Investigation of the Optical, Physical, and Chemical Interactions between Diammonium Hydrogen Phosphate (DAP) and Pigments

: This research investigates and evaluates the optical, physical, and chemical interactions between diammonium hydrogen phosphate (DAP) and seven pigments commonly encountered in archaeological and historic fresco and secco wall paintings and polychrome monuments. The pigments include cinnabar, French ochre, chalk, lapis lazuli, raw sienna, burnt umber, and red lead. The raw pigments were analyzed before and after the interaction with DAP, and the reaction products resulting from the contact of the pigments with the DAP solution were evaluated to obtain a comprehensive understanding of the e ﬀ ects of diammonium hydrogen phosphate on the color, morphology, and chemical composition of the pigments. The results indicated no signiﬁcant change of the color or of the chemistry of cinnabar, French ochre, and lapis lazuli. Carbonate-containing pigments, such as chalk and calcium carbonate, were transformed into calcium phosphate, though without a signiﬁcant change in color. Phase and strong color changes occurred only for the red lead pigment, associated with the transformation of red lead into hydroxypyromorphite. These data established the parameters and identiﬁed the risks of the direct application of DAP solutions on pigments. Further research will be undertaken to assess the potential use of DAP as a consolidant of wall paintings and other polychrome surfaces through testing on wall painting / polychromy mockups and on-site archaeological / historic painted surfaces.


Introduction
Cultural heritage materials including wall paintings and other forms of polychromy and painted architectural surfaces were central to the culture of ancient people. These complex, heterogeneous, and multilayer systems are usually composed of the paint layer (a binary system of pigment(s) and a binding medium) and the substrate (a rock surface or plaster(s) layers) [1,2]. In ancient and historical times, two different techniques were predominantly employed for painting on walls: the fresco (from the Italian, meaning 'fresh') and the secco (from the Italian, meaning 'dry') techniques. In the fresco technique, a small number of pigment powders-compatible with fresco application-are mixed with Reaction 1. Theoretical pathway of the formation of hydroxyapatite (HAP) using diammonium hydrogen phosphate as a precursor.
The superior qualities of HAP as a consolidating agent for calcium carbonate matrices lie in the fact that it has a much lower solubility (K sp = 1.6 × 10 −117 at 25 • C [30]) than calcite (K sp =3.4 × 10 −9 at 25 • C [31]). The lattice parameters of hydroxyapatite and calcite are relatively close, respectively, a = b = 9.43 Å and c = 6.88 Å for HAP [32], and a = b = 9.96 Å and c = 17.07 Å for calcite, considering two molecules per unit cell [33]. This indicates compatibility in the nucleation of the phosphate layer onto the surface of carbonate stones and strong bonding of the newly formed layer onto the substrate [21]. The other advantage is that hydroxyapatite is the least soluble and the most stable calcium phosphate phase in aqueous solutions at pH values higher than 4.2 [34,35]. Also, it has a dissolution rate about 4-5 orders of magnitude lower than that of calcite: R diss, HAP = 1 x 10 −14 moles·cm −2 ·s −1 , and R diss, calcite = 2 × 10 −10 moles·cm −2 ·s −1 at pH = 5.6; R diss, HAP = 3.7 × 10 −14 moles·cm −2 ·s −1 , and R diss, calcite = 5.4 × 10 −9 moles·cm −2 ·s −1 at pH = 4 [36,37]. It is therefore more stable in a range of pH and it is expected to provide additional protection against acid dissolution. In addition, the precursor ammonium phosphate is non-toxic, and a good penetration depth could be obtained in the consolidation treatment [21].
However, despite successful results for the consolidation of decohesive plaster layers as substrates/surface layers of fresco wall paintings [21] and regardless of the fact that some other consolidants, such as a nano calcium hydroxide suspension, have been tested on fresco wall painting mock-ups [38], this DAP-based method has not yet been tested on archaeological wall paintings nor has any thorough assessment been performed on pigments. This research aims to fill this gap of knowledge. Following from our previous research on the application of DAP for the consolidation of fresco plaster layers, here, as a first step, we systematically investigate and evaluate in laboratory-controlled conditions the optical, physical, and chemical effects of the ammonium phosphate precursor of HAP on selected pigments (mainly those compatible with fresco application). The aim is to have a fundamental understanding of the effects (mainly on color change and phase transformations) of this inorganic 'consolidant' precursor on pigments, prior to any testing of the consolidating effect on the paint layer (both fresco and secco) in wall painting mockups and archaeological/historic wall paintings and other polychrome monuments. More specifically, this research investigates the interactions between DAP solutions and seven pigments commonly found in wall paintings and other polychrome surfaces and focuses on answering the following questions: • Can DAP be considered as a potential precursor for a surface treatment of wall paintings (mainly fresco) and other monumental painted architectural surfaces? • Is there any obvious color change of pigments after contact with DAP solutions? • Are any chemical or morphological changes occurring? • What are the possible mechanisms leading to color change and/or other forms of physical and chemical phase transformation?

