μ‐Raman mapping to study calcium oxalate historical films

Abstract

This study is focused on the characterization of the oxalate films observed on the surfaces of several ancient monuments. They consist mostly of calcium oxalate in the common forms, namely whewellite (CaC2O4 • H2O) and weddellite (CaC2O4 • (2 + x) H2O), which chemically differ only by the number of water molecules contained in the crystalline unit cell. In order to ascertain the controversial origin of the calcium oxalate films, the settlement of their mineralogical composition and stratigraphy is essential. Because of the lack of analytical techniques able to discriminate, with an acceptable resolution, between whewellite and weddellite in cross‐sections, in the literature, very little attention has been paid to these data. μ‐Raman mapping allowed for the first time, to unequivocally identify the phases which occur in the calcium oxalate films and to obtain direct information, at high spatial resolution, on their distribution with respect to the material surface. μ‐Raman mapping carried out on polished cross‐sections of samples belonging to the façade of the Cathedral of Trento showed, for a yet unknown reason, either a random distribution of the two phases or a higher content of weddellite in the external portions of the calcium oxalate films. Meaningful differences in the mineralogical composition have been highlighted between areas of the monument sheltered from rainfall and those exposed to rainfall. A multivariate approach using principal components analysis has been used to analyze the huge number of spectra in the Raman maps. Copyright © 2012 John Wiley & Sons, Ltd.