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When Red Turns Black: Influence of the 79 AD Volcanic Eruption and
Burial Environment on the Blackening/Darkening of Pompeian
Cinnabar
Silvia Pérez-Diez,*Africa Pitarch Martí, Anastasia Giakoumaki, Nagore Prieto-Taboada,
Silvia Fdez-Ortiz de Vallejuelo, Alberta Martellone, Bruno De Nigris, Massimo Osanna,
Juan Manuel Madariaga, and Maite Maguregui*
Cite This: https://doi.org/10.1021/acs.analchem.1c02420
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ABSTRACT: It is widely known that the vivid hue of red cinnabar
can darken or turn black. Many authors have studied this
transformation, but only a few in the context of the archeological
site of Pompeii. In this work, the co-occurrence of different
degradation patterns associated with Pompeian cinnabar-contain-
ing fresco paintings (alone or in combination with red/yellow
ocher pigments) exposed to different types of environments (pre-
and post-79 AD atmosphere) is reported. Results obtained from
the in situ and laboratory multianalytical methodology revealed the
existence of diverse transformation products in the Pompeian
cinnabar, consistent with the impact of the environment. The effect
of hydrogen sulfide and sulfur dioxide emitted during the 79 AD
eruption on the cinnabar transformation was also evaluated by
comparing the experimental evidence found on paintings exposed and not exposed to the post-79 AD atmosphere. Our results
highlight that not all the darkened areas on the Pompeian cinnabar paintings are related to the transformation of the pigment itself,
as clear evidence of darkening associated with the presence of manganese and iron oxide formation (rock varnish) on fragments
buried before the 79 AD eruption has also been found.
The Roman city of Pompeii was destroyed after the
eruption of Mount Vesuvius in 79 AD. Although this was
an unfortunate natural and societal event, it resulted in a
remarkably good conservation of its remains, thanks to the
burial of the city under the pyroclastic flow. However, some of
the pigments applied on the walls of Pompeii experienced
transformations due to the eruption, such as the blackening
process of hematite (α-Fe2O3)
1
and the dehydration of yellow
ocher (goethite, α-FeOOH) into hematite.
2,3
A recent study
has shown that another reason for the degradation of the mural
paintings of Pompeii is the crystallization of salts coming from
the pyroclastic materials ejected in the 79 AD eruption.
4
In
addition, since the first archeological excavations in the 18th
century, the archeological park has suffered a continuous
decay, due to its exposure to the modern atmosphere and the
(former) application of restoration products that are no longer
used.
5
The study of ancient sources
6,7
and archeological records
demonstrates that red cinnabar (α-HgS) has been used as a
pigment since antiquity. This precious pigment, employed in
the mural paintings of the archeological site of Pompeii, suffers
from blackening. Hence, Vitruvius did not encourage its
application in open spaces (e.g., peristylia), since its exposure
to sunlight and moonlight
6
was already thought at that time to
be responsible for its deterioration.
Prominent examples of cinnabar blackening are found at the
Casa della Farnesina (Rome) or the Villa dei Misteri
(Pompeii).
8,9
This process occurs to a lesser degree in several
locations and can remain unnoticed by nonexperts.
After visual inspection, the color of the altered cinnabar from
Pompeii looks blacker
10
than the one on other discolored
cinnabar easel paintings.
11
In the latter, the altered cinnabar/
vermilion shows brownish to grayish hues.
11,12
The blackening of cinnabar has traditionally been attributed
to light exposure and transformation of red α-HgS (trigonal
crystal system) into black β-HgS metacinnabar (cubic crystal
structure), which is reported to take place at 344 ±2°C.
13
However, there exist scarce confirmations of black metacinna-
Received: June 8, 2021
Accepted: November 10, 2021
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bar detection in darkened cinnabar.
14,15
Since Raman spec-
troscopy cannot distinguish cinnabar from metacinnabar, other
techniques such as X-ray diffraction (XRD)
14
or pump-probe
microscopy
15
could be applied for that purpose. Nevertheless,
although different cinnabar-based mural paintings are exposed
to light, not all of them show the same degree of
transformation and even some areas do not present any sign
of darkening or blackening.
8,11
Hence, other variables, which
could contribute to this transformation, should be considered
for its complete explanation.
Further examples of the darkening or blackening process of
cinnabar in presence of Cl in easel and mural paintings
featuring mercury chlorides or Hg-S-Cl compounds (calomel:
Hg2Cl2, mercury (II) chloride: HgCl2, corderoite: α-Hg3S2Cl2,
terlinguaite: Hg2OCl, kenhsuite: γ-Hg3S2Cl2)havebeen
published in the past years.
10,11,16−18
Another plausible
degradation pathway that involves the formation of gypsum
crusts (possibly favored by the photodecomposition of
cinnabar)
10
has been proposed for the Vesuvian mural
paintings of Torre del Greco (Campania), in which calcite
acts as a binder: the formation of gypsum crusts as a result of
calcite sulfation,
10
favored by the photodecomposition of
cinnabar. The subsequent accumulation of airborne particulate
matter and organic pollutants inside the porous structure of
gypsum gives the crust its black color. Cotte et al.
10
mentioned
that calcite sulfation could take place due to the influence of
the SO2present in the polluted atmosphere or to the oxidized
S produced by the decomposition of HgS. This last hypothesis
might explain the failure to identify black crusts (gypsum crusts
with airborne particulate matter/organic pollutants) in murals
from the Vesuvian area decorated with pigments, other than
cinnabar.
