Study of dry- and wet-process amorphous arsenic sulfides: Synthesis, Raman reference spectra, and identification in historical art materials

Article abstract of DOI:10.1002/jrs.5534

This paper reports the dry and wet synthetic procedures and characterization by Raman spectroscopy of amorphous arsenic sulfide reference pigments. Reference spectra of two amorphous materials obtained by wet process methods and four dry process references of amorphous arsenic sulfide pigments of known composition are presented and discussed. While all materials present a main band characteristic for the amorphous pigment centered on 341?cm^?1, additional small contributions indicate the presence of sulfur, arsenic oxide, and crystalline nano phases embedded in the amorphous matrix. Although only the broad 341-cm^?1 peak is necessary to identify the arsenic sulfide as an amorphous material, the smaller additional features allow for the characterization of the various manufacturing processes and initial materials used. In ideal conditions, these small features also enable to assess the As/S ratio of the studied amorphous arsenic sulfide pigments based on their relative intensity. In this context, the latter reference spectra were used to characterize the amorphous arsenic sulfide pigments and their arsenic to sulfur elemental composition in four 18th- to 20th-century historical samples and compared with scanning electron microscopy with energy dispersive X-ray semiquantitative analyses. The identification of the amorphous arsenic sulfide used in these historical samples was compared with the description of the manufacturing processes reported in historical sources of the time, allowing for a better understanding of the evolution of the amorphous arsenic sulfide pigments manufacturing methods.

Full text of DOI:10.1002/jrs.5534

Received: 19 September 2018
DOI: 10.1002/jrs.5534
Revised: 12 November 2018
Accepted: 13 November 2018
R E S E A R C H A R T I C L E
Study of dry‐ and wet‐process amorphous arsenic sulfides:
Synthesis, Raman reference spectra, and identification in
historical art materials
Marc Vermeulen1
| Karel Palka2,3 | Miroslav Vlček3 | Jana Sanyova1
1 Polychrome Artefact Laboratory, Royal
Institute for Cultural Heritage, Brussels,
Belgium
2 Department of General and Inorganic
Chemistry, University of Pardubice,
Pardubice, Czech Republic
Abstract
This paper reports the dry and wet synthetic procedures and characterization
by Raman spectroscopy of amorphous arsenic sulfide reference pigments. Ref-
erence spectra of two amorphous materials obtained by wet process methods
and four dry process references of amorphous arsenic sulfide pigments of
known composition are presented and discussed.
3 Center of Materials and
Nanotechnologies, University of
Pardubice, Pardubice, Czech Republic
While all materials present a main band characteristic for the amorphous pig-
ment centered on 341 cm−1, additional small contributions indicate the pres-
ence of sulfur, arsenic oxide, and crystalline nano phases embedded in the
amorphous matrix. Although only the broad 341‐cm−1 peak is necessary to
identify the arsenic sulfide as an amorphous material, the smaller additional
features allow for the characterization of the various manufacturing processes
and initial materials used. In ideal conditions, these small features also enable
to assess the As/S ratio of the studied amorphous arsenic sulfide pigments
based on their relative intensity. In this context, the latter reference spectra
were used to characterize the amorphous arsenic sulfide pigments and their
arsenic to sulfur elemental composition in four 18th‐ to 20th‐century historical
samples and compared with scanning electron microscopy with energy
dispersive X‐ray semiquantitative analyses. The identification of the amor-
phous arsenic sulfide used in these historical samples was compared with the
description of the manufacturing processes reported in historical sources of
the time, allowing for a better understanding of the evolution of the amor-
phous arsenic sulfide pigments manufacturing methods.
Correspondence
Marc Vermeulen, Polychrome Artefact
Laboratory, Royal Institute for Cultural
Heritage, Parc du Cinquantenaire 1,
Brussels B‐1000, Belgium.
Email: marc.vermeulen@metmuseum.org
Present Address
Department of Scientific Research, The
Metropolitan Museum of Art, New York,
NY 10028, USA.
