3.2.3. Burnt Umber

The burnt umber pigment powder analyzed for this research (sample BUR-raw) contained hematite and manganese oxide (inferred by EDS point analysis) and minor phases of calcite and quartz (Figure 7). It exhibited particle sizes ranging from sub-micron to 20 μm (Figure 7b).

**Figure 7.** (**a**–**b**) Micrographs of the burnt umber pigment (BUR)-raw sample; (**c**–**d**) micrographs of the sample BUR-d28; (**e**) XRD pattern of the sample BUR-raw, BUR-d1, BUR-d28; (**f**) FTIR spectra of the BUR-raw and BUR-d28 samples; (**g**) FORS spectra of the samples BUR-raw, BUR-d1, BUR-d7, BUR-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, the formation of calcium phosphates was first estimated from the microstructural changes revealed by SEM–EDS analysis. XRD analysis (Figure 7e) of the sample BUR-raw showed that the raw burnt umber pigment consisted of hematite, quartz, and calcite; the latter was no longer detectable after 28 days in DAP solution (sample BUR-d28). FTIR

analysis (Figure 7f) showed bands at 1423 and 879 cm<sup>−</sup>1, corresponding to the vibration of CaCO3, and bands at 1030, 778, 797, and 463 cm−<sup>1</sup> corresponding to the vibration of the silicate (possibly silicate clay and SiO2) group. The bands at 532 and 463 cm<sup>−</sup><sup>1</sup> were indicative of the Fe–O vibration produced by hematite. After 28 days, no calcite could be detected by FTIR.

The FORS spectra of burned umber (Figure 7g) showed the same features as those collected for French ochre (Figure 7g–h), since the main component of both pigments is hematite.

## *3.3. Red Lead*

The red lead pigment powder analyzed in this study was found to be pure, consisting of minium (Pb3O4) with small and irregular particles (Figure 8a–b) ranging in size from 2 μm to 20 μm.

**Figure 8.** (**a**) Photomicrograph of the red lead (RED)-raw sample; (**b**) micrographs of the RED-raw; (**c**) photomicrograph of the sample RED-d28; (**d**) micrographs of the sample RED-d28, the elongated particle was identified as lead hydroxyapatite (Pb–HAP) by EDS point analysis; (**e**) photomicrograph of the sample RED-2m; (**f**) micrographs of the sample RED-2m; (**g**) XRD pattern of the samples RED-raw to RED-2m between 2θ of 20–38◦; (**h**) FORS spectra of the samples RED-raw, RED-d1, RED-d7, RED-d28. The intensity values of each XRD pattern and FORS spectra were normalized and offset for comparison purposes.

After dispersing pigment particles in 1M DAP for 28 days, part of the minium pigment was found to be converted into hydroxypyromorphite (also known as lead hydroxyapatite, Pb10(PO4)6(OH)2, JCPDS PDF No. 01-087-2477). The color of the pigment changed from orange red to brownish red after 28 days (Figure 8c). After two months, the color was further altered to dark brown (Figure 8e). The calculated ΔE\* value for the red lead pigment particles before and after the 28 days of immersion in DAP was found to be 30.6 (Table 1). This color change is significant and far beyond the threshold accepted in the field of conservation treatment (ΔE\* ≤ 5).

Microscopic observations of the sample RED-d28 showed that most particles remained the same, while some new elongated crystals could be detected (Figure 8d). EDS analysis on point 1 (see arrow in Figure 8d) confirmed the presence of Pb (24.65 at %), P (13.73 at %), and O (61.62 at %). The Pb/P/O atomic ratio was close to 5:3:13, indicating the presence of hydroxypyromorphite. After two months, a significant amount of the original pigment particles was transformed into hydroxypyromorphite (Figure 8f), which are believed to be responsible for the color change from originally red to brown.

XRD analysis (Figure 8g) indicated that the raw red lead pigment (sample RED-raw) solely consisted of minium (JCPDS PDF No. 41-1493). The formation of hydroxypyromorphite (JCPDS PDF No. 8–259) was observed to begin only one day after dispersing the pigment in 1M DAP solution (sample RED-d1). After two months, the lead hydroxyapatite became a dominant phase and was identified along with the precipitates of platternite (β-PbO2) and unreacted residual minium (RED-2m in Figure 8g). While phase transformations between the first day of reaction and after 28 days appeared similar, a more significant phase development was observed over a longer period (two months).

