A systematic study of the corrosion layers on excavated coins from varying historical periods

This research presents the results of a scientific study of archaeological copper and silver
coins from three collections belonging to Roman, Medieval and Modern periods discovered in
the UK. The morphology of the corrosion products was studied using Optical Microscopy
(OM) and Scanning Electron Microscopy (SEM) to understand the formation of corrosion.
Morphologically it was found that the surface corrosion of both metals was similar, a
uniform layer of corrosion was identified with a general attack of the surface. Both copper
and silver coins demonstrated a distinct layering with a metal core followed by an initial
oxide layer and an external layer bearing soil markers and secondary corrosion products
where present. X-ray diffraction, X-ray fluorescence and SEM-EDS were employed to
characterise the corrosion products and determine the basic elemental composition of the
corrosion layers. The study successfully identified the primary corrosion products present on
the copper coins as copper, cuprite and quartz with the inclusion of other compounds such
as kaolinite, cassiterite and malachite across some samples. Silver and Chlorargyrite were
identified as the main components of all silver surfaces in the study with hexagonal silver
oxide present in certain samples. The elemental composition, pH and texture of soil samples
from the areas of excavation in Brough, Nottinghamshire, England were studied alongside
the excavated coins and the direct influence the burial environment had on the corrosion
process discussed. A novel technique in the field of corrosion studies on artefacts, Medium
Energy Ion Scattering, was developed with the intention of attaining information about the
structure and composition of the samples surface; preliminary results could only provide
compositional information already attainable using existing techniques. This study provides
valuable information to both scientists and conservators to maintain and protect
archaeological objects; as well as providing significant information for corrosion science.