Raman microscopy in art history and conservation science
Gregory D. Smith and Robin J.H. Clark
Abstract
Numerous applications of Raman microscopy in art history and conservation science have
appeared in the literature, but unfortunately this work has gone largely unrecognized by
practitioners in those fields. This article assesses the causes of this situation and seeks to inform
conservators, art historians and archaeologists of the role that Raman microscopy is playing in the
analysis of historical materials. A brief description of the Raman scattering phenomenon and the
instrumentation used to collect Raman spectra is presented, and the capabilities and limitations of
the technique are discussed. Importantly, a comprehensive and critical review is provided of Raman
microscopy applications in the technical analysis of art and artefacts.
Introduction
Dispersive and Fourier transform (FT) Raman microscopy have become established analytical
techniques for the identification and study of cultural materials over the past 15 years. Surprisingly,
this development has occurred primarily in chemical and materials science departments in
academia, rather than in the museum or archaeometry laboratory. Although recently increased
interest among conservators, art historians and archaeometrists suggests that this situation may be
changing, the widespread acceptance and routine application of Raman microscopy in the fields of
art and archaeology have not yet occurred.
The absence of Raman microscopy in conservation and archaeometry laboratories is substantially
influenced by two factors. First, the instrumentation is expensive (£150,000/ US$225,000),
although this financial obstacle can be overcome through strategic budgeting if the desire to
incorporate Raman microscopy is sufficient. The second, and perhaps more fundamental, reason is
that conservation scientists are generally unfamiliar with the Raman technique and its widespread
application in their fields, despite the existence of a large number of publications on the topic. It is
this factor that will be dealt with in the present article.
The lack of awareness of Raman microscopy among conservation scientists is explicable. Although
many articles on the topic have appeared in recent years, only a handful has been published in
conservation journals, the balance having appeared in specialist spectroscopy and traditional
chemistry publications. Because most of this work originates from chemical science departments in
academia, there is pressure on the experimenters to justify their non-traditional research to a
somewhat indifferent science community. Moreover, funding for such activities is difficult to obtain
from traditional sources in the physical sciences. Therefore, scholars working
at the Arts-Science interface are required to defend their work as Science-based rather than Artsbased to both colleagues and funding institutions. This justification is manifested most often by
publishing their results in chemical journals that are read and endorsed by the scientists' peers but,
unfortunately, not by those in the art world. The end result is that, in most instances, the very
practitioners who would benefit from it miss a wealth of valuable information. A better record
exists for the appearance of Raman microscopy at recent conferences on conservation and
archaeometry, but such meetings rarely leave more than a generally inaccessible book of abstracts.
This article seeks to rectify the problem described above by providing those charged with the care
and study of our material culture with a basic understanding of Raman microscopy and its uses in
their fields. The Raman scattering phenomenon is treated briefly so as to provide sufficient
background information for the discussions that follow. Modern instruments for Raman microscopy
are described. The many features that make Raman microscopy so well suited for the analysis of
cultural materials are outlined, and the capabilities of the technique are discussed with comparisons
to other analytical techniques more familiar to archaeological and conservation scientists, namely
infra-red (IR) spectroscopy, optical and polarized light microscopy (PLM), ultraviolet-visible
(UV/VIS) spectroscopy, laser-induced breakdown spectroscopy (LIBS), scanning electron
microscopy with energy dispersive X-ray analysis (SEM/EDX), X-ray fluorescence (XRF), X-ray
photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and particle induced X-ray/γ-ray
emission (PIXE/PIGE). Finally, the paper seeks to provide a resource for those interested in Raman
microscopy as it applies to art and conservation studies by critically reviewing the relevant
literature.
The Raman scattering phenomenon
The theory of Raman scattering is treated here briefly and a further explanation can be found in the
general literature on the subject [1-3]. When light impinges upon matter, a small fraction of the
incident radiation is scattered, the rest being reflected, absorbed or transmitted by the sample.
Photon scattering can occur elastically, called Rayleigh scattering, or inelastically, called Raman
scattering. Both phenomena involve the immediate annihilation of an incoming photon of light by
the sample and the instantaneous emission of a second photon. In elastic scattering, the incoming
photon generates an emitted photon of the same energy, whereas in inelastic scattering, the
incoming photon generates a photon of either higher or lower energy. To understand the principles
governing Raman scattering, one must examine the momentary interaction of the incoming
radiation with the matter on which it is directed.
In Figure 1, the lowest energy levels represent the vibrational states of the ground electronic state of
a diatomic molecule, and the upper cluster of lines represents the vibrational states of the molecule
when in an excited electronic state. At room temperature, the majority of molecules exist in the
lowest vibrational state, while a small proportion occupy higher vibrational states of the ground
electronic state. An excitation beam of energy hv0 can be considered to excite the molecule into a
short-lived 'virtual' state. This higher energy state, caused by the momentary polarization of the
electrons in the molecule by the electric field of the incoming photon, is described as 'virtual'
because it does not have a fixed energy and is not therefore a true electronic excited state. The
instability of the virtual state leads to the immediate emission of a photon as the molecule returns to
a lower energy level.
The majority of the emitted photons are at the same energy, hv0, as the excitation beam and return
the molecule to the ground vibrational state of the ground electronic state via Rayleigh scattering
(Fig. la). However, a small proportion of the oscillating dipoles (approximately 1 in 10') emit
photons of either higher or lower energy. Those emitted at the lower energy, h0-hvt, are called
Stokes Raman photons (Fig. 1b), the energy difference hv1 arising from energy lost from the
incoming photon to promote the molecule into an excited vibrational level of the ground electronic
state. Anti-Stokes Raman photons (Fig. 1c), appear at the same energy difference in relation to the
excitation line, but on the high-energy side of the Rayleigh photons. The scattering centres
responsible for these photons are those few molecules that were originally in the first or, less
commonly, higher excited vibrational states of the ground electronic state. In these instances, the
energy of the excited vibrational state is transferred to the scattered photon as the molecule relaxes
from the virtual state to the ground vibrational state of the ground electronic state. The Boltzmann
law predicts that the number of vibrationally excited molecules giving rise to anti-Stokes scattering
is small at room temperature and so these Raman bands are rarely considered in the type of studies
discussed here.
Raman scattering can also arise from excitation of a molecule to virtual states lying near to its first
or higher excited electronic states [4]. In fact, excitation directly into one of these electronic
absorption bands can generate greatly enhanced Raman scattering in some molecules via a process
known as resonance
Fig. 1 An energy level diagram showing the concerted excitation-relaxation phenomena
responsible for (a) Rayleigh, (b) Stokes Raman and (c) anti-Stokes Raman scattering.
Raman scattering. It is impossible, however, to predict whether the effect will generate tangible
results in strongly absorbing materials since absorption of both the excitation beam and the resulting
Raman photons by the sample can negate the resonance enhancement and possibly lead to thermal
degradation of the sample. A more detailed description of the effect is outside the scope of this
discussion, although its value in the analysis of certain art pigments has been shown previously by a
number of authors [5-8].
Figure 1 describes a single vibrational mode of a diatomic molecule. Other photons of potentially
different energies, hvt, will be generated for a polyatomic molecule for each of its i normal
vibrational modes meeting the selection rules for Raman scattering. By spectrally sorting the Raman
scattered photons generated from these Raman-active modes, a highly characteristic vibrational
spectrum can be generated for the molecule being examined. As an example, Figure 2 shows the
Raman spectrum of vermilion (HgS) generated by a helium-neon (He-Ne) laser operating at 632.8
nm (-15,800 cm-1) and a power of 39 µW. The x-axis is labelled with both an absolute wavenumber
scale (top) and the wavenumber shift (bottom) from the excitation line, or Rayleigh peak, whose
residual intensity is seen at -15,800 cm-1 after extensive optical filtering. The wavenumber shift for
a Raman band is constant, regardless of the excitation line used, and directly relates to the energy of
the vibrational level being probed, hvt. It is therefore analogous to the wavenumber provided by IR
absorption spectroscopy. Although the vibrational information offered by Raman spectroscopy is
analogous to that of IR spectroscopy, it is not identical, but rather complementary, owing to the
different selection rules governing Raman scattering (mandatory change in polarizability) and IR
absorption (mandatory change in dipole moment).
Modern instrumentation for Raman and FT-Raman microscopy
A modern Raman microscope consists of four main components: an excitation source, an optical
microscope, a monochromator or interferometer, and a photon detector [3].
Fig. 2 The Stokes and anti-Stokes Raman spectra of vermilion (HgS) excited by the 632.8 nm
output of a He-Ne laser operating at 39 µW.
Figure 3 depicts a schematic drawing of these modules as combined in the latest line of dispersive
Raman microscopes. Many of the earlier limitations of Raman spectroscopy, e.g. poor quality
spectra and sample fluorescence, have been largely circumvented by improvements in
instrumentation.
