Surface Analysis Studies of Bidri Archaeomaterials from the Collection of the British Museum

Bidri ware is a class of inlaid Indian archaeological metalwork, made froma Zn-Cu alloy, and characterized by a black patina which contrasts strikingly with the shiny metallic inlays. While the chemical composition of the patina is well known, the reason for its black color is not, knowledge of such a reason being an essential pre-requisite for devising appropriate schemes for both restorative and conservative purposes. In order to ascertain whether the surface microchemistry of the patina is responsible for the black color, we undertook a thorough study of Bidri samples (courtesy of the British Museum) using XPS, scanning Auger microscopy, SIMS, and EDS as a means of analysis. We find that the patina, about 0.5 mm thick, contains Zn, Cu, O, Cl, and C along with trace amounts of Pb and Sn. The Zn/Cu atomic ratio is about 7 in the patina and increases considerably in the alloy, whereas the interface is high in Cl and poor in Cu. Both the copper and zinc are entirely oxidized at the patina, and laterally separated from each other to a few mm’s, whereas Cl lateral segregation is lower than 0.5 mm. The SIMS analyses has been performed using a magnetic sector mass spectrometrer (CAMECA 4f) in depth profile and immaging mode to obtain elemental maps. SEM/EDS were also cairred out to obtain morfological and elemental information from the surface samples and from the bottom of the SIMS craters. We propose a model which explains the black color of the patina as originating from a highly-dispersed ZnO-like phase which features a broad range of light absorption coefficients