In this work, blackened/darkened cinnabar paintings (alone
or in combination with red/yellow ocher pigments) have been
analyzed in situ and in the laboratory through a multianalytical
methodology. The main goals of this study were (i) to
determine the role of the 79 AD volcanic eruption in the
blackening of Pompeian murals decorated with cinnabar and
(ii) to evaluate whether different transformation phenomena
can be identified on samples protected from the pre- and post-
79 AD volcanic eruption.
To achieve these goals, three different kinds of cinnabar
paintings were compared: (i) painted areas impacted by the 79
AD eruption, excavated more than 150 years ago and exposed
to the modern atmosphere since then; (ii) painted panels
impacted by the 79 AD eruption, removed during the
excavations of the 19th century, stored at the Naples National
Archaeological Museum (MANN) and thus, protected from
the modern atmosphere; and (iii) painting fragments exposed
to the ancient atmosphere of Pompeii, presumably detached
after the 62 AD earthquake and deposited in a house pit since
then.
■EXPERIMENTAL SECTION
Samples and Studied Mural Paintings. Three Pom-
peian houses were selected for this study: House of Marcus
Lucretius (Regio IX, 5, 3/24), House of Ariadne (Regio VII, 4,
31/51), and House of the Golden Cupids (Regio VI, 16, 7)
(see Table S1). All the houses have suffered the influence of
the volcanic eruption and the preserved mural paintings have
been exposed to the modern atmosphere since their excavation
(19th century to beginning of 20th century).
Two samples (ATT2007/14 and 16/56) from the triclinium
of the House of Marcus Lucretius (see Table S1) were
considered. In the wall paintings of this room, the blackening
of hematite pigment was previously studied, being possible to
identify the presence of coquimbite/paracoquimbite
(Fe2(SO4)3·9H2O) as degradation product of the pigment.
1
Interestingly, this house also presented a deposit where earlier
detached mural decorations were abandoned and buried. This
deposit was used to cast aside detached fragments, possibly as a
consequence of the 62 AD earthquake that damaged the
murals of the house.
19
This waste pit was excavated during the
EPUH (Expeditio Pompeiana Universitatis Helsingiensis)
campaign in 2005. Since then, the recovered fragments have
been stored in the dark. In this work, two fragments from this
deposit (samples 3T, Red A) showing dark stains on the
cinnabar painting layer were considered. Additionally, panel
paintings extracted from the triclinia (panel references 9206,
9285, 8992, and 9103, the latter from the summer triclinium)
in the excavations of the 19th century and stored since then at
the MANN were also in situ analyzed. The three panels
belonging to the triclinium are surrounded by a blackened red
frame, which could have been painted with red cinnabar.
From the House of Ariadne, three samples were considered
(samples 6, 17, and 18; see Table S1).
Finally, in the House of the Golden Cupids, the blackened
cinnabar decorations from the exedra (Room G; see Table S1)
were studied. Due to sampling restrictions in this house, the
analyses were performed in situ, without taking any sample.
Portable and Benchtop Instrumentation. The in situ
molecular analysis was performed using a portable innoRam
Raman spectrometer (B&W Tek, Newark, USA) equipped
with a CleanLaze technology 785 nm excitation laser (<300
mW laser output power) and mounting the probe on a
motorized tripod (MICROBEAM S.A. Barcelona, Spain). For
the in situ elemental analysis, the XMET5100 (Oxford
Instruments, UK) Handheld Energy Dispersive X-ray Fluo-
rescence spectrometer (HH-EDXRF), equipped with an Rh X-
ray tube, was used. Details about the normalization procedure
to compare the S and Cl counts extracted from the walls and
panels under study can be reviewed in the Supporting
Information.
In the laboratory, the molecular study of the samples was
achieved using the inVia confocal Raman microscope
(Renishaw, Gloucestershire, UK). The main objective lens
used was the 50×one. Excitation lasers of 785 (nominal laser
power 350 mW) and 532 nm (nominal laser power 50 mW)
were employed for the acquisition of the spectra. The spectra
were acquired in the 60−1200 cm−1or 60−3000 cm−1spectral
range and accumulated 3, 5 to 10 times for 5−10 s.
To confirm molecular results, an elemental imaging study
was conducted on sample Red A. For that the M4 TORNADO
(Bruker Nano GmbH, Berlin, Germany) EDXRF spectrometer
was used. Elemental distribution maps were acquired at down
to 25 μm of lateral resolution using a use of polycapillary lens
and with the Rh X-ray tube working at 50 kV and 600 μA. The
spectral acquisition and data treatment were performed using
the ESPRIT software from Bruker.
To evaluate the composition of the black stains of the
sample 3T, X-ray Photoelectron Spectroscopy (XPS) and
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-
SIMS) were employed.
XPS analysis was conducted using a Thermo ScientificK-
Alpha ESCA instrument equipped with aluminum Kα1,2
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B
monochromatic radiation at 1486.6 eV. Neutralization of the
surface charge was achieved by using both a low energy flood
gun (electrons in the range 0−14 eV) and a low energy Ar-ions
gun. Photoelectrons were collected from a take-offangle of 90°
relative to the sample surface. The measurement was done in a
Constant Analyzer Energy mode (CAE) with a 100-eV pass
energy for survey spectra and 20-eV pass energy for high
resolution spectra. Charge referencing was done by setting the
lower binding energy C 1s photopeak at 285.0 eV C 1s
hydrocarbon peak. Surface elemental composition was
determined using the standard Scofield photoemission cross
sections.
A TOF-SIMS IV instrument from Ion-Tof GmbH Germany
was employed to collect the mass spectra and to conduct
mapping. A pulsed Bi3ion beam at 25 keV impacted the
sample, the generated secondary ions were extracted with a 10
kV voltage, and their TOF from the sample to the detector was
measured in a reflectron mass spectrometer. Pulsed Bi3beam
at 25 keV and incidence of 45°were used to scan 500 ×500
μm2areas.
