Page 354 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

tures could easily form by more than one process or by a mixture of processes that would be
genetically distinct. Keeping these difficulties in mind, a general conclusion can be made based
on recent studies that there are at least two distinct forms of pustular, or "warty" corrosion:
1. pustules of corrosion that are associated with cuprous chloride or with the copper tri­
hydroxychlorides that result from the subsequent transformation of the cuprous chlo­
ride; this cuprous chloride is often at the interface between the patina layer and the
base of the pustule (Tennent and Antonio 98i; Scott 1990); and
i
2. pustules of corrosion with cuprite and malachite but no chloride ion content; these
pustules may be associated with tin-oxide-enriched patinas in which the corrosion
process has proceeded in soils that do not contain significant amounts of chloride
ions; the preservation of metallic components, such as alpha+delta eutectoid phase,
may also be seen.

In the case of the Togati bronze, the corrosion pustules appear to originate by the forma­
tion of cuprite septa, filaments of cuprite that pass through the malachite and cuprite mass of
the pustule. These cuprite septa pass across the tin-oxide patina, allowing the diffusion of cop­
per to occur at particular locations. The tin-oxide layer that develops is probably hydrated, and
at varying times during burial it may be liable to dehydrate to some extent. Cracking of this
layer, which would accompany partial dehydration, could cause further loss of copper as cuprite
septa move through the tin-oxide patina and relay copper ions to the exterior.
Expansion of the corroded matrix of the original bronze metal is shown by the presence
of lead carbonates, which would have created a greatly expanded volume compared with that
of the original lead globules present in the metal, and particularly by the relict fragments of
alpha+delta eutectoid phase toward the outer surface of the pustule. The lead—which is pres­
ent in the alloy as discrete, small globules—is carried outward into the pustule where it becomes
carbonated and converted into cerrusite with considerable expansion in volume. This accounts
for the relatively large size of these lead-rich zones in the corrosion; they can range from about
10 to 35 μπι across. The tin-rich region of the eutectoid is preferentially preserved while the
copper-rich region is converted primarily into cuprite. FIGURE 11.3 is a schematic diagram com­
paring the first type of warty corrosion described earlier with the second type, which is found
in the corrosion pustules on the Togati bronze.
I CONCLUSIONS A hypothesis explaining the origin of the
type of corrosion seen on the Roman bronzes can be formulated based on research by Geilmann
(i956) and on the observed morphological characteristics: the tin-oxide enrichment in the
patina, the presence of mottled areas containing enhanced levels of iron, and the pseudo-
morphosis of structural details. According to this hypothesis, one possible environment for the
corrosion observed on these bronzes is burial in a porous soil that had both oxygen and carbon

S O M E A S P E C T S O F B R O N Z E PATINAS
337