Page 389 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Tel Nami, Israel. The objects were affected with cuprous chloride corrosion, and Fox found that
their surfaces were consolidated to some extent after treatment. One of the practical problems
associated with the use of alkaline dithionite is cost: the high-grade chemical is relatively expen­
sive, and the commercial-grade reagent tends to contain pungent-smelling impurities. Another
difficulty concerns the disposal of large volumes of partially spent solution. Apart from disposal
restrictions in different countries, the chemical will upset the microbiological balance of sewage
i
systems f poured down the drain.
MacLeod (i987a) found that for chloride ion removal, alkaline dithionite was the most
effective compared with inhibited citric acid, benzotriazole, sodium sesquicarbonate, or aque­
ous acetonitrile. As mentioned earlier, however, this does not necessarily imply that dithionite
solutions are the preferred treatment choice.
In another treatment of marine finds, aqueous acetonitrile, CH 3 CN, proved to be a highly
effective complexing reagent for copper (I) corrosion products, removing cuprous chloride from
objects with active bronze disease when used as a 50% (v/v) acetonitrile-water solution. Analy­
sis of the colorless treatment solution (MacLeod 1987a) showed that the molar ratio of chloride
to copper ions was ι : 1, which suggests that the following reaction is taking place:

CuCl + 4CH 3 CN = Cu(CH 3 CN) 4 Cl" 12.6
+
Washing regimes longer than six weeks, however, may result in discoloration of the patina.
Because of the greater solubility of molecular oxygen in the mixed solvent system, there is a ten­
dency for the resulting cuprite to be further oxidized to tenorite, which had developed on some
objects during a lengthy treatment. Because acetonitrile vapor is moderately toxic, care must be
taken to use the solution under a fume hood in a well-ventilated area, or to seal the containers.
In another evaluation along the same lines, Uminski and Guidetti (1995) used artificially
corroded bronze sheets to compare the effectiveness of acetonitrile with that of water and with
solutions of benzotriazole, AMT (5-amino-2-mercapto-i,3,4-thiadiazole), sodium carbonate,
or sodium bicarbonate. The acetonitrile was the most effective reagent for the removal of the
artificial cuprous chloride (nantokite) corrosion. Acetonitrile can complex with other copper (I)
i
species, so there may be the danger of disrupting cuprite layers, especially f they are inter-
layered with nantokite. This study needs to be evaluated on actual artifacts. Treating bronze
plates containing only nantokite corrosion cannot replicate the difficulties associated with
chloride-ion removal from antiquities, in which the chlorides may be deep-seated and not
exposed to the surface.
Treatment recommendations based on comparative evaluations depend on the objects stud­
ied and the methods employed for cleaning. For example, Thickett and Enderly (1997) examined
the range of chemical treatments discussed here to clean coin hoards, which can contain several
hundred coins corroded together as a mass. Surprisingly, the use of alkaline Rochelle salt caused
the least damage to the coins, 20 and alkaline dithionite was the most aggressive, although in all

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