Page 391 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
P. 391

R E P A T I N A T I O N  OF  C L E A N E D  S U R F A C E S

            The  repatination of cleaned  surfaces  is a reintegration technique  that  is often  necessary with
            corroded bronzes in the outdoor environment. Repatination  may be limited to damaged  areas
            of the remaining patina, or it may be applied to the entire bronze  sculpture, creating a new sur­
            face that hopefully will be maintained for the future. Repatination of an original bronze  surface,
                i
            even f badly corroded  and disfigured, is still  a contentious  issue because there is no certainty
            that the treatment  will  improve the preservation  of the bronze in the long term unless contin­
            ued maintenance  is provided.


            Outdoor bronzes          Weil  and  coworkers  maintained  that  most  outdoor  bronze
                                     sculpture was intended to be bronze in color and polished (Weil
            et al.  1982; Weil i985b) and that patinated or at least severely corroded bronze sculpture was best
            restored  to its original  appearance. This point of view was  exemplified by Weil's work  on a
            group  of  thirteen  bronzes  on  the  Washington  monument  by  Thomas  Crawford  (1814-57)
            and Randolph Rogers (1825-92)  erected  from  1844  to 1869 on the grounds  of  the state capítol in
            Richmond, Virginia. Weil used a variety of repatination treatments  to restore the sculptures  to
            the appearance they had when dedicated  and first placed outdoors. Each bronze figure was  first
            heated, area by area, with a large blowtorch until any water present on the statue vaporized. The
            patination mixture employed was  the  commonly used dilute solution of copper  nitrate, ferric
            nitrate, and potassium  sulfide. This was  applied in successive applications by brush  or  spray.
            The surface  was washed  frequently to remove  excess unreacted  chemicals.
               Weil also used the cold application of a patination solution called "French green," based on
            ammonium chloride, creating a principally atacamite  patina. This was followed by an applica­
            tion of dark pigmented wax whose  surface  appearance was modified by partial buffing  to better
            integrate  the surface  color and texture of the treated  regions with the untreated  surface. A coat­
            ing  of Incralac  (discussed  under  "Coatings  for Copper Alloys")  applied by spray could also be
            used over this "French  green" patina. This repatination solution was  commonly used on later
            French bronzes from  the foundries  of Barbedienne  and Rudier, according to Weil (i985a).
               In  another  example, the Museum of Art in Philadelphia cleaned the disfiguring surface lay­
            ers  on a version of Rodin's The Thinker in 1992  using a poultice of disodium ethylene  diamine
            tetra-acetic  acid (EDTA)  mixed with  cellulose powder. This complexing reagent is particularly
            efficient,  as are most chemical reagents, in removing cupric salts, but it attacks the cuprite layer
           very slowly. Consequently,  the outer corrosion layers on this sculpture were  removed down to
            a compact cuprite layer, which was then used as the base for chemical repatination using a solu­
            tion of ammonium chloride and potassium polysulfide. This was followed by hot waxing to pro­
            tect the new patina.






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