Additional details of the experimental aspects and data
treatment conducted using specific benchtop and portable
instruments are available in the Supporting Information.
■RESULTS AND DISCUSSION
Characterization of Blackened Cinnabar on Mural
Paintings Impacted by the 79 AD Eruption and
Nowadays Exposed to the Atmosphere. The eastern
and southern walls of the triclinium of the House of Marcus
Lucretius revealed the occurrence of Fe, Hg, and S, confirming
the presence of cinnabar, together with red and yellow
ochers.
20
In situ Raman measurements allowed the systematic
identification of calomel (Hg2Cl2) and gypsum (CaSO4·2H2O)
on the blackened cinnabar areas. Cl was also identified by HH-
EDXRF. Additional analytical details can be found in Table S2.
To evaluate the presence of additional compounds in the
triclinium of the House of Marcus Lucretius, samples from the
northern wall (ATT2007/14) and southern wall (16/56) were
analyzed in the laboratory by Raman microscopy (Table S2).
Calomel was present in gray-whitish particles of sample 16/56
(see Figure 1a). In sample ATT2007/14, extracted from the
upper red frame of the central panel, wax (band at 1062 cm−1),
cinnabar (weak band at 254 cm−1), calomel (Hg2Cl2, bands at
168 and 275 cm−1), and gypsum (CaSO4·2H2O, bands at 1008
and 1130 cm−1) were detected (see Figure 1b).
The 1062 cm−1Raman band was attributed to a wax applied
in the 19th century restorations
5
of the mural paintings and
not to the presence of nitratine (NaNO3), based on the
detection of a series of signals ascribable to an organic
compound
21
(see Figure S1, Supporting Information). The
presence of the 1734 cm−1band, assigned to the ν(CO)
vibrational mode, suggests the occurrence of a saturated wax.
The proposed assignment of the rest of the Raman bands is
shown in Table S3.
Both samples extracted from the triclinium of the House of
Marcus Lucretius show a dark crust on the top of the painting
layer (see for example the microscopic observation of sample
16/56, Figure 1a). To obtain further insights into the gypsum
and calomel distribution on these samples, cross sections were
studied by SEM-EDS. Figure 1a shows part of the stratigraphy
of sample 16/56, composed of a “black crust”layer, a pictorial
layer, and a plaster. An EDS map of the whole stratigraphy
(Figure 1c) reveals the accumulation of S, attributed to the
presence of gypsum in the “black crust”over the pictorial layer.
In the latter, it was possible to detect bright particles (marked
with a circle in Figure 1c,d) distributed throughout the layer.
The EDS analyses (Figure 1e) confirmed the detection of both
Hg and Cl in those particles (see Figure 1d), related to the
presence of calomel. The cross section of the sample
ATT2007/14 also revealed a black crust formed on the top
of a pictorial layer with Hg-rich particles and chlorine.
In a preliminary in situ Raman screening from the area
where sample 6 was obtained in the House of Ariadne, gypsum
and calomel had been detected. This last was confirmed later
in the laboratory with additional Raman analyses conducted on
sample 6. Furthermore, microscopic observations allowed the
identification of cinnabar as random pigment particles in a
yellow pictorial layer, as in the case of the samples from the
House of Marcus Lucretius (see Figure 2a). Raman analyses
performed on these particles (see Figure 2b) showed the
presence of goethite (FeOOH, bands at 301, 387, 483, 551,
and 686 cm−1), related to the yellow color (yellow ocher),
together with the presence of tridimite (high-temperature
Figure 1. (a) Optical micrograph of sample 16/56 (particle circled in
red in panels (c) and (d)). (b) Raman spectra of samples ATT2007/
14 and 16/56 (north and south walls of the triclinium, House of
Marcus Lucretius) showing the Raman bands of calomel (167/168,
275 cm−1), cinnabar (254, 283, 343 cm−1), gypsum (416, 1008, 1130
cm−1), wax (1062 cm−1), and calcite (1087 cm−1). (c) From top to
bottom: BSE micrograph, and EDS map of 16/56 cross section
showing the distribution of S. (d) From left to right: BSE micrographs
of a detail of 16/56 cross section and distribution maps of Hg and Cl.
(e) EDS spectrum acquired on the bright particle circled in red in
panels (c) and (d).
Figure 2. (a) Optical micrograph of sample 6 (southern wall of the
oecus, House of Ariadne). (b) Selected Raman spectrum showing the
bands of calomel, tridimite and goethite, and cinnabar acquired on the
area circled in white.
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C
polymorph of SiO2, 206, 401, 419 cm−1), cinnabar (254 cm−1),
and calomel (168 cm−1).
Calomel was also detected in sample 17, but not in sample
18. Gypsum had already been identified in situ by Raman
spectroscopy. Sample 18 corresponds to a white stripe painted
on a red background, which consisted of a mixture of cinnabar
and red ocher (see Figure 3a,b). In this underlying pictorial
layer, black-grayish metallic particles (around 15−20 ×5−10
μm2) were identified microscopically (Figure 3a,c). Raman and
EDS spectra acquired on those particles did not offer
additional information other than the signals related to
cinnabar (Hg and S detection). Interestingly, a previous
electrochemical study has demonstrated the formation of
metallic mercury as a degradation product of HgS upon the
influence of light and Cl−,
22
whereas a recent publication
concerning egg tempera painting has already proposed the
occurrence of metallic mercury on vermilion mock-ups.
23
The attributions of the in situ and laboratory-based Raman
spectra of the House of Ariadne are summarized in Table S2.
