Page 89 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

adding to the PEG solution a corrosion inhibitor, Hostacor KSI, based on an alkanolamine salt of
an aryl (sulfon) - amido carboxylic acid. The corrosion of copper alloys significantly increased
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in PEG 00 solutions (pH 5.6) compared with immersion in distilled water; rates between 25 and
41 μιη per year were determined. The corrosion rate in the amine-based polymer was lower than
in PEG—between 5 and 17 μιη per year. Because it is amine-based, this polymer may form a
soluble copper complex, which would increase the corrosion rates of brass alloys, especially
compared with their rates of corrosion in distilled water. Copper and wood components n
i
copper-wood test pieces treated with the same consolidant were stable six years later. In seawa­
ter with 15% v/v addition of this amine-based polymer, the corrosion of copper alloys was
significantly reduced to 0.4 μπι per year; this is a lower corrosion rate than that observed in dis­
tilled water or seawater alone, where the rates were 3 μιη and 6 μιη per year, respectively. It is
difficult to assess by laboratory experiment just how effective this consolidant would be in the
treatment of copper alloys covered with a thick corrosion crust. There is no ready way to evalu­
ate this without long-term treatment of waterlogged composite material.

C O P P E R IN C O N T A C T WITH O R G A N I C M A T E R I A L S

I have heard it said that bronzes that have been in the soil a long time are deeply impregnated with
the essence of the earth, and that this will maintain the beauty of flowers in their freshness and
brightness. Buds will open out more quickly and will wither more slowly. This nurturing strength
comes from the vase itself —YüAN HUNG-TAO 2 2

Copper ions, through their biocidal character, play an important role in the preservation of
associated burial materials, such as textiles, feathers, bone, wood, and leather. Copper may even
enhance the preservation of buds, as the Chinese literary theorist Yuan Hung-tao (i568-i6io)
notes, but it is doubtful that they will open more quickly, as he suggests. The organic matrix of
material buried with copper may also be replaced, in part or in whole, by copper salts. This is
the process of mineralization, which is the combination or replacement of the organic matrix
with an inorganic one (Gillard et al. 1994). The metallic ions of copper, iron, lead, and silver are
reported to be the principal inorganic corrosion species that participate in pseudomorphic
replacement processes.

Positive replacement In positive replacement, copper (or other) ions penetrate the
and mineralization fiber of organic materials and coordinate with the organic
of organic materials matrix. These coordination sites create additional nucleation
areas that attract more copper ions. Corrosion products then
gradually replace the fiber as it decays, forming a positive fiber cast composed primarily of cop­
per corrosion products, typically the basic chlorides or carbonates. Negative casts form when
the corrosion products deposit on the surface of the fiber, which then decays and leaves resid-

C H A P T E R O N E
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