Page 196 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

P L A T E 1 2 Photomicrograph of a cuprite crust on a Bronze
Age plain ax (unsocketed) from the area of Dublin, Ireland.
The fine lines retained in the cuprite layer represent strain
lines of the original copper grains produced during heavy
cold-working of the metal to harden the cutting edge.
The lighter-colored cuprite layer at the top of the image
does not preserve any pseudomorphic remnant structure
(magnification χ 6 2 5 ) .

P L A T E 1 3 Photomicrograph of a color-etched fragment
from a bronze ceremonial ax (see P L A T E 7 A) from the
Luristan region of Iran. The alpha+delta eutectoid phase,
visible at the center of the image, has darkened due to cor­
rosion; redeposited copper appears as twinned crystalline
phases, which are visible in this matrix as yellow, red, and
orange crystals. Note that the redeposited copper grows
around the cored alpha-phase dendrites and does not dis­
turb the alpha phase, which is etched a yellow orange
with reddish centers due to coring. The uncorroded eutec­
toid phase appears blue and silver between the dendrites
(etched in acidified thiosulfate, magnification χ625) .

P L A T E 1 4 Photomicrographs of two bronze objects:
A, cross section of a completely corroded toggle pin from
Iran (see P L A T E 7B), showing major and minor stratifi­
cation of the cuprite layers with malachite (unetched,
B
magnification χ 50); and , an example of a finely layered
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corrosion crust of malachite, cuprite, and tin oxides from
a British bronze palstave from Kent (unetched, magnifica­
tion X760 ; see also P L A T E 10) . The fine spacing within the
malachite layering of the toggle pin is strongly suggestive
of Liesegang banding, which results from the slow precipi­
tation of solid phases during the corrosion process.

P L A T E 1 5 Statuette of the God Ptah, Egyptian, ca. 6 6 4 -
52 5 B .c .E . Bronze, H : 17. 8 cm (not including base). The
figure shows two types of patination: parts of the head and
staff are black-patinated, and the body presents a typical
cuprite patina. Assuming that this sculpture has not been
re-treated in modern times, it represents one of the most
interesting examples of a black-patinated bronze to be
found. Such bronzes usually contain small amounts of gold
or other elements, such as arsenic, antimony, or iron; gold
and silver are the impurities or deliberate additions asso­
ciated with "Corinthian bronzes." Collections of the
Ashmolean Museum, University of Oxford (1986.50) .

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