Investigation of the Altered Layer on Ancient Chinese Bronze Mirrors and Model High-Tin Bronzes
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ABSTRACT Many ancient Chinese bronze mirrors have a smooth patina. An ingress of corrosion to a depth of approximately 100 tcm is found beneath the patina. The corrosion selectively replaces the Cu-rich a phase leaving the Sn-rich 6 phase intact. Previous work by x-ray diffraction has shown that the a-phase replacement product is poorly crystallized or nanocrystalline SnO Transmission electron microscopy was employed to further characterize the replacement product in both ancient mirror and replication samples. Nanocrystalline SnO, in the form of small spheroids has been found. Remnants of an original alloy phase appear to be interspersed with the tin oxide.
INTRODUCTION Well-preserved Chinese bronze mirrors have been the subject of scientific research since the early 12 1930's when interest was piqued by their smooth patinas. Most of the mirrors of interest date from the late Zhou and early Han dynasties. Some Tang mirrors have also been investigated. Chinese mirrors were cast with a typical composition of 70% copper, 25% tin and 5% lead by weight. Bronze of this composition takes on a two-phase microstructure consisting of the a- (Cu-rich) and 6- (Sn-rich) phases arranged as a-phase needles in a 6-phase matrix. Mirrors have also been reported with a dendrites 2 surrounded by the two-phase eutectoid which results from a slightly lower tin content. 3 The color and luster of the mirror surfaces have been the subject of many discussions. 16 Distinctions have been made between shiny and matte mirrors and between black and non-black mirrors. One common thread in all discussions is the report of an ingress of corrosion to a depth on the order of 100 ptm below the shiny patina. In this region, known as the "altered region," the corrosion attacks only the a-phase needles leaving the 8-phase matrix intact. Collins called this corrosion "rooted" in the cast alloy.' Several Most of the groups have characterized the corrosion product found in the surface regions.","3'6 characterization work has been done with x-ray diffraction (XRD), which yields patterns with broad peaks centered at lattice spacings corresponding to the mineral cassiterite. These results indicate that the surface product contains poorly crystallized or nanocrystalline Sn0 2. Si0 2 , Cu 2O and the two alloy phases have also been identified by XRD. The loss of a in the altered region takes place by a dissolution mechanism through which the copper 7 2 is selectively removed from the a phase and the tin is incorporated into an oxideS "'" It is possible to predict conditions under which selective dissolution will occur in bronze and thereby replicate the altered region using electrochemical methods. We have reproduced the altered region in two solutions with the 2 application of a constant potential. The surfaces on the model pieces have been shown to have a similar that found on ancient mirrors. I112 to composition and microstructure In the present work, transmission electron microscopy (TEM) has been used to investigate the altered layer replacement product in
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