Abstract
Limestone basements holding bronzes or other copper alloys artefacts such as sculptures, decorations and dedicatory inscriptions are frequently met both in modern and ancient monuments. In outdoor conditions, such a combination implies the corrosion products of the copper based alloy, directly exposed to rainwater, will be drained off and migrate through the porous surfaces, forming stains of different colours and intensities, finally causing the limestone structures to deteriorate.In this work we have analysed samples from two modern limestone monuments in Rome, the Botticino surfaces of the ‘Vittoriano’ (by G.Sacconi, 1885-1911- Piazza Venezia) and the travertine basement of the ‘Statua dello Studente’ (by A.Cataldi, 1920- University city, La Sapienza), and focussed our investigation on the chemical composition of the copper-stained zones using XPS (X-ray Photoelectron Spectroscopy) as a surface-specific technique.Based on observations reporting on the structure and bonding at the calcite surfaces we have identified copper complexes and mixed calcium/copper carbonates associated with the stains, as well as the chemical state of other elements therein included, and related the compositional changes with differences in chromatic characteristics and sampling locations.
Highlights
Outdoor bronzes are subjected to continual corrosion and dissolution processes, as well reported in literature [1,2,3]: associated with these processes are the coloured stains often observed on stone surfaces of outdoor monuments as the results of leaching from the attached bronze artefacts exposed to rainfall
- the major constituent of the coloured stains on both travertine and Botticino limestone is copper – the presence, always in minor quantity, of the other alloys elements in bronze artefacts is dependent on possible differences in their alloy composition and their relative distance and position with respect to the stone surface i. e. zinc was detected by XPS in the basement of the ‘Student statue’ but not in the wall surfaces of the ‘Vittoriano’ (Botticino limestone)
- copper is present in form of mixed compounds with calcium: hydroxysulfates, hydroxycarbonates, etc. , the relative amount of counter-anions and different degrees of hydration are related to the location of the monuments in the city and, within the same monuments, to the different sampling zones
Summary
Outdoor bronzes are subjected to continual corrosion and dissolution processes, as well reported in literature [1,2,3]: associated with these processes are the coloured stains often observed on stone surfaces of outdoor monuments as the results of leaching from the attached bronze artefacts exposed to rainfall. It is common, that the corrosion products, dissolved and washed by rain, can reach portions of the nearby stone surfaces. For the completion of this study, the XPS results as elaborated in this paper will be further rationalized after comparison with results obtained in parallel with Optical Microscopy, X-ray diffraction, Electronic Microscopy combined with Fluorescence analysis (EDXRF) and Micro-Raman spectroscopy [work in progress]
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