Page 252 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

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Assuming that metallic ions are precipitated from solution, the second mechanism that
could produce sulfides is conversion of the surface oxide to a sulfide, as expressed in the follow­
ing equation:

Cu 2 0 + xH 2S = 2CuS x + Υ 2χΗ 20 6.4

Duncan and Ganiaris (i987) exposed copper alloys with golden and black sulfide patinas to
80% RH, which is 20% higher than the level at which pyritic sulfides can become unstable. Color
changes were observed within seven days; after thirty-five days, all the objects had changed
color. This clearly demonstrates the inherent instability of these sulfide corrosion layers f
i
appropriate conservation measures are not taken during the treatment, storage, or display of
such material.

COPPER S U L F I D E S AND NIELLO

[0]n those that you want to niello, draw Greek foliage and engrave with a bold line; the
grounds of these you engrave with thin circles and fine workmanship. Then make up the niello
in this way. —THEOPHILUS 8

Niello is a black metallic compound of mixed sulfides used to fill in decorative incised designs
on metals, the term niello coming from the Latin niger for "black." The compound was prepared
in antiquity from a mixture of copper and silver sulfides. By the time Theophilus was writing,
in the twelfth century, lead had been added to the recipe as well, producing a mixture of copper,

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