## 3 Results and Discussion  
### Evaluation of the State of Conservation: Main Decay Phenomena  
An important aspect has been the technical-scientific support to the restoration interventions, mainly aimed at cleaning, through the diagnosis of the altering processes of the materials of the external cladding of the San Giovanni Bapistery, especially white marble. In Table S1, the main results of state of conservation of external sides of San Giovanni Baptistery (Florence) are reported. Many capitals, cornices, and decorated cornices of the windows and columns, beyond the intensity of the black crusts or deposits of dirt, were often affected by various decay phenomena, not only chemical-physical (e.g., sulfation), but also physical-mechanical (e.g., abrasion, decohesion, granular disintegration, with potential displacement and detachment, and differences in macro and micro porosity) (Figures 3 and 4). Laboratory analyzes of the patina and encrustations provided the main operational indications to the restoration technicians for cleaning: these phenomena are mainly attributable to the photochemical smog and to the use of protective organic products, which, with time, have degraded. The films are often decayed and discontinuous, presenting colors from pale pink to yellow, to ochre or brown. Regarding the identification of the compounds forming the patina, almost identical qualitative results were obtained for all samples analyzed (Figure 5). FT-IR spectra exhibited typical vibration bands of calcium sulphate hydrate, commonly called gypsum (CaSO\({}_{4}\cdot\)2H\({}_{2}\)O), centered at 1109, 669, and 596 cm\({}^{-1}\), as well as the stretching and deformation vibrations of the O-H bond of water at 3525, 3492, 3401 cm\({}^{-1}\), and at 1692 and 1627 cm\({}^{-1}\), respectively. In several analyzed samples, a representative band of silicates was present at 1036 and 1006 cm\({}^{-1}\) (Si-O stretching vibrations). Gypsum originates from the transformation of calcite in the presence of sulphur oxides, while silicate was subsequently embedded in the crust due to the deposition of wind-borne particles. Figure 4: (**a**) An example of a widespread granular disintegration phenomenon (see detail in (**b**)). Figure 3: Scarsella, south corner. (**a**) Angular-shaped human head in marble: loss of molded and black crusts; (**b**) a detail of the black crust (red square); (**c**) a cross section of a micro fragment of a black crust. Most notably, the prominent bands of calcium oxalate (CaC\({}_{2}\)O\({}_{4}\cdot\)nH\({}_{2}\)O) at 1324 (and 780 cm\({}^{-1}\) were present. The features determined in the spectra suggested the presence of the mineralogical phase of calcium oxalate monohydrate, whewelllite (CaC\({}_{2}\)O\({}_{4}\cdot\)H\({}_{2}\)O) (Figure 5). This identification was due to the presence of very specific bands at 3487, 3435, 3341, and 3068 (O-H stretching bands), 1624 and 1324 (C=O stretching vibration), and 780 and 674 cm\({}^{-1}\) (O-C-O bending vibrations). Calcium oxalate films are very stable, less soluble, and hardly affected by atmospheric pollution. Wherever the patinas, sometimes caused problematic removal, are discontinuous, especially near the friezes and decorative modules, the underlying marble appears detached, and a sugar-like crumbling can be observed. In several darker patinas, the presence of organic matter in the FT-IR spectra was highlighted exclusively by the weak bands at 2916 cm\({}^{-1}\) and 2841 cm\({}^{-1}\) (Figure 6a,b), attributed to the stretching asymmetric and symmetric vibrations, respectively, of methyl and methylene groups, the bands in the region (1470-1375 cm\({}^{-1}\)) due to CH\({}_{2}\) bending vibrations, and the methylene group rocking vibrations near 720 cm\({}^{-1}\). Figure 6b shows typical vibrations of the microcrystalline wax used as a protective treatment employed in the previous restorations.