A similar dissolution–precipitation mechanism was reported elsewhere [77,78]. The dissolution reaction of Pb3O4 begins to occur at the surface of Pb3O4 (Reaction 2). Pb3O4 first releases Pb2<sup>+</sup> species from a tetrahedrally coordinated Pb3(II,IV)O4 site through a ligand substitution, leaving unstable octahedral PbO2 fragments (octahedral arrangement hosting Pb4<sup>+</sup> ions in the crystalline structure of Pb3O4) in the solid. The precipitation of lead hydroxyapatite (Pb10(PO4)6(OH)2) observed after the immersion of Pb3O4 in DAP suggests a reaction between the supersaturated Pb2<sup>+</sup> ions released during Pb3O4 dissolution and the phosphate (PO4 <sup>3</sup>−) ions delivered through the DAP solution.

$$\text{Pb}\_3\text{O}\_4 + 2\text{H}\_2\text{O}\_{\text{(aq)}} \rightarrow 2\text{ Pb}^{2+}\_{\text{(aq)}} + \text{PbO}\_{2(s)} + 4\text{OH}^-\_{\text{(aq)}}$$

**Reaction 2.** Dissolution reaction of Pb3O4 in aqueous solution.

Following this step, two processes occur simultaneously:

(1) The unstable PbO2 fragments formed from the dissolution of Pb3O4 are reduced to Pb2+, as suggested by the Reaction 4.

$$\text{PbO}\_{2(\text{unstable})} + 2\text{e}^- + 2\text{H}\_2\text{O}\_{(\text{aq})} \rightarrow \text{Pb}^{2+}\_{(\text{aq})} + 4\text{OH}^-\_{(\text{aq})}$$

**Reaction 3.** Reduction of unstable PbO2 fragment into Pb2<sup>+</sup>.

(2) The nucleation of a newly stable β-PbO2 from Pb2<sup>+</sup> ions, as suggested by the Reaction 5:

$$\text{Pb}^{2+}\_{\text{(aq)}} + 4\text{OH}^-\_{\text{(aq)}} \rightarrow \text{PbO}\_{2(\text{stable})} + 2\text{e}^- + 2\text{H}\_2\text{O}\_{(\text{aq)}}$$

**Reaction 4.** Oxidation of Pb2<sup>+</sup> to stable β-PbO2 (platternite).

Since both Pb3O4 and β-PbO2 are semiconductors, the electrons can transfer between the solid phases. The driving force for the process described is provided by the decrease in both surface and lattice free energy, which results from the dissolution of the octahedral fragment of PbO2 (labelled as PbO2(unstable) above) in Pb3O4 and the precipitation of β-PbO2 [77].

During the dissolution reactions (Reaction 2 to Reaction 4) that occur on the surface of Pb3O4, a layer of very fine particles/precipitates of PbO2 forms during the earliest dissolution stages. Once formed, PbO2 can either remain as a spectator species or be reduced, as suggested by Reaction 5 [77,79,80], releasing more Pb2<sup>+</sup>.

$$\text{PbO}\_{2(s)} + \text{H}\_2\text{O}\_{(aq)} \rightarrow \text{Pb}^{2+}\_{(aq)} + 2\text{OH}^-\_{(aq)} + 0.5\text{ O}\_{2(aq)}$$

**Reaction 5.** Reductive dissolution of platternite (β-PbO2) in aqueous solution.

However, PbO2 formed on the surface of Pb3O4 is likely to passivate the substrate's surface, inhibiting further dissolution of Pb3O4. Still, no such particles/precipitates were detected using XRD during the first month, suggesting that the formation of PbO2 might have been limited to an amount

below the detection limit of XRD. In addition, owing to the very small porosity of the newly formed PbO2 layer on the Pb3O4 surface, the (NH4)2HPO4 solution required longer time to diffuse into the Pb3O4 substrate. On the basis of the XRD analysis, the β-PbO2 phase only became detectable after two months of reaction, which suggests that the dissolution of minium continued along with the constant formation of Pb–HAP and β-PbO2. However, the sudden increase in the precipitation of Pb–HAP and the kinetics of its precipitation rate between 28 days and two months will require further investigation. No previous research into the formation mechanism of lead hydroxyapatite in a comparable system has ever been published, and therefore future research is pivotal to understanding the reaction kinetics of that system.

#### *3.4. pH Value of the Supernantant Solutions*

The change of pH value of the DAP solutions as a function of time is shown in Figure 9.

**Figure 9.** The change of the pH value of the DAP solutions with time (the number on the x-axis means the number of days of measurement) with a standard error of ± 0.1.

The pH value of the French ochre, lapis lazuli, and cinnabar remained almost constant (~8.3) during the first 28 days of reaction. This is consistent with the fact that no significant color, phase, or morphological changes could be observed in these pigments upon exposure to the DAP solution.

By comparing the pH values of the solution at day 0 and day 28, an increase in the pH was observed for calcium carbonate-containing pigments, including chalk, bunt umber, raw sienna (for the latter two, as accessory mineral). This was due to the chemical reaction of calcium carbonate with DAP and the formation of phosphate phases that caused the increase in the pH value of the solution. A slight elevation was also observed in the pH value of the solution containing red lead after 28 days of reaction with DAP. This change is believed to be associated with the reaction of minium (Pb3O4) with diammonium hydrogen phosphate, which leads to the formation of hydroxypyromorphite and, hence, to the corresponding increase in the pH value.

