Page 388 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

for forty weeks, during which time the chloride-ion release rates were on the order of 3.7 and
2.5 ppm per hour. The study showed that the extraction of chlorides from these bronzes is
largely diffusion controlled. MacLeod also found that there were notable differences in extrac­
tion rates, depending on the type of alloy involved. The concentration of chloride ions in these
marine objects was greatest in aerobically corroded bronzes, somewhat less in brasses, and the
least for copper. This is due to the selective corrosion of bronze objects with correspondingly
greater absorption of chloride ions below the surface. The same principles apply to brasses,
although these are usually cored, dendritic, single-phase structures, since the zinc content is
I
usually insufficient to produce alpha-beta phase structures. f the brasses had possessed duplex
structures, then the corrosion would have been much greater.
One particular stabilization technique investigated by MacLeod (i987a) to clean marine
finds is the alkaline dithionite method, originally devised by MacLeod and North in 1979 for the
conservation of corroded silver. It is based on a treatment solution of 40 g/1 of sodium hydrox­
ide with 50 g/1 of sodium dithionite, Na 2 S 2 0 4 . The objects to be treated are added quickly to
this solution, and the container is sealed to eliminate atmospheric oxygen to the extent that this
is possible. When placed in alkaline dithionite, bronze objects change color from the blue green
of copper trihydroxychlorides, through a yellow orange of transient copper (I) hydroxide, to
a chocolate brown of finely divided metallic copper; all of this occurs within a few minutes
of immersion. The overall reaction is complex, but MacLeod resolves it into the following step­
wise process:
3Cu 2 (OH) 3 Cl + S 2 0 4 " + OH" = 6[Cu(OH)] + 3C1" + 2S0 4 " + 4 H + 12.3
2
2
6Cu(OH) + S 2 0 4 " = 6Cu + 2S0 4 ~ + 2 H 2 0 + 2 H + 12.4
2
2
3Cu 2 0 + S 2 0 4 " + OH" = 6Cu + 2S0 4 " + H + 12.5
2
2
The use of alkaline dithionite, which is a powerful reducing agent, will necessarily entail
patina alteration, so its use will depend on the aesthetic issues in question for the object.
MacLeod used this reagent to conserve several sets of Greco-Roman coins so badly corroded
that there was no discernible inscription. After treatment, the pustular corrosion was reduced
to a loosely adherent copper powder that could be brushed away, revealing the original inscrip­
tion. After treatment, the coins were washed for up to forty-eight hours in deionized water to
remove any residual chemicals. Monitoring the concentration of chloride, lead, tin, and zinc in
the cleaning solution showed that significant amounts of tin were present; this originated from
the corrosion crust rather than from the alloy itself.
MacLeod found that alkaline dithionite treatment can consolidate heavily corroded
bronzes and may be able to reconsolidate the original surface, although the method must be
i
used with care; some fragile archaeological bronzes may simply disintegrate f immersed in alka­
line dithionite. Fox (1995) successfully employed the method to treat several bronze coins from

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