Page 399 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Three recent reappraisals of the efficacy of AMT have been undertaken. Brunner (1993)
found that during treatment of a group of ancient bronze coins, only partial stabilization was
achieved. Hawley (i996) employed AMT in the conservation of a large group of Celtic finds from
Basel, Switzerland, and judged it to be quite effective at extracting chloride ions. It was difficult,
however, to remove the resulting sludge from the pores and crevices of the treated objects. Li
and coworkers (i998) carried out comparative studies of corrosion resistance that showed a pro­
tective film was indeed deposited on the surface of objects. The researchers found that the
reagent complexed effectively with chloride ions in the treatment of a Chinese bronze bell
and that AMT has excellent inhibitory effects in acidic media. They reported that a combination
of 5% citric acid and AMT efficiently cleaned surface deposits and that the use of this reagent
mixture avoided the development of light yellow precipitates within the surface patina of the
cleaned object. Judging from the black-and-white photographic illustrations published with this
article, however, it appears that the Chinese bell has been cleaned down to a cuprite patina by
this mixture. This level of cleaning may not always be necessary and may remove too much of
the original patina to be aesthetically desirable; in many cases, retention of the malachite patina
is preferable. Judgment of the merits of using this inhibitor must await further research. If,
indeed, it dissolves cuprous chloride selectively, then it may have some wider application. The
Ganorkar publication did not discuss one of the important issues of using organic inhibitors in
the conservation laboratory, namely safety. How safe is it to use, and is the compound a poten­
tial carcinogen?

C O A T I N G S FOR C O P P E R A L L O Y S

From time immemorial, waxes, oils, and natural resins have all been used to protect the surface
of copper alloys from tarnishing, corrosion, or as a means of impregnation —that is, to totally
immerse or saturate the object in the resin or wax to consolidate it and prevent disintegration
into mineralized fragments. Late-nineteenth- and early-twentieth-century methods continued
this tradition. Voss (1888), for example, used poppy seed oil and gum damar. Other materials
commonly employed were shellac, fish glue, and beeswax. Paraffin wax was often used as well
(Gilberg 1988), but it was eventually supplanted by cellulose nitrate and cellulose acetate, which
were recommended by Rathgen (1905).
Coatings for outdoor bronzes were traditionally based on natural waxes and oils. Research
into the suitability of these coatings was initiated during the i860 s by the Berlin Society for
the Encouragement of the Arts (see CHAPTER 1). Natural waxes and oils have their drawbacks,
though: they usually contain organic acids and esters that can interact with the bronze sub­
strate, creating their own specific range of corrosion products on the objects they are supposed
to be protecting. Until the advent of synthetic polymers and waxes, however, there was no other
option available to conservators in this field.

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