Metal surfaces of objet d'art were usually kept highly polished in the past and chemical patination as a decorative technique was rare before the 19th century. The best known exceptions are the Japanese irogane alloys and their occidental precursors. Another less well known patinated metal are the bidri wares of India. The items are sand castings of an alloy containing similar to 95% of zinc and 5% of copper. After casting the pieces are heavily inlaid with silver wires and foils and then treated to produce a rich black patina. Bidri wares are still made in quantity in the Deccan of central India, including at Bidar in the state of Karnataka, the traditional centre of the industry from which bidri takes its name. Zinc metal was first prepared in India similar to 1000 years ago and so an association between zinc production and zinc artefacts seems natural. However, this is not really the case, and the origins are really quite mysterious. Bidri has always been made in the Moslem parts of India by Moslem craftsmen who claim that the technique began in the Middle East, but where there was no zinc. Zinc was produced only at Zawar in the Hindu Rajput states of Rajasthan, well over 1000 km from the nearest bidri maker, and furthermore the few lead isotope analyses performed on the early bidri wares show the zinc did not come from Zawar. Thus the origins of the technique and the source of the zinc are shrouded in mystery, and a further mystery concerns the chemical nature of the patinated layer itself. None of the minerals identified in the patinated layer by X-ray diffraction should be black, but clearly some form of finely divided and probably amorphous copper oxide must be responsible. Whatever its origins and composition, bidri wares continue to be made and to provide a quintessential craft product of India.
