Page 57 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I MOISTURE, FIRE, AND ACIDITY Swedish researchers recently
carried out a detailed statistical study that examined aspects of bronze corrosion and the
burial environment for artifacts from the Bronze Age, the Viking period, and the early
Middle Ages (Mattsson et al. 199θ). The study, which used 170 parameters for the statistical
analysis, is one of the most detailed investigations of its kind to be published to date. The
researchers found that four important factors that correlated with accelerated corrosion of the
artifacts were the presence of chloride ions, soil moisture, phosphates, and carbonized material
in the burial environment.
The chloride content of the soil at Birka, Sweden, was found to promote corrosion of the
artifacts at that burial site. This soil had a pH of about 8. In contrast, bronzes from the burial
sites of Fresta, Valsta, or Sollentuna, where the soil pH ranged from 4 to 5, were in better con­
dition, although localized pitting corrosion had occurred on the Fresta material. The Swedish
researchers suggest that the corrosion products may not be very stable at pH 8, although it is not
immediately obvious why this should be the case. It is possible that a solid chloride-containing
crust may form at the lower pH values and so impede further corrosive attack. The severe local­
ized pitting of the bronzes from Fresta might be due to chlorides from salt used on the modern
road that crosses this cemetery site.
It is interesting to note that basic copper sulfates, mixed iron-copper sulfides, and copper
phosphates were found in many places on this recently excavated material. The copper sulfates
are not necessarily recent phenomena, however. This is shown by analyses that identified basic
copper sulfate on some of the 182 bronzes excavated between i87i and 1879 from the Birka site.
Many of these finds were made of brass rather than bronze, and most of the finds studied were
of Viking age, dating from about the tenth to the thirteenth century. 10
Soil moisture was shown in the Swedish statistical work to be a significant influence on
copper deterioration in burial environments. This corrosion is promoted in artifacts by deep
burial (but still above the water table); by burial at low height above sea level for coastal mate­
rial; by small pore size in the surrounding soil; and by burial in a barrow (burial mound). The
state of preservation of Bronze Age or Viking age materials in barrow burials was evaluated by
comparing the recently excavated materials with previously excavated objects kept in museum
storage. From basic corrosion principles it is known that the corrosivity of the soil is great­
est where the soil pores are partially filled with water, allowing exposure to both .oxygen and
groundwater in the burial environment.
Bronzes excavated from 1993 to 1994 revealed that a burned burial context, in which arti­
facts were associated with soot or cremated remains, was also deleterious. Similarly, artifacts
found near paths between houses, in areas where wastes of various kinds may have been depos­
ited, were more corroded than those bronzes buried inside or contiguous with house structures.

C H A P T E R O N E
40