3.2.1. Chalk

The pigment (CHA-raw) used for this research was a fine powder consisting of natural white calcium carbonate (CaCO3) (Figure 4a–c) and was prepared from pure microcrystalline chalk with particle sizes less than 5 μm. After 28 days of immersion in 1M DAP solution, the chalk (CaCO3) particles showed evident transformation into HAP (Ca10(PO4)6(OH)2) and octacalcium phosphate (OCP, Ca8H2(PO4)6·5H2O). The habit of the original calcium carbonate crystals had changed into "plate-like" crystals (Figure 4d–f). EDS mapping of the sample CHA-d28 showed that the major phases detected consisted of Ca, O, and P elements. This transformation continued even after a period of two months with more calcium carbonate crystals been transformed into calcium phosphate.

**Figure 4.** (**a**) Photomicrograph of the chalk (CHA)-raw sample; (**b**–**c**) micrographs of the CHA-raw sample; (**d**) photomicrograph of the sample CHA-d28; (**e**–**f**) micrographs of the CHA-d28 sample; (**g**–**j**) SE image and elemental mapping of the sample CHA-d28.

XRD analysis (Figure 5a) showed that the raw chalk pigment solely consisted of calcium carbonate (or calcite), while after 1 day and 28 days of immersion in DAP, some unreacted calcite, hydroxyapatite, and OCP were found to coexist. The consumption of calcite was not complete. FTIR analysis (Figure 5b) further confirmed the XRD results [65]. CaCO3 yielded bands at 712 cm−<sup>1</sup> (ν<sup>4</sup> in-plane bending vibration of CO3 <sup>2</sup>−), 873 cm−<sup>1</sup> (ν<sup>2</sup> out-of-plane bending vibration of CO3 <sup>2</sup>−), 1420 cm−<sup>1</sup> (ν<sup>3</sup> asymmetric stretching vibration of CO3 <sup>2</sup>−), and combination bands at 2513 cm−<sup>1</sup> and 1798 cm−1. In the sample CHA-d28, bands appeared at 468 cm−<sup>1</sup> (ν<sup>2</sup> bending mode of O–P–O bond), 562 cm−<sup>1</sup> (ν<sup>4</sup> bending mode of O–P–O bond), 601 cm−<sup>1</sup> (ν<sup>4</sup> bending mode of O–P–O bond), 957 cm−<sup>1</sup> (ν<sup>1</sup> symmetric stretching mode of P–O bond), and 1033 cm−<sup>1</sup> (ν<sup>3</sup> asymmetric stretching mode of P–O). These are the vibration modes associated with the phosphate group present in HAP and OCP. [66]. The fraction of unreacted calcite was further estimated through TGA analysis, with a weight loss recorded between 600 and 860 ◦C linked to the decomposition of calcite. Weight losses of 43.2 wt %, 16.9 wt %, and 11.55 wt % were observed for the samples CHA-raw, CHA-d1, and CHA-d28, respectively (Figure 5c–e). This roughly corresponded to calcite fractions of 98.2 wt %, 38.4 wt %, and 26.25 wt %, respectively. Most of the calcite was consumed on the first day of reaction with DAP.

**Figure 5.** (**a**) XRD pattern of the samples CHA-raw, CHA-d1, CHA-d28, and CHA-2m; (**b**) FTIR spectra of the CHA-raw, CHA-d1, and CHA-d28 samples; (**c**–**e**) TGA of the sample CHA-raw, CHA-d1, and CHA-d28. The intensity of each XRD pattern was normalized and offset for comparison purposes.

OCP is commonly found to be present as an intermediate phase in the conversion process from amorphous calcium phosphates (ACP) to HAP (hydroxyapatite) [67]. This transition could explain the co-existence of HAP and OCP within the mixtures. While the formation of these phases and the kinetics of transformation largely depend on the reaction conditions such as pH and presence of foreign ions, ultimately—i.e., at thermodynamic equilibrium—they are all expected to transform to HAP, which is thermodynamically the most stable phase [67,68].

