Page 394 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
P. 394
The use of a stabilization treatment was beneficial in creating a revised approach toward
the concepts of patina retention and humidity control as viable alternatives to the conservation
of ancient bronzes, which traditionally involved either patina removal or drastic alteration to
the existing patina. The precise mode of benzotriazole's action as a copper inhibitor has been
under discussion for several years. Since all copper surfaces have a thin film of copper (I) oxide
present, the polymer layer that is formed over the cuprite layer is thought to provide the major
anchoring sites for the benzotriazole multilayered complex. Hollander and May (i985) con
cluded from numerous prior studies that benzotriazole forms a protective film of predominantly
Cu(I) benzotriazole over the metal surface. This chemisorbed complex may form a polymeric
film up to thirty molecular layers thick that acts primarily to retard the cathodic reduction of
oxygen, although some studies also suggest mixed or anodic control. Detailed electron spectros
copy for chemical analysis (ESCA) and infrared work has shown that the protective film is a ι : ι
complex of Cu(I) and benzotriazole, probably polymeric, with benzotriazole bridging two cop
per atoms via the N x and N 3 , with the aromatic ring aligned parallel to the metal surface.
This Cu(I) Β TA complex is extremely insoluble, which is additional evidence for its poly
meric nature. The conditions under which the metal is pretreated affects the nature of the result
ing Β ΤΑ film. Pretreatment conditions include the nature of the patina, the degree of corrosion,
the presence of active chloride corrosion in pits, the composition of the patina, and how it is
cleaned prior to treatment. At low pH conditions, such as may be encountered with bronze dis
ease, BTA can form very thick films, but these are apparently caused by the partial precipitation
of benzotriazole from solution, since it becomes increasingly insoluble as the pH falls. Increas
ing the temperature used during treatment produces a more effective Β TA deposition.
BTA may also form a series of cupric complexes whose structures are currently unknown.
Using Fourier transform infrared spectroscopy, Brostoff (1997) found that the main triazo
stretching band in Cu(II) Β TA derivatives showed more variability in position and shape than in
the Cu(I) derivatives, suggesting that the divalent complexes are more variable and irregular
than the monovalent complexes. A series of studies were conducted by Brostoff on the inter
action between BTA and a variety of copper compounds, and the results are shown in TABLE 12.1.
Not unexpectedly, Cu(I) BTA predominates in reactions with cuprite and with copper powder.
Cu(II) BTA derivatives were identified from reactions with nantokite. These studies showed that
copper chloride salts strongly influence the copper-BTA reactions and that intermediates such
as CuCl 2 and unstable cuprous chloride-ΒΤΑ derivatives may be involved in the CUCI-BTA reac
tion mechanism.
When BTA reacts with cupric chloride, a cupric-ΒΤΑ derivative precipitates from solution;
this has been assigned the formula CU(BTA)C1. Under certain circumstances, there may be prob
lems with the stability of an artifact being treated f large amounts of cuprous chloride are pres
i
ent. For example, when workers at the Conservation Laboratory of the Museum of London
applied BTA to a particular object, a plume of reactants emerged from a chloride-containing cor
rosion pit. 3 1
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