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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