Funding information
Federaal Wetenschapsbeleid, Grant/
Award Number: SD/RI/04A; European
Regional Development Fund‐Project,
Grant/Award Number: CZ.02.1.01/0.0/
0.0/16_013/0001829; Ministry of Educa-
tion, Youth and Sports of the Czech
Republic, Grant/Award Number:
ED4.100/11.0251 LM2015082; Science for
a Sustainable Development – SDD, Grant/
Award Number: SD/RI/04A
KEYWORDS
amorphous arsenic sulfide, cultural heritage, pigment characterization, reference spectra, synthesis
1 | INTRODUCTION
is found as impurities in mineral ores used as color or
opacifier in glass and enamels,[7,8] it is found as a main
In the field of heritage science, the identification of art-
ists' pigments found in works of art often requires com-
parisons with reference material data, especially for
spectroscopic techniques.[1–6] However, although arsenic
element in a limited number of pigments. Therefore, arse-
nic sulfide pigments (yellow orpiment, As2S3; and
orangey‐red realgar, AsS) have often been identified
based on the color of the arsenic‐containing areas:
J Raman Spectrosc. 2018;1–11.
wileyonlinelibrary.com/journal/jrs
© 2018 John Wiley & Sons, Ltd.
1

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VERMEULEN ET AL.
Orpiment was identified in yellow areas whereas realgar
was to be expected in orangey‐red zones.[9–11] Neverthe-
less, the identification of pararealgar (AsS), the yellow
light‐induced degradation product of realgar, made the
identification solely based on elemental and color analysis
rather difficult. With the presence of such compounds,
arsenic‐rich yellow areas would not necessarily corre-
spond to orpiment. Consequently, other analytical tech-
niques such as X‐ray diffraction or Raman spectroscopy
were applied to the study of such pigments.[12–14] In that
regard, among the other techniques, Raman spectroscopy
proved to be a method of choice for a quick and easy differ-
entiation between natural orpiment and pararealgar, thus
overriding the limitations of elemental analysis.[15–17]
Despite being mentioned in artists' treatises as early as
the 14th century,[18,19] arsenic sulfide glasses (g‐AsxS100‐x),
the artificial amorphous forms of arsenic sulfide pig-
ments, were not identified in arsenic sulfide pigments
characterization studies. Instead, until recently, most
studies identified orpiment, realgar, or pararealgar as
being the arsenic sulfide pigments present.[20–23] All three
aforementioned pigments occur to be natural crystalline
arsenic sulfide materials with reference spectra available
in spectral databases and scientific literature.[24,25] Hence,
the lack of identification of amorphous arsenic sulfide
pigments in works of art may be due to the lack of refer-
ence materials to compare the result with as it may
appear difficult to differentiate between the crystalline
and amorphous forms based solely on microscopic and
elemental techniques.[26]
amorphous arsenic sulfide in works of art, reliable refer-
ence spectra are becoming a requirement.
In this article, the authors report the Raman spectra
and vibrational assignations of four pure homogeneous
amorphous arsenic sulfide pigments synthetized by dry
process and two amorphous pigments obtained by wet
process. Based on the reference spectra obtained, dry‐
process amorphous arsenic sulfide was identified in four
historical samples. Although the amorphous arsenic sul-
fide references given in this paper allow for the identifica-
tion of the pigments in historical paint samples, a closer
look to the smaller secondary bands also gives access to
more information regarding the synthesis processes of
the pigments identified in the same historical samples
such as their composition or the initial materials used
for their preparation.
To the best of our knowledge, this article reports for
the first time in the field of heritage science the Raman
spectra and vibrational assignations of four pure amor-
phous arsenic sulfide pigments of known compositions.
Furthermore, the satellite bands recorded for the four ref-
erences were used to estimate the presence of the realgar‐
like nano phases, free arsenic and sulfur in arsenic
sulfide‐based pigments of four 18th‐ to 20th‐century his-
torical samples.
2 | MATERIALS AND METHODS
2.1 | Raw materials
Nonetheless, in recent technical studies, optical and
electron microscopy allowed for the identification of
amorphous arsenic sulfide pigments.[27,28] The occur-
rence of such pigments was based on the presence of
almost perfectly round bright‐yellow particles with a
diameter of 2–5 μm interpreted as a purified form of arti-
ficial orpiment glass obtained by dry process as suggested
by the pioneering research on amorphous arsenic sulfide
by Grundman and Rötter.[29] In cases when samples
mounted as cross sections do not exhibit these almost
perfectly round particles, Raman spectroscopy proved to
be a powerful technique to highlight the presence of the
amorphous arsenic sulfide material based on its charac-
teristic vibrational signature, very distinct from its crystal-
line counterparts.[26] Therefore, in the past years, its
positive identification was reported several times in art
objects.[26,28,30–32] However, most of these identifications
are based on previously published so‐called reference
spectra for amorphous arsenic sulfides prepared in the
laboratory.[26] Although the amorphous nature of this
reference material is not questioned, its purity, homoge-
neity, and exact compositions remain unknown. Conse-
quently, due to the increasing identification of
Various compounds were considered as starting mate-
rials for the syntheses and references for the Raman
characterization. These materials include commercially
available natural orpiment (Kremer Pigmente GmbH &
Co, Aichstetten, Germany) and metalloid arsenic, arse-
nic oxide, and sulfur, all from Sigma‐Aldrich (St. Louis,
Missouri, USA).
