Page 56 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

This represents only one possibility. Another is that the initial form of the bronze object
is maintained by the corrosion of copper to cuprite, which remains as a pseudomorphic
remnant of the original copper or bronze surface. Tin compounds, such as tin (IV) oxide or
hydrated tin oxides, may be incorporated into this layer, representing an important part of
the patina. In cases where very extensive corrosion of the copper has occurred, all that may
be left of the bronze is a fragile relict of tin oxides that may look like a piece of bone. The shape
of the object is, however, remarkably preserved in many cases of this type of advanced corrosive
deterioration.
The circumstances of burial and corrosion of bronze objects, as opposed to unalloyed cop­
per, present an extraordinary panoply of events: copper alloys may be completely mineralized;
they may be almost uncorroded; they may be bloated with an extensive corrosion crust incor­
porating soil minerals; or they may be covered with a delicate patina that requires very little
cleaning. Even the least corrosive soil category in the BNFMRA study cited earlier may be con­
verted into a potent corrosive agent in archaeological contexts due to the disturbance of the soil
and the resulting alteration of the natural environment that may result. Association of copper
objects with a human cremation or burial site may result in increased corrosion of the copper
because of the leaching of numerous substances from the human remains into the surrounding
soil. On the other hand, bronzes may show very good preservation f the burial is in a relatively
i
dry environment, several meters below present-day ground level, where oxygen or bacterial
activity is absent. Disturbance of the stratigraphy of a site may result in some objects from the
same level being very well preserved while neighboring objects only a meter away may be in ter­
rible condition. Only a very detailed analysis of the finds from a particular site and of the site
environment can, hopefully, provide an answer to the question of differential preservation.
I THE HUMAN ELEMENT Human habitation increases soil levels
of phosphate and, often, calcium and iron, although the interpretation of elevated levels of phos­
phorus may not be straightforward. Some phosphorus sources, such as bone, are inorganic;
other sources, such as human excrement, animal dung, or food waste are principally organic.
The interesting phenomenon of "soil silhouette" 9 at a site in the United Kingdom is described
by Keeley, Hudson, and Evans (1977), who showed that levels of iron and manganese were
present in the altered soil at levels well exceeding those originally present in the interred
body. One possible explanation is that iron and manganese are accumulated by microbial activ­
ity. In another study, Lambert (1997) showed that soil adjacent to well-preserved skeletons
from the Middle Woodland period in west-central Illinois was depleted in iron and alumi­
num, while the skeletons had gained significant amounts of these elements; calcium was also
enriched in the soil and depleted in the bone. All of these chemical alterations will affect the cor­
rosion of buried bronzes associated with a habitation site; in some cases, soil disturbance will
encourage the formation of local electrochemical cells, resulting in greater corrosion than that
produced in virgin soil.

C O R R O S I O N AN D E N V I R O N M E N T
39