Transformation of ancient Chinese and model two-phase bronze surfaces to smooth adherent patinas

Many ancient Chinese bronze mirrors have survived with a patina that leaves the delicate surface decorations intact. The microstructure of these mirrors is two-phase: Cu-rich α and Sn-rich δ. Although the mirrors have been labeled ‘corrosion-resistant’, the smooth surface includes a corroded layer up to 100 µm thick. In this ‘altered’ layer, there is selective attack of the α phase; the α-phase regions are replaced by tin oxide with the δ phase remaining metallic. Here, electrochemical polarization has been used to drive the dealloying process in modern, cast bronze duplicating the microstructure on ancient bronzes. Timed experiments were performed to investigate the growth of the ‘altered’ region on the model samples. The altered layer forms quickly but the rate of growth slows over time. Theoretical calculations show that dissolution of copper from the α phase leaves a large volume for the tin oxide corrosion product permitting retention of the smooth surface.