2.2 | Raman spectroscopy
Micro‐Raman spectroscopy spectra were acquired with a
Renishaw inVia Raman microscope with a Peltier‐cooled
(203 K), near‐infrared enhanced, deep‐depletion CCD
detector (576 × 384 pixels) using a high power 785‐nm
diode laser (Innovative Photonic Solutions, New Jersey,
USA) in combination with a 1,200 l/mm grating. The
instrument was calibrated using a silicon wafer. Samples
were analyzed using a 50× objective in a direct‐coupled
Leica DMLM microscope with enclosure. To avoid degra-
dation or heat induced physical changes, the power on
the samples was reduced to 0.3 mW with neutral density
filters. A sample exposure time of 10 s (one accumulation)

VERMEULEN ET AL.
3
was employed, resulting in an adequate signal‐to‐noise
ratio. Spectra were acquired using the Wire 4 Raman soft-
ware and were baseline corrected and normalized into the
range [0,1] using the OriginPro 8 software.
was mixed with Prussian blue to obtain the desired green
color of the landscape from where the sample was taken
(Figure 1e). The second sample (H2) comes from a
Japanese leather paper or kinkarakawakami from the
musée Charlier in Saint‐Josse‐ten‐Noode in Belgium
(Figure 1b). In this sample, the yellow arsenic layer is
found between the leather and metallic layers (Figure 1f).
Samples H3 and H4 were taken on two decorative panels
of the Japanese tower found in Laeken, Brussels, Belgium
(Figure 1c,d). These samples present different application
processes: Sample H3 is found mixed with ultramarine
blue in the paint underlayer (Figure 1g) whereas sample
H4 presents two successive layers of arsenic sulfide
pigments of different tone (Figure 1h). The thinner upper
layer presents a brighter hue whereas the thicker deeper
layer presents a darker yellow tone (Figure 1e). The four
historical samples were prepared as embedded cross
sections in order to be analyzed.
To be cast as cross sections, the loose samples were
first fixed on a 1‐cm3 poly(methyl methacrylate) cube
using a white poly(vinyl acetate) glue, after which the
acrylic copolymer resin (Spofacryl®, SpofaDental, Prague)
was poured on the samples and a second poly(methyl
methacrylate) cube placed on top and left to dry. Cross
sections were polished until the sample surface was
exposed, first with wet polish and finished dry with
Micromesh® polishing cloths up to 8,000 mesh (Scientific
Instruments Services Inc., MN). Microphotographs were
2.3 | Scanning electron microscopy with
energy dispersive X‐ray analysis
Analyses were carried out using a Zeiss‐EVO LS15 scanning
electron microscopy (SEM) equipped with an Oxford
energy dispersive X‐Ray spectroscopy detection system, all
operated under variable pressure vacuum. Images, elemen-
tal maps, and semiquantitative analysis were recorded
using an acceleration voltage of 15 kV, and 8‐mm‐working
distance. The accuracy of the energy dispersive X‐ray (EDX)
semiquantitative analysis was assessed on the pure arsenic
sulfide glasses of known composition used as standards.
Data were collected and processed using the AZtecEnergy
software system, v. 2.1 (Oxford Instruments).
2.4 | Historical samples
For this study, arsenic sulfide pigments from four histori-
cal samples were investigated (Figure 1a–d). One was
taken on a French rococo Chinese‐inspired oil painting,
arbitrary untitled Chinoiserie (H1), which represents a
couple walking in a garden (Figure 1a). The arsenic sulfide
FIGURE 1 Photographs of (a) “Chinoiserie” (18th century), oil on canvas by an unknown artist (private collection), (b) Japanese
wallpaper (ca. 1900), gilt leather paper by unknown artist (Musée Charlier, Brussels), (c,d) decorative panels from the Japanese tower (ca.