The excitation source for Raman spectroscopy is almost universally the monochromatic output of a
laser. The frontal view in Figure 3 does not show the compact laser source because it is positioned
behind and parallel to the spectrometer unit. The laser beam enters the spectrometer in the bottom
right hand corner and is turned by an adjustable mirror to a beam expander, which decreases the
power density of the beam so that no optical components are damaged. Like other forms of
emission spectroscopy, the intensity of Raman scattering is directly related to the power of the
excitation source. However, the highly efficient optical components and sensitive photon detectors
available in modern Raman microscopes allow the use of relatively inexpensive, low-power lasers
[9]; the excitation power at the sample is seldom over 5 mW when using visible lasers, a situation
that virtually removes all risk of laser-induced degradation of the sample. The excitation line can be
of any energy so long as this is above that of the manifold of vibrational levels of the ground
electronic state of the material to be analysed. There are, however, several practical considerations
when selecting a laser for performing a Raman analysis, and a large selection of lasing lines is
desirable when examining a wide range of materials.
The scattering efficiency of a molecule is proportional to the fourth power of the frequency of the
scattered photon (v4 dependence), indicating that, other things being equal, shorter wavelength
lasers generate the more intense Raman scattering. Unfortunately, selecting an excitation
wavelength is not always simple; for instance, coloured materials are best analysed using laser lines
of similar colour, thereby limiting the absorption of the laser light, a process that competes with
Raman scattering and could, in unskilled hands, lead to sample degradation. Also, the choice of
excitation wavelength can be influenced by problematic electronic transitions in the sample, notably
fluorescence, which gives rise to red-shifted, broadband emission that is usually thousands of times
more intense than Raman scattering. Therefore, if the sample is fluorescent, the Raman spectrum is
often swamped by this more intense signal.
Sample fluorescence can be overcome by collecting the higher energy anti-Stokes Raman bands,
although this is normally
Fig. 3 A schematic drawing of a modern Raman spectrometer (Renishaw System 1000). The total
dimensions of the instrument are 168 cm x 65 cm x 75 cm while the footprint is 100 cm x 65 cm.
not practical due to their low intensities. More commonly, a long wavelength laser, often one in the
NIR region, such as a neodymium-doped yttrium-aluminium-garnet laser (Nd 3+:YAG, 1064 nm), is
employed as an excitation source. The energy of the Nd3+:YAG laser is too low to excite
fluorescence transitions in most samples. However, because NIR laser beams have low Raman
scattering efficiency (v4 dependence) and fall outside the sensitive region of Raman detectors, they
are normally used in an FT-Raman configuration with pyroelectric detectors so as to benefit from
all the advantages of interferometry over dispersive spectroscopy (see below) [2]. A recent
development is the availability of long wavelength diode lasers (e.g. the gallium-aluminiumarsenide (GaAlAs) diode laser, 780 nm) whose emission falls within the sensitive region of charge
coupled device (CCD) detectors. Using these lasers, the problem of fluorescence can be
circumvented without requiring interferometric systems and with increased Raman scattering
efficiency in comparison to the Nd3+:YAG laser.
The excitation beam is next passed through an optical microscope where it can be focused to an
area of ≤1 µm2. The Raman scattered photons that are generated at the focus are back-collected by
the same microscope objective and directed to the spectrometer. The ability of the microscope to
focus the radiation is ultimately constrained by the diffraction limit imposed by the excitation
wavelength. This can be a limitation when using NIR lasers in FT-Raman microscopy. In these
instances, the focus diameter is large, often -20 µm, thereby sacrificing one of the most important
advantages of Raman microscopy, that of high spatial resolution. The excitation depth similarly
varies with different laser wavelengths. In non-resonance conditions, the sampling depth is
normally of the order of the laser wavelength being used, and in resonance or near-resonance
conditions, it can be much less than this. Narrowing of the spectrometer slits (see Fig. 3), coupled
with a reduction in the active detector area, can allow for tight control over the sampled depth via a
confocal effect [3]. In this arrangement, only the Raman scattering generated at a very narrow slice
of the laser focus is able to pass through the slit and be properly focused onto the detector. Such
spatial discrimination has been used to profile stratified materials, such as crossed ink lines [10] or
paint and glaze layers in ceramics [11], and can also limit fluorescence interference from material
above or below the focal point.
A monochromator or interferometer is next used to sort spectrally the Raman scattered photons after
first rejecting the reflected and elastically scattered laser light. Because of their much larger relative
intensities, the laser and Rayleigh photons must be removed in order to avoid saturating the detector
system. In first-generation dispersive instruments, many of which are still being used to great effect,
a fore-monochromator is used to reject the interfering radiation before sending the Raman photons
to a second and even a third monochromator for spatial sorting. Although extremely large and slow,
these instruments are normally very efficient at eliminating Rayleigh scattering, and spectral data
often can be measured to within 10 cm-1 of the Rayleigh line. Portable instruments, such as that
shown in Figure 3, make use of small, narrow-band holographic notch filters to reject the Rayleigh
scattering. Although considerably smaller and faster, these instruments currently suffer because
spectral data cannot be obtained within 50 cm-1 of the laser line. Furthermore, a separate and
expensive notch filter assembly is required for each exciting line. A recently announced near
excitation tunable (NExT) notch filter should overcome both of these problems by allowing data
collection for any excitation wavelength to within 10 cm ' of the Rayleigh line.
Following the Rayleigh rejection filter, a diffraction grating allows the spatial separation of the
Raman scattered photons in a dispersive instrument (see Fig. 3). Spectral sorting in FT-Raman
instruments, however, is achieved by an interferometer [2]. These instruments often utilize the
Michelson interferometer of an existing FTIR spectrometer. The high optical throughput, multiplex
capability and high wavenumber accuracy of the interferometer are all retained in FT-Rarnan
microscopy. However, an additional complication is introduced in that the inelastically scattered
photons from a NIR laser source can fall within the absorption spectrum of water vapour, and so
these instruments must be purged with dry air constantly. This requirement cannot be met when
examining large objects under a microscope, and so small portions of the Raman spectrum are often
inaccessible due to this interference. Moreover, the NIR Raman photons can also be absorbed by
the sample via overtone and combination modes, possibly leading to thermal degradation of the
sample or to spectral intensity losses.
After rejecting the Rayleigh photons and spatially or interferometrically sorting the Raman photons,
the spectrum is recorded through the use of photovoltaic detectors in dispersive Raman microscopy
or a pyroelectric detector in FT systems. The former can be either a single photomultiplier tube
that records discrete wavenumber intensities as the monochromator is scanned or an array of
detector elements on which a region of the spectrum is dispersed. Two-dimensional detectors are
widely used now that inexpensive, thermoelectrically cooled, and highly sensitive CCD cameras are
available (see Fig. 3). Because current CCDs are insensitive in the NIR region of the spectrum, FTRaman microscopy again makes use of existing FTIR instrumentation, germanium and indiumgallium-arsenide (InGaAs) detectors, for recording the interferogram generated as the
interferometer is scanned. The need for liquid nitrogen cooling of some NIR detectors is an
additional complication of the FT-Raman technique.
Advantages and limitations of Raman microscopy
The characteristics of the Raman microscope that make it so well qualified for the analysis of
historical materials include its molecular specificity, speed, non-destructiveness, high spatial (< 1
µm) and spectral (< 1 cm-1) resolution, in situ analysis, applicability to unprepared samples of large
or non-uniform shape, and relative immunity to interference [7, 12]. These features are all highly
desirable in any analytical tool used to identify and study inhomogeneous and otherwise complex
samples such as those represented by museum objects. Although Raman microscopy is not suitable
for all forms of conservation and archaeological analysis (e.g. assaying of metal alloys,
Table 1 Strengths and weaknesses of analytical techniques commonly used in conservation science
Raman, visible laser Raman microscopy; IR, mid-infra-red reflectance microscopy; PLM,
polarising light microscopy; UV/VIS, ultraviolet/visible reflectance spectroscopy or fibre optic
reflectance spectroscopy (FORS); LIBS, laser-induced breakdown spectroscopy; SEM/EDX,
scanning electron microscopy with Be-windowed energy dispersive X-ray detection; XRF, X-ray
fluorescence spectroscopy; XPS, X-ray photoelectron spectroscopy, also called electron
spectroscopy for chemical analysis (ESCA); PIXE/PIGE, external beam proton-induced X-ray
emission/proton-induced γ-ray emission; XRD, powder X-ray diffraction.
a Sensitivity can be excellent under resonance conditions.
b Only possible with crystalline materials.
c Only atoms with atomic number Z ≥ 14 (Si) without an evacuated sample chamber.
d X-ray escape depth varies from nm to mm dimensions depending on the material and the
element being detected. Can lead
to loss of spatial resolution and to interference from sub-surface layers. e All atoms with Z ≥ 5
(B).
f Depth of sample is normally confined to several nanometres,
g All atoms with Z ≥ 1 l(Na). A proton beam (via PIXE) generates less bremsstrahlung
background radiation than other X- ray techniques, providing greater sensitivity. PIXE can also be
modified with different detectors to perform nuclear reaction analysis (NRA) and Rutherford backscattering (RBS) studies,
h Environmental SEM can analyse small, intact artefacts.
isotopic analysis, elemental fingerprinting, etc.) it compares well with other analytical techniques
commonly employed in the conservation or archaeometry laboratory for the identification of
molecular solids.
