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Application of µ-Raman spectroscopy to the study of the corrosion products of archaeological coins

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p>In this paper, a study of the corrosion products formed on archaeological bronze artefacts excavated in Tharros (Sardinia, Italy) is presented. The investigation was carried out by means of the combination of different analytical techniques, including optical microscopy, micro-Raman spectroscopy (µ-RS), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction. The artefacts under study are three bronze coins from the Phoenician–Punic period that are deeply corroded due to the chloride-rich soil of the Tharros excavation site. µ-Raman spectroscopy was chosen to investigate the corroded surfaces of the artefacts because it is a non-destructive technique, it has high spatial resolution, and it makes it possible to discriminate between polymorphs and correlate colour and chemical composition. Through µ-RS, it was possible to identify different mineralogical phases and different polymorphs, such as cuprite (Cu<sub>2</sub>O), copper trihydroxychloride [Cu<sub>2</sub>Cl(OH)<sub>3</sub>] polymorphs, hydroxy lead chloride laurionite [PbCl(OH)] and calcium carbonate polymorph aragonite. The experimental findings highlight that micro-Raman spectroscopy can be used to provide further knowledge regarding the environmental factors that may cause the degradation of archaeological bronzes in soil.</p
ACTA IMEKO
ISSN: 2221-870X
March 2021, Volume 10, Number 1, 234 - 240
ACTA IMEKO | www.imeko.org March 2021 | Volume 10 | Number 1 | 234
Application of -Raman spectroscopy to the study of the
corrosion products of archaeological coins
Tilde de Caro1, Emma Angelini2, Leila Es Sebar2
1 Istituto per lo Studio dei Materiali Nanostrutturati - National Research Council (ISMN-CNR), Rome, Italy
2 Dipartimento di Scienza Applicata e Tecnologia (DISAT), Politecnico di Torino, Torino, Italy
Section: RESEARCH PAPER
Keywords: Raman spectroscopy; bronze disease; corrosion, cultural heritage.
Citation: Tilde de Caro, Leila Es Sebar, Emma Angelini, Application of µ-Raman spectroscopy to the study of the corrosion products of archaeological coins,
Acta IMEKO, vol. 10, no. 1, article 31, March 2021, identifier: IMEKO-ACTA-10 (2021)-01-31
Editor: Ioan Tudosa, University of Sannio, Italy
Received May 25, 2020; In final form November 10, 2020; Published March 2021
Copyright: This is an open-access article distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use,
distribution, and reproduction in any medium, provided the original author and source are credited.
Corresponding author: Tilde de Caro, e-mail: tilde.decaro@cnr.it
1. INTRODUCTION
In the past few decades, a series of cutting-edge analytical
techniques have found an ever-growing application in the
cultural heritage conservation field, offering interesting insights
into the provenance, history and fabrication methods of cultural
heritage artefacts. Among these techniques, Raman spectroscopy
has become a fundamental tool in conservation science, as it does
not require sampling of the artefact under study and the analysis
can be performed in museum galleries, storage facilities and
conservation laboratories thanks to the use of portable
instrumentation [1], [2].
The Raman effect provides a quick and relatively
straightforward molecular identification of a material under
examination. A Raman spectrum is something like a fingerprint
that can be used to identify compounds against a database of
standard spectra [3].
This paper deals with the application of micro-Raman
spectroscopy (μ-RS) for the analysis of the corrosion products of
some metallic artefacts. μ-RS furnishes an identification of the
corrosion products by determining the vibration modes and thus
the bond vibrations in the structure [4].
Micro-Raman spectroscopy can be used to identify the
mineralogical nature of micro-phases and discriminate between
different polymorphs present in the corrosion products of the
patina 5. Furthermore, μ-RS has the advantage of being a fast
and non-destructive technique, and, in backscattering geometry,
it is especially suitable for analysing surfaces.
Due to its interesting features, μ-RS in addition to other
techniques such as X-ray diffraction (XRD) and scanning
electron microscopy coupled with energy dispersive X-ray
spectroscopy (SEMEDS) was utilised in order to investigate the
chemical and structural nature of the corrosion products (i.e. the
patina) grown on archaeological bronze artefacts. In particular,
three unreadable coins from the PhoenicianPunic period taken
from the Tharros excavation site were studied.
