## 3 Results and Discussion
### Pigments
#### 3.2.3 Blue  
The combination of XRF, FORS, and Raman spectroscopy allowed the detection of several blue pigments related to the original and restoration areas pertaining to various interventions, according to the imaging analysis results [17]. On the panels, both azurite  
Figure 3: Infrared spectra, acquired at different points of panel E, reveal the use of proteinaceous materials for the highlights (**a**,**b**) and lipids for the gildings (**c**,**d**). and ultramarine were, in fact, detected in different areas, as highlighted by the combined use of the spectroscopic techniques and reported in Table 1. In particular, XRF can detect high copper contents in azurite, while FORS can clearly distinguish the two different materials. It is interesting to note that, both in panel A and in panel D, the presence of ultramarine is related to a high presence of zinc, as indicated by the XRF spectra. It is worth noting that azurite was mapped using the UV fluorescence images [17], thanks to the binder, as this pigment was usually applied using the \"secco\" technique [24; 25]. On panels A and E, the typical infrared bands of azurite, at ca. 2592, 2557, and 2510 cm\({}^{-1}\) (v\({}_{1}\) + v\({}_{3}\) combination band of (CO\({}_{3}\))\({}^{2-}\)), along with the strong doublet at ca. 4380 and 4244 cm\({}^{-1}\) (combination v + 8 (OH) and overtone 3v\({}_{3}\)), were detected by ER-FTIR (Figure 4) [26]. In these areas, the XRF (high copper presence) and FORS spectroscopy indicated the use of this specific blue pigment. For blue pigments, as well as for green ones, a second restoration phase was highlighted by the use of Prussian blue. In fact, on panel A, the presence of the peak related to the carbon-nitrogen bond at ca. 2088 cm\({}^{-1}\) in the ER-FTIR spectra suggested the use of Prussian blue (Figure 4) [27; 28]. In this same area, the higher iron presence, along with the zinc signals in the XRF spectra, upholds a Prussian blue presence and confirms that it pertains to a modern restoration. Regarding the vault, instead, only XRF analyses were carried out, with the aim of investigating the composition of the blue background. However, due to problems relating to the height of the scaffold and the instrument placement, it was only possible to reach limited portions located in the lower part of the vault, while the top surfaces were not subjected to analysis. In all the spectra analyzed, as expected, high counts relating to calcium were observed. These are correlated to the Ca-rich matrix of the plaster. The calcium signals are always associated with those of strontium. Exceptions are the points analyzed in a single region of the vault (points V_08-V_12) in which the Sr signals are not detected; this could indicate that the calcium carbonate in that area was applied during restoration and shows a different geochemical signature. The XRF spectra also show intense counts relating to iron. The presence of this element, ubiquitously detected in the XRF spectra of all the areas of the vault analyzed, can be justified by the presence of a level of red preparation for the blue application, traditionally made with morellone [29]--an orange iron oxide and black carbon mixture applied on a calcite substratum. Copper signals are also always present, even if with very low intensities. These data allow us to hypothesize the presence  
Figure 4: ER-FTIR spectra, acquired on a figure with the blue clothing on panel A, reveal the spectral evidence of azurite in the original area (A05) and Prussian blue in the restored part (A08). of residual original azurite, which was subsequently integrated with modern materials. Regarding the identification of the blue pigment used in the repainting, a hypothesis can be formulated regarding the use of a mixture of Prussian blue and zinc white, the latter highlighted by the UVF images showing the lemon-yellow luminescence characteristic of the pigment [17], as already verified in the apsis panels.