1900), mixed medium on wood by unknown artists (Royal Museums of Art and History, Brussels) in which the sampling locations are
indicated (H1–H4). (e,h) Correspond to the microphotographs of respective cross sections under visible light showing the arsenic sulfide
pigment. Location of Raman analyses are indicated by blue circles [Colour figure can be viewed at wileyonlinelibrary.com]

4
VERMEULEN ET AL.
taken with an optical microscope Axio Imager 2 (Carl
Zeiss, Oberkochen, Germany) with incident light and
dark field with magnification up to 500 (Figure 1e–h).
of chalcogenide glasses and reaction yields are given in
Table 1.
mass of the final product ðgÞ
× XAs × MAs; (1)
mAs
¼
MAs × XAs þ Ms × XS
3 | RESULTS AND DISCUSSION
mass of the final product ðgÞ
MAs × XAs þ Ms × XS
where MAs = 74.9216 g.mol−1, MS = 32.06 g.mol−1
mS ¼
× XS × MS;
(2)
3.1 | Synthesis and characterization of
amorphous reference materials
,
and XAs and XS are the atomic fraction of arsenic and sul-
fur in the final products.
3.1.1 | Melt/quench dry process
Due to the limited commercial availability and high
prices of amorphous arsenic sulfide pigments, the synthe-
sis of reference materials with various composition was
undertaken. With the development and use of arsenic sul-
fide as a semiconducting material in the 20th century,
industrial recipes corresponding to a melt/quench of arse-
nic and sulfur mixture are accessible.[33–37] The obtained
amorphous masses are very homogeneous, and their
compositions are easily tunable due to a full control of
the experimental conditions, quantity, and purity of the
starting materials.
Following such procedures, four amorphous arsenic
sulfides were synthetized: g‐As40S60, g‐As34S66, g‐As30S70,
and g‐As25S75. These compositions were chosen for the
variety in shades they offer once ground, from bright yel-
low (g‐As25S75) to orangey‐red hues (g‐As40S60; Figure 2).
Ten grams of each four chalcogenide glasses were pre-
pared by standard melt‐quenching method.[33,38] High
purity (5N) arsenic and sulfur, which amounts were cal-
culated according to Equations 1 and 2, were loaded into
a cleaned quartz ampoule. The ampoule was evacuated
(~10−3 Pa) and sealed. The reagents were melted in a
rocking tube furnace at 800°C for 48 hr. Subsequently,
the ampoule with melted glass was quenched in cold
water and the amorphous masses collected.
The Raman spectra obtained of the four amorphous
arsenic sulfide glasses are presented in Figure 3 (1–4)
along with the Raman spectra of natural orpiment (5), sul-
fur (6), and arsenic oxide (7) reference materials. The main
bands wave numbers, relative intensities, and vibrational
assignments for all compounds can be found in Table 2.
The distinct Raman vibrations observed for the amor-
phous materials (1–4 in Figure 3) and the crystalline one
(5 in Figure 3) clearly highlight the difference in struc-
tures between the two types of materials. For the amor-
phous material (1–4 in Figure 3), all spectra are
characterized by a broad band centered around 341 cm
−1
corresponding to the symmetric stretching vibration
mode of AsS3/2 structural units.[40] Additional minor
bands are found at 153, 187, 219, 235, 473, and 493 cm
−1. The bands at 153, 219, and 473 cm−1 increase when
larger amounts of sulfur are found in the final glass prod-
ucts, and as suggested by the Raman spectrum of sulfur
reference (6 in Figure 3), these bands correspond to free
sulfur present in the nonstoichiometric materials. The
contribution at 493 cm−1 correspond to S‐S bonds,[39]
which is consistent with its higher intensity in the
sulfur‐rich arsenic sulfide glasses.
Bands at 187 and 235 cm−1 do not depend on the
material composition and cannot be associated with sul-
fur excess. The contribution at 187 cm−1 is only visible
for the g‐As40S60 (1 in Figure 3) whereas similar
The required masses of sulfur and arsenic necessary
for the synthesis of 10 g of each glass and the final mass
FIGURE 2 Photograph of ground (1) g‐As40S60, (2) g‐As34S66, (3) g‐As30S70, and (4) g‐As25S75 produced by melt/quench dry process,
showing the variation of hue from orange/red to bright yellow depending on the As/S ratios [Colour figure can be viewed at
wileyonlinelibrary.com]

VERMEULEN ET AL.