ABSTRACT
In this paper, a study of the corrosion products formed on archaeological bronze artefacts excavated in Tharros (Sardinia, Italy)
is presented. The investigation was carried out by means of the combination of different analytical techniques, including optical
microscopy, micro-Raman spectroscopy (µ-RS), scanning electron microscopy coupled with energy dispersive X-ray
spectroscopy and X-ray diffraction. The artefacts under study are three bronze coins from the PhoenicianPunic period that are
deeply corroded due to the chloride-rich soil of the Tharros excavation site. µ-Raman spectroscopy was chosen to investigate the
corroded surfaces of the artefacts because it is a non-destructive technique, it has high spatial resolution, and it makes it possible
to discriminate between polymorphs and correlate colour and chemical composition. Through µ-RS, it was possible to identify
different mineralogical phases and different polymorphs, such as cuprite (Cu2O), copper trihydroxychloride [Cu2Cl(OH)3]
polymorphs, hydroxy lead chloride laurionite [PbCl(OH)] and calcium carbonate polymorph aragonite. The experimental findings
highlight that micro-Raman spectroscopy can be used to provide further knowledge regarding the environmental factors that
may cause the degradation of archaeological bronzes in soil.
ACTA IMEKO | www.imeko.org March 2021 | Volume 10 | Number 1 | 235
The ancient city of Tharros is located along the north-western
coast of Sardinia (Italy) and was first established in the Nuragic
period. It is currently an archaeological site near the village of San
Giovanni di Sinis, municipality of Cabras, in the province of
Oristano. It is located on the southern shore of the Sinis
peninsula, which forms the northern cape of the Bay of Oristano,
as shown in Figure 1. The Phoenicians populated the area during
the 8th century BC, and then the city was governed by
Carthaginians until the Roman conquest in the 3rd century BC
5], [6. The remains of the town include foundations of temples,
Roman baths, a Roman Castellum Acquae, a PhoenicianPunic
tophet and an artisan quarter. The town was the capital of the
Judicate of Arborea until around 1070 AD, when it was
abandoned in favour of Oristano under the pressure of the Arab
incursions.
During archaeological excavations carried out in Tharros, a
large amount of Punic, Roman and Carthaginian bronze
artefacts, such as coins, nails, rings and everyday tools, was found
7. Due to the length of time they had been buried in the soil,
most of the bronze artefacts were covered by a thick layer of
corrosion products containing chlorides and were affected by
bronze disease, an irreversible and nearly inexorable corrosion
process that occurs when chlorides come into contact with
bronze or other copper-based alloys. The degradation process
induced by bronze disease is cyclic and self-sustaining and
destroys the artefacts, transforming them into a greenish powder
[8]-[10].
The main purpose of this study was to characterise, by means
of a multi-analytical approach, the three coins shown in Figure 2,
in order to identify the corrosion products, to ascertain the actual
state of conservation and provide recommendations for the
selection of tailored conservation approaches [11] and to
recommend materials to inhibit the degradation phenomena.
In Section 2, the different analytical techniques are presented. In
particular, optical microscopy (OM) and SEMEDS were
employed in order to investigate both the morphology of the
corrosion products and their elemental composition. This
combination provides the best results for the characterisation of
corrosion layers on ancient metal objects and is valid for
corrosion layers with thicknesses in the order of micrometres or
more.
Micro-Raman spectroscopy was performed to identify the
main corrosion products present on the surfaces; it has high
spatial resolution, and it can be used to discriminate between
polymorphs and to correlate colour and chemical composition
[10].
Finally, XRD was carried out to determine micro-structural
identification of the crystalline phases and to provide
complementary information with respect to the previous
techniques.
In Section 3, the results of the performed analyses are
reported and discussed. Finally, the conclusion section presents
the major contributions of this study.
2. INSTRUMENTS AND METHODS
The study of the corrosion mechanisms and the
characterisation of the layers of corrosion products on the coins
were mainly performed by means of μ-RS, which proved to be a
powerful technique for the identification of the different
corrosion products.
