>>Kerstin Eckstein writes:
>>Recently I was shown some fire gilded brass objects, covered in a
>> thick black corrosion layer which turned out to be mainly copper
>> oxide, Cu2O...
You say that the corrosion layer is black, but that the principal
component is copper oxide, Cu2O. Cu2O (cuprous oxide, cuprite) is
red-brown, and is commonly found as a natural copper corrosion
product (usually overlaid by green secondary corrosion products),
whereas cupric oxide (tenorite, CuO) is, for thermodynamic reasons,
rarely formed at temperatures below about 100C and is never a major
component of the corrosion products of bronzes from 'normal'
archaeological contexts. I have, however, seen bronzes with black
corrosion products containing some tenorite when heat treatment has
previously been used in an ill-advised attempt to remove corrosion
products. The presence of some sulphide corrosion products can also
cause such darkening.
As for the removal of oxide corrosion products, the only satisfactory
method is mechanical, requiring considerable skill and experience, but it
should be capable of revealing all surviving areas of gilding.
Nigel Seeley.
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