Bidri vessels have been manufactured in India from at least the 17th century until the present day, and are known to be made of a zinc alloy, decorated with inlays of silver, brass or, occasionally, gold. The inlays contrast strikingly with the matt black patina of the vessel. It is this patina, easily damaged by standard cleaning techniques used on the silver inlays, which prompted the investigation. A number of vessels were analyzed, and all were found to be alloys of zinc with between 2% and 10% copper (i.e. falling within the Epsilon + Eta phase field of the binary copper-zinc equilibrium diagram). Some have lead, tin, or iron also present in the alloy. The compound or compounds forming the patina have not been identified. However, x-ray fluorescence (XRF) analysis and replication experiments have proved that the copper content of the alloy is critical for the patina formation. The black patina forms instantaneously on immersion of the alloy in a warm solution of potassium nitrate (one part) and ammonium chloride (four parts). Standard brasses, as used for the inlay, are unaffected by the solution which induces only a whitish-gray patina on pure zinc. The ammonium chloride preferentially dissolves the zinc from the Bidri alloy. The rest of the process is unproven but it is suggested that the resulting copper-enriched surface is oxidized by potassium nitrate.