5
TABLE 1 Summary of the arsenic and sulfur masses used for the
preparation of the arsenic sulfide glass, the mass of the final prod-
ucts and the yields of the reaction
synthetized amorphous arsenic sulfides and may not be
fully representatives of amorphous arsenic sulfides found
in works of art. Furthermore, reaching conclusions can
be difficult when the amorphous arsenic sulfide is found
in small quantity or mixed with other pigments.
mAs
(grams)
mS
(grams)
mg‐AsS
(grams)
Yield
(%)
Glass
g‐As25S75
g‐As30S70
g‐As34S66
g‐As40S60
4.3785
5.0035
5.4625
6.0906
5.6215
4.9965
4.5375
3.9094
9.76
9.71
9.88
9.86
97.6
97.1
98.8
98.6
3.1.2 | Wet process
In the 19th and later in the early 20th century, several
sources describe the preparation of amorphous arsenic
sulfide materials through wet process chemistry.[42,43]
This novel method was made available through the
development of wet chemistry in the end of the 19th
century. For this synthesis process, a hydrochloric
solution of arsenic trioxide is treated with a stream of
hydrogen sulfide (Equation 3). The result will be a bright
yellow amorphous precipitate (Figure 4a) that can be
used as an artist pigment.
contribution intensities at 235 cm−1 are observed for the
three other compositions (2–4 in Figure 3) but not in
Spectrum 1. These contributions are attributed to As‐As
bending modes in realgar‐like nano phases, the latter
being crystalline structures included in the glassy
matrix.[34,39,40] These bands are often related to the exper-
imental setup such as the raw materials used, the temper-
ature program, or the quenching temperature.[41]
A
similar observation can be made with the shoulder to
the main band at ~310 cm−1 for g‐As40S60 (1 in Figure 3
) that is not observed for the other reference materials.
The differences observed in these reference materials
can be explained by the use of different sulfur and arsenic
raw materials batches for g‐As40S60 whereas the same
materials were used for the three other amorphous
glasses. Different batches of the same materials were used
because g‐As40S60 was synthetized at a different time than
the three other references.
In characterization study of arsenic sulfide pigments,
the satellite bands described here can be used as indica-
tors regarding the composition and intended color of
the amorphous pigment found in works of art. However,
it is important to keep in mind that the spectra presented
in Figure 3 were obtained on homogeneous laboratory‐
One of the source of hydrogen sulfide is the mineral
pyrite (FeS). The latter decomposes into the iron (II) chlo-
ride brown solid (FeCl2) and gaseous hydrogen sulfide
when placed in a 6N hydrochloric solution and heated
over a Bunsen burner (Equation 4). However, the H2S
gas produced need to be transported through the delivery
tube of a Kipps' apparatus to the arsenic oxide solution in
order to form the yellow precipitate (Equation 3). There-
fore, other hydrogen sulfide sources can be used.
Thioacetamide (CH3CSNH2), an organosulfur compound,
has been used due to the easy production of hydrogen
sulfide when present in aqueous solutions (Equation 5).
It offers the advantage of producing the hydrogen sulfide
gas directly in the 6N hydrochloric solution of arsenic tri-
oxide and form in situ the bright yellow amorphous arse-
nic sulfide according to Equation (3).
FIGURE 3 Raman spectra of (1) g‐As40S60, (2) g‐As34S66, (3) g‐As30S70, (4) g‐As25S75, (5) natural orpiment, (6) sulfur, and (7) arsenic oxide
references and the photographs of all four ground arsenic sulfide glasses analyzed (1–4). For the arsenic sulfide glasses (1–4), Raman bands
characteristic for sulfur are indicated in magenta whereas the bands characteristic for realgar‐like nano phases are indicated in cyan color
[Colour figure can be viewed at wileyonlinelibrary.com]

6
VERMEULEN ET AL.
TABLE 2 Raman shifts and vibrational assignments for natural
orpiment, arsenic sulfide glass, arsenic oxide, and sulfur
obtained by melt or sublimation for which the arsenic
oxide was considered to be the colorless matrix.[27,44]
When analyzing the final product of both preparation
methods with Raman spectroscopy, it appears clear that
both compounds are amorphous arsenic sulfide materials
based on their broad Raman scattering band centered on
341 cm−1 (Figure 4). However, when obtained with pyrite,
small bands at 185, 269, 371, and 562 cm−1 (Spectrum 1)
suggest the presence of arsenic oxide remnants. Such pres-
ence is most likely due to an incomplete purification and
elimination of the arsenic oxide excess by means of filtra-
tion. However, due to the preparation process, which does
not require the intervention of high temperature or
quenching, no distinctive realgar‐like nano phase bands
are observed in the Raman spectra for the amorphous
arsenic sulfide pigment obtained as suggested by the lack
of Raman bands at 187 and 235 cm−1. Nonetheless, the
asymmetry of the main band may indicate the presence
of As4S4. A more sensitive technique such as XRD analyses
would be require to confirm this observation.
