Page 401 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

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Despite the problems with cellulose nitrate as a coating, f it is kept for a long time at low
light levels, some polymer films made of this material have survived quite well for as long as
sixty years (Selwitz 1988). There are instances where objects coated with nitrocellulose lacquer
twenty-five years ago were delacquered with ease in 1996.

Resin coatings A resin coating popular from the early 1950 s was Bedacryl 221Χ ;
however, the acrylic polymer base, butyral methacrylate in tol­
uene, was not very stable and tended to cross-link on exposure to ultraviolet light. The use of
this coating has been discontinued, along with that of soluble nylon, which ended up not being
soluble at all. Another unsuitable material often used from the 1930s to the 1960s and now gen­
erally available only for thin films such as coin holders is polyvinyl chloride, or PVC. One com­
mon problem in using PVC coin holders is that bronze coins stored in them develop green stains.
This corrosion is caused by the chloride ions in the material and by the plasticizers (such as
phthalates), whose hydrolysis products can contain carboxylic acids. These holders have grad­
ually been phased out in most major museum collections and replaced with polyethylene hold­
ers or foams. Because of dioxins created during the manufacture of PVC, this polymer is now
banned from production in many European countries.
A much more satisfactory acrylic resin system is Paraloid Β 72, which has been manufac­
tured continuously for more than thirty years by Rohm and Haas Inc. of Philadelphia. Known
in the United States as Acryloid Β72, it is a copolymer of ethyl methacrylate and methyl meth­
acrylate and has much better aging properties than the butyl methacrylates. This resin is used
extensively in bronze conservation work, both as an adhesive and as a coating.

Incralac During the i960 s the International Copper Research and Devel­
opment Corporation in New York investigated suitable coatings
for copper alloys. The result was Incralac, a product consisting of epoxidized soybean oil (as
a leveling agent), benzotriazole (as a υ ν stabilizer—not, as is often stated, a corrosion inhibitor),
ethyl methacrylate and butyl acrylate copolymer (known as Β 44, a copolymer from Rohm and
Haas Inc.), toluene, and ethanol. The coating has become an industry standard for many out­
door applications, although it cannot be used as a coating for bronze sculpture without recog­
nition of the fact that it will eventually break down and become insoluble.
Incralac was included in a study of the effectiveness of various plastic and wax coatings in
protecting outdoor bronze sculpture (Beale and Smith 1987). Acrylic coatings performed well
in a series of evaluation trials, although wax formulations were also reasonably effective. In
this study two different environments were used: simulated acid rain and normal outdoor
conditions. Cast bronze relief panels, with composition of 85Cu5Sn5Zn5Pb, were coated with
the following:

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