The cinnabar used in the southern wall of the exedra of the
House of the Golden Cupids shows a totally black appearance,
suggesting that the blackening process is even more dramatic
than the one occurring in the well-known Villa dei Misteri
9
(see Figure 4a-c). In the painting fragments of the predella, the
intonaco described by Meyer-Graft,
24
based on a yellowish
granular lime with fine orange inclusions, is visible in some
areas (see Figure 4b). The Raman analysis of this mortar lead
to the identification of calcite (155, 711, 1086 cm−1) and
goethite (302, 308 cm−1). In addition, the 1062 cm−1Raman
band could correspond to a wax, as in the case of the House of
Marcus Lucretius (see Figure S1 and Table S3), probably
applied to the painting during the 20th century restorations of
the house.
24
The in situ measurements performed on the totally
blackened cinnabar from the predella showed the ubiquitous
presence of calomel and gypsum, together with cinnabar (see
Figure 4d). Both calomel and gypsum were also detected in the
whitish drips (see Figure 4a). In a previous study, thanks to
portable laser-induced breakdown spectroscopy (LIBS)
mapping of the mural paintings of the House of the Golden
Cupids,
25
it was possible to assess that this predella was the
most Cl-impacted painted surface among those considered in
the study.
Characterization of Blackened Cinnabar on Panel
Paintings Stored at Naples National Archaeological
Museum (MANN). Three panel paintings (9206, 9208, and
9255) extracted from the triclinium of the House of Marcus
Lucretius include a red frame that nowadays looks quite
blackened (see Figure S2). HH-EDXRF measurements
conducted in all the frames allowed the detection of Hg and
S together with high Fe contribution. These results pointed out
to the combined use of red ocher and cinnabar. Bands
associated to calomel or other Hg-Cl or Hg-S-Cl compounds
were not identified in any of the in situ Raman measurements
performed on the blackened frames. However, Cl was detected
by HH-EDXRF in the blackened frames (see the example of
panel 9206 in Figure S2).
Gypsum was also detected in the blackened cinnabar areas
(see the example of panel 9103 in Figure S3) of all the
considered panel paintings.
Interestingly, the identification of gypsum was not only
restricted to the blackened cinnabar areas. This sulfate has
been previously identified by infrared spectroscopy on the
same panel paintings.
26
To discard the intentional addition of gypsum to the plaster,
several measurements were conducted on the surface of the
panel paintings 9285, 9206, and 8992 by HH-EDXRF.
Moreover, additional measurements were performed on the
south and east walls of the triclinium, concretely on the
surrounding areas (left and right side) of the voids that the
panels left when they were removed during the first excavations
of the house (see Table S4 and Figure S4). The normalized net
counts of S and Cl obtained from each spectrum of each panel
paintings stored in the MANN were compared with the ones
obtained from the measurements in the walls from Pompeii.
Since the red frames are rich in HgS, these points were not
taken into account for the evaluation of S originating from
gypsum. According to the obtained values, the normalized
counts of S are higher in the panel paintings preserved at
MANN, than in their respective adjacent walls currently
exposed to the modern atmosphere (see Table S4 for
comparison).
The S decrease in the exposed walls may be associated to the
dissolution-mobilization-recrystallization of the formed sulfates
during the exposure to the open atmosphere. Moreover, the
restoration campaigns conducted in this room could have also
contributed to the reduction of the content of soluble salts
such as sulfates in the wall.
The S intensities are lower in the walls of Pompeii nowadays
exposed to the atmosphere than in the panel paintings
Figure 3. (a) Optical micrograph of the black-grayish metallic
particles in sample 18 (oecus, House of Ariadne). (b) Optical
micrograph of the cross section of the sample. (c) BSE micrograph of
the cross section.
Figure 4. In the House of the Golden Cupids: (a) Detail of the
whitish drips caused by rainwater percolation. (b) Close-up view of
the underlying yellow plaster. (c) General view of the blackened
predella of the southern wall. (d) Selected Raman spectra of the
blackened cinnabar of the exedra, the whitish drips, and the
underlying yellow plaster, marked with arrows in Figure 4a,b. The
spectra show the Raman bands of calomel (167 cm−1) and cinnabar
(252, 284, 343 cm−1); calomel, cinnabar, and gypsum (1006 cm−1);
and cinnabar, calcite (155, 711, 1086 cm−1), goethite (302, 388
cm−1), and a protective wax (1062 cm−1).
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D
preserved since the first excavations around 170 years ago.
Thus, it can be affirmed that, in the triclinium of the House of
Marcus Lucretius, the current atmospheric SO2is not playing a
crucial role in the sulfation of the calcium carbonate of the wall
paintings.
As regards the normalized Cl counts (see Table S4), they are
only slightly lower in the panel paintings (0.3 ±0.1 in panel
9206) than in the exposed walls (0.9 ±0.3 in the left side of
panel 9206, 0.5 ±0.2 in the right side of panel 9206). The only
exception is the right side of the void left by panel 9191 on the
southern wall (3.7 ±0.3). In this area, a prolonged direct
exposure to the marine aerosol is expected (see Figure S5,
Supporting Information) due to the strikingly intense Cl peak.
On the other hand, low Cl intensities, present both in the
stored panels and in the exposed walls, may be attributed to
the Cl emission of the volcanic eruption
27
and/or to a diffuse
exposure to marine aerosol.
Note that in some cases the standard deviation related to Cl
and S normalized counts is high (see Table S4). This is
associated with their heterogeneous distribution in the walls.
25
Characterization of Dark Stains on Cinnabar in
Painting Fragments Buried and Protected from the 79
AD Eruption. Samples 3T and Red A, recovered from the
deposit of the House of Marcus Lucretius, were also studied to
observe possible differences in the state of conservation of
cinnabar not exposed to the 79 AD eruption and to the
atmosphere since their recovery from the excavations.
