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sustainability
Article
Investigation of the Optical, Physical, and Chemical
Interactions between Diammonium Hydrogen
Phosphate (DAP) and Pigments
Xiao Ma 1, 2, *,†, Hélène Pasco 3, Magdalena Balonis 1and Ioanna Kakoulli 1,2,4
1Department of Materials Science and Engineering, University of California at Los Angeles, 410 Westwood
Plaza, Los Angeles, CA 90095-1595, USA
2Molecular and Nano Archaeology Laboratory, Henry Samueli School of Engineering and Applied Science,
Engineering V-1230B, 410 Westwood Plaza, Los Angeles, CA 90095-1595, USA
3Sorbonne Université, Facultédes Sciences et Ingénierie, UFR 926, F-75005 Paris, France
4UCLA/Getty Conservation Program, A210 Fowler Building, University of California at Los Angeles,
Los Angeles, CA 90095-1510, USA
*Correspondence: xiaomapurdue@gmail.com
†Current address: Scientific Research Department, National Gallery of Art, 2000B South Club Drive,
Landover, MD 20785, USA.
Received: 19 May 2019; Accepted: 8 July 2019; Published: 11 July 2019
Abstract:
This research investigates and evaluates the optical, physical, and chemical interactions
between a diammonium hydrogen phosphate (DAP) solution 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 the DAP solution, and the
reaction products resulting from the contact of the pigments with the DAP solution were evaluated
to obtain a comprehensive understanding of the effects of diammonium phosphate on the color,
morphology, and chemical composition of the pigments. The results indicated no significant changes of
the color or of the chemistry of cinnabar, French ochre, and lapis lazuli. Carbonate-containing primary
and secondary (found as impurities in earth pigments) 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.
Keywords:
hydroxyapatite; diammonium hydrogen phosphate; pigment alteration; wall painting
consolidation; cultural heritage
1. 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
binding media) 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 a fresco
application, a small number of pigment powders—compatible with fresco application—were mixed
Sustainability 2019,11, 3803; doi:10.3390/su11143803 www.mdpi.com/journal/sustainability
Sustainability 2019,11, 3803 2 of 20
with water and applied on a freshly laid or moist calcium hydroxide/lime (Ca(OH)
2
)-rich plaster layer.
Setting (hardening) of the lime plaster involved a chemical reaction between Ca(OH)
2
and carbon
dioxide (CO
2
) present in the atmosphere to form a calcium carbonate lattice within which the pigments
were ‘fixed’ and became an integral part of the wall. These chemical reactions helped produce durable
wall paintings. Owing to the high alkalinity of the lime and the exothermic reaction associated with
the setting of the lime, only a small number of pigments are compatible with the fresco technique,
and therefore ancient fresco paintings contain a limited palette of colors. The secco technique, on
the other hand, involves no chemical reaction for the fixation of the pigments. In a secco application,
the pigments are mixed with any film-forming binding medium such as egg, siccative oil, gum, and
others [
1
] and applied on any type of finished plaster layer including gypsum, earth-based plasters,
and lime plaster layers (fully carbonated).
As wall paintings constitute an integral part of the architectural ensemble where they are found,
they are inevitably exposed to an open system of events. As a result, the physical and chemical
attributes of the system and individual constituent materials (i.e., plaster(s), pigment(s), binding
media) can be impacted by fluctuations in the temperature, relative humidity, and presence of salts,
microorganisms, and pollution in the surrounding environment. These conditions can compromise the
stability of the system, resulting in the delamination of the plaster layers, staining, flaking, and losses
of the paint and plaster layers, and powdering [
1
,
3
]. For archaeological wall paintings in particular,
the risks for their preservation are even greater as the sudden change in the environmental conditions
at the time of the excavation—mainly of the temperature, relative humidity, and light—can cause
irreversible damage and degradation [1].
Over the course of the past decades, extensive studies have been carried out using a variety of
consolidation treatments to improve the condition and re-establish the lost cohesion of decorated
architectural surfaces and wall paintings [
4
–
12
]. These studies critically indicate that choosing a proper
consolidating agent for these porous materials, especially those found in situ, is challenging. An
appropriate consolidant for wall paintings needs to re-establish cohesion of the powdery layers at
the surface and subsurface levels and provide mechanical strength and abrasion resistance, without
causing any discernible color alteration [13].
