Page 104 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

corresponding to different deposition conditions in the cuprite as it formed during the corro­
sion of the object. PLATE 14B shows the finely layered corrosion crust of malachite, cuprite, and
tin oxides in a British bronze palstave from Kent.
It is difficult to prove that such multilayered structures are, in fact, attributable to Liesegang
phenomena, since the spacing of the layers rarely follows any mathematical equation that
relates to Liesegang structures. The principle, however, is surely applicable. Although the lay­
ered structures do not correspond to typical periodic cycles, such as years or seasons, they prob­
ably follow the slow cycle of interpénétration and deposition of corrosion products that occurs
during burial. In this process, the carbonate ions reach a level for precipitation as a solid crys­
tal phase; the zone of depleted carbonate or bicarbonate ions allows cuprite to re-form; and the
cycle can then be repeated (Scott 1985).
I ENVIRONMENTAL DISRUPTION OF CUPRITE PATINAS Cuprite
growth on bronze statues exposed to the outdoor environment in coastal areas may be exacer­
bated because of the higher concentration of atmospheric chlorides in these regions. In fact, it
has been demonstrated experimentally that cuprite patinas can be disrupted by sulfide ions and
chloride ions found in the soil and in marine environments as well. When copper plates are
exposed to a cupric chloride solution, a thin cuprite crust usually develops, with a green para-
tacamite layer on top. It is possible that the presence of additional or thickened cuprite layers
on outdoor bronzes may be associated with continued corrosion in a similar manner; further
research to clarify this issue is needed.
I CUPRITE REMOVAL Cuprite layers may resemble sugar f the
i
i
crystal size is large, or they may be dense and compact f finely crystalline. These dense lay­
ers can be shaved off with a scalpel when bronzes are mechanically cleaned. Some of these lay­
ers may be quite difficult to remove or to reduce in thickness, however, and this often leaves
clearly visible damage to the finer, protective cuprite layer beneath them f the work is not skill­
i
fully executed.
Chemical dissolution of cuprite layers without substantial attack on the metal or other
desirable surface products is difficult because most of the chelating agents employed will attack
cuprite only very slowly. This is often an advantage, since the fine layer of cuprite that is usu­
ally adjacent to the bare metal surface should not be removed. In some cases, however, a cuprite
i
corrosion crust may be unwanted, particularly f it has developed over gilding or silvering. In
such cases, it may be necessary to mechanically or chemically remove it. On gilded bronzes, a
10% aqueous solution of formic acid in water is a useful reagent to use initially, f mechanical
i
cleaning would cause too much disruption to the surface. Even with this solution, the dissolu­
tion of the cuprite crust is still very slow and may have to be aided by light rubbing with a cot­
ton swab dipped in the reagent.

O X I D E S AN D H Y D R O X I D E S
87