#### 3.2.2. Raw Sienna

Microscopic examination of the sample SIE-raw (Figure 6a–b) showed that the pigment consists of different particles sizes ranging from sub-micron to 50 μm. XRD analysis of the SIE-raw and SIE-d28 (Figure 6e) samples indicated that raw sienna consisted of goethite (α-FeOOH), gypsum (CaSO4•2H2O), calcite, quartz, and montmorillonite/clay. In the sample SIE-d1, gypsum was absent from the XRD pattern, whereas calcite could still be detected. This was due to the dissolution of gypsum into the DAP solution, while the transformation of calcite into HAP and/or OCP was not complete. For SIE-d28, however, the calcite peaks were absent, indicating that the amount of remaining calcite was probably below the detection limit (~2–3 wt %).

**Figure 6.** (**a**–**b**) Micrographs of the raw sienna pigment (SIE)-raw sample; (**c**–**d**) micrographs of the sample SIE-d28; (**e**) XRD pattern of the sample SIE-raw, SIE-d1, SIE-d28; (**f**) FTIR spectra of the SIE-raw and SIE-d28 samples; (**g**) FORS spectra of the samples SIE-raw, SIE-d1, SIE-d7, SIE-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.

After 28 days of immersion in DAP solution, newly formed phosphate-bearing phases were identified through the FTIR (Figure 6f) and SEM–EDS techniques. In the spectra of SIE-raw, gypsum yielded bands at 3541 and 3399 cm−<sup>1</sup> (ν<sup>3</sup> asymmetric stretching vibration and ν<sup>1</sup> symmetric stretching vibration of water molecule, respectively), 1685 and 1621 cm−<sup>1</sup> (O–H bending vibration), 1112 cm−<sup>1</sup> (ν<sup>3</sup> asymmetric stretching vibration of SO4 <sup>2</sup><sup>−</sup> tetrahedron), 669 cm−<sup>1</sup> (ν<sup>4</sup> asymmetric bending vibration of SO4 <sup>2</sup><sup>−</sup> tetrahedron), and 600 cm−<sup>1</sup> (ν<sup>4</sup> asymmetric bending vibration of SO4 <sup>2</sup>−tetrahedron). Quartz displayed characteristic bands at 1096 cm−<sup>1</sup> (Si–O–Si asymmetrical stretching vibration), 798 cm−<sup>1</sup> (Si–O–Si symmetrical stretching vibration), and 469 cm−<sup>1</sup> (Si–O–Si asymmetrical bending vibration), while silicate clay had bands at 3621 cm−<sup>1</sup> (O–H stretching vibration of structural hydroxyl group) and 1029 cm−<sup>1</sup> (Si–O–Si asymmetrical stretching vibration) and shared (with quartz) bands at 798 cm−<sup>1</sup>

and 469 cm<sup>−</sup>1. CaCO3 produced bands at 712 cm<sup>−</sup>1, 877 cm−1, 1425 cm−1, 1797 cm−1, and 2513 cm−1. Goethite gave a broad band centered at 3141 cm−<sup>1</sup> (broad, ν<sup>2</sup> stretching vibration of O–H) and bands at 899 cm−<sup>1</sup> (δO–H bending vibration) and 798 cm−<sup>1</sup> (γO–H bending vibration, overlapping with quartz and silicate clay) [69–76]. After 28 days of reaction with DAP, in the FTIR spectrum of SIE-d28, the calcite and gypsum bands disappeared with the appearance of the bands at 468 cm−1, 562 cm−1, 601 cm<sup>−</sup>1, and 1034 cm−1, corresponding to the vibration mode of the newly formed phosphate group. The bands of goethite, quartz, and silicate clay remained unchanged. The appearance of the phosphate group and the disappearance of gypsum and calcite in the FTIR spectrum further indicated that calcite and gypsum were converted into calcium phosphates.

Reflectance spectra of the yellow iron hydroxide pigment (goethite) showed the characteristic inflection point (maximum at its first derivative, Figure 6g) at around 545 nm and absorptions at 640 and ~900 nm (the latter was not visible in the spectrum) (Figure 6h).
