Page 41 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

confirmed by metallographic examination of a cross section through the copper, shown in
PLATE . Beneath the portrait head, the restorer had penned additional information for this con­
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ceit: "A. P. Ready Restorer, British Museum 1930."
Galvanic deposits of copper usually show columnar or twinned crystal structures, often
completely unlike anything that can be produced by either the casting or the hammering and
annealing of pure copper. Growth features in the deposit may be evident, and in these cases
there is usually a columnar aspect to the microstructure. The structures are often twinned
because, for some rather mysterious reason, this must represent the system of lowest energy
requirement for the transition of dissolved copper species to metallic deposition as a solid,
assuming that columnar growth is not dominant. Very often, copper that is redeposited from
solution during the corrosion of bronzes is also twinned. However, the twin lines in some
electrotypes may occur as markings within columnar structures that are quite unique to elec­
tro typed copper; some of these details can be seen in the photomicrograph of the modern relief
protome (see PLATE 4).
Another useful approach to the identification of electrotypes is that of Laue back-reflection
X-ray diffraction. Wharton (1984) was able to show that many electroformed reproductions
could be identified by this technique. Since the inception of Renaissance medals —introduced
in 1438 by the Italian painter Antonio Pisano, known as Pisanello (1395- ca. 1455) —various cast
or electrotyped copies have been produced from struck originals; Wharton's research sought
a nondestructive method to distinguish between them. In the Laue back-reflection X-ray dif­
fraction technique, a flat-plate camera is used with a beam of X rays in the back-reflection mode.
The number of spots that appear on the film then roughly correspond to the number of grains
that have diffracted the X rays. In Wharton's study, medals were placed 3 cm from the film in
a Laue diffraction cassette using an exposure of 35 KV, 15 mA for fifteen minutes. Since the
divided, columnar-type structure of most electrotypes comprises many discrete grains, the cor­
responding Laue pattern shows a much finer array of small spots than the struck or cast equiva­
lent. Obverse and reverse faces of the electrotypes do not necessarily show the small degree of
grain refinement, and a set of controls of cast and struck forms is an essential part of any detailed
investigation. The technique is an old one, and is well-known in the X-ray diffraction commu­
nity, but there appears to have been little application of the method in the conservation field fol­
lowing Wharton's seminal paper.
The microcrystalline nature of electrotyped copper is sensitive to a plethora of variables,
ranging from current density, presence of dirt or particulate matter in the solution, distance
from anode to cathode, temperature, degree of agitation, and the presence or absence of certain
additives; any of these could have a profound effect on the micromorphology of the deposited
copper. For example, it has been found over many years of empirical observation that com-
plexing reagents such as thiourea have a very beneficial effect on the grain refinement and com­
paction of the resulting deposit, ensuring a better quality reproduction of surface detail. The

C H A P T E R O N E
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