## 3 Results and Discussion
### Gold Varnish  
In the gold areas, ToF-SIMS experiments revealed the presence of sodium, magnesium, aluminum, silicon, potassium, silver, calcium, calcium-oxygen containing ions, lead, and lead-oxygen containing ions.  
MA-XRF elemental maps (Figure S3) also show lead widespread all over the gold and red portions of sample 1 and in correspondence of the gold area in sample 2.  
The presence of lead ions in the gold varnish can be explained with the use of lead as sicctive and/or an inorganic pigment. In this case, the inorganic pigment could be attributed to the presence of massicol (PbO), which is an inorganic yellow pigment.  
In the positive ToF-SIMS spectra (Figure 3) protonated linoleic (C\({}_{18}\)H\({}_{33}\)O\({}_{2}\)\({}^{+}\), _m/z_ 281), stearic (C\({}_{18}\)H\({}_{37}\)O\({}_{2}\)\({}^{+}\), _m/z_ 285), arachidonic (C\({}_{20}\)H\({}_{41}\)O\({}_{2}\)\({}^{+}\), _m/z_ 314), benenic (C\({}_{22}\)H\({}_{45}\)O\({}_{2}\)\({}^{+}\), _m/z_ 341), nervonic (C\({}_{24}\)H\({}_{47}\)O\({}_{2}\)\({}^{+}\), _m/z_ 367), and lignoercic (C\({}_{24}\)H\({}_{49}\)O\({}_{2}\)\({}^{+}\), _m/z_ 369) acid ion signals can be observed. The presence of these fatty acids was detected in most of the spectra collected on the different selected areas (gold, blue, green, and red). In some spectra, protonated arachidonic acid (C\({}_{20}\)H\({}_{33}\)O\({}_{2}\)\({}^{+}\), _m/z_ 305) was also detected (data not shown).  
The presence of linoleic, stearic, and arachidonic fatty acids together with the lead ions in the gold varnish seems to be in agreement with the ancient recipes for obtaining the golden surface by stating that this was usually constituted by heat-bodied linseed oil with lead components [1; 3]. However, further studies will be performed to validate the presence of other organic colorants in the gold area. In fact, arachidonic, linoleic, and lignoercic acids could be ascribed to the presence of aloe used as an organic colorant in the gold varnish [1; 3; 20].  
The ATR-FTIR spectra obtained in the region of 3500 to 200 cm\({}^{-1}\) for the gold varnish on sample 2 are shown in Figure 4a,b. Main spectral features are found at 1629 cm\({}^{-1}\) (\(\
u\)[C=O], in amide I), 1542 cm\({}^{-1}\) (\(\
u\)[C-N], and in-plane bending of N-H, both from amide II), 1459 cm\({}^{-1}\) (bending of C-H and \(\
u_{3}\) [C-O] from CO\({}_{3}\)\({}^{2-}\) group), 1316 cm\({}^{-1}\) (\(\
u\)[C-O]), 1031 cm\({}^{-1}\) (\(\rho\)[CH\({}_{3}\)]), and 1006 cm\({}^{-1}\) (\(\rho\)[CH\({}_{3}\)]) (extended attributions are reported in supplementary information and shown in Figure S4) [21; 22; 23; 24; 25; 26]. The found vibrations confirm the presence of a mixture of oils and proteins, the former being used as a painting medium and the latter constituting both a component of the medium itself and an adhesive layer between the leather substrate and the silver leaf. When observed at low magnification with the stereomicroscope, the silver leaf shows a large number of cracks and lacks exposure of the underlying layers. It allows for the IR absorption by the adhesive. No clear signal from pigments conferring the yellow coloration was recognized. ATR-FTIR spectra of the gold area in sample 3  
Figure 3: Part of positive ion mass spectra extracted from the gold area and showing the protonated fatty acids: linoleic (C\({}_{18}\)H\({}_{33}\)O\({}_{2}\)\({}^{+}\), _m/z_ 281), stearic (C\({}_{18}\)H\({}_{37}\)O\({}_{2}\)\({}^{+}\), _m/z_ 285), arachidonic (C\({}_{20}\)H\({}_{41}\)O\({}_{2}\)\({}^{+}\), _m/z_ 314), benenic (C\({}_{22}\)H\({}_{45}\)O\({}_{2}\)\({}^{+}\), _m/z_ 341), nervonic (C\({}_{24}\)H\({}_{47}\)O\({}_{2}\)\({}^{+}\), _m/z_ 367), and lignoercic (C\({}_{24}\)H\({}_{49}\)O\({}_{2}\)\({}^{+}\), _m/z_ 369).  
(Figure 4c) showed the presence of an additional compound. The comparison with a reference spectra of rosin oil from Pine-tree, comparable with those found in literature [8; 26; 27], led to attribute the unknown compound to the class of diterpenoid resins, such as colophony (attributions are reported in supplementary information).