Hyperspectral monitoring of marble in buildings: a case study of the Santa Maria del Fiore (Firenze, Italy) facades

Hyperspectral instruments, discriminating materials on the basis of their different patterns of wavelength-specific absorption, might be employed in the characterization of the components in exposed stone surfaces, and therefore contribute to understand their conservation state. A portable radiometer (ASD-FieldSpec Pro spectroradiometer), which continuously and rapidly acquires punctual reflectance spectra in the 350-2500 nm spectral range, has the potentialities to be used for monitoring the state of conservation of stone surfaces through the evaluation of the relative abundance of some components considered precursor symptoms of decay. Major advantages of this technique include its nondestructive and non invasive nature. In this study, gypsum is considered as a damage symptom in ancient building facades realized in marble. A method to unravel qualitative and semiquantitative information about the degree of alteration of the calcite to gypsum is proposed. This method is based on the spectral decomposition in individual spectral components assigned to calcite and gypsum, during the analysis of spectra of synthetic mixtures of microcrystalline calcite and gypsum. Advantages, perspectives and limits of the method are discussed. Furthermore, a case study is considered. The analysis of 24 areas of the white Carrara marble stones, was carried out on the facades of the Santa Maria del Fiore church (Firenze, Italy), cathedral of Florence belonging to the UNESCO world heritage monuments. The preliminary analysis of the data are presented and discussed.