Table 1 assesses the relative strengths and weaknesses of Raman microscopy and nine other
spectroscopic and microscopic techniques with which the conservation scientist will be familiar.
The evaluation of each instrument is necessarily approximate and is based on its typical
configuration as found in conservation laboratories across the world, it being well understood that
specialized operating conditions and accessories can be used to enhance the performance in each
case. The information contained in Table 1 was gleaned from overviews of these analytical methods
[13-16]. The comparisons in Table 1 make a strong argument for the role that Raman microscopy
can play in the conservation laboratory, and several of the important strengths of the technique are
highlighted below. It is worth noting, though, that the complexity of cultural materials inevitably
requires the use of several analytical tools for a complete and confident analysis; no single
instrument can answer all the questions that may arise in the laboratory. Particularly informative
investigations of artefacts have resulted from the powerful combinations of Raman microscopy and
total reflection XRF [17, 18], LIBS [19], PIXE [20] and FTIR reflection microscopy [21].
The Raman technique is shown in Table 1 to be highly effective in identifying unknown materials,
both organic and inorganic. Molecular specificity is inherent in vibrational spectroscopy, since
molecular group frequencies and crystal lattice modes lead to unique vibrational spectra for
different materials, and even for materials that are compositionally identical but differ in either
connectivity, e.g. realgar versus pararealgar (a and 7-As4S4, respectively) [22, 23], or in their crystal
structure, e.g. massicot versus litharge (orthorhombic and tetragonal PbO, respectively) [24].
Comparing the Raman spectrum of a sample to a library of spectra of possible materials usually
yields an unambiguous match that leads to the identification of the otherwise unknown compound.
Raman spectra databases for minerals [25-28], plant fibres [29], historical pigments [24, 30],
modern synthetic pigments [31], enamel/glaze pigments [32], modern ballpoint inks [10],
archaeologically significant gums [33] and waxes [34, 35], and artistic varnishes, resins, and
binders [30, 36] are available in the literature, and databases of Raman spectra of thousands of
organic and inorganic materials are offered commercially. Moreover, Raman spectra respond
predictably to structural and environmental changes in a molecule in terms of the number of Raman
bands observed, their wavenumbers, intensities and bandwidths. This makes the technique
appropriate for the study of reactions or of physical changes in materials. Although the technique is
well suited to qualitative study, quantitative data are more difficult to obtain due to their
dependence on individual instrumental parameters; consequently, only a few researchers examining
historical materials have attempted to obtain them [37, 38].
As seen in Table 1, several other techniques also provide superb molecular specificity. IR
microscopy, as mentioned previously, generates data analogous to that of Raman spectroscopy.
However, as recently as 1994 Mills and White could claim, 'The Raman effect is however a very
weak one and good spectra are not easy to obtain. It seems unlikely to be able to compete with
FTIR and the infra-red microscope' [39, p. 22]. Even then, Raman instrumentation was improving
rapidly, and today the high spectral quality, ease of use and speed of modern Raman spectrometers
is comparable to, and often exceeds, that of IR microscopes. For example, each technique generates
a characteristic vibrational spectrum for a material, but the low wavenumber region - critical for the
identification of inorganic materials - is more easily studied by Raman microscopy. Furthermore,
water, a ubiquitous component adsorbed to or structurally associated with archaeological and
artistic materials, is a serious interference in IR spectroscopy but not in Raman spectroscopy.
Despite these hindrances, IR microscopy is often capable of detecting thin organic layers that can
often be highly fluorescent, such as pigment binders [21], a task that is typically more difficult for
Raman microscopy. As such, IR microscopy is not likely to be replaced in the analysis of artists'
materials, but it certainly no longer overshadows Raman spectroscopy.
Powder XRD, a superbly specific technique for identifying compounds based on their atomic
structures, is only suitable for highly crystalline samples and is limited, except for the most recent
and most expensive diffractometers, to the analysis of ex situ samples, normally material sampled
from a small area on the surface of the artefact. By comparison, Raman microscopy can be used
successfully to identify even amorphous materials and can be used in situ on large, non-uniformly
shaped objects, such as statuary and codices. XRF has been widely used to identify in situ mineral
pigments based on their elemental profiles, but it cannot detect elements lighter than silicon without
the benefit of an evacuated sample chamber, nor can it distinguish between pigments with identical
or even similar elemental compositions. The latter limitation might explain why the yellow mineral
pararealgar had never been detected as a pigment on a manuscript illumination until Raman
microscopy became widely used in this area [40]; XRF analysts would have assigned all yellow
arsenic sulfide pigments to the commonly used mineral orpiment (As2S3) [22, 23]. Optical
microscopy, perhaps the most utilized technique in conservation, requires a highly skilled operator
and sometimes considerable sample preparation [13]. Even then, it often fails to provide conclusive
evidence for many of the important materials in archaeology and art, especially those that are
organic. Although these structural, elemental and optical techniques have a well-deserved place in
the conservation laboratory, many of the tasks of identification for which they are normally used
can be done equally well and much more quickly by Raman spectroscopy, either as the principal
technique or as a confirmatory method.
Raman microscopy offers high spatial resolution for in situ analyses. For complex mixtures, as in
paintings where the mixing of several pigments generates the final colour, the ability to resolve
spatially and identify pigment grains of different composition is highly desirable. Often
unrecognized is the added benefit that high spatial resolution yields in removing interferences in
Raman microscopy, such as fluorescence from binders or varnishes. The ability to focus the
excitation beam on an isolated, exposed pigment grain effectively reduces the interfering signal
from the associated material, often making an apparently hopeless analysis feasible.
Another popular attribute of the Raman microscope is its small size, and therefore its portability.
Advances in Rayleigh rejection filters and laser miniaturization are leading to instruments that can
be moved easily. This development is allowing more extensive collaboration between academic
research groups and museums that had been hesitant to loan materials to laboratories. The authors
have on numerous occasions disassembled, transported and reassembled their research-grade
Raman microscope in order to work on-site at London museums and libraries on objects either too
large or too valuable to be brought to the laboratory. In one instance, a portable instrument was
shipped from England to the United States in order to perform an analysis of the Vinland Map at
Yale University [41]. Portability and increasingly rugged instrument design suggest that Raman
microscopy is likely to expand into field-based applications in conservation, archaeology and
geology in the near future.
Raman microscopy in art analysis and conservation studies
The plenitude of material in art and archaeology to which Raman spectroscopy has contributed
precludes a comprehensive treatment of both areas here. Rather, an attempt has been made to focus
on those articles concerned with conservation science and the analysis of artists' materials while
reserving those dealing with traditionally archaeological objects, e.g. glass and glazes, lithics,
ceramics, cave art, etc., for coverage in a separate review. It is well recognized that the separation
between these fields is tenuous and somewhat arbitrary, but it is unfortunately necessary in order to
treat the material in any way other than that of providing a brief catalogue (see, for example, [42,
43]).
In this review, every effort has been made to cover all of the art and conservation research articles
in English from those peer-reviewed journals that are widely available. An attempt is made to assess
the work critically and to highlight the historical or scientific importance of the results. Those
applications in which Raman spectroscopy was used without the benefit of a microscope have been
included, it being understood that the research could have been performed as well or better with a
microscope attachment had one been available. Similarly, applications using fibre optic sampling
accessories are also reported. This area is recognized as being one that is developing rapidly and
that promises to increase the applicability of Raman spectroscopy to extremely large or otherwise
inaccessible objects [44, 45]. The review is separated into sections devoted to the analysis of a
particular class of material or to a specific research goal. This division is intended to demonstrate
the different ways in which Raman analysis is being used and to highlight the various instrumental
concerns dictated by different materials.
Authentication of artefacts and artwork
Raman microscopy has been used to 'authenticate' painted objects by identifying the palette used in
the creation of the object and noting any date-marker pigments. The latter are synthetic pigments for
which the date of first manufacture or availability is well known; their identification therefore
provides a terminus post quern for the decoration of the object which can be compared with the
purported date of the work. It is important to note that the analysis of the palette of an object can
identify materials inconsistent with its suggested date of manufacture, thereby providing evidence
of the object's inauthenticity, but can never definitively prove that an object is authentic. An
example of such negative evidence of forgery was generated during the Raman analysis of an
unusually illuminated thirteenth-century Qur'an for a London auction house; only natural, period
pigments were identified [46]. A perfect forgery, although not suspected in this instance, might
make use of pigments known in antiquity and other aged materials in order to appear 'authentic,'
even if the object were freshly constructed.