In the study of corrosion, μ-RS is widely employed to assess
the composition of corrosion products in a non-destructive way.
A monochromatic light, usually from a laser in the visible range,
is employed to determine the vibration mode of the molecules
and identify them based on the shift in energy.
Micro-Raman analyses were performed at room temperature
using a Renishaw RM2000 equipped with a Peltier-cooled
charge-coupled device camera in conjunction with a Leica optical
microscope with 10 ×, 20 ×, 50 × and 100 × objectives.
Measurements were performed using the 50 × objective (laser
spot diameter of about 1 μm) and the 785 nm line of a laser
diode. Two edge filters blocked the Rayleigh-scattered light
below 100 cm1. For this reason, the study of ultra-low
wavenumber Raman spectra in the region < 100 cm1 is
overlooked. In order to avoid damaging the patina and to prevent
the fluorescence from covering the Raman signal, the laser power
was lowered. No baseline was subtracted from the recorded
spectra. The spectra obtained were compared with GRAMS
spectroscopy software and databases available in the literature
[12].
Moreover, an investigation of the chemical composition and
morphology of the corrosion product layers was carried out by
optical and electron microscopy and X-ray diffraction.
An optical microscope uses visible light and a system of lenses
to generate magnified images. OM measurements can be easily
performed without any sampling and can be used to study the
morphology and distribution of corrosion products on the
artefact surface, allowing for identification based on colour.
SEM uses a beam of accelerated electrons to scan an object’s
surface and obtain higher magnification images. Generally, the
microscope is coupled with EDS, which makes it possible to
obtain information on the chemical composition of the sample.
EDS characterisation capabilities are due to the fundamental
principle that each element has a unique atomic structure that
appears as a unique set of peaks on its electromagnetic emission
spectrum.
Figure 1. The archaeological site of Tharros (Sardinia, Italy).
Figure 2. The PhoenicianPunic coins under study found in the archaeological
site of Tharros (Sardinia, Italy): (a) THT CLO1; (b) THT CLO2; (c) THT CLO3.
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Optical microscopy investigations were performed using a
Leica microscope M 125C equipped with a digital camera. SEM
and EDS were carried out by a Cambridge 360 scanning electron
microscope equipped with a LaB6 filament and with an INCA
250 energy-dispersive spectrometer and a four-quadrant back-
scattered electron detector (BSE). SEM images were recorded in
BSE mode at an acceleration voltage of 20 kV and a working
distance of 25 mm.
Micro-structural identification of the crystalline phases
formed on the bronze coins was determined by XRD analysis by
means of a Siemens 5000 X-ray powder diffractometer using Ni-
filtered Cu Kα radiation (λ 1.5418 Å). Angular values in the range
of 20 ° 60 ° in additive mode, a step size of 0.05 ° and a sampling
time of 2 s were the experimental parameters used for first data
acquisition.
3. RESULTS AND DISCUSSION
The three PhoenicianPunic bronze coins selected for this
study, whose visual appearance is shown in Figure 2, were found
in a sewer of the archaeological site of Tharros. The coins,
identified as THT CLO1, THT CLO2 and THT CLO3, appeared
to be poorly conserved and were analysed before any cleaning or
preservation treatment.
Copper-based alloys are more vulnerable to attack by chloride
compared to other corrosive species, especially when they are
exposed to marine or coastal environments or buried in chloride-
enriched soils. This is the situation experienced by the metallic
artefacts from Tharros. Artefacts are mainly endangered by
bronze disease when pitting corrosion develops in conditions of
high relative humidity and chloride supply. Moreover, phase
segregation phenomena contribute to surface heterogeneity and
induce local galvanic cells.
At visual examination, the surface patinas of all the coins
appear greenish and dusty, as shown in the OM images of Figure
3, Figure 4 and Figure 5, where the SEM images accompanied by
the energy-dispersive spectra and the XDR diffractograms are
also reported. EDS spectra reveal the presence of oxygen, copper
and chlorine on the coins surfaces [13].
The XRD spectra of the patinas of the three archaeological
artefacts highlight the presence of dangerous copper-containing
corrosion products such as trihydroxychlorides [Cu2Cl(OH)3]
[13]-[16].
