Page 365 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

It is difficult to fully understand this recipe as recorded in this translation. Cuprite and
tenorite would be expected to form during the melting down of the old bronze vessels; the roast­
ing stage mentioned later in the recipe would have had the same effect. The reliance on mercury
in several of these recipes is interesting, and additional associations are made between the color
of cinnabar and the color of the cuprite corrosion of the bronzes. This indicates that ancient con­
noisseurs clearly recognized that the desirable red color of cuprite corrosion could be imitated
by the more readily available cinnabar.
Chase (1994) reports an archaeologically interesting group of bronzes from the Western
Han dynasty tombs of Lui Sheng and Duo Win in Man-cheng, northwest of Beijing. These
tombs, which date to around 175 B.C.E., were cut in solid rock, and cast iron was poured into the
prepared cavity to seal them. The chambers were never filled with earth. The bronzes preserved
in these tombs present a variety of patinas. A sword, for example, has what looks like a trans­
parent shiny patina over deep yellow, bronze-colored metal. Another appears to have silvery
edges and a black center. Age dagger-ax has a surface pattern of black dots on a bronze-colored
background. Mirrors of three different colors were found. Most of the bronzes were dark and
shiny, but the famous boshanlu censer from these tombs is a matte black that contrasts well with
the gold inlay. Chase hypothesizes that these bronzes already had a variety of different patinas
when they were placed in the rock-cut tombs.
I AN INDIAN PATINA A blackened surface used for decorative

metalwork in India is found on the well-known bidri ware, which is made of zinc-copper alloy
often inlaid with silver or brass wires. The earliest surviving examples of Indian bidri ware date
from the sixteenth to the seventeenth century. Most of the bidri alloys contain zinc, copper, lead,
and tin; typically the composition falls in the range of 0-95% zinc, 3-10% copper, -8% lead,
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and 1-2% tin with a trace of iron (Stronge 1993). After manufacture, this high-zinc brass is pati­
nated with a mixture of mud or clay paste containing one part ammonium chloride mixed with
one-quarter part unrefined potassium nitrate and the same amount of salt. f the mixture is hot,
I
the patina develops within a few seconds. The object can be either immersed in boiling slurry
or first heated and then coated with the patinating mixture. To date the analytical characteriza­
tion of the bidri ware patina has met with only partial success. It is assumed that mixed oxides
of both copper and zinc are involved since the potassium nitrate will ensure a highly oxidizing
environment at the metal surface; copper or zinc chlorides are discounted since they are not
black. A study by LaNiece and Martin (1987) found that pure zinc treated in the bidri patinating
solution developed only a pale gray patina of zinc oxide and chloride.
Analyses of bidri ware in the Victoria and Albert Museum showed the typical composition
range quoted earlier. Black patina samples from twelve objects were analyzed by X-ray diffrac­
tion. In all cases, simonkolleite; zinc-hydroxide-chloride hydrate, Zn 5 (OH) 8 Cl 2 -H 2 0 (ICDD
7-155); and zinc oxide, ZnO (ICDD 5-664), were present in the patina. In some cases, cuprite was
found as well. The proportions of zinc and copper in the alloy appear to be a critical factor for

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