## 3 Results and Discussion
### Leather Support and Silver Leaf  
The first decorative layer applied on the leather surface is a silver leaf. In this case, ToF-SIMS results obtained in a positive ion mode, show the presence of sodium, magnesium, aluminum, silicon, potassium, calcium and oxygenated calcium ions, and silver and iron sulfates (FeSO\({}_{4}\)\({}^{-}\), _m/\(\gtrsim\)_ 152 and FeSO\({}_{5}\)H\({}^{-}\), _m/\(\gtrsim\)_ 169).  
Complementary elemental maps obtained from MA-XRF show calcium and iron equally distributed throughout the MA-XRF sampled regions from leather samples 1 and 2 (Figure S1). The presence of calcium can be attributed to the un-hairing process of leather tanning, which traditionally makes use of alkaline solutions such as lime or soda ash [4]. Regarding what concerns the iron, it should be considered that potash alum was commonly used in the tanning process in the past. Impure alum usually contained a great amount of iron sulfate. Moreover, potash alum obtained from naturally occurring alunite or alum-stone by roasting followed by lixiviation and crystallization can present a brownish or pinkish hue due to the presence of Fe\({}_{2}\)O\({}_{3}\)[19]. In this context, iron is likely to be related to the tanning process itself.  
Moreover, in the areas where the gold varnish cracked, oxidation damages of the exposed silver surface were revealed. In these regions, in fact, ToF-SIMS experiments detected clusters of silver, silver oxides, silver chlorides, and silver sulfides (Peaks and spectra are shown in Figure S2). In Figure 2, the corresponding positive ion maps are reported. The presence of CN\({}^{-}\) (_m/\(\gtrsim\)_ 26), P\({}^{-}\) (_m/\(\gtrsim\)_ 31), CNO\({}^{-}\) (_m/\(\gtrsim\)_ 42), PO\({}^{-}\) (_m/\(\gtrsim\)_ 47), PO\({}^{-}\) (_m/\(\gtrsim\)_ 63), and PO\({}_{3}\)\({}^{-}\) (_m/\(\gtrsim\)_ 79) in the negative ion mass spectra may suggest the use of proteinaceous compounds applied on the surface layers or arising from the animal glue used to adhere the silver leaf to the leather support. In fact, CN\({}^{-}\) and CNO\({}^{-}\) ions are characteristic of the peptide backbone fragmentation. Phosphate ions are usually detected in the presence of cellular material. In the same area, Cl\({}^{-}\) (_m/\(\gtrsim\)_ 35) and S\({}^{-}\) (_m/\(\gtrsim\)_ 32) were also detected.  
Figure 2: Silver-containing compounds in the silver leaf. (**a**) Macro camera view of the area where ToF-SIMS images were acquired (area of 200 \(\upmu\)m \(\times\) 200 \(\upmu\)m). (**b–f**) TOF-SIMS ion images were recorded in the positive ion mode, (**b**) total ion image, (**c**) ion image of the sum of silver ions (sum of: Ag\({}^{+}\), _m/\(\gtrsim\)_ 107, Ag\({}^{+}\), _m/\(\gtrsim\)_ 214; Ag\({}^{+}\), _m/\(\gtrsim\)_ 321, Ag\({}^{+}\), _m/\(\gtrsim\)_ 354), (**d**) ion image of the sum of silver oxides (sum of: Ag\({}_{3}\)O\({}^{+}\), _m/\(\gtrsim\)_ 353; Ag\({}_{3}\)O\({}^{+}\), _m/\(\gtrsim\)_ 567), (**e**) ion image of the sum of silver chlorides (sum of: Ag\({}_{2}\)Cl\({}^{+}\), _m/\(\gtrsim\)_ 249, Ag\({}_{3}\)Cl\({}^{+}\), _m/\(\gtrsim\)_ 391, Ag\({}_{4}\)Cl\({}^{+}\), _m/\(\gtrsim\)_ 463), and (**f**) ion image of the sum of silver sulfides (AgS\({}^{+}\), _m/\(\gtrsim\)_ 139, Ag\({}^{+}\), _m/\(\gtrsim\)_ 246, Ag\({}_{3}\)S\({}^{+}\), _m/\(\gtrsim\)_ 353).