Page 363 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

above 357 °C with tin diffusion into the copper substrate. A thin layer of nu intermetallic com­
i
pound develops at the outer surface over a thick layer of epsilon intermetallic compound; f the
surface of the object is heated above 520 °C, a layer of alpha+delta eutectoid phase develops
under this layer and is contiguous with the copper substrate.
Very little evidence exists to support the use of tinned surfaces made by applying a
mercury-tin amalgam to ancient Chinese mirrors. These mirrors were usually made in leaded
bronze with a high tin content, and there would be no need to create extra tin-rich copper
phases by chemically treating the surface. Meeks (1993 a) discovered, however, that a cold,
mechanical application of a mercury-tin amalgam to the polished surface of a 25% tin bronze
did produce a subtle surface effect. The treatment involved rubbing the polished bronze surface
with a small crushed pellet of the amalgam. After two minutes, the treated surface appeared
more noticeably silver colored. Optical microscopy revealed that a silver-colored film obscured
all but a faint trace of the morphology of the underlying eutectoid microstructure. Examination
of the surface with a scanning electron microscope confirmed that this layer is very thin, per­
haps 10-100 nm, and appears to consist of tin with some mercury. Meeks thinks that this
extremely thin coating may result from localized heating of the amalgam when it is rubbed on
the surface; the heating breaks down the gamma compound into its constituent parts, allowing
the tin to bond to the surface. Thus, xuan xi could have been used to enhance the silvery finish
of the mirror.
It is interesting that this Song dynasty recipe is not designed to produce genuine corrosion
products of any depth on the surface of the bronze. Instead, it relies on a purely superficial
deception. The same is also partially true of the recipe given below from the Ming dynasty man­
uscript called the Hsin-ju wei-tso. It invokes a concoction of truly alchemical proportions for fak­
ing a patina, as follows:

[A]fter [the bronze vessels] are cast they are scraped and polished until they are clean
and shining; where the decor has not been cast clearly, it is engraved with tools. Then the
vessel is soaked for a time in a mixture of morning-fresh well-water, clay and alumina, it
is taken out and baked, again immersed, and again baked. This is done three times and is
termed "making the basic colour." When the vessel is dry, a solution of ammonium chlo­
ride, copper sulphate, sodium chloride and "gold-thread alumina" is applied in green brine
using a clean brush two or three times and after one or two days is washed away; again
dried and again washed. The whole process is in adjusting the surface colour and the
amount of washing may have to be done three or five times before it is settled. Next an oven
is dug into the earth, red-hot charcoal is heaped in it and strong vinegar is sprinkled onto
it; the bronze vessels are placed inside and still more vinegar is thrown over them and they
are completely covered with earth and left buried for three days. When taken out and exam­
ined they are all found to have grown the colours of ancient patina mould; wax is rubbed

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