Raman
shift (cm−1) Intensity Vibrational assignment
Arsenic sulfide glass[39]
233
341
w
Realgar‐like nano phase
vs, br
As‐S‐As stretching
vibration in AsS3/2
495
w
S‐S vibration
Natural orpiment (As2S3)[24]
106
136
154
179
203
vw
w
Unknown
Bending pyramids AsS3/2
Skeletal deformation
Bending pyramids AsS3/2
m
w
m
Symmetric stretching of As‐S‐As
bridges
293
311
s
Symmetric stretching of
pyramids AsS3/2
vs
Asymmetric stretching of
pyramids AsS3/2
3.2 | Historical samples
355
384
vs
m
Stretching of pyramids AsS3/2
Asymmetric stretching of
Arsenic sulfide pigments were identified and localized in
various historical samples based on the presence of arse-
nic and sulfur by SEM–EDX mapping (Figure 5). Due to
the yellow color of the pigment particles, natural orpi-
ment is expected. However, it remains difficult to con-
clude on the nature of arsenic sulfide pigments based
on microscopic and elemental analyses only as the simul-
taneous presence of arsenic and sulfur may correspond to
several arsenic sulfide pigments. Therefore, due to the
distinctive vibrational signatures of arsenic sulfide pig-
ments,[23,26,45–47] Raman measurements on the arsenic
sulfide particles were undertaken (Figure 6).
Sample H1 distinguishes itself from the other three
analyzed samples by the presence of sharp bands charac-
teristic for crystalline arsenic sulfides (yellow dotted lines
in Spectra 1 and 2 in Figure 6). However, if Spectrum 2
presents the unambiguous signal of natural orpiment,
Spectrum 1 appears more complex. In addition to the
sharp bands of crystalline orpiment, the spectrum looks
broaden around 341 cm−1, which indicates the presence
of amorphous arsenic sulfide along with crystalline orpi-
ment. The simultaneous presence of both compounds in
H1 may suggest an incomplete melt or sublimation of
crystalline natural orpiment into the amorphous form of
the pigment. Such sublimation processes were known in
medieval time for the preparation of artificial pigments
such as vermillion, for which mercury and sulfur were
heated together.[48–50] However, vermilion was not the
only pigment produced by sublimation processes.[51] The
As‐S‐As bridges
Sulfur
152
w to vw
w to vw
w to s
218
471
Arsenic (III) oxide (As2O3)
183
268
369
470
560
sm
s
As‐O‐As bending vibration
As‐O‐As wagging vibration
As‐O‐As bending vibration
As‐O stretching vibration
As‐O stretching vibration
vs
s
m
Note. w, m, s: weak, medium, strong; v: very; sh: shoulder; br: broad.
As2O3 ðaqÞ þ 3 H2S →As2S3 ðsÞ þ 3 H2O
(3)
(4)
ðgÞ
ðlÞ;
Δ
FeSðsÞ þ 2 HCl
H S ;
!
2
ðgÞ
ðaqÞ
CH3CSNH2 ðsÞ þ H2O →CH3CONH2 ðaqÞ þ H2S : (5)
ðlÞ
ðgÞ
The yellow precipitate produced in both procedures
corresponds to fine amorphous arsenic sulfide particles,
require filtering and rinsing to eliminate traces of residual
arsenic oxide. This latter operation may explain the obser-
vation made in the 19th century, which reported the wet
chemistry arsenic sulfide to be purer than that the one

VERMEULEN ET AL.
7
FIGURE 4 (a) Photograph of amorphous arsenic sulfide produced through bubbling of FeS‐induced H2S in an arsenic oxide solution and
(b) Raman spectra of amorphous arsenic sulfide produced by wet chemistry using (1) FeS and (2) thioacetamide as source of H2S [Colour
figure can be viewed at wileyonlinelibrary.com]
FIGURE 5 Backscattered electron images of (a) H2, (b) H3, and (c) H4 samples and their respective scanning electron microscopy with
energy dispersive X‐ray analysis highlighting the use of arsenic sulfide pigments in the yellow‐containing areas [Colour figure can be
viewed at wileyonlinelibrary.com]
dry process preparation of amorphous arsenic sulfide
pigments has been described in several artists' treatises
such as Experimenta de coloribus (Jehan Le Begue, 15th
century) or the Paduan Manuscript (author unknown,
16th century).[19,48,51] In these manuscripts, the authors
often describe how to melt natural orpiment alone or
mixed with a sulfur source (described as “powdered red
sulfur” in Jehan Le Begue's manuscript) in order “to
make a most beautiful orange yellow color”.[19] The
sulfur added to the mix is most likely used to modify

8
VERMEULEN ET AL.
