## 3 Results and Discussion
### Blue Areas  
In the blue areas of sample 2, several inorganic ions were detected: sodium, potassium, calcium, and calcium-oxygen containing ions (Figure S5). In addition, lead ion, a cluster of lead oxides, and lead hydroxides were found (Peaks and spectra are shown in Figure S6). The positive ion ToF-SIMS spectrum (Figure 5) of the blue area in sample 2 shows peaks related to the indigotin protonated molecular ion ([M + H\({}^{+}\)]) at _n_/\(\geq\) 263 (C\({}_{16}\)H\({}_{11}\)N\({}_{2}\)O\({}^{+}\)) and to its characteristic fragmentation at _m_/\(\geq\) 247 (C\({}_{16}\)H\({}_{11}\)N\({}_{2}\)O\({}^{+}\)). These two ion peaks indicate the use of indigo as a pigment for the blue areas. Moreover, protonated linoleic (C\({}_{18}\)H\({}_{33}\)O\({}^{2+}\)), stearic (C\({}_{18}\)H\({}_{37}\)O\({}_{2}\)\({}^{+}\)), and lignoercic (C\({}_{24}\)H\({}_{49}\)O\({}^{2+}\)) acids were observed (Figure 5). The diacylglycerols of stearic and palmitic acids, respectively, at _m_/\(\geq\) 523 and _m_/\(\geq\) 551, were also found (data not shown). The fragmentation of these molecules usually generates the signals of the corresponding fatty acids. In this case, no signals related to the palmitic acid were revealed. In addition, in this area, the presence of lead soaps of linoleic acid at _m_/\(\geq\) 489 was revealed (data not shown). Figure 4: ATR-FTIR spectra of the gold varnish in the range 3500 \(\div\) 500 cm\({}^{-1}\) (**a**) and in the range, 1600 + 200 cm\({}^{-1}\) (**b**), showing spectral features compatible with a mixture of proteins and oils. Comparison between the gold varnish from sample 1 and 3 with colophony reference spectrum (**c**), confirming its presence in sample 3. The lead soaps formation is a topic of major importance in conservation and restoration interventions since, among others, it is indicated as the cause of the increased transparency of the paint layers [28; 29; 30]. Furthermore, the exposure to environmental conditions can be responsible for this phenomenon since it can lead to the formation of superficial crusts due to the remineralization of compounds through the reactions with the environment itself [30; 31]. On the other hand, in the negative mode, several clusters of lead oxides and lead hydroxides were detected (peaks and spectra are shown in Figure S7). All these ions suggest the presence of lead white as a substrate of indigo. Additionally, when observing the MA-XRF elemental maps of lead, it is possible to appreciate a higher concentration of lead in the blue pigment area (Figure 6). The results obtained indicate that the indigo pigment was used in the oil binder and it was mixed with lead white. Lastly, the negative ion mass spectra (Figure 7) shows peaks at _m_/_z_ 259, _m_/_z_ 278, and _m_/_z_ 313. The signal at _m_/_z_ 313 could be attributable to the 7-oxo-dehydroabietic acid of the colophony and the peaks at _m_/_z_ 259 and _m_/_z_ 278 to its fragmentation process [32]. The presence of colophony as resin found in the leather fragments was detected by ATR-FTIR analysis in the other samples. Figure 5: Part of the positive ion mass spectra extracted from the blue area and showing indigotin protonated ion (C\({}_{16}\)H\({}_{11}\)N\({}_{2}\)O\({}_{2}\)\({}^{+}\), _m_/_z_ 263), indigotin fragment ion (C\({}_{16}\)H\({}_{11}\)N\({}_{2}\)O\({}^{+}\), _m_/_z_ 247), and protonated fatty acids: linoleic (C\({}_{18}\)H\({}_{33}\)O\({}_{2}\)\({}^{+}\), _m_/_z_ 281), stearic (C\({}_{18}\)H\({}_{37}\)O\({}_{2}\)\({}^{+}\), _m_/_z_ 285), and lignoercic (C\({}_{24}\)H\({}_{49}\)O\({}_{2}\)\({}^{+}\), _m_/_z_ 369). Figure 6: MA-XRF elemental map of Pb from leather sample 2 (zone 2a). ATR-FTIR spectra of the blue area (Figure 8) show spectral features common to those obtained from gold varnish.