Prior to their possible auction, six supposedly thirteenth- to first-century BC illuminated papyri
were examined by Raman microscopy and shown conclusively to be fakes [47]. The paintings were
found throughout to contain white anatase TiO2, phthalocyanine blue and green, Hansa yellow and
Prussian blue; all of these pigments are post-1704 synthetic artists' materials and therefore betray
the modern date of the works. A second authentication study by the same authors on a Book of
Frames on parchment sought to identify authentic mid-sixteenth-century Ghent/Bruges scatter
borders from possible later additions and copies [48]. The collection of eight frames was shown to
consist primarily of period mineral pigments with the exception of chrome yellow (PbCrO4), which
was only used as a pigment from the beginning of the nineteenth century [24]. However, its
presence may indicate extensive restoration after 1846 when the frames were known to have
suffered from fire and water damage. This point could only be verified by an art specialist.
Unsurprisingly, researchers have capitalized on the structural specificity of Raman microscopy as a
quick, non-destructive means of uncovering faked artefacts and faux jewellery by detecting the use
of alternative materials in their fabrication. Raman spectra from purportedly eighteenth- and
nineteenth-century scrimshaw (whale ivory carvings) can be used to distinguish those composed of
authentic animal ivory from contemporary 'forgeries' composed of substitute materials, such as
bone, and from modern fakes made of composite materials [49, 50]. Similar work with precious
stones and jewellery has discerned polystyrene, acrylate and polyurethane costume jewellery from
true amber, ivory artefacts from simulated ivory, real pearls from faux beads, and true coral from
glass imitations [51, 52]. Raman microscopy has been used to verify the presence of genuine ruby
and diamond rather than coloured glass on the famous Armada Jewel [53]. Among these precious
gems, it is even possible to use Raman spectroscopy to discriminate between true geological
specimens and their modern synthetic counterparts, for example those produced using hydrothermal
or anhydrous dissolution methods [54].
Pigments on manuscripts and fine art
Most work involving pigment identification on manuscripts and fine artworks has been undertaken
as a prelude to restoration or conservation efforts, for authentication purposes (see above) or in the
pursuit of art historical information. Because these applications are so numerous and constitute the
vast majority of the published research in which Raman microscopy has been used to analyse art,
they cannot all be dealt with fully here. Instead, a thematic treatment of the rich information
generated by the Raman analysis of artistic palettes will be presented below.
The principal advantages brought to pigment analysis by Raman microscopy are its ease of
application, and therefore speed, and its non-destructive nature. The former means that the palettes
of large numbers of artworks can be examined in order to test hypotheses regarding the use and
history of certain pigments or the techniques and careers of specific artists. The non-destructiveness
has allowed pigment analyses to proceed independently of restoration or conservation treatments
since the technique is often capable of identifying pigments without the removal of varnishes, either
by analyzing serendipitously exposed pigment grains or by collecting the Raman spectrum directly
through the varnish [45]. Taken together, these qualities have helped to assuage many of the
concerns of curators regarding destructive artefact sampling and long-term removal of an object
from display.
The ability to identify pigments with confidence using a Raman microscope requires the availability
of a large, reliable database of reference spectra from genuine historical materials. A significant
obstacle to the widespread application of the technique in art analysis was overcome with the
publication of the first large spectral library of mineral and organic pigments [24]. However, even
before its availability, a few investigations had already taken place as the potential of the technique
was realized by several researchers working at the Arts-Science interface [7, 55-57]. The first items
to be investigated, and still the most frequently examined, were ancient manuscripts [7, 8, 12, 17,
18, 20, 21, 36, 40, 44, 46, 55-74]. This was primarily due to the dominant use of strongly scattering
mineral pigments on these artefacts and their watercolour application without an overlayer of
varnish. It did not take long, however, before the Raman analysis of all fine art forms had been
attempted, including watercolours [31, 53, 75-77], oil paintings [45, 75, 78-80], lithographs [75, 81]
and chalk drawings [6].
The extensive application of Raman microscopy to manuscripts is allowing the compilation of
typical regional palettes for Europe [53, 66, 71], Iceland [62], Scandinavia [69], Persia [82], East
Asia [8, 64, 68] and the Middle East [40, 46, 67]. Present work by the authors and others will
further expand this list and continue to refine these palettes both geographically and temporally. It is
expected that these results will add palette identification as an objective criterion to stylistic analysis
when attributing problematic manuscript illuminations to specific schools, scriptoria or cultures.
One such instance in which preliminary work has been completed concerns the Qazwini
manuscripts held by the British Library [44]. In these works, Arabic text is accompanied by a
confusing mixture of illuminations in variously Indian, Persian and Arabic styles. Pigment analysis
is expected to add to the codicological information regarding the chronology and cultural origin of
these illustrations with respect to the script.
Another benefit of defining regional palettes will be the possibility of relating their differences to
specific technological, cultural, geological and archaeological circumstances. The anomalous
absence of lead-containing pigments, e.g. lead white (2PbCO3.Pb(OH)2) and red lead (Pb3O4),
ubiquitous pigments in European illuminations, from a medieval Icelandic work is one such
example [62]. Although it is not yet resolved, this peculiarity might indicate inconstant or limited
trade links with the European mainland, an observation of potential archaeological importance,
rather than simply the artistic prerogative of the Icelandic ateliers.
Pigment analysis using Raman spectroscopy has afforded solid evidence to corroborate and to
controvert textual references in treatises that record various illumination techniques. Spectroscopic
identification of a thinly applied layer of lapis lazuli (Na8[Al6Si6O24]Sn ) over azurite
(2CuCO3.Cu(OH)2) confirms mention of this method of medieval artistic economy
for maximizing the effect of small quantities of the expensive material lazurite, which was difficult
to obtain and demanding to refine [7]. Similarly, the medieval concept of hierarchy, and therefore
intrinsic value, has been shown to be an important consideration in the artists' choice of certain
pigments for the illumination of specific subjects [18, 21, 57]. For instance, in Commentary on
Ezekiel, an ecclesiastical manuscript of the eleventh century AD, an abbot and saint are shown
together; the abbot's robe is painted in the inexpensive organic dye woad while the revered saint's
vestments are coloured richly with lapis lazuli [57].
The identification of lapis lazuli on this manuscript and chronologically related ones from other
abbeys in France is important since it establishes the use of that mineral pigment in Europe nearly
two hundred years earlier than had previously been thought [59, 83]. The examination of blue
pigments on illuminations from this period shows a transition in which the usage of indigo begins to
wane as the dominant blue pigment in manuscript art. Clearly, trade at the turn of the millennium
between the East - Afghanistan being the accepted source of ancient lapis lazuli - and Europe was
far more extensive than previously believed. The possible earlier availability of this material in
northern Europe will be investigated shortly when the blue pigments on the Lindisfarne Gospels
from the eighth century AD and other contemporary Anglo-Saxon manuscripts are examined by
Raman microscopy through collaboration between University College London and the British
Library.
The accuracy of textual evidence for contemporary artistic practices is contestable, and Raman
analyses have shown that the information in historical treatises, though of extreme importance,
should not be accepted unequivocally. Such a deviation from historical accounts was confirmed by
the pigment analysis of the limning in the Armada Jewel given to Elizabeth I by Richard Heneage
after the defeat of the Spanish Armada [53]. Although the magnum opus of the foremost limner of
the time, Nicholas Hilliard (1547-1619), is quoted as warning against the use of 'ill-smelling
colours, all ill-tasting, as orpiment' [53, p. 186], in fact, orpiment was found to have been used on
this prized locket.
Although the aforementioned applications of Raman microscopy to the identification of mineral
pigments were met with almost immediate success, progress in the detection of organic pigments
has been much slower. Organic pigments, dyes and binders on artefacts may suffer from poor
Raman scattering efficiency, susceptibility to photo-degradation and intrinsic fluorescence. Only
recently has Raman analysis developed to the point of surmounting these obstacles. Significant
improvement in the detectability of organic materials has accompanied the recent availability of
high throughput spectrometers and more sensitive detectors (see, for example, the improvement in
the Raman spectrum of indigo between [60] and [68]). Luminescence interference from binders and
substrates can be minimized through the use of clever spectral manipulations, such as subtracted
shifted Raman spectroscopy (SSRS), which has allowed the detection and characterization of
yellow huangbo dye on highly fluorescent papers from the library at Dunhuang, China [8].
Moreover, the increasing use of long wavelength lasers in dispersive and FT-Raman microscopy
has further reduced the obstacle imposed by fluorescent species. A wealth of reference spectra for
organic materials has been generated using these systems [29, 33-36], and its availability has been
lauded as a prelude to the widespread identification of binders, varnishes and organic pigments in
artwork using the technique. In some instances, it has even been suggested that the exact species of
plant generating the artists' material could be identified from subtle spectral differences [84].
Despite the enthusiasm, these assertions have been shown by experiment to be overly optimistic for
the current state of the art in Raman microscopy as applied to real historical samples. One cause of
the discrepancy between the claims made for this type of organic analysis and its successful
implementation lies in the reference samples used in the construction of the spectral libraries. In
these databases, the reference materials are in most instances single examples of modern specimens
that have been analysed in bulk. The organic pigments are represented in the libraries by spectra of
the pure chromophore compounds rather than the lakes or dyed substrates likely to be encountered
in real samples. Numerous complications therefore arise when one considers that the compositional
complexity of the materials, their natural biodiversity, their low concentrations in real artwork, the
structural effects of ageing and the effects of ancient methods of preparation are not taken into
consideration. These concerns are less significant for mineral compounds since they are rarely
affected by such physical and chemical changes from their native state to that encountered in
pigments.
