Characterisation of surface layers formed under natural environmental conditions on medieval stained glass and ancient copper alloys using SEM, SIMS and atomic force microscopy

Abstract

Atmospheric corrosion is the result of interactions between a material and the surrounding environment. It involves a number of physical, chemical and electrochemical processes in the interfacial region ranging from the contacting atmosphere over the aqueous adlayer to the material itself. In this contribution surface analytical techniques such as scanning electron microscopy, in combination with energy dispersive X-ray microanalysis (SEM-EDX) and secondary ion mass spectrometry (SIMS) were applied to characterise the corrosion phenomena occurring on medieval stained glass and ancient bronze artefacts. A so-called leached layer has been formed on the glass surfaces due to an ion exchange process, where the potassium and calcium of the glass are replaced by hydrogen bearing species from the moist air. Subsequently, chemical reactions of the leached glass constituents K and Ca with acidifying gases in the ambient atmosphere has led to the formation of a weathering crust. On the surfaces of bronze artefacts a cuprous oxide (mainly) has been built up. Further chemical reactions are leading to crystalline weathering products such as brochantite or malachite depending on the environmental conditions. Additionally, tapping mode atomic force microscopy (TM-AFM) has been applied to study the initial stages of the weathering processes on glass with medieval composition and on pure copper. The task of the present work was not only to develop new analytical strategies and methods, but also to gain additional information of the surface processes involved. Protection against atmospheric corrosion requires such detailed understanding of the role of different corrosion stimulating constituents in the environment, such as humidity, gaseous pollutants and aerosol particulates.