Recent studies [
14
–
25
] have demonstrated a considerable potential for improving consolidation
methods of degraded calcium-carbonate matrices of a technique consisting in bio-mimicking the growth
of hydroxyapatite (HAP, with the formula Ca
5
(PO
4
)
3
(OH) but usually written as Ca
10
(PO
4
)
6
(OH)
2
to
denote that the crystal unit cell comprises two formula units), the main mineralogical component of
teeth and bones [
15
,
26
,
27
]. HAP was proved to be effective in binding grain boundaries and improving
the mechanical properties of limestone, such as tensile strength, ultrasonic pulse velocity, resistance to
abrasion [16,19,20].
HAP is formed in situ by activating reactions between Ca in calcium carbonate (CaCO
3
)-rich
layers and ammonium phosphate precursors. The theoretical chemical pathway of HAP formation
using diammonium hydrogen phosphate (DAP) as the precursor is presented below (Reaction 1) [
28
].
The resulting hydroxyapatite network is expected to improve the cohesion between loose particles at
the surface and subsurface of a wall painting [21,29].
10 CaCO3+6(NH4)2HPO4→Ca10(PO4)6(OH)2+10 CO2↑+12 NH3↑+8 H2O
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
Sustainability 2019,11, 3803 3 of 20
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 despite 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 investigated and evaluated 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 was 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 investigated the interactions between DAP
solutions and seven pigments commonly found in wall paintings and other polychrome surfaces and
focused on answering the following questions:
•
Could 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?
2. Materials and Methods
2.1. 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.
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:
±
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
Sustainability 2019,11, 3803 4 of 20
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-K
α
radiation,
λ
=1.5404 Å, voltage 40 kV, beam current
40 mA, and a 2–80
◦
2
θ
exploration range with a step size of 0.014
◦
2
θ
. 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 analysis was performed on a JASCO FT/IR-420 Fourier-Transform Infrared
Spectrometer using the KBr pellet method. The sample 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) spectra were measured 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:
∆E∗= [(∆L∗)2+(∆a∗)2+(∆b∗)2]1/2(1)
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 [16,41–48].
Sustainability 2019,11, 3803 5 of 20
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
∆
E* 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 (
∆
E*
≤
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
µ
m (Figure 1a–d).
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–d).
Sustainability 2019, 11, x FOR PEER REVIEW 5 of 21
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). 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–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.
3.1.2. French Ochre
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.
Sustainability 2019,11, 3803 6 of 20
3.1.2. French Ochre
Based on the XRD, FTIR, and FORS analysis (Figure 2), no detectable phase transformations were
observed in the pigment particles subjected to the immersion in DAP solution (FRE-d28) as compared
to the untreated powders (FRE-raw).
Sustainability 2019, 11, x FOR PEER REVIEW 6 of 21
Based on the XRD, FTIR, and FORS analysis (Figure 2), no detectable phase transformations
were observed in the pigment particles subjected to the immersion in DAP solution (FRE-d28) as
compared to the untreated powders (FRE-raw).
Figure 2. (a) Photomicrograph of the French ochre (FRE)-raw sample; (b) micrographs of the FRE-
raw. Qtz stands for quartz and Kao stands for kaolinite; (c) photomicrograph of the sample FRE-d28;
(d) micrographs of the FRE-d28; (e) XRD pattern of the samples FRE-raw and FRE-d28; (f) FTIR
spectra of the FRE-raw, FRE-d1, and FRE-d28 samples; (g) FORS spectra of the French ochre samples
FRE-raw, FRE-d1, FRE-d7, FRE-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.