Even in the pure specimens used to construct the databases, the clear, distinct differences professed
to exist between the Raman spectra of compounds within a specific class of organic materials are
often neither clear nor distinct; compare the reference spectra of poppy-seed, walnut and sunflower
oils in Vandenabeele et al. [36]. Furthermore, this type of analysis is not as simple as the
identification of mineral pigments; the extreme similarity among some spectra of related organic
materials requires the use of sophisticated self-deconvolution analysis in order to provide
characteristic features suitable for discriminating between different samples [33]. It remains to be
seen whether such chemometric analysis will continue to function successfully on real samples. As
a consequence of these complications, successful attempts to distinguish between organic binders
by Raman spectroscopy are scant, there being only one instance in which the presence of a beeswax
coating on paper has been clearly identified [36]. In most instances, it is only possible to provide a
general description of the material present, e.g. a 'resin' [74, 85], based on the functional groups
revealed in the spectrum.
Although organic analysis has proved to be neither as effortless nor as successful as originally
claimed, the ability to distinguish between even broad classes of binder while simultaneously
acquiring data on the pigments and dyes contained in an artwork is highly useful and likely to
improve with advances in instrumentation and in the quality of spectral databases. As large
databases of compositionally similar organic pigments and artists' materials are created, laudable
efforts are also being made towards the development of automated search routines for objectively
selecting matches between unknown and reference spectra [80]. Various parameterized and nonparameterized modelling algorithms have been tested for the automated deconvolution of pigment
mixtures for both qualitative and quantitative analysis of paint layers in artwork; however, the
results are not yet convincing [86].
Even with suitable reference databases, the identification of artists' materials is not trivial.
Numerous pigments and pigment
degradation products are known to be highly sensitive to laser radiation and could transform readily
at powers beyond a limiting value. Examples include many of the manganese [87], iron [27, 28] and
lead [88] oxides, hydroxides and sulfides. This laser sensitivity has led to the mistaken association
of a compound with that of its thermally-induced degradation species [40, 67, 73]. In a number of
instances, these mistaken identifications have been discovered and corrected afterwards [89, 90].
When examining the important prehistoric black pigment pyrolusite (β-MnO2) however, the laser
sensitivity of the material seems to have been unrecognized [74, 91-94]. Various spectra have been
presented as genuine β~MnO2, but which deviate from those collected cautiously at low laser power
[28, 87].
Painted statuary, icons and architecture
Few differences exist between the analysis of pigments on fine art and manuscripts and those on
three dimensional art and architecture except for considerations of the sometimes immense size and
irregular shape of the latter. These physical constraints have meant that samples were normally
taken from objects either too large or irregularly shaped to fit easily under a traditional microscope.
However, fibre optics for remote laser Raman spectroscopy, side-looking microscope objectives
and customized microscope stages are alleviating this stricture.
Raman microscopic analysis of pigments used to decorate two Egyptian cartonnage masks found a
diverse palette in use in Egypt in the third to fourth centuries BC that included realgar, pararealgar,
cinnabar (HgS), haematite (α-Fe2O)), Egyptian blue (CaCuSi4O|0), calcite (trigonal CaCO3) and
gypsum (CaSO4.2H2O) [95]. The pararealgar appears to have been intentionally applied rather than
present as a degradation product of realgar; no residual realgar was detected in admixture with
pararealgar although it was present elsewhere on the mask. Whether the ancient Egyptians realized
that they were using pararealgar as a yellow pigment or whether they mistook it for the more
common orpiment is not known. Proof of an early appreciation by the Egyptians for this material as
a pigment per se awaits further identifications, but already such evidence is mounting [22, 23, 40,
47, 67, 95, 96]. Guineau has shown that a similar, but much older yellow funerary mask from
thirteenth-dynasty Egypt (1780 to 1680 BC) was in fact painted with orpiment and calcite [58].
The compound specificity of Raman microscopy has recently been coupled to the elemental depth
profiling of LIBS for the analysis of a wooden Rococo altarpiece [97], a Byzantine icon [19], a
Greek icon [98] and two Venetian miniatures executed on ivory [98]. The LIBS technique uses laser
radiation to generate a plasma at the surface of the sample and then collects the atomic emission of
the atomized pigment. Multiple laser shots ablate successive layers of paint, thereby obtaining a
micro-destructive, in situ cross-sectional elemental profile of the painting materials. Figure 4 shows
the combined results of a LIBS-Raman analysis of a brown painted area on a Russian icon [19]. The
topmost paint layer failed to give a Raman spectrum using a 780 nm excitation line, but the LIBS
data showed the presence of Fe, which, in combination with the negative result from the Raman
analysis, suggested the use of an iron earth pigment. The brown earth pigments are known to be
weakly scattering samples in the NIR region. LIBS spectra from successive laser pulses revealed
that the brown paint was applied over a very thin silver foil. Elemental analysis can be especially
important in iconography where silver and gold leaf, as well as pigments used to imitate these
metals, are used extensively. Pure metallic pigments are Raman silent, and so the coupling of the
two techniques is particularly powerful for providing a comprehensive analysis of an artist's palette.
Finally, the lowest layers of the painting revealed large concentrations of Ca (Fig. 4, bottom left),
suggesting a gypsum, calcite or mixed ground layer. In instances where the elemental profiles are
inconclusive as to the identity of a particular pigment, the Raman technique is definitive; the Raman
spectrum from this area (Fig. 4, bottom right) proves that the ground was in fact a mixture of
gypsum and anhydrite (CaSO4). Although the studies mentioned above involved the sequential use
of the two methods, a tandem LIBS-Raman spectrometer has been reported [99].
A thirteenth-century Spanish statue from Sasamon has been shown by FT-Raman spectroscopy to
have been painted using several complex mixtures of pigments including cinnabar and red lead to
form an orange-red colour as well as red lead possibly mixed with aurum musivum (SnS2, mosaic
gold) and litharge or massicot to form a golden brown colour [38]. The absence of bands due to aquartz (SiO2) and the presence of those due to calcite in the spectrum of the HgS have been
interpreted as indicating a local source of cinnabar, the Tarna mines. Pigment obtained from the
other large Roman-period mine at Almaden, Spain, always bears tell-tale traces of quartz due to its
volcanic origin, while calcite has been found as a
component of cinnabar ore from Tarna. Although such interpretations are worth noting, one must be
cautious, since the purposeful addition of quartz sand as an aid to pulverization of the pigment, or
the adulteration of cinnabar with chalk either to lighten the colour or as a profiteering trick of the
medieval apothecary, cannot be ruled out. Furthermore, the appearance of either of these ancillary
components in the paint as a byproduct of its application to lime plaster or surfaces otherwise
prepared to accept pigments would not be out of the ordinary, and therefore provenance based on
these indicators alone could be misleading.
The aureate pigment mixture is noteworthy because of the suggested presence of mosaic gold.
However, this conclusion is surprising since the identification of SnS2 was based on a single, weak
Raman band at the same wavenumber as a weak band of red lead, 313 cm-1, shown unequivocally to
be present in the mixture. Definitive elemental data were not collected, and so the identification of
mosaic gold in this instance must be taken with caution. Massicot or even pale litharge could
equally well account for the yellowish colour of the pigment without invoking the presence of
mosaic gold. Although SnS2 itself has a metallic lustre, it was discovered that the grinding of that
material with a lead oxide produced a rich golden mirror-like surface. Upon examination, the
lustrous material was thought to be a Pb-Sn alloy since it failed to give a Raman spectrum,
suggesting that the original pigment mixture, if composed of red lead and mosaic gold as claimed,
could indicate
Fig. 4 LIBS spectra (left) from successive laser pulses reveal the elemental makeup of paint
layers on a Russian icon. The first two layers did not yield Raman spectra (right), while the bottom
layer is shown by its Raman spectrum (a) to be composed of (b) gypsum (CaSO4.2H2O) and (c)
anhydrite (CaSO4) [19].
a lost technology for forming a faux gilding material. However, why it apparently existed on the
statue in its mixed components rather than as the Raman-silent alloy discovered in the grinding
experiments was not addressed.
Marble from the facade of the Certosa of Pavia was sampled for analysis by Raman microscopy to
identify the nature of red stains [100]. These had been thought to arise from bacterial growth, but
were instead shown to be red lead, most likely from oxidation of lead salts originating from the
degradation of lead architectural elements. Small green spots affecting the surface were also
identified by the Raman technique as being Chlorophyta micro-organisms based on their association
with carotenoids as indicated in the Raman spectrum. In another study, the identification of PbO
and red lead in underlayers of gilded stucco provided data on the Baroque technique for applying
gold to plaster [101]. The lead compounds were used as siccatives to aid the drying of linseed oil
mordants for the gold leaf.
