## 4 Results
### Pigments and Dyes  
#### 4.4.1 Blue and Violet Hues  
According to PLM and ESEM-EDX investigations, the pale blue mountain of the _Martyrdom of Saint Sebastian_ panel consists of a thin painting layer (50 \(\upmu\)m thick), mainly composed of lead white, large azurite particles with Ag impurities, calcium carbonate and silicates (sample GSM5: Figure S6a-c, Table S2).  
FTIR and FTIR-ATR analysis carried out on the cross-sections of the blue samples (GSM5 and GSM10) confirmed the occurrence of azurite mixed with lead white in different proportions and with small additions of other pigments depending on the hues. In the acquired spectra of the blue grains of sample GSM5, the presence of azurite can be inferred by the characteristic bands at 3419 cm\({}^{-1}\) (stretching of the O-H bond), and at 1513 and 1414 cm\({}^{-1}\) due to the stretching \(\
u_{3}\) of the CO\({}_{3}\)\({}^{2-}\) ion. In addition, the bands at 956 cm\({}^{-1}\) ascribed to the out-of-plane bending of the O-H bond, and at 836 cm\({}^{-1}\), due to the stretching\(\
u_{2}\) of the CO\({}_{3}\)\({}^{2-}\) ion, were detected (Figure 3, Table S2) [40]. Bands and peaks indicative of the use of a lipoprotein binder (assigned to egg) were also observed (2924, 2854, 1735, 1648, 1538 cm\({}^{-1}\)) (Figure 3). The relatively broad peak at 1708 cm\({}^{-1}\) was ascribed to carboxylic acids derived from the hydrolysis of eggs' esters. Lead white, detected on the pictorial layer (a white matrix that includes blue crystals) by FTIR-ATR, was identified by IR spectral features associated with the CO\({}_{3}\)\({}^{2-}\) rocking deformations (683 and 692 cm\({}^{-1}\)) and the symmetric CO\({}_{3}\)\({}^{-}\) stretching vibration at 1044 cm\({}^{-1}\). Additionally, the antisymmetric CO\({}_{3}\)\({}^{-}\) stretching vibration (1407 cm\({}^{-1}\)) and the hydroxyl stretching vibration at 3535 cm\({}^{-1}\) can be observed [32].  
The presence of gypsum (peaks at 1118 and 1139 cm\({}^{-1}\)) and quartz (peaks at 1080 and 460 cm\({}^{-1}\)) was also detected [41; 42].  
The violet-blue hue of [PERSON]'s loncloth (sample GSM7, Table S2) is composed of a thin layer (50 \(\upmu\)m) of azurite, silicates lead white, bone black and red lake particles (Figure S6d,e). PLM and ESEM-EDX allowed to distinguish the red lake particles due to the high peak of Al [43] as confirmed by previous reflectance spectroscopy measurements carried out in that area [6]. FTIR analysis allowed the identification of azurite and egg as a binder.  
In the _Visitation_ altarpiece, the shadow of the Virgin's blue mantle consists of a layer (90 \(\upmu\)m thick) composed of finely grounded azurite, silica and rare hematite particles (sample SF6: Figure S4a, Table S3) [44]. Micro-FTIR analysis confirmed the use of azurite (Figure S6f).  
Two painting layers were identified in the Archangel [PERSON]'s violet robe (sample GS84, Table S4): a first one (10 \(\upmu\)m) composed of a red lake followed by a thicker layer (40 \(\upmu\)m) mainly made of lead white, red lake particles and natural ultramarine. The red lake presence [43] was also confirmed by previous reflectance spectroscopy measurements [6]. FTIR-ATR analysis confirmed the presence of natural ultramarine (peak of Si, Al-O asym stretch at 979 cm\({}^{-1}\)) [45; 46], lead white, small amounts of esters of carboxylic acids, lead carboxylates (peak at 1526 cm\({}^{-1}\)) and proteinaceous material (due to amide I at 1650 cm\({}^{-1}\)). The presence of lead carboxylates with very small amounts of non-hydrolyzed esters suggests the use of oil as a binder; however, the occurrence of the proteinaceous material could be related to egg binder.