Page 356 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Chinese bronze mirrors Some of the most puzzling and difficult quandaries to unravel
in regard to patinas are those found on excavated Chinese bronze
mirrors. These mirrors exhibit a wide range of surface preservation and include smooth, shiny,
silver-colored patinas; lustrous black patinas; smooth green patinas with a translucent aspect;
and the more common patinas, such as those found on any buried bronze, with pustules of cor-
rosion, eruptions of cuprite or nantokite, or partially disrupted surface layers with loss of detail.
From about the tenth century, an important trading center was established at Kota Cina,
Sumatra, and fragments of bronze mirrors made by the sophisticated beta bronze technology
occur in related areas, such as Java. One such example, a beta-quenched Javanese bronze mir-
18
ror with a reflective green-black surface, is shown in PLATE 7 7 A . The microstructure of this
object is illustrated in PLATE 77B, which reveals a well-developed beta-phase morphology
in which the islands of alpha are twinned, revealing that some hot-working of the alloy must
have occurred during the fabrication process. Working these kinds of alloys at high temperature
demands great skill.
Two examples of representative patinas on Chinese mirrors from the Freer Gallery of
Art in Washington, D.C, are shown in PLATES 78 and 79. Characteristic microstructures of
the alloys and patinas for these types of bronzes are shown in PLATE so. The earliest Chinese
bronze mirror was excavated from Guiñan county, Qinghai province, and dates to the Xia
dynasty (2100-1600 B.C.E.), according to Zhu and He (1993). These mirrors were prized for
more than two thousand years, starting in the Warring States Period (475-221 B.C.E.) of the
Eastern Zhou dynasty and continuing into the period of the Qing dynasty (1644-1911).
I COMPOSITION Some evidence suggests that Chinese metal-
smiths employed many fire-refinings 19 of the copper used to make high-quality mirrors before
alloying with lead and tin. From the Warring States Period to the Tang dynasty (6I8-907 C.E.),
these mirrors had a composition of about 69-72% copper, 22-25% tin, and 4-6% lead. Mirrors
made after the Song dynasty (960-1279) tended to have more variable composition and less
tin content. Most fine-quality mirrors that have been excavated have either a silver-colored sur-
face under a corrosion layer or a black-colored surface, which may itself represent a corrosion
layer. Both kinds of surfaces have given rise to extensive speculation about their origins. Were
they created artificially, or naturally in a burial environment over long periods of time? The
corrosion that occurs on the silver-colored surface could have formed by the diffusion of cop-
per ions through the surface layer; therefore, it is not clear how such a surface is either pre-
served or formed as a possible result of corrosion. The same kind of speculation occurs in regard
to the black layer. The first scientific paper on Chinese and Japanese bronzes appears to be that
of Morin (1874). The most cogent investigations on the subject are those reported by Meeks
(i988a,b; i993a,b).

S O M E A S P E C T S O F B R O N Z E PATINAS
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