In this case, the samples did not show a widespread black
appearance of the cinnabar layer, but only certain dark stains or
patches (see the species identified by Raman spectroscopy in
this work in Table S2). In previous studies conducted using
Raman spectroscopy, calomel had been detected.
20
Interestingly, the stratigraphic analysis of sample 3T shows
that cinnabar was applied over a pictorial layer composed by
Egyptian Blue (Raman bands at 112, 137, 164, 192, 378, 400,
431, 473, 568, 763, 788, 985, 1010, and 1083 cm−1)
28
(see
Figure S6) and goethite (300 and 386 cm−1, spectrum not
shown). This suggests either a previous redecoration of the
area from which these fragments were detached or the
application of cinnabar as overlying color on a greenish blue
background.
19
Raman measurements performed on the dark spots (see
Figure S7a, Supporting Information) unveiled the presence of
a broad band at around 683 cm−1(see an example of it in the
measurements performed in sample Red A, Figure S7b).
Bearing in mind that only cinnabar was clearly detected in this
sample and no abundant evidences of hematite were identified,
this band cannot be associated only with magnetite (Fe3O4),
29
as it could happen in some measurements acquired in the
darkened hematite areas of the triclinium in the House of
Marcus Lucretius (Fe3O4, Raman band at 661 cm−1, see Figure
S7c,d). Moreover, considering the width of the 683 cm−1band,
it is complicated to attribute it to a specific mineral phase being
most probable the presence of a mixture of several ones.
To further investigate this issue, XPS and TOF-SIMS
measurements were performed on sample 3T for an in-depth
study of the dark patches (around 50−250 μm, see Figures S8
and 5). XPS was preferred for line analysis on altered (dark
areas) and intact cinnabar areas, while TOF-SIMS was more
adequate to perform mapping, due to the sample roughness
and the better depth and lateral resolution of the technique.
The XPS analyses (see Figure S8) and TOF-SIMS maps
(see Figure 5) on the dark stains revealed an increment in
manganese and iron oxides or oxide hydroxides. Therefore, the
broad band identified in the Raman measurements of the dark
stains (see Figure S7, Supporting Information) could be
related to the presence of a mixture of manganese and iron
oxides or oxide hydroxides.
29−31
In the literature, many references can be found regarding the
formation of dark colored coatings composed mostly of
manganese and iron oxides. This dark discoloration process is
usually called “rock varnish”. Mn and Fe present in “rock
varnish”could come from various sources, including dust and
soil.
32,33
Considering that these painting fragments have been
buried for more than 2000 years, the occurrence of these
metals could be readily elucidated. To explain the dissolution
of Mn and Fe from the soil and their subsequent precipitation
as oxides, pH and Eh changes in the burial environment should
take place.
34
Moreover, water should be present to favor the
process. This is also guaranteed due to the previously assessed
influence of groundwater in this archeological site.
4,25
Whereas certain authors concluded that this phenomenon
takes place under abiotic conditions, others held that
microorganisms control Mn precipitation
35
(biomineralization
of Mn).
Although most of the “rock varnish”examples are located in
desert environments,
32
in the last years, different examples
have been published regarding archeological contexts
36
and
19th century buildings.
37
Together with manganese oxides, whitish stains were also
visible, related to the formation of a calcareous (calcite) patina
(caused by the dissolution and recrystallization of the binder,
see Figure S9). This result reinforces the influence of a water
source in the dissolution−recrystallization process.
TOF-SIMS maps (area of 342 ×342 μm2) also showed the
occurrence of F−and Cl−in the surface, while Hg-SCl−
(related to a Hg-S-Cl compound, such as corderoite, α-
Hg3S2Cl2, or kehnsuite, γ-Hg3S2Cl2) was more abundant in the
red areas not affected by the dark stains (see Figure 5). This
Figure 5. TOF-SIMS maps of negative ions (S−, MnO2
−, HgSCl−,
Cl−,F
−) acquired in position 1 (a) and 2 (b) on sample 3T. In all
scales, black color represents the absence of and specific lighter color
represents a higher abundance of the negative ion represented.
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Anal. Chem. XXXX, XXX, XXX−XXX
E
result suggests that the presence of Cl−is not always strictly
related to the darkening of the cinnabar pigment, as already
proposed by certain authors.
18
The TOF-SIMS detection of F−, a halide of volcanic origin,
4
reinforces the hypothesis of a leaching process (favored by
groundwater) of the volcanic soil that covered the fragments,
contributing to the increase in fluorine. Moreover, the
contribution of groundwater rich in F−and Cl−
4,25
could
also favor the relatively prominent presence of these halides in
the cinnabar pictorial layer.
To confirm the occurrence of manganese oxides on the dark
patches of sample Red A detected by Raman spectroscopy,
EDXRF imaging was conducted. As in sample 3T, the Mn
distribution coincided with the dark areas present on the red
cinnabar pictorial layer (Figure 6), verifying the presence of
manganese oxides. Besides, Fe accumulations on these areas
were detected, as expected according to the XPS analyses of
sample 3T (Figure 6). In this case, Fe is scarcely distributed
comparing with Mn, suggesting that iron oxide has been
formed to a lesser extent over the cinnabar layer.
■CONCLUSIONS
This work shows how the state of conservation of the
Pompeian cinnabar pigment varies depending on its protection
against the pre- and post-79 AD atmospheres.
