Page 244 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

TABLE 6.1 CHARACTERISTICS OF SOME COPPER SULFIDE MINERALS

MINERAL FORMULA CRYSTAL COLOR MOHS
NAME SYSTEM HARDNESS

anilite Cu 7 S 4 orthorhombic metallic bluish gray 3
chalcocite, low Cu 2S hexagonal metallic blackish gray 2.5-3
chalcocite, high Cu 2S tetragonal metallic black gray ρ
chalcocite Cu 2S hexagonal metallic blackish gray 2.5-3
covellite CuS hexagonal submetallic blue 1.5-2
digenite, low Cu 1>8S rhombohedral blue/black 2.5-3
djurleite monoclinic metallic gray 2.5-3
geerite Cu 1>6S cubic metallic bluish white ρ
roxbyite Cu 1>78 S monoclinic metallic blue gray 2.5
spionkopite Cu 1>32 S rhombohedral metallic blue gray 2.5
yarrowite Cu 1>2S rhombohedral metallic bluish gray 2.5

C O R R O S I O N E N V I R O N M E N T S AND COPPER S U L F I D E P R O D U C T I O N
Sulfide formation in On objects buried in seawater sediments, removed from oxy-
reducing environments genated conditions, copper sulfides are a common corrosion
product. In this environment, the whole range of compositions
from covellite, CuS, to chalcocite, Cu 2S, is represented.
In burial conditions where reduction of sulfate by bacteria is possible, sulfide ions are pro­
duced essentially by the reaction

2S0 4 " = S° + S 2 + + 4 0 2 6.1
2
The bacteria utilize the oxygen for oxidative enzymatic activity, and one of the end products in
seawater or sediments is hydrogen sulfide,

2 H + + S " = H 2 S 6.2
2
which can form insoluble complexes with trace elements in oxygen-deficient sediments.
Removal of hydrogen from the cathodic areas results in depolarization of the cathode,
allowing corrosion reactions to occur in the absence of oxygen. Cathodic areas may occur
on bronze objects, but they may also represent contiguous materials that are functioning as
cathodic regions. Hydrogen sulfide liberation accelerates the corrosion of copper alloys by

C O P P E R S U L F I D E S
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