Page 400 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Tatti (i985) and Veloz (1994) are of the opinion that minimal cleaning and hot waxing over
the remaining corrosion layers continues to be an effective treatment. The approach taken by
Tatti is to first clean away surface dirt using water and a nonionic detergent, rinse with clean
water, and then eliminate entrapped moisture by heating the sculpture with a propane torch.
The wax used is made from 85% Bareco Victory microcrystalline wax, 10% Bareco 2000 poly­
ethylene wax, and 5% Cosmoloid 80H wax. (Similar wax mixtures are commonly used by the
U.S. National Park Service for conservation of the monuments in its care.) Saturation of the
patina with this wax minimizes the inherent color contrast on partially disfigured surfaces.
Three coatings of wax are applied. After the final coat has dried overnight, the surface is buffed
by hand with brushes and cloths, which allows highlights to be polished preferentially for aes­
thetic reasons. This approach retains the original patina, which is now covered with a coherent
wax outer layer. Routine annual maintenance is necessary, however, to ensure that the treatment
continues to be effective over time.
The history of some typical coatings used from 1939 to 1997 is documented by Shorer (1997).
Beginning in 1939 an adhesive was made from Perspex (polymethyl methacrylate) by dissolving
it in chloroform. By 1965 the hazards of chloroform had become widely known, and the solvent
was changed to methylene dichloride. By the 1970s methylene dichloride was also found to be
hazardous, and the use of dissolved Perspex gradually ceased. The following sections summa­
rize the variety of coatings used since 1939.

Shellacs and lacquers A common protective coating used in the 1940s was brown
copal shellac dissolved in blue methylated spirits. By the 1950s,
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however, this coating was found to be inadvisable because of the toxicity of pyridine, the dye,
and other substances used, and Shorer changed the recipe to white shellac and colorless indus­
trial methylated spirits IMS). 3 6
(
I
Another common lacquer used for metalwork before World War I was cellulose acetate
made by dissolving celluloid strip film in acetone and iso-amyl acetate. This coating was bor­
rowed from the aircraft industry, where shellac or celluloid was used to impregnate the cloth
used in the manufacture of coverings for airplane wings.
Ercalene and Frigilene nitrocellulose lacquers (Agateen in the U.S.) were introduced in the
1960s and have continued to be popular in museum conservation for coating bronze, brass, and
silver. Frigilene was supposed to be sulfur free, making it suitable for coating silver alloys. One
of the attractions of these nitrocellulose (or cellulose nitrate) lacquers is that they can be easily
applied by spraying, which produces an even polymer film; however, the polymer film is sub­
ject to degradation with exposure to sunlight or ultraviolet light, so these lacquers have gradu­
H
ally been replaced by acrylic resins. There is now an acrylic MG analog, but its viscosity is
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hardly ideal.

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
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