that bronze finds excavated 50-100 years ago seem to be less corroded than those found today. Recent anthropogenic pollution is suspected to have accelerated the corrosion, but there are also many other factors involved. An interdisciplinary project has been started in Sweden with the aim of correlating deterioration, corrosion products, soil chemistry, and general archaeological and environmental conditions. From four archaeological sites near Stockholm, 66 bronze artefacts and about 200 soil samples have been investigated. Corrosion products and remaining metal cores were analysed by scanning electron microscopy and X-ray microanalysis (SEM/EDS), and by X-ray powder diffraction (XRD). Each soil sample has been geologically classified, and a number of chemical analyses have been undertaken of : pH, resistivity, loss of ignition, certain anions and cations. About 20,000 numerical data have been compiled in Excel data files. Multivariate statistical evaluation utilizing the Simca-S system indicate that moist-retaining conditions around the object, as well as high concentrations of soot or phosphate in the soil, seem to have accelerated the corrosion. Acidic conditions and certain archaeological variables are also important.