Page 362 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

applied to the bronze object will not produce a malachite patina; the mixture's primary corro­
sive agents are chloride and sulfate anions, which would result in a mixed atacamite-brochantite
patina at best. f the charcoal is able to burn off this crust to make a cuprite-tenorite layer, how­
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ever, this may be partially converted to a copper acetate patina. Burial could perhaps then par­
tially convert this new patina to a paratacamite-malachite crust, depending on the soil's acidity
and other parameters.
Barnard (i96i) provides examples of other historical recipes for the alteration of surface
appearance, including the following one from the Tung-fien ch'ing-lu, a tenth-century scroll
from the Song dynasty:

The method of faking archaic bronzes is achieved by an application of quicksilver and tin-
powder—the chemical mixture now used to coat mirrors. This is firstly applied uniformly
onto the surface of the new bronze vessel, afterwards a mixture of strong vinegar and fine
sand powder is applied evenly by brush; it is left until the surface colour is like that of dried
tea, then it is immediately immersed into fresh water and fully soaked. It thereby becomes
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permanently the colour of dried tea; f it is left until it turns a lacquer-like colour and imme­
diately immersed into fresh water and soaked, it thereby becomes permanently the colour
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of lacquer. f the soaking is delayed the colour will change. f it is not immersed in water it
will then turn into a pure kingfisher-green colour. In each of these three cases the vessel is
rubbed with a new cloth to give it lustre. Its bronze malodour is covered by the quicksilver
and never appears; however, the sound of old bronze is dainty and clear, whilst the sound
of new bronze is turbid and clamorous — this cannot escape the observation of the connois­
seur. (Barnard 19 61:214)

Use of the mercury-tin amalgam described here may have been an attempt to imitate the
lustrous, shiny, silver-colored surfaces that are sometimes found on old mirrors and that may
appear to be corroded to some extent by the application of corrosive agents to artificially pit and
mar the surface. Such attempts at patina replication, however, would not be expected to deceive
the connoisseur.
Analytical studies rarely show the presence of mercury in the patina of ancient Chi­
nese mirrors, despite the fact that mercury and tin are frequently mentioned in association with
their finish. Part of the explanation can be found in an ancient recipe for a polishing compound
known as xuan xi, which was essentially a mercury-tin amalgam (Zhu and He 1993). Investi­
gations by Meeks (i988a,b; 1993 a,b) have helped to clarify the issue. When mercury and tin are
mixed together, a very sluggish reaction occurs at room temperature, and the mixture must be
heated to form a mercury-tin amalgam; heating at 200-250 °C produces a pasty gamma-phase
HgSn 6 . The gamma phase decomposes at 214 °C, and mercury can be driven off by heating

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