Gypsum has been systematically identified in the blackened
areas of the pigment exposed to the pre- and post-79 AD
atmospheres (e.g., House of the Golden Cupids). This result
could suggest that the blackening might be related to the
formation of a gypsum layer, either due to the polluted
atmosphere or due to the sulfur emissions of the volcanic
eruption. This layer would be subsequently enriched with
airborne particulate matter, responsible for the final black color
of the painting surface (“black crust”formation).
The experimental evidence agrees with what Cotte et al.
10
previously suggested. In this last case, the sulfation of calcite in
the fresco painting was explained by the oxidation of S coming
from the decomposition of the HgS pigment. Nevertheless, in
the mural paintings of the House of Marcus Lucretius here
presented, the cinnabar proportion is much lower than the one
of yellow ocher, and thus the extended formation of gypsum
cannot be explained according to this hypothesis. In the future,
additional painting stratigraphies, other than pure cinnabar or
cinnabar mixed with ocher, should be investigated in order to
track the formation of “black crusts”on other decorated/
nondecorated areas of Pompeii.
The lower S intensities detected in the mural paintings of
the triclinium of the House of Marcus Lucretius (exposed to
the atmosphere since the 19th century excavations) compared
with the panels of the same room preserved at the MANN,
suggest that the H2S and SO2emitted in the 79 AD eruption
are crucial in the sulfation process.
26
Another evidence of the
impact owing to the eruption is the clear transformation of
yellow ocher into red hematite in specific areas of the
cubiculum annexed to the triclinium of this house.
2
This room
of the house is covered by a roof, protecting the mural
paintings from the direct influence of polluted atmosphere,
reducing the effects of this environmental agent in the sulfation
process.
Regarding the protection of the cinnabar pigment when
mixed with other pigments,
11,38
this work demonstrates that
cinnabar can be altered (calomel identification) even when
blended with an ocher pigment. In addition, visually altered
cinnabar particles were also identified in a red hematite
pigment layer covered by a superficial one (calcite and
dolomite). The absence of cinnabar transformation products in
the metallic-like cinnabar particles identified could suggest that
they either belong to metallic mercury, metacinnabar or even
an amorphous cinnabar phase. This hypothesis should be
confirmed in the future with the use of adequate
instrumentation, which can offer sub-micrometric resolution,
such as synchrotron assisted μXANES at the Hg L3-edge.
In the buried Pompeian cinnabar-based fresco fragments and
not exposed to the 79 AD eruption, well preserved areas and
dark stains/patches were identified. In the nondarkened areas
Hg-Cl and Hg-Cl-S compounds were detected. These results
confirm that such compounds can be formed independently of
the pigment darkening or blackening process, as already stated
by various authors.
18
Moreover, in the darkened and not
darkened areas of the samples, it was not possible to identify
the presence of gypsum, since they were exposed neither to the
H2S and SO2gases of the eruption nor to the postexcavation
atmosphere. On the contrary, the dark patches/stains are rich
in manganese and iron oxide hydroxides, and do not belong to
the conventional blackening process of the cinnabar. There-
fore, for conservation purposes, when a cinnabar mural
painting/fragment is recovered from an archeological context,
an in-depth characterization of the dark/black formations on
the cinnabar is necessary to conclude whether the cinnabar
pigment is transformed or just affected by “rock varnish”or by
the precipitation of other colored crusts.
Furthermore, this work also demonstrates that the color of
the transformed Pompeian cinnabar may suggest different
pigment degradation prompted by the impact of a number of
environmental agents. The main transformation occurred after
its exposure to the pre- and post-79 AD atmosphere is the
blackening process connected to the formation of calomel and
gypsum. On the other hand, buried Pompeian cinnabar could
experience darkening due to the formation of black/brownish
Mn/Fe stains and not to the raw pigment transformation itself.
In the future, accelerated weathering experiments using
cinnabar fresco mock-ups reproducing the pre/post-79 AD
Figure 6. (a) View of sample Red A, featuring dark spots on the
surface. (b) Selected μ-EDXRF spectra corresponding to the
unaltered pictorial layer (HgS), a dark coating area rich in Fe
(HgS_Fe), and a dark coating area rich in Mn (HgS_Mn). (c) μ-
EDXRF elemental map showing the distribution of Hg and Mn on the
fragment. (d) μ-EDXRF elemental map showing the distribution of
Mn and Fe on the fragment.
Analytical Chemistry pubs.acs.org/ac Article
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Anal. Chem. XXXX, XXX, XXX−XXX
F
atmosphere impact and burial environment will help delve into
the chemical reactivity leading to these transformation
products. It will thus allow the development of conservation
protocols, which will protect and preserve the original red
color of this pigment.
■ASSOCIATED CONTENT
*
sıSupporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.analchem.1c02420.
