Page 397 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

with coplanar BTA and chloride ligands or a fairly low degree of polymerization. The adhesion
of these films, however, even f they formed as oligomers, should be beneficial to the preserva­
i
tion of the treated bronzes, because these films will insulate the active corrosion products from
the external environment. Stock notes that neither conventional benzotriazole treatment nor
3 2
control of the relative humidity effectively conserved bronzes with extensive chalconatronite
corrosion in the collections of the Royal Ontario Museum; after treatment, bright blue powder
continued to be produced from the affected objects. Further research in this area is needed,
although it is often not possible to confirm or reproduce some of the difficulties reported by con­
servators during their treatment of particular objects.
The overall process of benzotriazole interaction with objects affected by bronze disease is
complex. Significant amounts of chloride ions may accumulate from soluble salts, from cor­
rosion reactions, and from Cu(II) BTA reaction products. MacLeod (i987b) demonstrated that
Cu(II) BTA reaction products accumulated when bronze objects were immersed for two hours
in a % (w/v) aqueous BTA solution. The results of this study showed that oxygen and water may
i
be present in sufficient quantities, even in alcohol, to activate corrosion reactions, and this has
a direct bearing on the observations by Faltermeier described above.
Tennent and Antonio (i98i) have established that the complex formed between BTA and
cuprous chloride may not be stabilized against further reaction and that its conversion to a BTA-
atacamite complex occurred after seventy-two hours of reaction with water. There is some
evidence that the cuprous chloride-BTA complex is stable to a higher relative humidity than
cuprous chloride alone, which tends to transform to one of the copper trihydroxychlorides at
over 45% relative humidity (Scott and O'Hanlon 1987; Tennent and Antonio i98i).
I STANDARD PRACTICE AND RECOMMENDATIONS It IS now
standard practice for conservation treatment to immerse bronzes in a 3% (w/v) benzotriazole
solution in ethanol at room temperature, often under vacuum, for up to twenty-four hours. 33
The object is then removed from the solution, excess BTA crystals are rinsed from the object with
clean ethanol, and the object is dried. The BTA film that remains on the object is often given a
protective coating, either with a 5% solution of Paraloid Β72 (Acryloid Β72 in the U.S.) in ethanol
or with a solution of Incralac lacquer diluted in ethanol or toluene (see the following section,
"Coatings for Copper Alloys"). Often aerogel silica is added to the Incralac mixture as a matting
agent to prevent the surface from appearing too plastic; the aerogel silica particles reduce sur­
face gloss substantially. More than one coat of the Paraloid Β 72 polymer film is usually applied
to increase its protective capacity.
The results of the studies discussed in this section suggest that pretreatment regimes, such
as rinsing and exposure to buffering agents to remove soluble chlorides and neutralize acid­
ity, may be beneficial before using BTA solutions. Relatively short immersion times and the use
of fresh BTA solutions are advisable. The latter advice is particularly noteworthy, since most
conservation laboratories using benzotriazole as a 3% w/v solution in ethanol tend to reuse the

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