Page 380 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

General treatments ELECTROLYTIC REDUCTION At the end of the nineteenth
century, electrolytic reduction represented a state-of-the-art
application of science, perhaps similar to the recent fascination with plasma reduction or the
laser cleaning of artifacts. In 1905 Rathgen employed electrolytic reduction, using a 2% potas­
sium cyanide solution in water as electrolyte and platinum anodes, to eradicate unstable cuprous
chloride from objects. Later, less dangerous electrolytes and less expensive anodes came into use
for museum treatments.
If used with skill and care, electrolytic reduction is capable of surprisingly good results.
Fink (i948), for example, illustrated the results of electrolytic reduction on an inlaid Egyptian
bronze in the collections of the Metropolitan Museum of Art in New York. Fink's photographs
of the bronze before and after treatment showed that no grossly deleterious effects had been
caused by this cleaning method. This, however, is the exception rather than the rule.
Plenderleith (1956) recommended using sodium hydroxide as the electrolyte and stainless
steel for the anodes. This treatment regimen continued into the 1970s in the United Kingdom
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where molybdenum-stabilized stainless steel anodes were used with the sodium hydroxide solu­
tion. On anything but very thin patinas over sound metal, this technique essentially strips away
both the patina and deeper zones of metallic corrosion. Stripping the metal bare in this way may
result in severe loss of the surface and possibly the original shape of the object.
I CHEMICAL ALTERATION I N THE LABORATORY Laboratory-
induced chemical alteration of one copper mineral to another often forms the basis for conserva­
tion treatment. Thus cuprous chloride may be completely eliminated by electrolytic reduction,
or it can be altered to a mineral phase that is more stable by immersing the object in a variety of
chemical reagents. Scott (1921), for example, recommended the use of a 5% solution of sodium
sesquicarbonate, NaHC0 3 -Na 2 C0 3 , as an immersion solution for chloride removal from both
iron and bronze artifacts. The sesquicarbonate solution has a pH of about 10; at this alkalin­
ity level, cuprous chloride is unstable and can be converted to cuprite. The hydrochloric acid
released is then neutralized by the carbonate, forming sodium chloride.
Oddy and Hughes (1970) reassessed this method in greater detail and recommended
extended periods of immersion, sometimes longer than a year, for bronze objects. This long-
term treatment did succeed in washing chloride ions out of corroded bronze surfaces, but it
was also found to cause mineralogical changes to the patina, sometimes drastically altering
the appearance of the bronze. A secondary layer of malachite also sometimes formed as a pre­
cipitate on the object, even though the ion [Cu(C0 3 ) 2 ] ~ is supposed to be stable in the pres­
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ence of bicarbonate ion. Conservators were advised to change the solution f this precipitation
occurred, but it was often difficult to adequately monitor the solution over long periods.
The chemical basis for this treatment is that when cuprous chloride is allowed to react with
water, there is a steady increase in chloride ion concentration in solution as a function of time,

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