The XRD analysis of all samples (Figure 2e) revealed the presence of quartz, muscovite, kaolinite,
and hematite. FTIR spectroscopy analysis (Figure 2f) further corroborated the results. Kaolinite
(Al
2
Si
2
O
5
(OH)
4
) disclosed vibrational bands at 3696, 3670, 3653 cm
−1
(surface hydroxyl groups
stretching vibration), 3621 cm
−1
(inner hydroxyl groups stretching vibration), 1030 cm
−1
(Si–O–Si
stretching vibration), 1008 cm
−1
(Si–O–Al stretching vibration), 938 and 912 cm
−1
(Al–OH deformation
vibration), 693 cm
−1
(Si–O–Si symmetrical bending vibration), 538 cm
−1
(Si–O–Al stretching vibration),
and 469 cm
−1
(Si–O–Si asymmetrical bending vibration). Quartz (SiO
2
) exhibited vibrational bands at
1162 cm
−1
(Si–O–Si rocking vibration), 1096 cm
−1
(Si–O–Si asymmetrical stretching vibration), doublets
at 777 and 799 cm
−1
(Si–O–Si symmetrical stretching vibration) and at 693 cm
−1
and 469 cm
−1
(Si–O–Si
symmetrical and asymmetrical bending vibration, overlapping with kaolinite). It should be noted
that the Fe–O vibration of hematite which yields vibrational bands at 538 cm
−1
and 469 cm
−1
, were
overlapping with the Si–O–Al stretching vibration of kaolinite and the Si–O–Si bending vibration of
kaolinite/quartz, respectively [50–56]. The bands at 3432 cm
−1
and 1626 cm
−1
corresponded to the
O–
H stretching and O–H bending of surface-absorbed water. The bands at 3130 cm
−1
and 1399 cm
−1
were
Figure 2.
(
a
) Photomicrograph of the French ochre (FRE)-raw sample; (
b
) micrographs of the FRE-raw.
Qtz stands for quartz and Kao stands for kaolinite; (
c
) photomicrograph of the sample FRE-d28;
(
d
) micrographs of the FRE-d28; (
e
) XRD pattern of the samples FRE-raw and FRE-d28; (
f
) FTIR
spectra of the FRE-raw, FRE-d1, and FRE-d28 samples; (
g
) FORS spectra of the French ochre samples
FRE-raw, FRE-d1, FRE-d7, FRE-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.
The XRD analysis of all samples (Figure 2e) revealed the presence of quartz, muscovite, kaolinite,
and hematite. FTIR spectroscopy analysis (Figure 2f) further corroborated the results. Kaolinite
(Al
2
Si
2
O
5
(OH)
4
) disclosed vibrational bands at 3696, 3670, 3653 cm
−1
(surface hydroxyl groups
stretching vibration), 3621 cm
−1
(inner hydroxyl groups stretching vibration), 1030 cm
−1
(Si–O–Si
stretching vibration), 1008 cm
−1
(Si–O–Al stretching vibration), 938 and 912 cm
−1
(Al–OH deformation
vibration), 693 cm
−1
(Si–O–Si symmetrical bending vibration), 538 cm
−1
(Si–O–Al stretching vibration),
and 469 cm
−1
(Si–O–Si asymmetrical bending vibration). Quartz (SiO
2
) exhibited vibrational bands at
1162 cm
−1
(Si–O–Si rocking vibration), 1096 cm
−1
(Si–O–Si asymmetrical stretching vibration), doublets
at 777 and 799 cm
−1
(Si–O–Si symmetrical stretching vibration) and at 693 cm
−1
and 469 cm
−1
(Si–O–Si
symmetrical and asymmetrical bending vibration, overlapping with kaolinite). It should be noted
that the Fe–O vibration of hematite which yields vibrational bands at 538 cm
−1
and 469 cm
−1
, were
overlapping with the Si–O–Al stretching vibration of kaolinite and the Si–O–Si bending vibration of
Sustainability 2019,11, 3803 7 of 20
kaolinite/quartz, respectively [
50
–
56
]. The bands at 3432 cm
−1
and 1626 cm
−1
corresponded to the O–H
stretching and O–H bending of surface-absorbed water. The bands at 3130 cm
−1
and 1399 cm
−1
were
present probably due to the
ν3
stretching vibration and the
ν4
bending vibration of surface-adsorbed
NH4+[57–59].
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 offat 800 nm). These were attributed to ligand-to-metal
charge transfer transitions in hematite [60].
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.
Sustainability 2019, 11, x FOR PEER REVIEW 7 of 21
present probably due to the ν
3
stretching vibration and the ν
4
bending vibration of surface-adsorbed
NH
4+
[57–59].