### *3.5. Summary of Color and Phase Changes in the Pigments*

The color values of the pigments before immersion into the DAP solutions and after 28 days of reaction with DAP, as well as the ΔE\* values, are listed in Table 1. In this research, it was demonstrated that, while the color difference ΔE\* of most pigments tested, including cinnabar (deep red), French ochre (yellow), lapis lazuli (blue), chalk (white), and raw sienna (yellow), were above the threshold detected by the human eye (ΔE\* > 2), with the exception of burnt umber (brown) which showed no detectable color change (ΔE\* < 2), they all showed ΔE\* values below the accepted threshold (ΔE\* ≤ 5) for cultural heritage studies [16,41–48]. Slightly darkening (−ΔL\*) was observed for most pigments, except raw sienna. Red lead, however, showed a significant color change, with ΔE\* = 30.643, which is well above the accepted level.

**Table 1.** Changes in color values of the examined pigments before and after 28 days of immersion in DAP solutions.


Pigments such as chalk and calcite, found as impurity or accessory mineral in some of the colored pigments, also underwent evident phase changes from calcium carbonate into calcium phosphates such as hydroxyapatite. In this case, however, these mineralogical phase changes could be considered as 'favorable', given that they provide an additional binding mechanism which is beneficial to the overall consolidation effect.

Conversely, the changes that occurred in the red lead (Pb3O4) pigment can be characterized as 'non-favorable', resulting in significant color alteration from bright orange to brown (with a ΔE\* = 30.6). Associated phase transformation from lead tetroxide into lead hydroxyapatite possibly occurred via the dissolution–precipitation mechanism described above. As a result, the exposure to DAP caused irreversible color damage in the red lead pigment. The phase transformation and significant color change of red lead caused by the DAP precursor poses significant concerns regarding this consolidation treatment for artifacts painted with this pigment, and therefore DAP-based consolidation would not be recommended.

#### **4. Conclusions**

The optical, physical, and chemical interactions between DAP and six pigments commonly employed in fresco applications (cinnabar, French ochre, chalk, lapis lazuli, raw sienna, and burnt umber) and one additional pigment (red lead) often used for secco applications in wall paintings and other polychrome paintings, were investigated. To study the effects of the application of the DAP precursor on the pigments' color, morphology, and mineralogy, the raw pigments (before treatment) and the reaction products after 28 days of exposure to DAP were evaluated using different and complementary characterization techniques including DM, XRD, FTIR, TGA, SEM–EDS, and FORS.

While color changes seemed to occur for most of the pigments analyzed, the majority of these were below the accepted color change threshold established for cultural heritage surface treatments. Evident phase transformations into HAP were identified only in the pigments containing calcium

carbonate (calcite), such as the chalk pigment (main coloring phase of white pigment) and the pigments raw sienna and burnt umber, where calcite was identified as an accessory mineral. The formation of the HAP network in this context did not affect the overall color of these pigments. A significant color and phase change were only observed in the red lead pigment with the transformation of red lead (lead tetroxide) into hydroxypyromorphite. The DAP treatment on painted surfaces pigmented with red lead could therefore cause serious and irreversible damage to the artwork, both chromatically and chemically. For this reason, surface treatments using DAP solutions should be avoided when red lead is present. As demonstrated, measurable color differences and phase transformations of pigments, occurring immediately after the application of the DAP solution and after two months under controlled environmental exposure conditions, allowed for the assessment of the direct impact of the DAP solution on the color and mineralogy of pigments commonly encountered in archaeological and historic materials of cultural importance.

While this research did not directly evaluate the consolidation effect of DAP for wall paintings and other polychrome paintings, from our previous research evaluating the effects of DAP on calcium hydroxide-rich plaster layers [21] and the current research investigating the interactions between DAP and pigments, it can be inferred that for fresco wall paintings, where pigments are applied with water on the surface of a moist calcium hydroxide-rich plaster layer and are 'fixed' in place by the newly formed calcium carbonate crystals 'embedding' them into the 'surface skin' of the plaster layer, DAP precursors could also have a consolidating effect, without causing any phase or significant color change. As a proof of concept, further research, testing, and long-term monitoring will be conducted on mockups of fresco paintings and on site, where some other steps such as cleaning [81] and de-salination might be necessary prior to consolidation. Additional investigations will also be carried out on the effect of DAP on different organic binding media, a larger number of pigments, and secco wall paintings mockups to assess the extent of the use of DAP as a surface treatment for polychrome surfaces.

**Author Contributions:** Conceptualization, design of experiment, I.K. and X.M.; supervision and project administration: I.K., writing-original draft: X.M. and H.P., writing-review and editing: X.M., H.P., M.B., and I.K., investigation and formal analysis: X.M. and H.P., funding acquisition: I.K.

**Funding:** The work was supported financially by the National Science Foundation (NSF) (Award # 1139227, Solid State and Materials Chemistry program, Division of Materials Research).

**Acknowledgments:** The authors would like to acknowledge the support provided by the Molecular and Nano Archaeology Laboratory at UCLA. The co-author, Hélène Pasco, was a visiting student to the Archaeomaterials Group at UCLA during 2016. The authors would acknowledge the assistance of Christian Fischer and Roxanne Radpour from the department of materials science and engineering of UCLA in FORS measurement and interpretation of the spectra.

**Conflicts of Interest:** The authors declare no conflict of interest.