Furthermore, the OM images show the presence of red ruby
crystals that can be attributed to cuprite crystals mixed with a
small amount of soil components and other corrosion products
[16]. XRD spectra confirm the presence of cuprite (Cu2O) mixed
with small amounts of soil components including quartz (SiO2)
and calcite (CaCO3).
After these characterisations, the coins patinas were analysed
by means of -Raman spectroscopy [16]-[18]. As evidenced in
Figure 6, the spectra confirm that the main aggressive agent is
represented by chloride-containing species, which lead to the
formation of copper chloride- and oxy-chloride-based corrosion
products. The Raman spectra allow differentiation between two
different polymorphs of [Cu2Cl(OH)3]: clinoatacamite and
atacamite [19]. The spectra of these two mineralogical species are
very similar: the region where the difference can be observed is
highlighted by a red line, while the vibrational assignments are
listed in Table 1.
As explained above, bronze disease is a cyclic reaction that
occurs when the archaeological artefact interacts with chlorides
in the soil that, in the presence of moisture, produce light green
powdery copper chloride [nantokite (CuCl) and
hydroxychlorides (atacamite and its polymorph clinoatacamite)
[20]. The outermost green layer can be constituted by atacamite
and its polymorphs, botallackite and clinoatacamite.
Thermodynamic data show that clinoatacamite is the most stable
Figure 3. Coin THT CLO1: OM image (A), XRD spectrum (B), SEM image (C) and
EDS spectrum (D).
Figure 4. Coin THT CLO2: OM image (A), XRD spectrum (B), SEM image (C) and
EDS spectrum (D).
Figure 5. Coin THT CLO3: OM image (A), XRD spectrum (B), SEM image (C) and
EDS spectrum (D).
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phase of the three polymorphs, and botallackite, which forms
first, is the least stable. Therefore, when atacamite is found on an
archaeological artefact, this indicates the process is in an
intermediate stage, while clinoatacamite represents the
completion of the sequence [21].
The nantokite phase, which is very unstable, is the first phase
that forms, and it is found as a corrosion product only under dry
environmental conditions. From this information, it can be
inferred that some corrosion products, such as clinoatacamite,
are stable, and therefore, the artefact once excavated will not
encounter serious degradation processes. Other products, on the
contrary, such as atacamite, change their crystalline structure
over time, increasing their volume, for example, and these
processes can lead to rapid degradation of the artefact.
The broad bands of the spectra suggest that these minerals
have a disordered structure and are not well crystallised.
The cuprite layer, evidenced in the Raman spectra shown in
Figure 7 and Table 2, should work as an electrolytic membrane
allowing the transport of anions such as Cl and O2 inside and
cations such as Cu+ outside. The large amounts of chlorine and
oxygen in the interface metal/patina can be interpreted as an
autocatalytic reaction that facilitates the oxidation of copper and
the accumulation of chloride ions, allowing the formation of
cuprite and copper chloride.
It is interesting to observe that the crystal structure of cuprous
oxide is still preserved after the incorporation of a small amount
of other cations into the lattice. Indeed, the passive layer of Cu2O
could incorporate up to about 2 at. % of Sn into the lattice,
forming a defective layer of copper oxide. The substitution of Cu
by Sn in the Cu2O phase is noticeable, as confirmed by the
presence of the overtone peak at 218cm1. The increase in the
intensity of the band indicates that Cu2O is disordered and not
well crystallised. Furthermore, the presence of Sn, which has
replaced some Cu atoms in the lattice structure, could be linked
to a small red shift of the band at 640cm−1 [22].
The Raman spectrum, shown in Figure 8 and Table 3,
highlights the presence of laurionite [PbCl(OH)], which is related
Table 1. Results of vibrational spectra (*wavenumber/cm1) and Raman
bands attribution of the polymorphs atacamite and clinoatacamite.
atacamite
Cu2Cl(OH)3
clinoatacamite
Cu2Cl(OH)3
Raman bands
attribution
975*
928*
hydroxyl deformation
910
893
845
820
800
580
CuO stretching vibration
512
513
451
444
CuCl stretching vibration
424
354
365
239
259
200
169
147
142
OCuO bending modes
116
Figure 6. Raman spectra of copper trihydroxychlorides [Cu2Cl(OH)3]
atacamite and clinoatacamite on the coins found at Tharros.
