Page 352 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

F I G U R E íi.i Cross section of a
corrosion pustule on the Togati bronze
(see P L A T E S 73 and 75), showing the
complex structure of the corrosion
products. The lighter gray phase is
cuprite; the darker gray is malachite;
the light, solid particles are lead car­
bonates; and the light particles are
alpha+delta eutectoid. The area of the
pustule that is in contact with the metal
surface is toward the bottom of the pic­
ture. Viewed under the electron micro-
probe in backscattered mode. Scale bar
represents 100 μιη.

F I G U R E 11.2 Backscattered electron
image of part of the pustule shown
in F I G U R E 11.1. Attachment of the
pustule to the mineralized surface
zone, which includes tin oxides, is
visible at the bottom of the image.
The lighter gray fragments just above
the attachment are euhedral crystals
of cuprite. Above this are more mas­
sive cuprite layers. Lighter gray par­
ticles are alpha+delta eutectoid; the
very light particles are basic lead car­
bonates (magnification χ 3 2 θ ) .

cleaned, the figures have a smooth, sometimes mottled patina. Overlying this layer are dissemi­
nated fragments of a copper- and lead-rich phase whose globular shape is oriented in a pattern
suggesting interdendritic regions. These globular particles can be easily cleaved from the patina
with a scalpel, using a gentle cutting motion parallel to the patinated surface. The surface con­
tinuity is periodically interrupted by pustules of corrosion, as illustrated in FIGURE 11.1. These
pustules have an apparently layered structure; beneath each pustule, the smooth patina is
interrupted by a zone of cuprite. An overall backscattered electron image of one such pustule is
shown in FIGURE 11.2.

S O M E A S P E C T S O F B R O N Z E P A T I N A S
335