3.1.2. French Ochre

Based on the XRD, FTIR, and FORS analysis (Figure 2), no detectable phase transformations were observed in the pigment particles subjected to the immersion in DAP solution (FRE-d28) as compared to the untreated powders (FRE-raw).

**Figure 2.** (**a**) Photomicrograph of the French ochre (FRE)-raw sample; (**b**) micrographs of the FRE-raw. Qtz stands for quartz and Kao stands for kaolinite; (**c**) photomicrograph of the sample FRE-d28; (**d**) micrographs of the FRE-d28; (**e**) XRD pattern of the samples FRE-raw and FRE-d28; (**f**) FTIR spectra of the FRE-raw, FRE-d1, and FRE-d28 samples; (**g**) FORS spectra of the French ochre samples FRE-raw, FRE-d1, FRE-d7, FRE-d28; (**h**) first derivative of the FORS spectra in (**g**). The intensity values of each XRD pattern, FORS spectra, and its first derivative plots were normalized and offset for comparison purposes.

XRD analysis (Figure 2e) revealed the presence of quartz, muscovite, kaolinite, and hematite. FTIR spectroscopy (Figure 2f) further corroborated the results. Kaolinite (Al2Si2O5(OH)4) disclosed vibrational bands at 3696, 3670, 3653 cm−<sup>1</sup> (surface hydroxyl groups stretching vibration), 3621 cm−<sup>1</sup> (inner hydroxyl groups stretching vibration), 1030 cm−<sup>1</sup> (Si–O–Si stretching vibration), 1008 cm−<sup>1</sup> (Si–O–Al stretching vibration), 938 and 912 cm−<sup>1</sup> (Al–OH deformation vibration), 693 cm−<sup>1</sup> (Si–O–Si symmetrical bending vibration), 538 cm−<sup>1</sup> (Si–O–Al stretching vibration), and 469 cm−<sup>1</sup> (Si–O–Si asymmetrical bending vibration). Quartz (SiO2) exhibited vibrational bands at 1162 cm−1(Si–O–Si rocking vibration), 1096 cm−<sup>1</sup> (Si–O–Si asymmetrical stretching vibration), doublets at 777 and 799 cm<sup>−</sup>1(Si–O–Si symmetrical stretching vibration) and at 693 cm−<sup>1</sup> and 469 cm−<sup>1</sup> (Si–O–Si symmetrical and asymmetrical bending vibration, overlapping with kaolinite). It should be noted that the Fe–O vibration of hematite which yields vibrational bands at 538 cm−<sup>1</sup> and 469 cm<sup>−</sup>1, were overlapping with the Si–O–Al stretching vibration of kaolinite and the Si–O–Si bending vibration of kaolinite/quartz,

respectively [50–56]. The bands at 3432 cm−<sup>1</sup> and 1626 cm−<sup>1</sup> corresponded to the O–H stretching and O–H bending of surface-absorbed water. The bands at 3130 cm−<sup>1</sup> and 1399 cm−<sup>1</sup> were present probably due to the ν<sup>3</sup> stretching vibration and the ν<sup>4</sup> bending vibration of surface-adsorbed NH4 <sup>+</sup> [57–59].

FORS showed the characteristic inflection point (maximum at its first derivative (Figure 2g) of hematite at around 580 nm (Figure 2h). The broad absorption at ~875 nm also characteristic of hematite, could not be seen in this spectrum (cut off at 800 nm). These were attributed to ligand-to-metal charge transfer transitions in hematite [60].
