Chinese Bronze Mirrors
Bronze mirrors were introduced in China in the 6th century BC to ward off evil spirits by light reflection. Chinese archaeologists have found the buried bronze mirrors, which had black surfaces, resisted corrosion typical of copper/tin alloys. This remarkable chemical stability has stimulated metallurgical analyses for sixty years. Although it has been found that the near-surface region is depleted of copper, leaving tin oxide, and that a thin glassy layer coats the surface, there is debate over whether these alterations were performed deliberately two thousand years ago or are the result of long burial. A single bronze mirror on loan from a Chinese colleague is the focus of this investigation.
Microchemical and microstructural analyses, including X-ray diffraction, SEM/EDS, Auger electron spectroscopy and X-ray photoelectron spectroscopy (XPS), were used to address the following questions:
- What is the nature of the black surface?
- Did the makers of the mirrors prepare the black surface to protect the underlying bronze, or did it develop from burial in soil for over 2000 years?
Results: "In addition to the well-known bronze mirror near-surface compounds..cassiterite, cuprite, and delta-phase, another compound developed from bronze was found: romarchite (SnO). Several others based on rocks and minerals appeared in the glassy surface matrix: anorthite, tridymite, fayalite, rankinite, calc-flinta, magnetite, cuprite and romarchite. The data collected from several techniques also confirmed that both front and reverse mirror surfaces contain 7-8 atomic % iron. Since iron was removed from copper by successive smelting processes when the mirrors were produced, its presence must be the result of soil adherence to the buried mirror and diffusion into oxidizing surface. Cassiterite (tin oxide) containing iron impurity is known to be black, as is the case with dozens of other minerals having Fe+2. This argument and the presence of particles of rock/mineral compounds embedded in the matrix combine to support the position that the black, oxidized surface resulted from burial."
excerpted from an abstract of a poster presented at the Budapest International Symposium on Archaeometry in 1998 entitled "Interpretation of Black Surface of Ancient Chinese Bronze Mirrors," by Wendell Williams(Physics, UIUC), Pankaj Sarin(Materials Science and Engineering, UIUC), Changsui Wang(University of Science and Technology, Hefei, China), and Sarah Wisseman(ATAM, UIUC).