Scanning Auger microscopy and x-ray photoelectron spectroscopy studies of Roman bronzes

Abstract

Scanning Auger microscopy and x-ray photoelectron spectroscopy (XPS) techniques are used to study the surface and interface microchemical composition of a series of Roman leaded bronzes of the third century B.C. The materials’ surface composition, as obtained from large-area XPS analysis, compares fairly well with that of the bulk, except for a significant surface lead segregation observed in one case. We find that, in the sample with the highest tin content, tin plays a sacrificial role in preventing the corrosion of copper. The tin is present mainly in the oxidized form, even at a depth of about 100 Å, whereas the copper is virtually all in the metallic form. Evidence is given for the presence of ‘‘white lead’’ [2PbCO3⋅Pb (OH)2] at the surface of the sample with the highest lead content. Auger point analysis evidences significant lateral inhomogeneities in the microchemistry of the materials, as well as sulfur segregation in the original patina. Auger images bring to light tin and lead lateral segregations with spatial resolutions of about 0.5 and 0.1 μm, respectively.