Page 341 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Richter also makes the observation that bronzes from the same locality are usually covered
with the same kind of patina. Although this is not always the case, it is a useful general prin­
ciple. Some bronzes in the British Museum, for example, may be distinguished by the color of
their patinas: some Roman bronzes from Campania have a bright apple-green color; the Etrus­
can bronzes from the Lake of Falterona are covered with a brownish green patina; bronzes from
the site of Falerii, Italy, are apt to show a smooth turquoise blue; the Boscoreale bronzes (from
Bosco, Italy) have a rough green patina with dark blue patches; and the bronzes of Dodona are
almost invariably distinguished by a patina of great beauty and finish.
A freshly polished bronze or even brass surface might have been used to counterfeit gold,
which was always so much more expensive and unattainable. In fact, the aesthetics of a polished
surface may have applied to the majority of ancient bronzes, but variations in surface finish had
not been identified by 1915, when Richter was writing, and her assertions in this regard have
since been modified. For example, recent scientific studies have verified the existence of bronzes
whose surfaces were deliberately given a black patina. Pliny refers to them as Corinthian
bronzes, recalling the account by Plutarch (see CHAPTER 2, on cuprite patinas), who recorded
that when Corinth was destroyed by the Roman army under Lucias Mummius in 146 B.C.E. and
many metal objects were subjected to fire, the melting of copper with some gold and silver pro­
duced an alloy whose oxidized patina was an attractive blue black. The existence of black-
patinated Egyptian bronzes, known as hsmn-km, from the first millenium B.C.E. indicates that
this kind of patina originated well before the destruction of Corinth (Cooney 1966) and reveals
that Pliny's account is apocryphal.
Penny (1993) suggests that in color and finish these black-surfaced bronzes would have
resembled polished hard stones, such as basanite and basalt, that were used so extensively for
ancient Egyptian and Greek sculpture. Craddock and Giumlia-Mair (1993), who analyzed black-
surfaced bronzes in the collections of the British Museum, showed that some of these bronzes
have a composition of about 94% copper, 4% gold, 1% arsenic, and 1% silver. This is very similar
to the later Roman Corinthian bronzes, which contain a characteristic small percentage of gold.
Plutarch claims that Silanion of Athens, working around 325 B.C.E., used a small amount of sil­
ver in the alloy for the head of his sculpture of the dying Jocasta to impart a pallid appearance.
This may be so, since silver-copper alloys were well known at the time. The silver-copper alloys
generally contained only 1-5% copper to harden the silver, but the progressive dilution of
the rose-pink color of copper with a small percentage of silver would also have been known;
alloys with 10% silver are already noticeably lighter in color. The microstructure of an alloy with
this kind of composition would consist of an alpha-phase, copper-rich dendritic matrix with an
infill of the alpha+beta eutectic. Since these alloys are so heavily segregated, however, the
eutectic may also be represented by silver-rich beta-phase particles in a cored dendritic matrix
(Scott 1991).

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