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-to-
metal charge transfer transitions in hematite [60].
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
−1
region
that could be assigned to overlapping of Al, Si–O
4
tetrahedra asymmetric stretching vibration of
lazurite and O–Si–O asymmetric stretching vibration of wollastonite, as well as bands in the 700–600
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
−1
region that
Sustainability 2019,11, 3803 8 of 20
could be assigned to overlapping of Al, Si–O
4
tetrahedra asymmetric stretching vibration of lazurite
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–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
∆
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
–
48
]. The
color change of burnt umber pigment remained below the detection limit of human eye.
3.2.1. 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
µ
m. 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.
Sustainability 2019, 11, x FOR PEER REVIEW 8 of 21
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–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 S
3−
(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.
3.2.1. 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.
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.
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.
Sustainability 2019,11, 3803 9 of 20
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
(
ν4
in-plane bending
vibration of CO
32−
), 873 cm
−1
(
ν2
out-of-plane bending vibration of CO
32−
), 1420 cm
−1
(
ν3
asymmetric
stretching vibration of CO
32−
), and combination bands at 2513 cm
−1
and 1798 cm
−1
. In the sample
CHA-d28, bands appeared at 468 cm
−1
(
ν2
bending mode of O–P–O bond), 562 cm
−1
(
ν4
bending mode
of O–P–O bond), 601 cm
−1
(
ν4
bending mode of O–P–O bond), 957 cm
−1
(
ν1
symmetric stretching
mode of P–O bond), and 1033 cm
−1
(
ν3
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.
Sustainability 2019, 11, x FOR PEER REVIEW 9 of 21
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
32−
), 873 cm
−1
(ν
2
out-of-plane bending vibration of CO
32−
), 1420 cm
−1
(ν
3
asymmetric stretching vibration of CO
32−
), and combination bands at 2513 cm
−1
and 1798 cm
−1
. In the
sample CHA-d28, bands appeared at 468 cm
−1
(ν
2
bending mode of O–P–O bond), 562 cm
−1
(ν
4
bending mode of O–P–O bond), 601 cm
−1
(ν
4
bending mode of O–P–O bond), 957 cm
−1
(ν
1
symmetric
stretching mode of P–O bond), and 1033 cm
−1
(ν
3
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
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
Sustainability 2019,11, 3803 10 of 20
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
µ
m. 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].
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
(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 %).
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
−1
(ν
3
asymmetric stretching vibration and ν
1
symmetric stretching
vibration of water molecule, respectively), 1685 and 1621 cm
−1
(O–H bending vibration), 1112 cm
−1
(ν
3
asymmetric stretching vibration of SO
42-
tetrahedron), 669 cm
−1
(ν
4
asymmetric bending vibration of
SO
42−
tetrahedron), and 600 cm
−1
(ν
4
asymmetric bending vibration of SO
42-
tetrahedron). Quartz
displayed characteristic bands at 1096 cm
−1
(Si–O–Si asymmetrical stretching vibration), 798 cm
−1
(Si–
O–Si symmetrical stretching vibration), and 469 cm
−1
(Si–O–Si asymmetrical bending vibration),
while silicate clay had bands at 3621 cm
−1
(O–H stretching vibration of structural hydroxyl group)
and 1029 cm
−1
(Si–O–Si asymmetrical stretching vibration) and shared (with quartz) bands at 798 cm
−1
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
−1
(
ν3
asymmetric stretching vibration and
ν1
symmetric stretching
vibration of water molecule, respectively), 1685 and 1621 cm
−1
(O–H bending vibration), 1112 cm
−1
(
ν3
asymmetric stretching vibration of SO
42−
tetrahedron), 669 cm
−1
(
ν4
asymmetric bending vibration
of SO
42−
tetrahedron), and 600 cm
−1
(
ν4
asymmetric bending vibration of SO
42−
tetrahedron). Quartz
displayed characteristic bands at 1096 cm
−1
(Si–O–Si asymmetrical stretching vibration), 798 cm
−1
(Si–O–Si symmetrical stretching vibration), and 469 cm
−1
(Si–O–Si asymmetrical bending vibration),
while silicate clay had bands at 3621 cm
−1
(O–H stretching vibration of structural hydroxyl group) and
Sustainability 2019,11, 3803 11 of 20
1029 cm
−1
(Si–O–Si asymmetrical stretching vibration) and shared (with quartz) bands at 798 cm
−1
and 469 cm
−1
. CaCO
3
produced bands at 712 cm
−1
, 877 cm
−1
, 1425 cm
−1
, 1797 cm
−1
, and 2513 cm
−1
.
