Page 256 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

and seventh centuries. On objects with gilt-silver surfaces, the niello was probably applied first,
since it had to be heated to at least 600 °C to fuse it into position, whereas application of the gild­
ing amalgam took place at temperatures no greater than 350 °C.
A gold buckle recovered from the Anglo-Saxon ship burial at Sutton Hoo was decorated
with a monometallic silver sulfide niello, while a mixed silver-copper sulfide was used on a
hanging bowl from the same site. The mixed silver-copper sulfide was identified as stromeyer­
ite. The ICDD files assign the formula Ag 0 Cu 1 0 7 S to this compound, but there is some latitude
9 3
in the solid-solution composition between copper and silver, which probably results in fluctua­
tions in composition and, therefore, in slightly varying d-spacings. Stromeyerite was also found
on an Irish bell shrine from the eleventh century and on a crozier from Inisfallen, Ireland,
which has a second, different inlay of niello consisting of acanthite. Only one object examined
from the British Museum collections, a thirteenth-century Hugo crozier, was decorated with
niello that contained lead. This niello is a mixture of stromeyerite with galena, PbS.
A problem with the X-ray diffraction data for niello is that one cannot be certain whether
some of the minerals identified are the original components of the niello, the products of chemi­
cal alteration during burial, and /or the possible result of inappropriate conservation treatment.
For example, chlorargyrite (silver chloride), AgCl, was identified as a surface alteration product
on the silver sulfide niello applied to one of the Sutton Hoo finds. Chlorargyrite is not unex­
pected as a corrosion product derived from the original silver sulfide, since silver sulfide layers
are easily disrupted during corrosion by chloride (or bromide) ions and converted into silver
chloride or silver bromide, both of which are quite insoluble and relatively stable. These layers
may be partially decomposed by light, producing localized alteration to metallic silver, although
they are not washed away and so remain in situ. For this reason, care must be taken to ensure
that any sample examined is representative of the niello as a whole and not just a surface scrap­
ing, which may provide ambiguous or erroneous information.

Notes
1 Chakrapani Dutta Chakradatta (Ray 1956:110). king. Discovered in 1939, this Germanic burial
2 Pliny the Elder Natural History 34.22 (Pliny 1979). site was one of the richest found in Europe, as it
3 Pliny 34.23. contained a ship fully equipped for the afterlife
4 Ibid. (but with no body).
5 Geber is the pen name of an unknown author of 7 Nigel J. Seeley, e-mail message to the author,
several books that were among the most influen­ 8 April 1997.
tial works on alchemy and metallurgy during the 8 Presbyter Theophilus De diversis artibus 1.35.27
fourteenth and fifteenth centuries. (Theophilus 19 i).
6
6 Sutton Hoo estate at Woodbridge, near the Deben 9 Pliny 33.46.
River, Suffolk, England, is the site of a seventh- 10 Theophilus 1.35.28.
century grave or cenotaph of an Anglo-Saxon 11 Theophilus 1.35.29.

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