On the degradation mechanisms under the influence of pedological factors through the study of archeological bronze patina
Creators: name: Quaranta, Marta <1980>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2009
Subjects: CHIM/12 Chimica dell'ambiente e dei beni culturali

description: This experimental thesis concerns the study of the long-term behaviour of ancient bronzes
recently excavated from burial conditions. The scientific interest is to clarify the effect of soil
parameters on the degradation mechanisms of ancient bronze alloy. The work took into
consideration bronzes recovered from the archaeological sites in the region of Dobrudja,
Romania.
The first part of research work was dedicated to the characterization of bronze artefacts using
non destructive (micro-FTIR, reflectance mode) and micro-destructive (based on sampling
and analysis of a stratigraphical section by OM and SEM-EDX) methods. Burial soils were
geologically classified and analyzed by chemical methods (pH, conductivity, anions content).
Most of objects analyzed showed a coarse and inhomogeneous corroded structure, often made
up of several corrosion layers. This has been explained by the silt nature of soils, which
contain low amount of clay and are, therefore, quite accessible to water and air. The main
cause of a high dissolution rate of bronze alloys is the alternate water saturation and
instauration of the soil, for example on a seasonal scale. Moreover, due to the vicinity of the
Black Sea, the detrimental effect of chlorine has been evidenced for few objects, which were
affected by the bronze disease.
A general classification of corrosion layers was achieved by comparing values of the ratio
Cu/Sn in the alloy and in the patina. Decuprification is a general trend, and enrichment of
copper within the corrosion layers, due to the formation of thick layers of cuprite (Cu2O), is
pointed out as well. Uncommon corrosion products and degradation patterns were presented
as well, and they are probably due to peculiar local conditions taking place during the burial
time, such as anaerobic conditions or fluctuating environmental conditions.
In order to acquire a better insight into the corrosion mechanisms, the second part of the thesis
has regarded simulation experiments, which were conducted on commercial Cu-Sn alloys,
whose composition resembles those of ancient artefacts one. Electrochemical measurements
were conducted in natural electrolytes, such as solutions extracted from natural soil (sampled
at the archaeological sites) and seawater. Cyclic potentiodynamic experiments allowed
appreciating the mechanism of corrosion in both cases. Soil extract’s electrolyte has been
evaluated being a non aggressive medium, while artificial solution prepared by increasing the
concentration of anions caused the pitting corrosion of the alloy, which is demonstrated by
optical observations.
In particular, electrochemical impedance spectroscopy allows assessing qualitatively the
nature of corroded structures formed in soil and seawater. A double-structured layer is
proposed, which differ, in the two cases, for the nature of the internal passive layer, which
result defectiveness and porous in case of seawater.