Page 321 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

metal or left them honeycombed like a sponge. The majority of these objects are now kept in
museum storage vaults as embarrassing reminders of excessively interventionist treatment. The
work by Evans foreshadowed many other developments, notably the use of local reduction
methods (Aldaz et al. 1986) and the use of sodium carbonate treatments (Weisser 1987), although
these treatments do not seem to be particularly recommended. 29

Treatment residues and the Schrenk (1991) carried out a detailed examination of so-called
formation of copper salts Benin bronzes from the Republic of Benin in West Africa that
revealed the presence of organometallic copper salts on the sur­
faces caused by ill-advised conservation treatments in the past. These Benin bronzes are, in fact,
made of copper-zinc alloys and should therefore be referred to as Benin brasses. Some of these
objects reached Europe after 1897, when they were appropriated by British forces. Several Benin
brasses now in the collection of the Pitt-Rivers Museum, Oxford, were apparently treated many
decades ago with neat's-foot oil, although there is no evidence for the use of this substance.
FTiR spectroscopy suggests that gum tragacanth, a substance similar to neat's-foot oil, was used
instead (Moffett 1996). These same objects, which are now in the collections of the National
Museum of African Art, Washington, D.C., exhibited a dark olive-green coating with tarry black
deposits in interstices after conservation treatment. The original aesthetic of the Benin art had
been drastically altered by the treatment.
Turquoise-colored corrosion was observed on the surfaces of Benin objects from at least
three other European collections, although there were individual differences, FTIR spectroscopy
revealed maxima that were characteristic of fatty acids. The spectra of the bright turquoise-
colored corrosion matched those of copper fatty acid salts and occasionally zinc fatty acid salts,
and the probable presence of copper oléate and copper stéarate was deduced from X-ray
diffraction studies. Brick-red inclusions within the turquoise-colored samples were shown
by X-ray diffraction to be cuprite, but there is still the possibility that this cuprite may be part
of the original patina that is now detached from the surface by growth of the copper organo­
metallic salts. Moffett (i996) carried out another study on the same cultural material. One Benin
bronze from the National Museum of African Art had developed a white surface haze, and
Moffett reported that it was difficult to remove the Incralac (see CHAPTER 12) and the poly­
ethylene and microcrystalline waxes from the treated surface. Toluene was tried first, but it left
a brittle translucent white residue. Only hot xylene was able to remove this component of the
wax. A yellow material was also removed from the surface, and this was identified by FTIR spec­
troscopy as a copper carboxylic acid, probably a residue from a previous coating removed dur­
ing treatment in 1984.

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