Page 382 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

products formed. The gray spots could later be color-matched to the patina f desired (see
i
PLATES 84 and 85 for an example of pitting corrosion before and after cleaning, respectively). 11
The chemical reactions that result in this seal start with hydrolysis of the CuCl in con­
tact with the moist zinc dust. This results in cuprite formation and the generation of a slightly
acidic solution that favors the production of more zinc ions. These zinc ions then react to
form basic zinc hydroxide chloride, which provides a more effective treatment than conser­
vation with silver oxide paste. At least two basic zinc hydroxide chlorides—6Zn(OH) 2 -ZnCl 2
and 4Zn(OH) 2 ZnCl 2 —form in the presence of chlorides and zinc ions at a concentration
greater than o.oi M and at a pH less than 7; crystalline zinc hydroxide, Zn(OH) 2 , also forms.
The layered structure of 4Zn(OH) 2 ZnCl 2 is beneficial to the anticorrosion behavior of zinc
coatings in general.
There are two drawbacks to the zinc-dust treatment as published. One is the amount of
repetitive work needed to moisten the pits ten to twelve times per day; it may be that this can be
substantially reduced without affecting the efficacy of the treatment. The second drawback is the
need to cosmetically color-match the filled pits either with copper carbonates as a paint, as sug­
gested by Sharma, Shankar Lai, and Nair (1995), or with acrylics in a suitable medium, such as
acrylic emulsion or polyvinyl acetate emulsion. f there are many pits to be treated on an object,
I
the work may become highly labor intensive, and some conservators may still prefer the silver-
oxide paste treatment, 12 especially since the dark brown-black color of the silver oxide paste
blends well with most patinas.
Another approach to localized treatment was taken by Aldaz and coworkers (i986), who
developed a simple electrolytic tool for corrosion removal. With their apparatus, the object
being treated is used as the cathode in an electrolytic cell; the anode is made of carbon or stain­
less steel; and the electrolyte is a solution of sodium hydroxide. The anode rod and electrolyte
are housed in a plastic cylinder that ends in a porous tip, such as sintered glass. A drop of the
solution is placed on the area to be treated, and the tip of the cylinder is applied to it; the sur­
rounding metallic area becomes the anode, and the two areas are connected by a copper wire to
a battery, forming a circuit. The authors claim good results using this method on Roman bronze
coins from Santa Pola, Alicante, Spain. There are problems with this method, however, such as
the application of the electrolyte to the object's surface, since chemical changes to the surface
corrosion products may result, and it may be difficult to make a circuit function properly on
heavily corroded bronzes. It may also be difficult to limit the electrolytic action precisely to the
spot under treatment; there is always a danger that the solution will affect the region immedi­
ately surrounding the area under treatment, causing it to darken or undergo other alteration. As
a treatment method, this technique does not appear to be worse than some others, but it has not
been mentioned in the literature again since it was published in 1986, which limits the evalua­
tion of its efficacy.

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
365