FIGURE 6 Raman spectra of the arsenic sulfide particles from (1 and 2) the French Chinoiserie (H1), (3) the Japanese wallpaper (H2), and
(4, 5, and 6) the decorative panels from the Brussels's Japanese tower (H3 and H4). Insert corresponds to the 330‐ to 400‐cm−1 area of
Spectrum 2 presenting the increased 359‐cm−1 contribution compared with unaltered natural orpiment (black spectrum) [Colour figure can
be viewed at wileyonlinelibrary.com]
the hue of the final product as a greater amount of sulfur
will lead to a brighter yellow tone (Figure 2) or allowing a
purer orpiment to be produced.[52] Once cooled down, the
amorphous material can be conserved and ground when
needed. The occurrence of sulfur in H1 (band at 221 cm
−1) tends to indicate that sulfur was most likely added
to the crystalline pigment to brighten its hue as described
in the medieval recipes.
Similar Raman results have previously been observed
for arsenic sulfide pigments identified on 13th‐century
Portuguese illuminated manuscripts.[53] Although the
authors identified the amorphous arsenic sulfide as a deg-
radation product of the orpiment, the heat required to
transform orpiment into amorphous arsenic sulfide tends
to suggest a similar incomplete sublimation of natural
orpiment rather than a degradation product. A similar
process has previously been identified in 18th‐century
Dutch painting in which the artificial arsenic sulfide
was identified by Raman spectroscopy as partially sub-
limed pararealgar.[54]
Arsenic oxide, a common degradation product of nat-
ural orpiment,[51,55,56] was not identified by means of
Raman spectroscopy in the pigments particles of H1.
Although this may indicate that no degradation of the
arsenic sulfide may have happened, it may also be present
under the detection limit of the technique or could have
been removed due to weathering or conservation
interventions.
However, a broadening of the main band of orpiment
due to an increase of the 359 cm−1 band contribution
(Figure 6 insert) suggests that the natural orpiment has
been subjected to light alteration.[57] Nonetheless, such
modification is not observed for the natural orpiment
contribution found along the amorphous arsenic sulfide.
This further strengthens the greater stability of amor-
phous arsenic sulfides compared with the natural crystal-
line forms—at least in the initial degradation phase—as
already highlighted in previous research.[45,58]
Samples H2, H3, and H4 do not present the character-
istic bands for crystalline arsenic sulfide. Instead, they
present a main broad band centered on 341 cm−1, very
similar to the Raman signatures of the pure reference
materials presented in Figure 3. Spectra 3 and 4 (samples
H2 and H3, Figure 1f,g) present satellite bands at 234,
471, and 494 cm−1, whereas Spectrum 6 (upper bright yel-
low layer in sample H4, Figure 1h) presents sharp bands
at 152, 219, and 473 cm−1. No significant satellite bands
are observed in Spectrum 5 (darker layer in sample H4,
Figure 1h). Whereas the satellite band at 234 cm−1 in
Spectra 3 and 4 suggests realgar‐like nano phases, their
471 and 494 cm−1 bands and the sharp contributions
observed for Spectrum 6 correspond to sulfur remnants.
All bands indicate amorphous arsenic sulfide obtained
by dry process. The presence of sulfur in these samples
indicates the use of sulfur in the dry process synthesis.
However, it remains difficult to conclude on the nature
of the arsenic source used as being metalloid arsenic or
arsenic oxide. Nevertheless, similar dry processes involv-
ing sublimation or melt/quench of arsenic oxide and sul-
fur are found in 18th‐century historical literature.[59,60] In
these treatises, it was advised to mix together sulfur with
a powder arsenic (also described as white arsenic, which
could correspond to arsenic oxide or small pieces of oxi-
dized metalloid arsenic) and sublimate the mixture. The
achieved materials are described as never as perfect and
beautiful as the crystalline form found in nature.[60] From

VERMEULEN ET AL.
9
these treatises, it appears that pigments manufacturers
were aware that the quantity of sulfur contained in the
final material had an influence on its hue as observed
during the dry process manufacturing procedure. The
absence of satellite bands observed for Spectrum 5 may
indicate a different synthetic process than the ones used
for the other samples. A similar profile—without
realgar‐like nano phases or sulfur remnants—is also
observed for the amorphous arsenic sulfide obtained by
wet process (Spectrum 2 in Figure 4). This may represent
the first occurrence of wet‐process arsenic sulfide in
works of art. However, all amorphous arsenic sulfide
obtained by wet process is described as very fine (0,4–
1 μm in diameter),[29,55] which does not appear to be
the case for H4 (Figure 5c). The lack of realgar‐like nano
phases may just inform that this particular amorphous
arsenic sulfide was obtained following different experi-
mental procedures than the other pigments analyzed.