Table 2. Results of vibrational spectra (wavenumber/cm1) and Raman bands
attribution of cuprite.
cuprite (Cu2O)
Raman bands attribution
637
Raman symmetry allowed
529
Raman symmetry allowed
416
overtone
218*
overtone-defective Cu2O*
149
Raman symmetry allowed
Figure 7. Raman spectra of cuprite (Cu2O) on the coins found at Tharros. The
Raman spectra collected in a Cu‐rich area evidence the presence of poorly
crystallised cuprite.
ACTA IMEKO | www.imeko.org March 2021 | Volume 10 | Number 1 | 238
to the interaction of lead in the alloy with chloride in the soil.
Laurionite is a mineral that is thermodynamically stable over a
wide range of pH values and chloride concentrations [24], [24].
Lead was commonly added to bronze in the ancient manufacture
of artefacts characterised by low mechanical properties such as
coins. Indeed, an addition of lead up to 2 % improves the fluidity
of the melted bronze alloy.
Figure 9 and Table 4 show the Raman spectra and bands
attribution of aragonite, a polymorph of CaCO3, in the patina of
coin THT CLO3. For the calcium carbonate polymorphs, the
stability and consequently the solubility increases as follows:
amorphous calcium carbonate (ACC) > vaterite > aragonite >
calcite [26], [26]. Despite the fact that aragonite is unstable at
room temperature and atmospheric pressure, the presence of this
mineral is not unusual on Cu base alloy patinas found in marine
and/or coastal environments. This can be due to the formation
of microclimates on the surface of the metal with different pH
levels and temperatures and the presence of Mg2+ ions, which are
abundant in sea water. Moreover, previous studies have noted
that the ratio of Mg2+ to Ca2+ ions in solutions could facilitate
the formation of aragonite over calcite [28], [28].
The experimental findings of the adopted multi-analytical
approach reveal the deep interaction between soil components
and corrosion processes and products and also evidence the
relevant presence of chlorides in the patina of the archaeological
Cu-based artefacts found in Tharros. This latter occurrence is
considered dangerous because it could induce the previously
mentioned bronze disease, a cyclic and self-sustaining corrosion
reaction of copper that would disfigure the artefact.
These corrosion products have to be neutralised, and their
harmful and irreversible actions have to be stopped in order to
preserve the artefact [30]-[32].
Several steps can be taken to both prevent and treat bronze
disease: i) removing moisture from the artefacts by placing them
in the oven on low heat in order to dry them out, with the
unwanted possible effect of darkening the surfaces, ii) soaking
the artefact in either distilled water or a solution of sodium
carbonate and sodium bicarbonate, a non-permanent fix that will
only halt the reaction until the cuprous chloride comes into
contact with moisture in the air, iii) using corrosion inhibitors
such as benzotriazole (C6H5N3), a highly carcinogenic
complexing agent, or iiii) coating the artefact with a varnish, wax
or resin to prevent the recurrence of corrosion; however, if the
protective layer lacks adhesion and if the bronze disease hasn’t
been completely eliminated, the metal will continue to corrode
beneath the coating [33]-[36].
Curtailing bronze disease is still an open problem, along
with the question of whether humidity control could serve as an
alternative method to reversible surface modifications of
artefacts. An environmental condition in which relative humidity
is maintained below 39 % is ideal for bronze storage and display.
However, humidity control is costly and sometimes impractical
in a display setting. Consequently, the effects of bronze disease
have to be controlled with due precaution, and careful periodic
examination of artefacts has to be carried out. To this aim, micro-
Raman spectroscopy has been shown to be a powerful non-
destructive tool to characterise the surfaces of archaeological
artefacts.
4. CONCLUSIONS
By means of -Raman spectroscopy, the micro-chemical
structure of long-term corrosion products growing on bronze
archaeological artefacts found at Tharros was investigated.