Goethite gave a broad band centered at 3141 cm
−1
(broad,
ν2
stretching vibration of O–H) and bands
at 899 cm
−1
(
δ
O–H bending vibration) and 798 cm
−1
(
γ
O–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
−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).
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).
Sustainability 2019, 11, x FOR PEER REVIEW 11 of 21
and 469 cm
−1
. CaCO
3
produced bands at 712 cm
−1
, 877 cm
−1
, 1425 cm
−1
, 1797 cm
−1
, and 2513 cm
−1
.
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) [69–76]. 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).
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
−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
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
Sustainability 2019,11, 3803 12 of 20
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.
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.
Sustainability 2019, 11, x FOR PEER REVIEW 12 of 21
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.
3.3. 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.
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,
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 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
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 2
θ
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 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,
Sustainability 2019,11, 3803 13 of 20
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
-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
]. The dissolution
reaction of Pb
3
O
4
begins to occur at the surface of Pb
3
O
4
(Reaction 2). Pb
3
O
4
first releases Pb
2+
species
from a tetrahedrally coordinated Pb
3
(II,IV)O
4
site through a ligand substitution, leaving unstable
octahedral PbO
2
fragments (octahedral arrangement hosting Pb
4+
ions in the crystalline structure of
Pb
3
O
4
) in the solid. The precipitation of lead hydroxyapatite (Pb
10
(PO
4
)
6
(OH)
2
) observed after the
immersion of Pb
3
O
4
in DAP suggests a reaction between the supersaturated Pb
2+
ions released during
Pb3O4dissolution and the phosphate (PO43−) ions delivered through the DAP solution.
Pb3O4+2H2O(aq)→2 Pb2+
(aq)+PbO2(s)+4OH−
(aq)
Reaction 2. Dissolution reaction of Pb3O4in aqueous solution.
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.
PbO2(unstable)+2e−+2H2O(aq)→Pb2+
(aq)+4OH−
(aq)
Reaction 3. Reduction of unstable PbO2fragment into Pb2+.
(2)
The nucleation of a newly stable β-PbO2from Pb2+ions, as suggested by the Reaction 5:
Pb2+
(aq)+4OH−
(aq)→PbO2(stable)+2e−+2H2O(aq)
Reaction 4. Oxidation of Pb2+to stable β-PbO2(platternite).
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
PbO2(unstable) above) in Pb3O4and the precipitation of β-PbO2[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 Pb2+.
PbO2(s)+H2O(aq)→Pb2+
(aq)+2OH−
(aq)+0.5 O2(aq)
Reaction 5. Reductive dissolution of platternite (β-PbO2) in aqueous solution.
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
Sustainability 2019,11, 3803 14 of 20
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.
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.
Sustainability 2019, 11, x FOR PEER REVIEW 14 of 21
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.
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 (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.
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
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 (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.
Sustainability 2019,11, 3803 15 of 20
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
∆
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
–
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.
Table 1.
Changes in color values of the examined pigments before and after 28 days of immersion in
DAP solutions.
Pigment Name
Value Change after 28 Days Reaction with DAP Change in Color
∆E*
∆L∆a* ∆b*
Cinnabar −1.759 −1.874 −2.429 3.5
French Ochre −1.043 −0.399 −3.201 3.4
Lapis Lazuli −2.844 1.246 2.773 4.2
Chalk −4.424 0.508 −2.028 4.9
Raw Sienna 1.989 −1.48 −0.629 2.6
Burnt Umber −0.865 1.224 0.709 1.7
Red lead −10.492 −18.057 −22.425 30.6
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.
4. Conclusions
The optical, physical, and chemical interactions between a DAP solution 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, most 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
Sustainability 2019,11, 3803 16 of 20
(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, H
é
l
è
ne 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.
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