Page 392 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
P. 392

At  the Fogg Museum of  Art  in 1985,  Crane is reported to have used partial repatination with
          ferric  nitrate  and  potassium  permanganate  to blend  a  new,  cleaned  bronze  surface  with  the
          existing patinated areas. 23
             Another  more  complicated  cleaning  and  repatination  treatment  was  carried  out  by
          Puhringer  and Johnsson  (1990), who  first  cleaned  an outdoor bronze  sculpture with  a poultice
          of  Calgon  (sodium polyphosphate)  mixed with  clay. The  surface  color was  then  modified  by
          application of another  poultice made from  clay mixed with  "K-Fe cyanide" solution; it is not
          clear whether this was potassium ferricyanide or potassium ferrocyanide. Next, concentrates of
          alkyltrialkoxysilanes with  catalysts of metallic acid esters were applied to the surface  as part of
          a protective coating. The authors  claim that the polymerized siloxanes  can be removed by the
          application of packs of monomeric alkylalkoxysilanes in silica or clay.  This treatment  requires
                                                                 24
          further evaluation as to its long-term efficacy. Calgon can be a harsh cleaning reagent, and it can
          slowly attack cuprite layers. The nature of the  artificial patination also needs to be more  fully
          characterized in terms of the longevity of the silane coating.


         An  "indoor"              Bronzes  intended for display indoors may  sometimes  have an
          bronze outdoors          "outdoor" patina f exposed  to  the  elements,  as  illustrated by
                                                 i
                                                                               I
                                  Jugging  Figure,  shown in PLATE  86, executed in about 610-15
         by  the  Dutch  artist  Adriaen  de  Vries  (i546-i626)  and  now  on  display  at  the  J.  Paul  Getty
         Museum. In this instance,  the outdoor patina resulted from  the bronze  having been mistaken
          earlier for a piece of garden  sculpture. 25
             The patina of  the bronze revealed a corrosion layer that was 20  -100  μπι thick and had rem­
          nant  areas of eutectoid phase of the copper-tin system immersed in a striated corrosion crust
         incorporating copper oxides, chlorides, and sulfates. Only brochantite, however, was  identified
         by  powder X-ray diffraction  (see APPENDIX  D,  TABLE  29). The corrosion contains  a variety of
          components  as  revealed  by PLATES  87 and  88, which  show X-ray fluorescence  elemental dis­
          tribution  maps  (each  identified  by  the  element  symbol)  along with  secondary-electron  and
         backscattered-electron  images. The maps in PLATE  87  show the presence of sulfur,  tin,  and lead
         in  the patina. The map for carbon, which includes the edge of  the resin used to mount the cross
         section of  the sample for polishing visible in the upper right corner, shows that carbonate  is not
         prevalent in  the patina. PLATE  88  reveals, most significantly, the distribution of copper, chlorine,
         and oxygen. These illustrations show that tin compounds  are present in the lower layers of the
         patina, closer to the metal, while chlorides occur toward the outer layers. This contrasts with the
         more common occurrence  of chlorides contiguous with the metallic surface.
             The aesthetic appearance of Juggling Figure was quite problematic in terms of returning it
         to  a more  appropriate  hue  for  museum  display and  disguising the  blackish and  contrasting
         green  areas of the principally brochantite  surface.  A cosmetic treatment  was recommended  by
         Stone,  who believed that treatment with a pigmented wax mixture would wear  off  in  the long
               26


                                    C O N S E R V A T I O N  T R E A T M E N T S  F O R  B R O N Z E  O B J E C T S
                                                                    375
   387   388   389   390   391   392   393   394   395   396   397