Chromatic ‘Alterations’ on Stone Surfaces in Contact with Bronze or Copper Monuments: Spectroscopic Characterization and Statistic Analyses

The chromatic alterations visible on surfaces of monuments, induced by coexistence of stone and metallic components, are indicative of damages, which are essentially aesthetic, but also require the cleaning procedures to be carefully studied in order to avoid any further damage to the physicalchemical and structural properties of the surface of monuments themselves. Within a research programme devoted to optimise analytical procedures for the removal of stains on travertine basements of bronze monuments, it has been shown how the chemical state of copper influences the efficiency of cleaning. Not withstanding the clear evidence of the stains, that were also analysed by colorimetric methods, their analytical characterization has turned to be hard, the low concentration of the compounds responsible of the discoloration being at the limits of detection of the most common diagnostic means. Here the investigation on three different typologies of Roman monuments is reported, using a multi-techniques approach: X-ray Diffraction (XRD), X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM) and X-ray Micro Analysis (EDS) combined with PCA-data elaboration, Micro Raman, X-ray Photoelectron Spectroscopy (XPS). The last two resulted the most suitable spectroscopic techniques.