## 3 Results
### Silicate Pigments

Table 2: Silicate-based pigments analysed by ATR-FTIR and DRIFT modalities. Bands (cm\({}^{-1}\)) and assignment (vibration mode and mineral associated) are included where possible. See Table 1 for the explanation of the pigment identification codes. Figure 1: Absorbance spectra of Egyptian blue (EGB-S) obtained by ATR-FTIR (**a**) and DRIFT (**b**) modalities. See Table 1 for the explanation of the pigment identification codes. The blue colour in blue ultramarine pigments is due to sodalite group minerals, mainly lazurite and sodalite [53], as detected by XRD and is present in both variants (natural and synthetic). Additionally, natural blue ultramarine (UL-N) was also composed of calcite, diopside, pyrite, albite, muscovite and wollastonite, all mineral phases related to the mineral ore [53]. As for synthetic blue ultramarine (UL-S), repheline (another sodalite-group mineral) and kaolinite were detected, the latter related to its manufacturing process [53]. Their characteristic IR characteristic bands of lazurite/sodalite were present in both spectra (Table 2), regardless of the modality (ATR-FTIR or DRIFT) and the pigment's nature (natural-N or synthetic-S). These bands were positioned at similar wavenumbers (\(\pm\)5 cm\({}^{-1}\)) in both ATR-FTIR and DRIFT. This similarity was also reported by [PERSON] et al. [10] in a study comparing DRIFT and transmission modalities. Still, whilst the ATR-FTIR spectra of UL-N (Figure 2a) and UL-S (Figure 2c) were similar, DRIFT spectra showed differences between them (Figure 2b,d, respectively). UL-N showed a _Reststrahlen_ band in the DRIFT spectrum at 1030 cm\({}^{-1}\) assigned to lazurite (marked with a black arrow in Figure 2b), not present in the UL-S DRIFT spectrum [15; 36; 37]. Additionally, bands in the region 2200-1600 cm\({}^{-1}\) in the UL-N spectrum (marked with a black dashed-line rectangle in Figure 2b) suggested the presence of organic matter (OM) [26]. Indeed, as reported by the supplier, the method of obtaining this pigment is by grinding the mineral (lapis lazuli) and mixing it with melted wax, resins and oils; a complex process reported by medieval authors such as [PERSON] [54]. Lastly, DRIFT spectra of both UL-N and UL-S showed bands between 1350 and 1200 cm\({}^{-1}\) (marked with a black rectangle in Figure 2b,d) related to Si-O or Si, Al-O overlapping [36; 37], not present in the ATR-FTIR spectrum. In green earth pigments, the green colour is due to mica-group phases, mainly glau-conite/celadonite [55], as detected by XRD. These were observed in both modalities (Table 2). Other mica-group minerals were identified by XRD, such as muscovite, together with calcite, clinochlore, albite and anorthite. Moreover, XRD of oriented-aggregates allowed the identification of clay minerals such as montmorillonite and kaolinite. All these impurities are common among commercial green-earth pigments [47; 56]. In the FTIR spectra, the main difference was observed through the presence of a _Reststrahlen_ band at 1065 cm\({}^{-1}\) (marked with a black arrow in Figure 3b) in the DRIFT spectrum, assigned to glauconite [15]. In addition, DRIFT modality showed a higher number of bands when compared to the ATR-FTIR spectra, all ascribable to the silicate pigment impurities (Table 2). Lastly, CHR-N was not only composed by chrysocolla by XRD; malachite and quartz were also detected, mineral phases associated to chrysocolla [49]. These phases were also identified by FTIR analysis, as observed in Table 2. ATR-FTIR (Figure 4a) showed silicate characteristic bands assigned to chrysocolla and quartz, as well as carbonate group bands ascribable to malachite. These were also present in the DRIFT spectrum (Figure 4b), in addition to a _Reststrahlen_ band at 1056 cm\({}^{-1}\) (marked with a black arrow in Figure 4b). Overall, when comparing ATR-FTIR and DRIFT modalities, the silicate characteristic bands were present at higher wavenumbers in the later.