Materials
A 1M DAP solution was prepared by adding the appropriate amount of DAP (Fisher Scientific, Hampton, NH, USA, purity: 99 + %, used as received) to deionized (DI) water. To study the chemical reaction between the DAP solution and the pigments, seven commercial inorganic pigments purchased from Kremer Pigments Inc. were tested, including six pigments commonly used for fresco application, such as cinnabar (Kremer No.10620), lapis lazuli (Kremer No. 10562), white chalk (Kremer No. 58000), French ochre (Kremer No. 40090), burnt umber (Kremer No. 40710), raw sienna (Kremer No. 40400), and one pigment, red lead (Kremer No. 42500), frequently encountered in secco paintings.

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: ± 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 NanoSEM TM 230 scanning electron microscope (SEM) with field emission gun (FEG) and variable pressure (VP) capabilities, equipped with a Thermo Scientific TM NORAN TM 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 −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 (∆E*) were calculated with the following formula (Equation (1)) as recommended by the CIE: where ∆L*, ∆a*, and ∆b* are the differences in L*, a*, and b* values before and after immersion in the DAP solution. ∆L* describes the change in luminance, ∆a* the change in red/green components, and ∆b* the change in yellow/blue components. While generally ∆E* ≤ 2 is widely acceptable as the value detectable by the human eye [40], a color difference of ∆E* ≤ 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][42][43][44][45][46][47][48].

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).

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). 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).

Cinnabar
Cinnabar has a deep red color with angular particles of various sizes up to 100 μm (Figure 1ad). This observation was based on XRD results ( Figure 1e) and collected FORS spectra (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      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].

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.
Sustainability 2019, 11, x FOR PEER REVIEW 7 of 21 present probably due to the ν3 stretching vibration and the ν4 bending vibration of surface-adsorbed NH4 + [57][58][59]. FORS spectra 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-tometal charge transfer transitions in hematite [60].

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.  and O-Si-O asymmetric stretching vibration of wollastonite, as well as bands in the 700-600 cm −1 region, which could be linked to an overlapping of Al, Si-O 4 tetrahedra symmetric stretching vibration of lazurite and O-Si-O symmetric stretching vibration of wollastonite [61]. The band at 568 cm −1 and the band at 452 cm −1 represent the terminal -O-Si-Obonds bending vibration and Si-O-Si bending vibration, respectively [62][63][64].
The visible spectrum of the lapis lazuli was dominated by an absorption band around 600 nm, corresponding to the electronic transitions for S 3 − (see Figure 3g).

Chalk, Sienna, Burnt Umber
The calculated ∆E* 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 (∆E* ≤ 5) accepted for consolidation applications in cultural heritage [16,[41][42][43][44][45][46][47][48]. The color change of burnt umber pigment remained below the detection limit of human eye.

Chalk
The pigment (CHA-raw) used for this research was a fine powder consisting of natural white calcium carbonate (CaCO 3 ) (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 (CaCO 3 ) particles showed evident transformation into HAP (Ca 10 (PO 4 ) 6 (OH) 2 ) and octacalcium phosphate (OCP, Ca 8 H 2 (PO 4 ) 6 ·5H 2 O). 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. cm −1 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 −1 and the band at 452 cm −1 represent the terminal -O-Si-Obonds bending vibration and Si-O-Si bending vibration, respectively [62][63][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).