Wall paintings and fresco
Wall paintings and fresco are normally unlike other artistic works, due to their large size and by
their having been exposed to the elements, light and pollution. As such, mural art rarely survives
intact. Raman and FT-Raman spectroscopy has been used to examine the effects of lichen
encrustations on fresco art [102-104] and on ecclesiastic architecture [105]. The lichen Dirina
massiliensis forma sorediata has been shown to biodeteriorate frescoes aggressively through the
chelating action of oxalic acid (H2C2O4) produced by its burrowing hyphae. Calcium oxalate
monohydrate (CaC2O4.H2O, whewellite) was found to be present several millimetres below the
surface of the frescos, and significant amounts of substratal material, normally calcite or gypsum,
have been established to incorporate into the thallus of the lichen, thus adding further disruption to
the artwork. The Raman spectra of some biodegraded painted areas suggest that the lichen might
also be capable of neutralizing toxic heavy metals from pigments by incorporating them into other
oxalate species [106]. The same Dirina mycobiont was also found to be responsible for the white
crust, originally thought to be the remnants of an early conservation effort, covering the limestone
exterior of an English church [105|. Here, however, the biodeterioration product was found to be
calcium oxalate dihydrate (CaC2O4.2H2O, weddellite), suggesting alternative survival strategies of
the lichen, possibly related to water storage, depending on its host substrate or its environment.
The identification of pigments by Raman spectroscopy has been performed on samples of wall
paintings from Roman Palestine [107], Byzantine Greece [28,108], medieval England [109] and
Spain [85, 110, 111] and Renaissance Italy [106]. Many of the same artistic strategies discussed for
manuscript illuminations and fine art were shown to have been employed for the painting of murals
and frescos. Instances of pigment economy have been suggested by the adulteration of the
expensive pigment cinnabar with red lead [37, 85, 110] or red ochre [109] as revealed by Raman
analysis. The hierarchical use of costly cinnabar and lapis lazuli for paintings of the Holy Family
has been reported in Spanish frescoes whose ancillary figures were painted with diluted cinnabar
and an arguably cheaper organic red, possibly a resin [37, 85]. In the study of another fresco bearing
geometric designs [37, 110], however, different pigment mixtures ranging from pure cinnabar to
mostly red lead were used indiscriminately, suggesting in this instance that the use of pigments of
varying quality was arbitrary or possibly due to numerous artistic hands.
Textiles and plant fibres
FT-Raman analysis has been performed on a number of archaeologically important natural plant
fibres, leading to the construction of a library of reference spectra [29]. Attempts have been made to
identify the structural changes in archaeological linen that lead to its brittleness and dark colour
after long periods of interment [112, 113]. Despite its age and burial conditions, the analysis has
shown that a 4000-year-old Egyptian mummy wrapping is in quite good structural condition by
comparison of its Raman spectrum with that of a sample of modern linen. However, spectral
changes between the mummy linen and a badly deteriorated linen sample from a burial near the
Dead Sea (614 AD) are slight, and provide little information as to the nature of the colour change
and friability of these textiles [112]. The presence of carotenoid-like compounds in water-soluble
extracts of the linen 'crumbs', that is the physical detritus of the material, suggests tentatively that
this process might result from enzymatic damage to the cellulose structure via fungal or microbial
activity [113].
An analysis of contemporary 'jeans' in a textile museum collection has identified the dye used in the
manufacturing process [114]. In almost all instances where the style and cut of the garments match
the fashion definition of jeans, the Raman spectrum of indigotin, the main component along with
indirubin of indigo dye, was observed. Interestingly, in some textile samples, Raman-inactive bu
modes of indigotin gave rise to detectable bands, indicating that the dyeing process had led to the
loss of planarity and centrosymmetry in the molecule. Similarly, mordants were also shown to
affect the Raman spectra of haemateine dye [83], but no structural changes were detected between
neat madder and the same dye applied to wool with various mordants [115].
The analyses of jeans mentioned previously [114] showed that the technique lacked the sensitivity
to detect the spectrum of indirubin [60], a compound known through chromatography to exist in
small quantities in the native dye. Although the technique was unable in this instance to detect dye
compounds that exist in extremely small quantities, a related application using FT-Raman
spectroscopy and a fibre sampling accessory identified small amounts of cobalt blue and red
matador dyes (1-2% w/w) on acrylic textile fibres [116]. In this work, the spectrum of the polymer
was successfully subtracted from that of the fibre and dye to yield a high quality spectrum of the
dyestuff alone. An identical manipulation using diffuse reflectance FTIR spectra was less
successful.
Conservation and ancient technology studies
The characteristics of Raman microscopy that make it so well suited for the applied research
mentioned above also allow it to be used to great effect for basic research in conservation and
archaeology. This type of work involves understanding the chemistry and degradation mechanisms
of historical materials and pigments, assessing the effectiveness of conservation procedures and
rediscovering the ancient technologies required for the production of certain pigments. The
structural sensitivity of Raman microscopy has been used to specify S3-., S2-. and another
unidentified species, as the principal chromophores in a series of differently-coloured ultramarine
pigments [5]; to investigate the transformation process for the light-induced conversion of realgar to
pararealgar through an intermediate phase [23]; and to optimize the experimental conditions for the
non-destructive analysis of various lead pigments by Raman microscopy [88, 89]. Further
investigation into the thermal decomposition of lead compounds by several techniques including
Raman spectroscopy has elucidated better the ancient process by which red lead was generated from
the roasting of lead white [117]. Notably, a previously unknown intermediate phase of lead white
was discovered. The formation of Pb3O4 was found to depend on the amount of oxygen, the
temperature and the duration of heating, and these conditions were shown to effect the production
of lead(II) oxides that either superseded the formation of red lead or greatly influenced the final
shade of that pigment.
The investigation of degraded organic materials has also been pursued. The ability of FT-Raman
spectroscopy to monitor structural differences between ancient vellums and parchments has been
shown, although the results suggest that the ability of this technique to indicate their state of
degradation is questionable [118]. A similar Raman analysis combined with IR and Brillouin
spectroscopies examined the molecular changes imposed on parchment that has undergone burning
and restoration [119]. Although the Raman technique is particularly well suited to monitoring
changes in the inorganic components of the manuscript substrates, the analysis of the organic
components relied heavily on the IR data. A study of the structural effects of various bleaching
agents on ancient papers also utilized FTIR and Raman spectroscopy [120]. In this study, the IR
data were significantly complicated by the high water content of the papers both before and after
treatment with aqueous bleaching solutions. Moreover, the IR analysis required that portions of the
paper be destroyed in making IR-transmissive KBr pellets. The Raman data showed more clearly in
this instance the apparent oxidation of cellulose to form carbonyl moieties. These spectra also
revealed residual carbonate ions from treatment of the papers with 2% sodium percarbonate
(2Na2CO3.3H2O2) solution, even after washing.
The examination of an ink print suffering from a powdery white bloom revealed that the
efflorescence was p-toluenesulfonamide, the active ingredient in the bleach Chloramine-T, most
likely a residue of a recorded, but unspecified, previous restoration process [121]. The same
analysis showed the presence of CaCO3, which could have been formed on the surface of the print
from atmospheric action on Ca(OH)2 remaining from a de-acidification treatment.
In a clever experiment, spectroscopists working with conservators have used Raman microscopy to
examine the surface of faded and tarnished daguerreotypes [122]. These artefacts, the earliest form
of photography, are composed of copper plates with a nano-particulate coating of silver/mercury
amalgam that was used to record the image. The density of the metal particles provides the shading
of the photograph. Disappointingly, it was found that conventional Raman spectra collected from
tarnished areas of the daguerreotypes provided only weak spectral features consistent with the
presence of silver oxidation products. However, deposition of dye molecules on the sub-micrometre
structured layer of the image generated surface-enhanced Raman spectra (SERS, [2]) of the dye in
contact with the nanoparticles. SERS spectra of brilliant cresyl blue and fast cresyl violet dyes were
collected and suggest that the technique might be used for enhancing or retrieving faded images on
degraded daguerreotypes.
Much attention has been raised by the possibility of using lasers in the cleaning of paintings and
stone artefacts. Raman microscopy has been employed in a study to test the sensitivity of various
pigments to UV laser light [123] and to monitor the integrity of underlying stone when being
cleaned with pulsed NIR radiation [124]. Others have determined the limits of detection for Raman
spectroscopy when analysing mixtures of calcite, gypsum and aragonite (orthorhombic CaCO3)
[125]. The technique is shown to be superior to XRD as a means of monitoring marble degradation
by sulfurous pollution, as demonstrated on a marble sample from the Athens National Garden.
