## 3 Results and Discussion
### Green Areas  
ToF-SIMS results from the green areas detected the presence of sodium, magnesium, aluminum, silicon, potassium, calcium ions (Ca\({}^{+}\) and CaOH\({}^{+}\)), silver (\(m\)/\(z\) 107), lead, and lead-oxygen containing ions (Pb\({}^{+}\) and PbOH\({}^{+}\)). All these ions are characteristics of the surrounding gold varnish, silver leaf, and leather, as mentioned previously. Figure 8: ATR-FTIR spectra of the blue areas in the range 3500 \(\div\) 500 cm\({}^{-1}\) (**a**) and in the range 1600 \(\div\) 200 cm\({}^{-1}\) (**b**). In addition to spectral features common with gold varnish, characteristic peaks and shoulders (1079, 1044, 690, 680, 671, 658, 616, and 596 cm\({}^{-1}\)) attribute the blue pigment as indigo on a lead carbonate substrate, which is coupled with the presence of sulphates. Figure 7: Part of negative ion mass spectra extracted from the blue area and showing the 7-oxo-dehydroabietic acid (\(m\)/\(z\) 313) of the colophony and its fragments ions (\(m\)/\(z\) 259 and \(m\)/\(z\) 278). Furthermore, the detection and localization of Cu\({}^{+}\) (_m/z_ 63) (Figure 9) in the green areas suggest the use of a copper-based green pigment. The signal distribution appears strongly influenced by the surface roughness of the sample. Thus, the mapping of the copper ion signal was normalized to the total ion image in order to minimize topological effects. The presence of copper is further confirmed by the MA-XRF elemental maps where it is possible to observe that this element is present as a major component only in the green areas (Figure 10). On the other hand, the negative ion maps of the green area (Figure 11) show the presence of two fragments (CHO2\({}^{-}\), _m/z_ 45, and CuOH\({}^{-}\), _m/z_ 80) that could be ascribed to the presence of verdigris or copper resinate, which are two typical green pigments largely used in oil paint [1; 3]. Figure 10: MA-XRF elemental maps of Cu obtained from leather samples 1 and 2 (zones 1, 2a, and 2b from left to right). Figure 9: ToF-SIMS ion images are recorded in the positive ion mode in the green area. (**a**) Macro camera view of the area where ToF-SIMS images were acquired (area of 500 \(\upmu\)m \(\times\) 500 \(\upmu\)m), (**b**) total ion image, and (**c**) ion image of the copper ion (Cu\({}^{+}\), _m/z_ 63). Besides CN\({}^{-}\), P\({}^{-}\), CNO\({}^{-}\), PO\({}^{-}\), PO\({}_{2}\)\({}^{-}\), and PO\({}_{3}\)\({}^{-}\) ions in the negative ion mass spectra were detected, which suggests the use of proteinaceous compounds. In the same area, especially in the regions where the silver surface is more exposed, Cl\({}^{-}\), S\({}^{-}\), and silver chlorides (_m_/_z_ 321 and 463) were found. ATR-FTIR spectra for green areas are shown in Figure 12. Spectral features common to gold varnish and blue areas can be found, but the main peak is centered at 1609 cm\({}^{-1}\) (v[COO] asymmetric) instead of 1629 cm\({}^{-1}\) and the peak at 1316 cm\({}^{-1}\) is much more intense. Characteristic peaks are at 1362 cm\({}^{-1}\) (v[C-O] from oxalate and \(\delta\)[CH\({}_{3}\)]), 821 cm\({}^{-1}\) (\(\delta\)[O-C-O]), 494 cm\({}^{-1}\) (v[Cu-O]), and 376 cm\({}^{-1}\) (v[Cu-O]) [35; 36; 37]. All these features suggest that green is a mixture of verdigris (copper acetate, as pigment) and oils (as a medium). The sharp peaks at 1362 and 821 cm\({}^{-1}\) could be related to the presence of oxalate salts [38], which are not present in the other colored areas. Oxalate ions have been proposed as the resulting product of the degradation of the binding matrix (oily painting medium) [39; 40], especially in the presence of lead-based and copper-based pigments [41]. The strong correlation between the presence of copper-based pigments and formation of oxalate salts has also been reported in the literature [40]. This gave reason for the selective presence of oxalate signals only in some of the areas analyzed. Figure 11: TOF-SIMS ion images recorded in the negative ion mode in the green area.