Page 245 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

substitution of more oxidized, protective films. Because these films tend to be less well ordered,
their protective properties can be less effective as a barrier to corrosion than surface patinas that
are more oxidized. Additional bacterial growth may occur under concreted growths; this in turn
may promote further corrosion. f the hydrogen sulfide presence on the copper surfaces is dis­
I
continuous, then pitting of the copper may occur above a potential of 15o mV. General corrosion
rates of copper alloys can increase by up to five times in a hydrogen sulfide concentration of
4 ppm, with a concentration of 50 ppm being not uncommon in these types of deposits, accel­
erating corrosion even further (Florian 1987).
One of the earliest studies of the copper sulfides as a corrosion product was that of Daubree
(1875), who examined the patina of some Roman coins and medals recovered many years ear­
lier from French mineral springs. Daubree was able to identify chalcocite, Cu 2 S; chalcopyrite,
CuFeS 2; bornite, Cu 5 FeS 4 ; tetrahedrite, (Cu,Fe) 12 Sb 4 S 13 ; and covellite, CuS, as present on the
coins, medals, and other small finds. Daubree believed that the sulfide in the thermal springs
originated from the reduction of soluble sulfates by the action of bacteria on vegetable matter.
In 1879 Von Hochstetter, quoted by Daubree (1881), also gave an early report on the occurrence
of covellite as a patina component of a Celtic bronze ax from excavations at Salzburg, Austria.
Examination of several fragments of a corroded gunpowder canister recovered from the
wreck of the Herminie, the flagship of the French West Indies fleet that sank off the coast of
Bermuda in 1838, showed that the copper had altered to a brittle blue-black mass identified by
X-ray diffraction as covellite (Gettens 1964). Lacroix (1910) found covellite mixed with chalco­
cite on copper nails from a late Hellenistic shipwreck off Madia, Tunisia. In a related study,
Lacroix (1909) described a black chalcocite, which was sectile and locally crystalline, on Roman
bronze coins found at a thermal spring in Saone-et-Loire, France. Daubree (1881) noticed a black
variety of chalcocite on copper coins from thermal springs as well, while Palache, Berman, and
Frondel (1951) give the color as blackish lead-gray; obviously there is some color variation pres­
ent. Gettens (i963a) notes that in i96i, Périnet identified chalcocite and digenite from the cor­
rosion crust of a copper nail found in an ancient shipwreck in the Mediterranean sea off Grande
Congloue, France.
MacLeod (1991) found a number of copper and silver sulfides on coins recovered from the
1811 wreck of the Rapid near the coast of Western Australia. The major copper sulfides found
were chalcocite with small amounts of djurleite and digenite. Of interest are the mixed copper-
silver sulfides, jalpaite, Ag 1<55 Cu 0i45 S, and stromeyerite, CuAgS. These mixed copper-silver sul­
fides were found on silver coins that had some copper content. Relatively few analyses of silver
corrosion products are available, and the existence of these minerals is probably more common
than present data would suggest. Many silver objects contain minor alloying additions of cop­
per, so there is every reason to believe that more examples of jalpaite and stromeyerite will be
found by future analytical studies. Typical working conditions for the recovery of these small

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