It can be precarious to compare historic naturally aged
pigments with newly synthetized ones due to the potential
differences that can exist, mainly the reference materials
being perfectly homogeneous and the historic ones being
unknown. However, the pigment particles studied and
the Raman beam are small and large enough to cover most
of the particles, therefore inducing a Raman spectrum rep-
resentative of the entire amorphous particle. Raman spec-
tra obtained for the amorphous arsenic sulfide found in
works of art can then be compared with the reference
materials synthetized during this research in order to
extrapolate information regarding the elemental composi-
tion with enough confidence. By comparing Spectra 3, 4,
and 5 from Figure 6 (Spectrum 6 clearly present an excess
of free sulfur) to the reference Spectra 1–4 from Figure 3
and giving a special attention to the sulfur satellite band
at 494 cm−1, we could expect the As/S ratio to be similar
to 30/70 for samples H2 and H3 (Spectra 3 and 4) and to
34/66 for H4 (darker lower part, Spectrum 5). These values
compared well with the SEM–EDX semiquantitative anal-
yses performed on the same samples, which read respec-
tively (atomic % arsenic) 30.14 1.79, 29.35 2.36, and
34.82 1.79. The results demonstrate that good quality
Raman spectra of amorphous arsenic sulfides could be
used to approximate the elemental composition of the
pigments, even though it is important to bear in mind that
elemental composition may be influenced by degradation
processes of the pigments over time and by homogeneity
of the pigments studied.
requires a multianalytical approach, which includes
microscopic and spectroscopic techniques. However, in
recent studies, Raman spectroscopy has allowed to char-
acterize amorphous arsenic sulfide pigment based on its
As‐S‐As stretching vibration in AsS3/2 structural units.
Most studies use previously published reference spectra
for amorphous arsenic sulfide as reference material and
Raman spectra that can be used to compare the historical
samples remain scarce. Even though the amorphous
nature of this reference material is not contested, its
unknown composition and homogeneity make it nonfully
trustworthy for further investigations. Therefore, more
homogeneous and reliable references were required due
to the steady Raman‐based identification of amorphous
arsenic sulfide pigments in works of art. This article fills
this gap by reporting the Raman spectra of four dry‐
process homogeneous amorphous arsenic sulfide pig-
ments of known composition along with the Raman spec-
tra of two wet process pigments. The article also
investigates the various dry‐ and wet‐process synthesis
procedures available through times to better understand
the possible preparation of arsenic sulfide pigments found
in works of art, applying it to amorphous arsenic sulfide
found in three 18th‐ to 20th‐century decorative objects.
Although the main Raman broad band at 341 cm−1 is
enough to conclude on the amorphous nature of the
arsenic sulfide pigments, the satellite bands provide infor-
mation on the materials used and the preparation condi-
tions, such as the use of sulfur or arsenic oxide in excess
or the dry preparation when realgar‐like nano phases are
found in the amorphous material.
Furthermore, when working on high signal‐to‐noise
ratio, well‐baselined Raman spectra, and suitable refer-
ence materials, the samples' Raman spectra can provide
information regarding the As/S ratios based on the satel-
lite bands (especially at 494 cm−1) without requiring
access to SEM–EDX semiquantitative analyses. This can
be very useful to explain various shades of arsenic sulfide
pigments within one object.
ACKNOWLEDGEMENTS
This research is made possible with the support of the
Belgian Science Policy Office (BELSPO, Brussels) through
the research program Science for a Sustainable Develop-
ment – SDD, “Long‐term role and fate of metal‐sulfides
in painted works of art – S2ART” (SD/RI/04A). K. P.
and M. V. appreciate support from Ministry of Education,
Youth and Sports of the Czech Republic (LM2015082 and
ED4.100/11.0251) and European Regional Development
Fund‐Project “Modernization and upgrade of the
CEMNAT” (CZ.02.1.01/0.0/0.0/16_013/0001829).
4 | CONCLUSIONS
Identification of amorphous arsenic sulfide in works of
art is not as straight forward as it may seem. It often

10
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ORCID
Marc Vermeulen https://orcid.org/0000-0003-1000-3259
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