Page 59 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I BUFFERING GAPACITY OF SOILS Soils vary greatly in their
buffering capacity, and there is a range of factors responsible for this, from carbonate buffers at
pH > 6.2 to ferrihydrite buffers at pH < 3.2. The most poorly buffered soils are those origi­
nating from the weathering of siliceous bedrock, such as quartzites, feldspathic gneisses, and
granites; these soils have a low capacity to replenish base cations lost in acid neutralization reac­
tions. They also have a low ability to retain sulfate, which explains why Scandinavia was the first
European area to suffer widespread acidification of lakes—its geology is dominated by gneiss,
schist, and quartzite.
Central European soils have been more resistant to acidification, but with sulfur-deposition
I
rates up to ten times greater than in the past, their buffering capacity is in jeopardy. f the agri­
cultural use of the land ceases, the addition of lime may also cease, reducing the pH level from
6 to 5 in clay soils, or to about 4-4.5 in sandy soils. At the same time, draining wetlands may
cause dramatic acidification of soils because of the quantities of sulfur compounds that would
be released and the resulting oxidation of sulfides.
Some soil components are still not well understood. One of these is soil humus, which
influences buffering capacity, metal-binding capacity, and water retention, as well as the absorp­
tion of hydrophobic organic compounds. The study by Geilmann (i956) is a classic account of
the influence of humic acid on bronze corrosion (see CHAPTER 11). The complex structure of
humus has only recendy been elucidated. According to Wershaw (1992), it consists of ordered
aggregates of amphiphiles, which are molecules with separate polar and nonpolar parts com­
posed mainly of relatively unaltered plant polymer segments attached to carboxylic acid groups.
These react with soil mineral grains to form membranelike coatings with highly charged exte­
rior surfaces that act as separate ion-exchange phases.
I PROBLEMS FOR CONSERVATION RESEARCH The difficulties of
either inferences based on soil conditions or on deductions derived from the examination of
objects is well illustrated by the work of Chase and Quanyu (1997), who studied a suite of exca­
vated bronze fragments from the site of Tienma-Qu in China (Western Zhou dynasty, 1000-
650 B.C.E.). Some of these bronzes were totally corroded while others were quite well preserved.
The authors were unable to assign these differences to particular tombs or environments by
examination of the soil samples. This is not an isolated case. Highly detailed statistical studies
along the lines of the Swedish study would be necessary to accurately characterize and correlate
soil corrosivity with artifact condition.
The work by Wagner and coworkers (1997), funded by the European Environment Pro­
gramme of the Directorate General XII for Science, Research and Development under the gen­
eral aegis of research for the "Protection and Conservation of the European Cultural Heritage,"
is currently at an early stage of development. It will be interesting to see what develops from this
collaborative project and whether the research will augment the findings of the Swedish study
coordinated by Mattsson.

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