Page 112 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

T E N O R I T E

Tenorite, sometimes called melaconite in older textbooks, is usually a dull black. The ICDD files
describe the color a metallic gray black, possibly grayish white in reflected light. The mineral
has a Mohs hardness of 3.5. Like cuprite, tenorite is insoluble in water. The type locality for
tenorite and other important copper minerals is Mount Vesuvius near Naples, Italy.
Tenorite crystallizes in the monoclinic system and may occur as a crystalline deposit on the
surface of corroded objects. The difference in the crystal lattice between copper and tenorite,
however, makes it much more difficult to retain pseudomorphic information concerning the
structure of the original artifact.

Tenorite formation Tenorite forms when copper is slowly heated in air. At first, the
copper develops a cuprite film that, as it thickens, exhibits a suc­
cession of interference colors up to the fourth order. As the film continues to grow, small black
spots of tenorite appear, giving the film a sooty appearance. As the thickness increases beyond
the interference color range, the black tenorite layer spreads over the entire surface of the cop­
per. A chip of this patina would reveal that the initial red cuprite layer is retained below the
tenorite (Evans i960).
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The oxidation of most copper compounds in air ill eventually produce tenorite on heat­
ing; the compounds will decompose to cupric oxide between 400 °C and 600 °C. One of the
ways that Egyptians made black eye paints during the Old Kingdom period (2649 - 2575 B.C.E)
was by roasting malachite to obtain tenorite (Partington 1935). 10
Tenorite is a rare component of natural patinas. In most terrestrial, marine, and exposed
environments, the red cuprite layer is the first to form. When tenorite is present as a patina con­
stituent, it usually indicates that the object has been subjected to heating (by fire, conflagration,
etc.) before or during burial. PLATE 17 illustrates a first-century Roman thymiaterion (incense
burner) in the form of a comic actor seated on an altar. X-ray diffraction studies of this object,
which is of bronze with silver inlay, revealed substantial amounts of tenorite in the patina. This
would be expected because heat from the burning incense would have produced the conditions
necessary for tenorite formation.
Pourbaix diagrams suggest that tenorite should be found in many different environments.
In nature, however, the paucity of tenorite occurrences is controlled by kinetic and other factors
that limit the mineral's formation to a few specific conditions—for example, high-temperature
oxidation and high pH.
MacLeod (1991) found only two instances of tenorite formation in more than five thousand
objects examined from Australian shipwreck sites. Tenorite was the primary corrosion product
on a copper nail from the Rapid. The ship had been burned to the waterline, and the nail was
surrounded by charred oak. In the second instance, tenorite was identified on the copper bore
of a composite cannon, which had been fired, from the wreck of the Batavia.

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