Page 99 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

TABLE 2.1 CHARACTERISTICS OF SOME COPPER OXIDE
AND COPPER HYDROXIDE MINERALS

MINERAL FORMULA CRYSTAL COLOR MOHS
NAME SYSTEM HARDNESS

cuprite Cu 2 0 cubic submetallic red 3.5- 4
tenorite CuO monoclinic metallic gray black 3.5
spertiniite Cu(OH) 2 often amorphous blue green 1-2 ?

C U P R I T E
Properties of cuprite Cuprous oxide typically has a dark red to orange-red color and
a sometimes adamantine luster. f finely crystalline, cuprite may
I
also appear orange yellow. In fact, cuprite may exhibit a range of colors —yellow, orange, red,
or dark brown—depending on its impurities, nonstoichiometry, and particle size (Gmelin 1965).
The mineral has a Mohs hardness of 3.5-4 and is insoluble in water. Cuprite crystallizes in the
cubic system, and cubes or interpenetrating cubes are often seen on the surfaces of corroded
bronzes. Sage (1779) was the first to equate the cuprous oxide corrosion product of ancient cop­
per alloys with the natural mineral cuprite. By the 1820s several workers—Noggerath (1825), for
example—were also aware that the corrosion product on copper and the natural mineral were
one and the same.
When metal reacts with the gaseous environment, metallic oxides are the first interface to
form, developing into a thin film or thick scale over the exposed metal surface, thereby slowing
the rate of further oxidation. These oxide layers are often interference films, in which light is
reflected back from the top and bottom of the film. The oxidation temperature may alter the
plasticity of oxide films. 2
During the growth of copper oxide films, oxygen atoms migrate inward and metal atoms
outward, with the mode of migration dependent on the defect structure of the oxide lattice. A
cuprite film contains slightly less copper than the formula Cu 2 0 suggests. The oxygen lattice
is stoichiometric, but there are a small number of vacant sites in the copper lattice. To main­
tain the electrical neutrality of the oxide as a whole, some copper (II) ions may be present in
the Cu 2 0 lattice. Cuprite is, therefore, a defective oxide structure; the missing electrons make
cuprite a p-type semiconductor, since each missing electron is equivalent to a positive hole in
the electron band structure. This structure is important for cuprite's role in the corrosion of COP-

C H A P T E R T W O
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