Additional experimental details, description of the
samples and paintings analyzed and the obtained results
(PDF)
■AUTHOR INFORMATION
Corresponding Authors
Silvia Pérez-Diez −Department of Analytical Chemistry,
Faculty of Science and Technology, University of the Basque
Country UPV/EHU, 48080 Bilbao, Basque Country, Spain;
orcid.org/0000-0001-7986-7843; Email: silvia.perezd@
ehu.eus
Maite Maguregui −Department of Analytical Chemistry,
Faculty of Pharmacy, University of the Basque Country
UPV/EHU, 01008 Vitoria-Gasteiz, Basque Country, Spain;
orcid.org/0000-0001-6011-3590;
Email: maite.maguregui@ehu.eus
Authors
Africa Pitarch Martí −Departament d’Arts i Conservació-
Restauració, Facultat de Belles Arts, Universitat de Barcelona,
08028 Barcelona, Catalonia, Spain; IAUB. Institut
d’Arqueologia UB, Facultat de Geografia i Història, 08001
Barcelona, Catalonia, Spain; orcid.org/0000-0002-8396-
9487
Anastasia Giakoumaki −Department of Analytical Chemistry,
Faculty of Science and Technology, University of the Basque
Country UPV/EHU, 48080 Bilbao, Basque Country, Spain;
Institute of Electronic Structure and Laser −Foundation for
Research and Technology, 70013 Heraklion, Crete, Greece
Nagore Prieto-Taboada −Department of Analytical
Chemistry, Faculty of Science and Technology, University of
the Basque Country UPV/EHU, 48080 Bilbao, Basque
Country, Spain; orcid.org/0000-0003-4649-2381
Silvia Fdez-Ortiz de Vallejuelo −Department of Analytical
Chemistry, Faculty of Science and Technology, University of
the Basque Country UPV/EHU, 48080 Bilbao, Basque
Country, Spain
Alberta Martellone −Applied Research Laboratory of the
Archaeological Park of Pompeii, 80045 Pompeii, Naples,
Italy
Bruno De Nigris −Applied Research Laboratory of the
Archaeological Park of Pompeii, 80045 Pompeii, Naples,
Italy
Massimo Osanna −Former General Director of the
Archaeological Park of Pompeii, 80045 Pompeii, Naples,
Italy; Director-General of the Directorate-General of
Museums, 00153 Rome, Italy
Juan Manuel Madariaga −Department of Analytical
Chemistry, Faculty of Science and Technology, University of
the Basque Country UPV/EHU, 48080 Bilbao, Basque
Country, Spain; Unesco Chair on Cultural Landscape and
Heritage, University of the Basque Country UPV/EHU,
01008 Vitoria-Gasteiz, Basque Country, Spain; orcid.org/
0000-0002-1685-6335
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.analchem.1c02420
Author Contributions
M.M., A.P.M., A.G., N.P.T., S.F.O.V., and J.M.M. designed the
in situ studies. S.P.D. and M.M. designed the laboratory
studies. In situ analyses were performed by M.M., A.P.M., A.G.,
N.P.T., S.F.O.V., and J.M.M. EDXRF and Raman laboratory
analysis were performed by S.P.D. A.M, B.D.N., and M.O.
provided access to the archeological site and supervise the in
situ analyses. S.P.D. wrote the first draft of the manuscript and
all authors have reviewed the text and given their approval to
the final version of the manuscript.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The research leading to these results has received funding from
“la Caixa”Foundation (Silvia Pérez-Diez, ID 100010434,
Fellowship code LCF/BQ/ES18/11670017). A.P.M. is a Serra
Húnter fellow. A.P.M’s research was supported by a Beatriu de
Pinós postdoctoral grant (2017 BP-A 00046) of the Govern-
ment of Catalonia’s Secretariat for Universities & Research of
the Ministry of Economy and Knowledge. This work has been
supported by the project MADyLIN (BIA2017-87063-P)
funded by the Spanish Agency for Research AEI (MINECO-
FEDER/UE). The authors thank for the funding provided by
University of the Basque Country through the Institutionally
Sponsored Open Access. The analytical campaign carried out
at the House of Golden Cupids was conducted within the
cooperation agreement between the Archaeological Park of
Pompeii and the University of the Basque Country (UPV/
EHU) through the research group IBeA of the Department of
Analytical Chemistry. The authors thank for technical and
human support provided by the Molecules and Material Unit
of the General X-ray Service of SGIker (UPV/EHU/ERDF,
EU) and the Nanotechnology and Surface Analysis Service of
the University of Vigo (CACTI). The authors would like to
thank Ulla Knnutinen (University of Jyväskylä and University
of Helsinki) for her participation in the in situ studies. The
members of Expeditio Pompeiana Universitatis Helsingiensis
(EPUH) and Naples National Archaeological Museum
(MANN) are also gratefully acknowledged for putting at our
disposal the fragments and the panel paintings of the House of
Marcus Lucretius under study, respectively. Livio Ferraza and
Gemma María Contreras Zamorano, from the Institut Valencià
de Conservació, Restauració i Investigació (IVCR+i), are
kindly acknowledged for providing the samples of the House of
Ariadne. The suggestions given by Francesco Caruso (SIK-
ISEA) to improve the manuscript, and his language revision
and editing of the text are also gratefully acknowledged.
■REFERENCES
(1) Maguregui, M.; Knuutinen, U.; Martínez-Arkarazo, I.; Castro, K.;
Madariaga, J. M. Anal. Chem. 2011,83, 3319−3326.
(2) Marcaida, I.; Maguregui, M.; Fdez-Ortiz de Vallejuelo, S.;
Morillas, H.; Prieto-Taboada, N.; Veneranda, M.; Castro, K.;
Madariaga, J. M. Anal. Bioanal. Chem. 2017,409, 3853−3860.
(3) Marcaida, I.; Maguregui, M.; Morillas, H.; Perez-Diez, S.;
Madariaga, J. M. Anal. Bioanal. Chem. 2019,411, 7585−7593.
Analytical Chemistry pubs.acs.org/ac Article
https://doi.org/10.1021/acs.analchem.1c02420
Anal. Chem. XXXX, XXX, XXX−XXX
G
(4) Pérez-Diez, S.; Fernández-Menéndez, L. J.; Morillas, H.;
Martellone, A.; Nigris, B. D.; Osanna, M.; Bordel, N.; Caruso, F.;
Madariaga, J. M.; Maguregui, M. Angew. Chem. Int. Ed. 2021,60,
3028−3036.
(5) Millán Sañudo, E. J. La Técnica Parietal Romana. Análisis del
Proceso Técnico Mural Romano en el Área Vesubiana; PhD Thesis,
Universidad de Sevilla, 2011.
