Page 387 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

involved repairs that would today be carried out much less intrusively the bronze patina
retained the shape of the original cast figure quite well, as seen in PLATE 83.
The Riace bronzes (see CHAPTER 11), recovered from sea burial with a dark sulfidic patina,
were conserved after careful mechanical cleaning by immersion in deionized water until the
chloride ion release (from the iron and the bronze components) abated after sixty days. This was
followed by an unconventional conservation treatment called the "B70 method" (Formigli 1991)
for which a covered tank was made to the specific measurements of the bronze being treated. A
controlled relative humidity chamber was also built to subject the treated bronzes to higher lev­
els of RH, allowing an assessment of stability after treatment. One of the Riace bronzes was then
placed in the tank and covered with a solution of ammonia in methanol. After two hours in this
solution, it was immersed for another two hours in a second bath of hydrogen peroxide in meth­
anol. Following this treatment, the bronze was placed in a humidity chamber; after twenty
hours, a bloom of paratacamite had formed on parts of the statue.
This conservation procedure appears inadvisable in view of the fact that the casting core
had not yet been removed and remained salt-laden, which surely counteracts the purpose of the
humidity treatment. The core was subsequently removed after cutting away lead fills on the base
of the feet. Treatment with ammonia in methanol and hydrogen peroxide in methanol was then
employed on the inside of the bronze by temporarily plugging the opening in the feet. After dry­
ing , the humidity-chamber test was again applied. Eventually, after further air-abrasive clean­
ing and RH testing, the bronze was locally treated with benzotriazole in ethanol. Only three
areas were lacquered, and the cleaned surface was left otherwise uncoated. This particular strat­
egy remains questionable, however, since electrochemical effects may result in anode and cath­
ode areas becoming active on the untreated surface.

Stabilization problems MacLeod (i98i, 1982, 1987b) has addressed some of the diffi­
and techniques culties of stabilizing marine copper alloys, based on experience
dealing with the conservation of over twelve thousand copper-
based artifacts. Most of the objects studied were from ten shipwreck sites off the West Australian
coast. The ages of the sites range from the wreck of the Batavia in I629 to that of the Macedón in
1883. After excavation and treatment with a variety of conservation methods over several weeks,
most of the objects, which had been stored at 20 °C and 55% RH, still showed signs of instabil­
ity. This indicated that a few weeks of treatment were insufficient to stabilize heavily corroded
copper alloys, and extended washing regimes were then employed to extract as much chloride
ion as possible from the objects.
The treatments were based on solutions made with distilled water or sodium sesquicarbon-
ate, since these were the least damaging to the patina. Some pieces of bronze were removed from
the Batavia and used as test strips. These were first cleaned in 10% citric acid inhibited with 2%
thiourea for two weeks. The strips were then treated in a 2% sodium sesquicarbonate solution
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