Corrosion of Ancient Chinese Bronze Money Trees Studied with Raman Microscopy

Lowell McCann, Karen Trentelman^*, Theresa Possley^†, and Brage Golding

^*Detroit Institute of Arts, ^†Kalamazoo College

Metal artifacts that have been buried underground or exposed to the atmosphere undergo corrosion that can eventually destroy the piece. The types of corrosion that occur depend critically on the environment in which the artifact was kept and therefore may provide information about the original burial site. We have used Raman spectroscopy to identify many of the corrosion products on the Detroit Institute of Arts Chinese bronze money tree (left, item 1996.29). These stylized trees, yaoqianshu, have branches containing coins, Daoist immortals, and other figures. They are found in tombs dating from the Eastern Han Dynasty (25 – 220 A.D.) and were intended to bring prosperity to the dead in the afterlife.

Cross-sections of these branches reveal (center, image size: 2 x 3 mm) a variety of corrosion products surrounding the uncorroded metallic bronze core. Using Raman microscopy, we have identified a number of corrosion products, including Cu₂O, CuCO₃·Cu(OH)₂, PbO, PbCO₃, and PbSO₄. Two-dimensional Raman maps can be made showing the spatial extent of the corrosion products. Here, a backscatter Scanning Electron Micrograph (right top, image size: 60 x 56 mm) shows a corroded spot in one cross-section that contains Pb (the bright region). The Raman map (right bottom) identifies the Pb region as PbSO₄ (blue), and the surrounding corrosion in the Cu-Sn alloy as Cu₂O (red).

Photograph by Paul Cooney, Detroit Institute of Arts