(6) Gwilt, J. The Architecture Of Marcus Vitruvius Pollio. In Ten
Books; Translator; Priestley and Weale: London, 1826.
(7) The Natural History of Pliny; Bostock, J., Riley, H. T.,
Translators; Bohn, H. G.: London, 1855.
(8) Cagiano de Azevedo, M. Boll. Dell Ist. Cent. Restauro 1951,7−8,
33−34.
(9) Harris, C. D. Cinnabar: The Symbolic, Seductive, Sublethal Shade
of Pompeii; Brandeis University, 2015.
(10) Cotte, M.; Susini, J.; Metrich, N.; Moscato, A.; Gratziu, C.;
Bertagnini, A.; Pagano, M. Anal. Chem. 2006,78, 7484−7492.
(11) Spring, M.; Grout, R. Natl. Gallery Tech. Bull. 2002,23,50−61.
(12) Radepont, M. Understanding of Chemical Reactions Involved in
Pigment Discoloration, in Particular in Mercury Sulfide (HgS)
Blackening. PhD Thesis, Paris 6, 2013.
(13) Dickson, F. W.; Tunnel, G. Am. Mineral. 1959,44, 471−487.
(14) Istudor, I.; Dina, A.; Rosu, G.; Seclaman, D.; Niculescu, G.
e_conserv. magaz. 2007,2,24−33.
(15) Yu, J.; Warren, W. S.; Fischer, M. C. Sci. Adv. 2019,5,
No. eaaw3136.
(16) McCormack, J. K. Miner. Deposita 2000,35, 796−798.
(17) Keune, K.; Boon, J. J. Anal. Chem. 2005,77, 4742−4750.
(18) Radepont, M.; De Nolf, W.; Janssens, K.; Van Der Snickt, G.;
Coquinot, Y.; Klaassen, L.; Cotte, M. J. Anal. Spectrom. 2011,26,
959−968.
(19) Castrén, P.; Andrews, J.; Berg, R. Domus pompeiana: una casa a
Pompei; Otava: Helsinki, 2008.
(20) Maguregui, M.; Knuutinen, U.; Castro, K.; Madariaga, J. M. J.
Raman Spectrosc. 2010,41, 1400−1409.
(21) Edwards, H. G. M.; Falk, M. J. P. Spectrochim. Acta A Mol.
Biomol. Spectrosc. 1997,53, 2685−2694.
(22) Anaf, W.; Janssens, K.; De Wael, K. Angew. Chem. Int. Ed. 2013,
52, 12568−12571.
(23) Elert, K.; Cardell, C. Spectrochim. Acta A Mol. Biomol. Spectrosc.
2019,216, 236−248.
(24) Seiler, F.; Grunwald, P.; Gut, W.; Diederichs, H.; Sellers, J.
Casa Degli Amorini Dorati (VI 16, 7, 38);Ha
̈user in Pompeji; Hirmer:
München, 1992.
(25) Pérez-Diez, S.; Fernández-Menéndez, L. J.; Veneranda, M.;
Morillas, H.; Prieto-Taboada, N.; Fdez-Ortiz de Vallejuelo, S.; Bordel,
N.; Martellone, A.; De Nigris, B.; Osanna, M.; Madariaga, J. M.;
Maguregui, M. Anal. Chim. Acta 2021,1168, No. 338565.
(26) Madariaga, J. M.; Maguregui, M.; Castro, K.; Knuutinen, U.;
Martínez-Arkarazo, I. Appl. Spectrosc. 2016,70, 137−146.
(27) Shea, T.; Hellebrand, E.; Gurioli, L.; Tuffen, H. J. Petrol. 2014,
55, 315−344.
(28) Luque, L. D. M.; Ruiz, J. R. Antiquitas 2015,27,69−85.
(29) de Faria, D. L. A.; Venâncio Silva, S.; de Oliveira, M. T. J.
Raman Spectrosc. 1997,28, 873−878.
(30) Gao, T.; Glerup, M.; Krumeich, F.; Nesper, R.; Fjellvåg, H.;
Norby, P. J. Phys. Chem. C 2008,112, 13134−13140.
(31) Sepúlveda, M.; Gutiérrez, S.; Vallette, M. C.; Standen, V. G.;
Arriaza, B. T.; Cárcamo-Vega, J. J. Herit. Sci. 2015,3,1−6.
(32) Krumbein, W. E.; Jens, K. Oecologia 1981,50,25−38.
(33) Thiagarajan, N.; Aeolus Lee, C.-T. Earth Planet. Sci. Lett. 2004,
224, 131−141.
(34) Broecker, W. S.; Liu, T. GSA Today 2001,11,4.
(35) Tebo, B. M.; Bargar, J. R.; Clement, B. G.; Dick, G. J.; Murray,
K. J.; Parker, D.; Verity, R.; Webb, S. M. Annu. Rev. Earth Planet. Sci.
2004,32, 287−328.
(36) Uchida, E.; Watanabe, R.; Osawa, S. Herit. Sci. 2016,4, 16.
(37) Vicenzi, E. P.; Grissom, C. A.; Livingston, R. A.; Weldon-
Yochim, Z. Herit. Sci. 2016,4, 26.
(38) του εκ Φουρνά(tou ek Fourna). (Dionisiou). Ερμηνει ́
α της
ζωγραφικήςτε
́χνης (Erminia tis zografikis technis); Papadopoulou-
Kerameos, A., Ed.; Petroupoli (Athens), 1900.
Analytical Chemistry pubs.acs.org/ac Article
https://doi.org/10.1021/acs.analchem.1c02420
Anal. Chem. XXXX, XXX, XXX−XXX
H