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][42][43][44][45][46][47][48]. The color change of burnt umber pigment remained below the detection limit of human eye.

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.  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]. CaCO 3 yielded bands at 712 cm −1 (ν 4 in-plane bending vibration of CO 3 2− ), 873 cm −1 (ν 2 out-of-plane bending vibration of CO 3  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 −1 (ν4 inplane bending vibration of CO3 2− ), 873 cm −1 (ν2 out-of-plane bending vibration of CO3 2− ), 1420 cm −1 (ν3 asymmetric stretching vibration of CO3 2− ), and combination bands at 2513 cm −1 and 1798 cm −1 . In the sample CHA-d28, bands appeared at 468 cm  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 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].

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 (CaSO 4 •2H 2 O), 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 %).
Sustainability 2019, 11, x FOR PEER REVIEW 10 of 21 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].

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 %). Goethite gave a broad band centered at 3141 cm −1 (broad, ν 2 stretching vibration of O-H) and bands at 899 cm −1 (δO-H bending vibration) and 798 cm −1 (γO-H bending vibration, overlapping with quartz and silicate clay) [69][70][71][72][73][74][75][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 −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).

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).  [69][70][71][72][73][74][75][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 −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).

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). (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)   (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 −1 , corresponding to the vibration of CaCO 3 , and bands at 1030, 778, 797, and 463 cm −1 corresponding to the vibration of the silicate (possibly silicate clay and SiO 2 ) group. The bands at 532 and 463 cm −1 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.

Red Lead
The red lead pigment powder analyzed in this study was found to be pure, consisting of minium (Pb 3 O 4 ) with small and irregular particles (Figure 8a-b) ranging in size from 2 µm to 20 µm. 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.

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. 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 (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, Pb 10 (PO 4 ) 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 ∆E* 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 (∆E* ≤ 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 (Figure 8g) indicated that the raw red lead pigment (sample RED-raw) solely consisted of minium (JCPDS PDF No. . 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-PbO 2 ) 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]. Following this step, two processes occur simultaneously: (1) The unstable PbO 2 fragments formed from the dissolution of Pb 3 O 4 are reduced to Pb 2+ , as suggested by the Reaction 4.
Since both Pb 3 O 4 and β-PbO 2 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 PbO 2 (labelled as PbO 2(unstable) above) in Pb 3 O 4 and the precipitation of β-PbO 2 [77].
During the dissolution reactions (Reaction 2 to Reaction 4) that occur on the surface of Pb 3 O 4 , a layer of very fine particles/precipitates of PbO 2 forms during the earliest dissolution stages. Once formed, PbO 2 can either remain as a spectator species or be reduced, as suggested by Reaction 5 [77,79,80], releasing more Pb 2+ .
However, PbO 2 formed on the surface of Pb 3 O 4 is likely to passivate the substrate's surface, inhibiting further dissolution of Pb 3 O 4 . Still, no such particles/precipitates were detected using XRD during the first month, suggesting that the formation of PbO 2 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 PbO 2 layer on the Pb 3 O 4 surface, the (NH 4 ) 2 HPO 4 solution required longer time to diffuse into the Pb 3 O 4 substrate. On the basis of the XRD analysis, the β-PbO 2 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-PbO 2 . 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.

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. 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.

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. 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.

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 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 (Pb 3 O 4 ) with diammonium hydrogen phosphate, which leads to the formation of hydroxypyromorphite and, hence, to the corresponding increase in the pH value.

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 ∆E* values, are listed in Table 1. In this research, it was demonstrated that, while the color difference ∆E* 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 (∆E* > 2), with the exception of burnt umber (brown) which showed no detectable color change (∆E* < 2), they all showed ∆E* values below the accepted threshold (∆E* ≤ 5) for cultural heritage studies [16,[41][42][43][44][45][46][47][48]. Slightly darkening (−∆L*) was observed for most pigments, except raw sienna. Red lead, however, showed a significant color change, with ∆E* = 30.643, which is well above the accepted level. 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 (Pb 3 O 4 ) pigment can be characterized as 'non-favorable', resulting in significant color alteration from bright orange to brown (with a ∆E* = 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.

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.