Raman spectroscopy was used in an investigation of the mechanisms responsible for the curing and
degradation of oil paint films [126]. The temporal evolution of Raman spectra taken from white
paint films (ZnO or TiO2 in safflower oil) exposed to UV radiation did not reveal evidence of either
radical termination reactions or Diels-Alder type crosslinking, the two mechanisms which are
believed to bring about the 'drying' of oil paints. Rather, the spectral responses - a decrease in the
intensity of the C=C stretch (1660 cm"1) and a temporary increase in that of the RO-OH stretch
(870 cm"') - have been interpreted as evidence for a hydroperoxide addition reaction to carboncarbon double bonds as the principal curing mechanism for these oil paints. However, ZnO and
TiO2 are known to generate both singlet oxygen and OH radicals upon irradiation [127], and
therefore these pigments, chosen for their resistance to heating from the UV lamps, in fact add
further variables that should have been considered. Although the Raman technique is thus shown to
be applicable to the investigation of complex reactions in the ageing of oil paints, the possible
influence of these powerful oxidants on the paint samples renders the conclusions of this study
suspect.
Conclusion
A strong argument can be made for the inclusion of modern Raman microscopes among the
analytical tools commonly found in the museum laboratory. In order to realize this role, however,
many of the historical perceptions of Raman microscopy as a spectroscopic novelty with limited
applications and numerous instrumental limitations must be dismissed. Furthermore, future work
using Raman microscopy must move beyond the proof-of-concept stage that currently dominates
the literature and begin to tackle in-depth art historical projects and pressing scientific problems
where its many advantages can be realized. This work must also begin to integrate fully the
expertise of the spectroscopist, curator and conservator and result in publications that ultimately
reach the intended audience, either by appearing in conservation journals or through a new
awareness among the conservation community of the developing location for these articles in
applied spectroscopy journals. As the review portion of this article shows, Raman microscopy
already has a strong record of applications in conservation research, and recent increasing interest
among conservation scientists is likely to lead to greater numbers of museums investing in this
powerful technique.
principal chromophores in a series of differently-coloured ultramarine pigments [5]; to investigate
the transformation process for the light-induced conversion of realgar to pararealgar through an
intermediate phase [23]; and to optimize the experimental conditions for the non-destructive
analysis of various lead pigments by Raman microscopy [88, 89]. Further investigation into the
thermal decomposition of lead compounds by several techniques including Raman spectroscopy has
elucidated better the ancient process by which red lead was generated from the roasting of lead
white [117]. Notably, a previously unknown intermediate phase of lead white was discovered. The
formation of Pb3O4 was found to depend on the amount of oxygen, the temperature and the duration
of heating, and these conditions were shown to effect the production of lead(II) oxides that either
superseded the formation of red lead or greatly influenced the final shade of that pigment.
The investigation of degraded organic materials has also been pursued. The ability of FT-Raman
spectroscopy to monitor structural differences between ancient vellums and parchments has been
shown, although the results suggest that the ability of this technique to indicate their state of
degradation is questionable [118]. A similar Raman analysis combined with IR and Brillouin
spectroscopies examined the molecular changes imposed on parchment that has undergone burning
and restoration [119]. Although the Raman technique is particularly well suited to monitoring
changes in the inorganic components of the manuscript substrates, the analysis of the organic
components relied heavily on the IR data. A study of the structural effects of various bleaching
agents on ancient papers also utilized FTIR and Raman spectroscopy [120]. In this study, the IR
data were significantly complicated by the high water content of the papers both before and after
treatment with aqueous bleaching solutions. Moreover, the IR analysis required that portions of the
paper be destroyed in making IR-transmissive KBr pellets. The Raman data showed more clearly in
this instance the apparent oxidation of cellulose to form carbonyl moieties. These spectra also
revealed residual carbonate ions from treatment of the papers with 2% sodium percarbonate
(2Na2CO3.3H2O2) solution, even after washing.
The examination of an ink print suffering from a powdery white bloom revealed that the
efflorescence was p-toluenesulfonamide, the active ingredient in the bleach Chloramine-T, most
likely a residue of a recorded, but unspecified, previous restoration process [121]. The same
analysis showed the presence of CaCO3, which could have been formed on the surface of the print
from atmospheric action on Ca(OH)2 remaining from a de-acidification treatment.
In a clever experiment, spectroscopists working with conservators have used Raman microscopy to
examine the surface of faded and tarnished daguerreotypes [122]. These artefacts, the earliest form
of photography, are composed of copper plates with a nano-particulate coating of silver/mercury
amalgam that was used to record the image. The density of the metal particles provides the shading
of the photograph. Disappointingly, it was found that conventional Raman spectra collected from
tarnished areas of the daguerreotypes provided only weak spectral features consistent with the
presence of silver oxidation products. However, deposition of dye molecules on the sub-micrometre
structured layer of the image generated surface-enhanced Raman spectra (SERS, [2]) of the dye in
contact with the nanoparticles. SERS spectra of brilliant cresyl blue and fast cresyl violet dyes were
collected and suggest that the technique might be used for enhancing or retrieving faded images on
degraded daguerreotypes.
Much attention has been raised by the possibility of using lasers in the cleaning of paintings and
stone artefacts. Raman microscopy has been employed in a study to test the sensitivity of various
pigments to UV laser light [123] and to monitor the integrity of underlying stone when being
cleaned with pulsed NIR radiation [124]. Others have determined the limits of detection for Raman
spectroscopy when analysing mixtures of calcite, gypsum and aragonite (orthorhombic CaCO3)
[125]. The technique is shown to be superior to XRD as a means of monitoring marble degradation
by sulfurous pollution, as demonstrated on a marble sample from the Athens National Garden.
Raman spectroscopy was used in an investigation of the mechanisms responsible for the curing and
degradation of oil paint films [126]. The temporal evolution of Raman spectra taken from white
paint films (ZnO or TiO2 in safflower oil) exposed to UV radiation did not reveal evidence of either
radical termination reactions or Diels-Alder type crosslinking, the two mechanisms which are
believed to bring about the 'drying' of oil paints. Rather, the spectral responses - a decrease in the
intensity of the C=C stretch (1660 cm"1) and a temporary increase in that of the RO-OH stretch
(870 cm"') - have been interpreted as evidence for a hydroperoxide addition reaction to carboncarbon double bonds as the principal curing mechanism for these oil paints. However, ZnO and
TiO2 are known to generate both singlet oxygen and OH radicals upon irradiation [127], and
therefore these pigments, chosen for their resistance to heating from the UV lamps, in fact add
further variables that should have been considered. Although the Raman technique is thus shown to
be applicable to the investigation of complex reactions in the ageing of oil paints, the possible
influence of these powerful oxidants on the paint samples renders the conclusions of this study
suspect.
Conclusion
A strong argument can be made for the inclusion of modern Raman microscopes among the
analytical tools commonly found in the museum laboratory. In order to realize this role, however,
many of the historical perceptions of Raman microscopy as a spectroscopic novelty with limited
applications and numerous instrumental limitations must be dismissed. Furthermore, future work
using Raman microscopy must move beyond the proof-of-concept stage that currently dominates
the literature and begin to tackle in-depth art historical projects and pressing scientific problems
where its many advantages can be realized. This work must also begin to integrate fully the
expertise of the spectroscopist, curator and conservator and result in publications that ultimately
reach the intended audience, either by appearing in conservation journals or through a new
awareness among the conservation community of the developing location for these articles in
applied spectroscopy journals. As the review portion of this article shows, Raman microscopy
already has a strong record of applications in conservation research, and recent increasing interest
among conservation scientists is likely to lead to greater numbers of museums investing in this
powerful technique.
38 Edwards, H.G.M., Farwell, D.W., Newton, E.M., Perez, F.R. and Villar, S.J., 'Raman
Spectroscopic Studies of a 13th Century Polychrome Statue: Identification of a 'Forgotten' Pigment',
Journal of Raman Spectroscopy 31, 2000, pp. 407-13.
39 Mills, J.S. and White, R., Organic Chemistry of Museum Objects, Butterworth-Heinemann,
Oxford, 1994.
40 Clark, R.J.H. and Gibbs, P.J., 'Identification of Lead(II) Sulfide and Pararealgar on a 13th
Century Manuscript by Raman Microscopy', Chemical Communications 1997, pp. 1003-4.
41 Clark, R.J.H., 'Pigment Identification by Spectroscopic Means: An Arts/Science Interface',
Comptes Rendus Chimie 5, 2002, pp. 7-20; Brown, K.L. and Clark, R.J.H., to be published.
42 Cariati, F. and Bruni, S., 'Raman Spectroscopy', in Ciliberto, E. and Spoto, G., eds, Modern
Analytical Methods in Art and Archaeology, Wiley, New York, 2000, pp. 255-78.
43 Chalmers, J. and Griffiths, P.R., eds, The Handbook of Vibrational Spectroscopy, Wiley, New
York, 2001.
44 Clark, R.J.H. and Gibbs, P.J., 'Analysis of 16th Century Qazwini Manuscripts by Raman
Microscopy and Remote Laser Raman Microscopy', Journal of Archaeological Sciences 25, 1998,
pp. 621-9.
45 Vandenabeele, P., Verpoort, F. and Moens, L., 'Non-destructive Analysis of Paintings Using
Fourier Transform Raman Spectroscopy with Fibre Optics', Journal of Raman Spectroscopy 32,
2001, pp. 263-9.