Through the multi-analytical approach proposed in this study
it was possible to identify the presence of Cu (I) species such as
cuprite (Cu2O) and of copper trihydroxychlorides [Cu2Cl(OH)3]
polymorphs. Furthermore, the presence of the hydroxy lead
chloride laurionite [PbCl(OH)] and of the calcium carbonate
polymorph aragonite was detected. Taking into account that the
artefacts were found in a soil rich in chlorides due to the location
Table 3. Results of vibrational spectra (wavenumber/cm-1) and Raman bands
attribution of laurionite.
laurionite
PbCl(OH)
Raman bands
attribution
612
OH deformation modes
327
PbCl stretching vibration
279
Figure 8. Raman spectrum of laurionite [PbCl(OH)] on coin THT CLO1.
Table 4. Results of vibrational spectra (wavenumber/cm1) and Raman bands
attribution of aragonite.
Raman bands attribution
C-O symmetric stretching in CO32-
C-O asymmetric bending mode in CO32-
Ca- CO32- stretching
Figure 9. Raman spectrum of aragonite (CaCO3) on coin THT CLO3.
ACTA IMEKO | www.imeko.org March 2021 | Volume 10 | Number 1 | 239
of the archaeological site along the western coast of Sardinia, it
was possible to correlate the corrosion phases to the chemical
composition of both the artefacts and the burial context.
The obtained results confirmed that Raman spectroscopy is
an effective, versatile and non-invasive technique that allows for
the fast detection of mixed polymorphs in the outermost layers
of the patina before and after the restoration of the artefact and
also during the storage and exhibition period. This is of particular
interest in the determination of focused conservation methods
for these materials.
In fact, Raman data are an indispensable part of the multi-
analytical approach adopted for the study of the coins. The high
spatial resolution makes it possible to obtain details regarding the
physicochemical interrelationships of mixed phases and
corrosion products of a specific metallic artefact a single
micrometric-sized crystal may be analysed. This property is of
particular interest in the corrosion field because most of the
corrosion products are distributed unevenly on the surface of the
artefacts corresponding to localised corrosion formed by holes
or craters of micrometric depth.
Indeed, once the corrosion phases have been detected,
precautions have to be taken in order to assure the long-lasting
preservation of artefacts. According to the authors’ knowledge,
to achieve this aim, the transformation of dangerous copper
chloride or oxy-chlorides into some stable phase is the most
suitable approach.
It is worth noting that conservators have developed several
procedures to neutralise and stop these degradation processes,
ranging from inhibitors to coatings [37]. However, before using
a new restoration product, it is necessary to know in advance the
way the patina will respond once exposed to the new
compounds. A corrosion-inhibiting product must stop the
aggressive substances and at the same time preserve the colour
and composition of the patina in order to avoid altering the
appearance of the archaeological artefact.
ACKNOWLEDGEMENT
The authors gratefully acknowledge Gianni Chiozzini and
Claudio Veroli (ISMN-CNR) for their skilful technical assistance
with the SEM and XRD.
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... Archaeometric and diagnostic studies on ancient numismatic materials using traditional analytical techniques have provided important aspectsrelated to minting technologies, economic trends, political interactions, and numismatic and cultural exchanges between ancient states and groups [1]. For instance, Light Microscopy (LM) and Scanning Electron Microscopy (SEM) are commonly employed to investigate the surface morphology of coins, while X-ray Fluorescence (XRF), Atomic Absorption Spectroscopy (AAS), Inductively Coupled Plasma (ICP) Analysis, and X-ray Photoelectron Spectroscopy (XPS) are used to analyze their chemical composition and infer the metals used in their production; X-Ray Diffraction (XRD) and μ-Raman spectroscopy are generally used to identifying the mineralogical composition of alteration products [2][3][4][5][6][7][8][9][10][11]. Additionally, the microstructural information about the inner core of the coins obtained by cutting-edge X-rays and neutron methods expands the possibilities of traditional studies of coins, providing new insight into coinage manufacturing andcoins degradation like internal corrosion tracks [12][13][14][15]. ...
... Cuprite is the initial product formed in the corrosion process of copper-based alloys and generally constitutes the layer most adhered to the alloy surface [10]. The attribution of peaks corresponding to cuprite is supported by the literature [13]. ...
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