Page 390 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
P. 390

cases, the effects were not great. Alkaline glycerol and formic acid treatments  adversely affected
            coins containing lead.
               Another  use of Rochelle salt for cleaning an important, exposed  bronze  surface  is worth
            noting here. Around  1990  Ghiberti's famous  bronze  doors on the Baptistry in Florence, Italy,
            were taken indoors and replaced by replicas, while the conservation studios of the Opificio delle
            Pietre Dure in Florence undertook conservation work  on their gypsum-encrusted  surfaces. 21
            Rochelle salt was shown to be the most effective treatment for this large bronze relief. The salt
            was used in neutral aqueous solution, however, without any alkali additions, at a concentration
            of about  30%  (w/v). This solution effectively  treated some of the individual gilded door  panels
            without disrupting the cuprite surface immediately under the gold. In other areas of the  surface,
            however, brochantite and antlerite existed within small pits under the amalgam gilding, result­
            ing  in the gold  surface  being blistered with  numerous  small mounds.  Some of these blistered
            gilded  areas burst  open  to  reveal  copper  sulfates;  beneath  these  sulfates  some  copper  chlo­
            rides — such as clinoatacamite and paratacamite, or even nantokite — may exist. These open blis­
            ters could not be fully  treated.
               After  treatment of the door panels,  distilled water was used  to wash away residues  of the
            Rochelle salt. The panels  were then dewatered with  acetone and  finally  heated  slightly to dry.
            The doors are now on exhibit in nitrogen-filled display cases in Florence.
               Immersing objects in large tanks of aqueous solutions is difficult  and can create practical
            problems. Aqueous gels based on cleaning reagents that incorporate Polyacrylamides such  as
            buffered  citric  acid or  tartrates  with  B TA  additions may eventually prove useful  for cleaning
            bronze  objects. 22
               Washing marine finds in distilled water  has  also been  attempted, but, although this may
            wash out chlorides by hydrolysis of cuprous chloride, the process is very slow. MacLeod (i987a)
                                                          i
            estimates  that effective stabilization can be attained only f the washing period is from  two to
            four  years, which  is generally impractical. During  such long periods of immersion,  reactions
            with other corrosion products might occur, and these would not be controllable. For these rea­
            sons, this method cannot be recommended.
               Another approach  to the washing process for bronze  objects is immersion in sodium ses­
            quicarbonate solutions, subject to the patina alteration problems previously discussed. Sodium
            sesquicarbonate  does remove more chloride ions than distilled water, although at  5% (w/v) in
            water this solution also slowly removes copper from  the sound metallic regions of an object  as
            well  as from  the corrosion crust.
               MacLeod  (1987a)  also  noted  that  a solution of  1% (w/v) of aqueous benzotriazole  with
            5%  (v/v)  of ethanol is capable of displacing a large proportion of chloride-containing corrosion
            products. Some of these treatments  have not been  successful,  however, possibly due to  insuffi­
            cient impregnation time. Immersion in this solution for a few months, rather than a few days,
            may  lead to better corrosion resistance  of the treated artifacts.



                                      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
                                                                      373
   385   386   387   388   389   390   391   392   393   394   395