Page 392 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
P. 392
At the Fogg Museum of Art in 1985, Crane is reported to have used partial repatination with
ferric nitrate and potassium permanganate to blend a new, cleaned bronze surface with the
existing patinated areas. 23
Another more complicated cleaning and repatination treatment was carried out by
Puhringer and Johnsson (1990), who first cleaned an outdoor bronze sculpture with a poultice
of Calgon (sodium polyphosphate) mixed with clay. The surface color was then modified by
application of another poultice made from clay mixed with "K-Fe cyanide" solution; it is not
clear whether this was potassium ferricyanide or potassium ferrocyanide. Next, concentrates of
alkyltrialkoxysilanes with catalysts of metallic acid esters were applied to the surface as part of
a protective coating. The authors claim that the polymerized siloxanes can be removed by the
application of packs of monomeric alkylalkoxysilanes in silica or clay. This treatment requires
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further evaluation as to its long-term efficacy. Calgon can be a harsh cleaning reagent, and it can
slowly attack cuprite layers. The nature of the artificial patination also needs to be more fully
characterized in terms of the longevity of the silane coating.
An "indoor" Bronzes intended for display indoors may sometimes have an
bronze outdoors "outdoor" patina f exposed to the elements, as illustrated by
i
I
Jugging Figure, shown in PLATE 86, executed in about 610-15
by the Dutch artist Adriaen de Vries (i546-i626) and now on display at the J. Paul Getty
Museum. In this instance, the outdoor patina resulted from the bronze having been mistaken
earlier for a piece of garden sculpture. 25
The patina of the bronze revealed a corrosion layer that was 20 -100 μπι thick and had rem
nant areas of eutectoid phase of the copper-tin system immersed in a striated corrosion crust
incorporating copper oxides, chlorides, and sulfates. Only brochantite, however, was identified
by powder X-ray diffraction (see APPENDIX D, TABLE 29). The corrosion contains a variety of
components as revealed by PLATES 87 and 88, which show X-ray fluorescence elemental dis
tribution maps (each identified by the element symbol) along with secondary-electron and
backscattered-electron images. The maps in PLATE 87 show the presence of sulfur, tin, and lead
in the patina. The map for carbon, which includes the edge of the resin used to mount the cross
section of the sample for polishing visible in the upper right corner, shows that carbonate is not
prevalent in the patina. PLATE 88 reveals, most significantly, the distribution of copper, chlorine,
and oxygen. These illustrations show that tin compounds are present in the lower layers of the
patina, closer to the metal, while chlorides occur toward the outer layers. This contrasts with the
more common occurrence of chlorides contiguous with the metallic surface.
The aesthetic appearance of Juggling Figure was quite problematic in terms of returning it
to a more appropriate hue for museum display and disguising the blackish and contrasting
green areas of the principally brochantite surface. A cosmetic treatment was recommended by
Stone, who believed that treatment with a pigmented wax mixture would wear off in the long
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