Page 58 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The Swedish study also found a statistical correlation between the degree of corrosion and
the presence of phosphates in the soil. This finding may seem surprising at first, since phos­
phates are often used as passivating agents and corrosion inhibitors in modern treatments. In
this case, however, the relative molar volumes of the phosphates were quite high, and the cop­
per phosphate minerals tended to form a noncoherent corrosion crust. This situation may exac­
erbate corrosion processes because of uneven exposure of the corroding metal surface and
possible effects due to differential oxygen concentrations and galvanic reactions.
The study showed that the bronzes from the Malaren region, however, had a lower degree
of deterioration compared with bronzes from the west coast of Sweden, where the acidifica­
tion of the soil is more extensive. Statistical analysis showed that recently excavated bronzes
from the Malaren region, however, were in a worse state of preservation than the same type of
bronzes from the same site excavated from i87i to 1879 and kept in museum collections. What
appears to be accelerated corrosion over the last hundred years is ascribed to the increased acid­
ity of the soil due to acid rain and pollution.
This is a disturbing conclusion. f artifacts are going to deteriorate at the accelerated rates
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found in the Swedish study, at least for regions of the Swedish environment, the finding implies
that reburial of extant material may be deleterious compared with indoor storage and, further,
that excavation may actually be preferable to leaving buried bronzes in situ.
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The association between increased soil acidity and increased corrosion is echoed n a
supplemental report by Fjaestad, Nord, and Tronner (1997). The researchers reported a signifi­
cant statistical correlation between advanced deterioration of buried bronzes and elevated
concentrations of sulfates and phosphates in Swedish soils. A similar observation regarding
recent accelerated deterioration of bronzes was made by Scharff and Huesmann (1997), who
concluded that many freshly excavated German bronzes were more deteriorated than similar
material excavated fifty to a hundred years earlier. A large number of coins of Celtic auxiliaries
from the Roman camp of Delbrück-Anreppen, recently excavated in Westphalia, were in worse
condition than the same material excavated in 1968. A possible cause is increased soil acidity in
later years. Since 1970, liquid animal waste with an ammoniacal content was used as manure in
the area. Apart from acid rain, this could account for the increased acid levels in the soil, which
is sandy with a very limited buffering capacity. Although lime has been added to the fields in
Anreppen in recent years, the soil pH measured 4.2-4.8 in 1997. Indeed, Wagner and colleagues
(1997) alarmingly conclude that the acid buffering capacity of many European soils has either
been exceeded or will be within a few decades. Although it is impractical to suggest that it
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would be better to excavate archaeological sites in such regions than to leave them alone, there
now may be a greater imperative for monitoring these sites and, in particular cases, selecting
some for excavation.

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