*2.2. Characterization of Pigment–DAP Interaction*

For the experimental application, 10 g of each pigment were dispersed in 100 mL of 1M DAP solution or in 100 mL of DI water, which was used as a control sample, and the dispersions were subsequently sealed in a glass bottle. The bottles were kept in the dark to avoid any photochemical reaction. The room temperature (T) was maintained at ~22 ◦C, and the relative humidity (RH) at ~50%. Using an Oakton EcoTestr® pH2 Waterproof pH Tester (standard error: <sup>±</sup> 0.1), pH measurements were taken of the 1M DAP solution and of each pigment dispersion on day 0, a few minutes after the pigments were dispersed in the DAP solution, and subsequently at regular intervals: every 24 h between day 1 and 7 and then on day 14, 21, and 28. Monitoring of color/phase change of those

pigments was carried out in the first 28 days. Red lead and chalk, however, showed phase and color change after two months of immersion in the DAP solution. For these two pigments, further monitoring will be required.

Prior to subjecting the samples to the measurements, all the powders were rinsed using DI water and left to dry overnight on filter paper. The powders were analyzed every 24 h between day 1 and 7, and then on day 14, 21, and 28, following the dispersion into 1M DAP solution. The samples listed were named using the abbreviation of the pigment name and the immersion time. For instance, CIN-raw stands for cinnabar pigment prior to the analysis, whereas CIN-d28 stands for cinnabar precipitate collected 28 days after dispersion in 1M DAP solution.

All powders were first examined using a Keyence VHX-1000 Digital Optical Microscope, using a magnification between 20× and 200×.

XRD measurements on the pigment powders were performed using a Bruker D8 diffractometer with the following measurement parameters: Cu-Kα radiation, λ= 1.5404 Å, voltage 40 kV, beam current 40 mA, and a 2–80◦ 2θ exploration range with a step size of 0.014◦ 2θ. The mineral phases were identified by using the ICDD database (International Center for Diffraction Data, Newtown Square, PA, USA).

TGA analysis was performed on selected pigment powders using a Perkin Elmer Pyris Diamond TG/DTA (Thermogravimetric/Differential Thermal Analyzer). The temperatures were scanned in the range between 40 ◦C to 900 ◦C, at the heating rate of 20 ◦C/min, in a flowing Ar atmosphere.

Microstructural and elemental analyses of the powders were performed on a FEI Nova NanoSEMTM 230 scanning electron microscope (SEM) with field emission gun (FEG) and variable pressure (VP) capabilities, equipped with a Thermo ScientificTM NORANTM System 7 X-ray energy dispersive spectrometer (EDS). Gold (Au) coating to improve the electrical conductivity was applied using a Hummer® 6.2 sputtering system (Anatech Ltd., Battle Creek, MI, USA). Secondary electron (SE) imaging was performed in vacuum using the Everhart–Thornley detector (ETD). The elemental composition of single spots and area elemental maps were acquired using EDS.

FTIR spectroscopy was performed on a JASCO FT/IR-420 Fourier-Transform Infrared Spectrometer using the KBr pellet method. Pigment powders were ground and dispersed in a KBr matrix at a concentration around 0.5 wt % and then pressed into a pellet. All spectra were collected at 64 scans with a spectral resolution of 4 cm<sup>−</sup>1, from 4000 to 400 cm−1. The spectra were matched against the spectral database of the Infrared and Raman Users Group (IRUG, Philadelphia, PA, USA) and published literature data.

FORS (Fiber Optic Reflectance Spectroscopy) was conducted using an Ocean Optics USB 2000+ fiber optical spectrophotometer and the FieldSpec3® Spectroradiometer (Analytical Spectral Devices Inc., Boulder, CO, USA). The spectro-colorimetric measurements allowed for the quantification of incident and reflected radiation intensities, which roughly equal human color perception. During the measurement, a white diffuse reference standard was measured every 30 min. Color values were recorded in the L\*a\*b\* color space defined in 1976 by CIE (Commission Internationale de l'Eclairage, Vienna, Austria) [39]. Changes in color/color difference (ΔE\*) were calculated with the following formula (Equation (1)) as recommended by the CIE:

$$
\Delta \mathbf{E}^\* = \left[ (\Delta \mathbf{L}^\*)^2 + (\Delta \mathbf{a}^\*)^2 + (\Delta \mathbf{b}^\*)^2 \right]^{1/2} \tag{1}
$$

where ΔL\*, Δa\*, and Δb\* are the differences in L\*, a\*, and b\* values before and after immersion in the DAP solution. ΔL\* describes the change in luminance, Δa\* the change in red/green components, and Δb\* the change in yellow/blue components. While generally ΔE\* ≤ 2 is widely acceptable as the value detectable by the human eye [40], a color difference of ΔE\* ≤ 5 has been established as the threshold in the field of cultural heritage to evaluate color change after a conservation intervention such as consolidation treatment [16,41–48].

#### **3. Results and Discussion**

After 28 days of immersion of the pigments in the DAP solution, the pigments were assessed on the basis of phase transformations and color change. Three main groups were revealed: (1) pigments that showed no chemical and/or optical interaction (no phase or significant color change) with DAP (i.e., cinnabar, French ochre, and lapis lazuli); (2) pigments that showed phase transformation without significant color change (i.e., chalk, raw sienna, and burnt umber); and (3) pigment with strong phase and color change (i.e., red lead).

#### *3.1. Cinnabar, French Ochre, Lapis Lazuli*

The calculated ΔE\* values for cinnabar, French Ochre, and lapis lazuli pigment particles before and after the 28 days of immersion in DAP were determined to be 3.5, 3.4, and 4.2, respectively (Table 1). Though these values are above the threshold of color change detected by the human eye [40], they are still below the established value (ΔE\* ≤ 5) accepted for cultural heritage consolidation treatments [16,41–48].

#### 3.1.1. Cinnabar

Cinnabar has a deep red color with angular particles of various sizes up to 100 μm (Figure 1a–d). Its identification was based on XRD analysis (Figure 1e) and FORS (Figure 1f) which showed consistently the characteristic sigmoid-shaped spectrum with an inflection point (maximum at its first derivative, Figure 1g) at ~614 nm corresponding to the bandgap of cinnabar [49]. No obvious change in shape or size of the cinnabar pigment particles (inferred by SEM–EDS analysis) was observed (Figure 1a–d).

**Figure 1.** (**a**) Photomicrograph of the cinnabar (CIN)-raw sample; (**b**) secondary electron (SE) micrographs of the CIN-raw sample; (**c**) DM (Digital Micrograph) photomicrograph of the sample CIN-d28; (**d**) SE micrographs of the sample CIN-d28; (**e**) XRD pattern of CIN-raw and CIN-d28 ; (**f**) FORS spectra of cinnabar: CIN-raw, CIN-d1, CIN-d7, CIN-d28; (**g**) first derivative of the FORS spectra in (**f**). The intensity values of each XRD pattern, FORS spectra, and its first derivative plots were normalized and offset for comparison purposes.
