3.1.3. Lapis Lazuli

The lapis lazuli pigment powder analyzed for this research (sample LAP-raw) was found to contain various minerals including lazurite, wollastonite, cancrinite, and feldspars (Figure 3), with particle sizes ranging from 2 to 50 μm.

**Figure 3.** (**a**–**b**) Micrographs of the lapis lazuli pigment (LAP)-raw sample; (**c**–**d**) micrographs of the sample LAP-d28; (**e**) XRD pattern of the samples LAP-raw, LAP-d28; (**f**) FTIR spectra of the LAP-raw and LAP-d28 samples; (**g**) FORS spectra of the samples LAP-raw, LAP-d1, LAP-d7, LAP-d28. The intensity values of each XRD pattern and FORS spectra were normalized and offset for comparison purposes.

XRD (Figure 3e) and FTIR analysis (Figure 3f) showed no detectable phase changes resulting from the immersion in the DAP solution. The FTIR spectra showed bands in the 1100–900 cm−<sup>1</sup> region that could be assigned to overlapping of Al, Si–O4 tetrahedra asymmetric stretching vibration of lazurite and O–Si–O asymmetric stretching vibration of wollastonite, as well as bands in the 700–600 cm−<sup>1</sup> region, which could be linked to an overlapping of Al, Si–O4 tetrahedra symmetric stretching vibration of lazurite and O–Si–O symmetric stretching vibration of wollastonite [61]. The band at 568 cm−<sup>1</sup> and the band at 452 cm−<sup>1</sup> represent the terminal –O–Si–O– bonds bending vibration and Si–O–Si bending vibration, respectively [62–64].

The visible spectrum of the lapis lazuli was dominated by an absorption band around 600 nm, corresponding to the electronic transitions for S3 − (see Figure 3g).

#### *3.2. Chalk, Sienna, Burnt Umber*

The calculated ΔE\* values for the chalk, raw sienna, and burnt umber pigment particles before and after 28 days of immersion in DAP were 4.9, 2.6, and 1.7, respectively (Table 1). Though the value of chalk was above the threshold of color change detected by the human eye [40], it was lower than the established value (ΔE\* ≤ 5) accepted for consolidation applications in cultural heritage [16,41–48]. The color change of burnt umber pigment remained below the detection limit of human eye.
