Page 393 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
P. 393

run and that, in any case, the sheen of the waxed surface  that is commonly used  as a protective
           coating on bronzes  of this kind would not be desirable in this particular case. Instead,  he  rec­
           ommended  the use of a tinted shellac applied to the green areas to reduce  the contrast  between
           the green  and the black coloration. During the  first  conservation  treatment,  the sculpture  was
           given a diluted coating of Incralac. The appearance was not greatly improved, however, and the
           bronze was retreated in 1992 with Arkon p-90,  a fully  saturated  alicyclic hydrocarbon soluble
                                                2 7
           in petroleum naphtha,  used  as a  15% solution. During the four years that the treated  sculpture
           was on exhibit, this coating became more plastic-looking and developed a slight milkiness. The
           decision was made to re-treat the surface  to try to achieve better reintegration. The Arkon p-90
           was removed with Shellsol i,  which was followed by cleaning with naphtha, but this failed to
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                                  2 8
           remove all of the coating. Xylene applied with cotton swabs  finally  succeeded in removing the
           remaining coating.
              A  new  treatment  using Laropal aldehyde  resin  in mineral spirits was  tried  with  good
                                                     2 9
           results, but it left  too much surface  gloss  that would  have necessitated matting out areas with
           powder pigments. Most acrylic resins that were tried were also too glossy. Finally, tinted super-
           blonde shellac was applied. This varied the opacity and  finish of the resin to suit different areas
           of the variegated surface  (Bassett  1996); this is the  finish  that the sculpture now has.
              In  all these examples,  any earlier patina  still on the bronze  after  cleaning was allowed to
           remain there when the sculpture was repatinated. There are, of course, conservation approaches
           that involve totally stripping away an existing patina, using both chemical and mechanical tech­
           niques, before a new one is applied. This may be justified in particular cases, since every bronze
           must be considered  as an individual object with specific conservation requirements. There is no
          universal panacea.


       THE  U S E OF  C O R R O S I O N  I N H I B I T O R S

          A  number  of different organic  compounds  that  complex with  copper  have been proposed  as
           corrosion inhibitors for ancient copper alloys or exposed bronze statuary. Corrosion inhibitors
          act to retard the corrosion of metals by forming a stable surface  complex that interferes with the
          anodic reaction, the cathodic reaction, or both. In the case of copper  alloys, complexes  can  also
          be formed between benzotriazole  and copper-corrosion products. The most well known com­
          pound is benzotriazole, which is reviewed here first and is followed by a brief account of other
          corrosion inhibitors.


          Benzotriazole             In  1967 Madsen  introduced the  use  of the  corrosion inhibitor
                                    benzotriazole  (BTA)  to conservation practice. In fact, benzotri­
          azole had been the subject of a British patent for use  as a corrosion inhibitor in 1947 (Cotton and
          Scholes  1967), and it had been used in commercial applications  for twenty years before  the
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          conservation community became aware of its benefits  for the stabilization of ancient  bronzes.


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