46 Clark, R.J.H. and Huxley, K., 'Raman Spectroscopic Study of the Pigments on a Large
Illuminated Qur'an Circa Thirteenth Century', Science and Technology for Cultural Heritage
5,1996, pp. 95-101.
47 Burgio, L. and Clark, R.J.H., 'Comparative Pigment Analysis of Six Modern Egyptian Papyri
and an Authentic One of the 13th Century BC by Raman Microscopy and Other Techniques',
Journal of Raman Spectroscopy 31, 2000, pp. 395-401.
48 Burgio, L., Clark, R.J.H. and Williams, K.P.J., 'The Use of Raman Spectroscopy in the Art
World', in McCrone, W.C. and Weiss, R.J., eds, Fakebusters II, Scientific Detection of Fakery in
Art, SPIE, Chicago, 2001, pp. 138-49.
49 Edwards, H.G.M., Farwell, D.W., Seddon, T. and Tait, J.K.F., 'Scrimshaw: Real or Fake? A
Fourier-Transform Raman Diagnostic Study', Journal of Raman Spectroscopy 26,1995, pp. 623-8.
50 Edwards, H.G.M. and Farwell, D.W., 'Ivory and Simulated Ivory Artefacts: Fourier Transform
Raman Diagnostic Study', Spectrochimica Ada A 51, 1995, pp. 2073-81.
51 Edwards, H.G.M. and Farwell, D.W., 'Fourier Transform-Raman Spectroscopy of Amber',
Spectrochimica Ada A 52, 1996, pp. 1119-25.
52 Schrader, B., Schulz, H., Andreev, G.N., Klump, H.H. and Sawatzki, J., 'Non-destructive NIRFT-Raman Spectroscopy of Plant and Animal Tissues, of Food and Works of Art', Talanta 53,
2000, pp. 35-45.
53 Derbyshire, A. and Withnall, R., 'Pigment Analysis of Portrait Miniatures Using Raman
Microscopy', Journal of Raman Spectroscopy 30, 1999, pp. 185-8.
54 Corset, J., Dhamelincourt, P. and Barbillat, J., 'Raman Microscopy', Chemistry in Britain 1989,
pp. 612-6.
55 Guineau, B., 'Analyse Non Destructive des Pigments par Microsonde Raman Laser: Exemples
de l'Azurite et de la Malachite', Studies in Conservation 29, 1983, pp. 35-41.
56 Vezin, J., 'La Microsonde Raman Laser: Un Nouvel Instrument d'Analyse des Pigments dans
les Enluminures', Scriptorium 38, 1984, pp. 325-6.
57 Hughes, S., 'Blues for the Chemist', New Scientist 22/29, 1990, pp. 21-4.
58 Guineau, B., 'Microanalysis of Painted Manuscripts and of Colored Archaeological Materials
by Raman Laser Microprobe', Journal of Forensic Science 29, 1984, pp. 471-85.
59 Guineau, B., Coupry, C, Gousset, M.T., Forgerit, J.P. and Vezin, J., 'Identification de Bleu de
Lapis-Lazuli dans Six Manuscrits a Peintures du XHth Siecle Provenant de PAbbaye de Corbie',
Scriptorium 40, 1986, pp. 157-71.
60 Withnall, R., Clark, R.J.H., Cooksey, C.J. and Daniels, M.A.M., 'Non-destructive, In Situ
Identification of Indigo/Woad and Shellfish Purple by Raman Microscopy and Visible Reflectance
Spectroscopy', Dyes in History and Archaeology 11, 1993, pp. 19-24.
61 Best, S., Clark, R., Daniels, M. and Withnall, R., 'A Bible Laid Open', Chemistry in Britain
29, 1993, pp. 118-22.
62 Best, S.P., Clark, R.J.H., Daniels, M.A.M., Porter, C.A. and Withnall, R., 'Identification by
Raman Microscopy and Visible Reflectance Spectroscopy of Pigments on an Icelandic Manuscript',
Studies in Conservation 40, 1995, pp. 31-40.
63 Clark, R.J.H., 'Raman Microscopy: Application to the Identification of Pigments on Medieval
Manuscripts', Chemical Society Reviews 24, 1995, pp. 187-96.
64 Clark, R.J.H., Gibbs, P.J., Seddon, K.R., Brovenko, N.M. and Petrosyan, Y.A., 'NonDestructive in situ Identification of Cinnabar on Ancient Chinese Manuscripts', Journal of Raman
Spectroscopy 28, 1997, pp. 91-4.
65 Burgio, L., Ciomartan, D.A. and Clark, R.J.H., 'Pigment Identification on Medieval
Manuscripts, Paintings and Other Artefacts by Raman Microscopy: Applications to the Study of
Three German Manuscripts', Journal of Molecular Structure 405, 1997, pp. 1-11.
66 Burgio, L., Ciomartan, D.A. and Clark, R.J.H., 'Raman Microscopy Study of the Pigments on
Three Illuminated Mediaeval Latin Manuscripts', Journal of Raman Spectroscopy 28, 1997, pp. 7983.
67 Clark, R.J.H. and Gibbs, P.J., 'Raman Microscopy of a 13th-century Illuminated Text',
Analytical Chemistry 70, 1998, pp. 99A-104A.
68 Burgio, L., Clark, R.J.H. and Gibbs, P.J., 'Pigment Identification Studies in situ of Javanese,
Thai, Korean, Chinese, and Uighur Manuscripts by Raman Microscopy', Journal of Raman
Spectroscopy 30, 1999, pp. 181-4.
69 Burgio, L., Clark, R.J.H. and Toftlund, H., 'The Identification of Pigments Used on Illuminated
Plates from 'Flora Danica' by Raman Microscopy', Ada Chemica Scandinavica 53, 1999, pp. 181-7.
70 Clark, R.J.H., 'Raman Microscopy: Sensitive Probe of Pigments on Manuscripts, Paintings and
Other Artefacts', Journal of Molecular Structure 480-481, 1999, pp. 15-20.
71 Wehling, B., Vandenabeele, P., Moens, L., Klockenkamper, R., von Bohlen, A., Van
Hooydonk, G. and de Reu, M., 'Investigation of Pigments in Medieval Manuscripts by Micro
Raman Spectroscopy and Total Reflection X-Ray Fluorescence Spectrometry', Mikrochimica Ada
130, 1999, pp. 253-60.
72 Bicchieri, M., Nardone, M. and Sodo, A., 'Application of Micro-Raman Spectroscopy to the
Study of an Illuminated Medieval Manuscript', Journal of Cultural Heritage 1, Suppl. 1, 2000, pp.
S277-9.
73 Andalo, C, Bicchieri, M., Bocchini, P., Casu, G., Galletti, G.C., Mando, P.A., Nardone, M.,
Soda, A. and Plossi Zappala, M., 'The Beautiful "Trionfo d'Amore" Attributed to Botticelli: A
Chemical Characterisation by Proton-Induced X-ray Emission and Micro-Raman Spectroscopy',
Analytica Chimica Ada 429, 2001, pp. 279-86.
74 Edwards, H.G.M., Farwell, D.W., Perez, F.R. and Garcia, J.M., 'Mediaeval Cantorals in the
Valladolid Biblioteca: FT-Raman Spectroscopic Study', The Analyst 126, 2001, pp. 383-8.
75 Davey, R., Gardiner, D.J., Singer, B.W. and Spokes, M., 'Examples of Analysis of Pigments
from Fine Art Objects by Raman Microscopy', Journal of Raman Spectroscopy 25, 1993, pp. 53-7.
76 Singer, B.W., Gardiner, D.J. and Derow, J.P., 'Analysis of White and Blue Pigments from
Watercolours by Raman Microscopy', The Paper Conservator 17, 1993, pp. 13-19.
77
Coupry, C, Lautie, A., Revault, M. and Dufilho, J., 'Contribution of Raman Spectroscopy to Art and History', Journal of Raman Spectroscopy
25, 1994, pp. 89-94.
78 Clark, R.J.H., Cridland, L, Kariuki, B.M., Harris, K.D.M. and Withnall, R., 'Synthesis,
Structural Characterisation and Raman Spectroscopy of the Inorganic Pigments Lead Tin Yellow
Types I and II and Lead Antimonate Yellow: Their Identification on Medieval Paintings and
Manuscripts', Journal of the Chemical Society, Dalton Transactions 1995, pp. 2577-82.
79 de Oliveira, L.F.C., Boscan, J.D.R.P., Santos, P.S. and Temperini, M.L.A., 'Identification of
Pigments from Candido Portinari's Oil Painting "Portrait of Murilo Mendes" by Raman
Microscopy', Quimica Nova 21, 1998, pp. 172-5.
80 Vandenabeele, P., Hardy, A., Edwards, H.G.M. and Moens, L., 'Evaluation of a Principal
Components-Based Searching Algorithm for Raman Spectroscopic Identification of Organic
Pigments in 20th Century Artwork', Applied Spectroscopy 55, 2001, pp. 525-33.
81 Marcolli, C. and Wiedemann, H.G., 'Distinction of Original and Forged Lithographs by Means
of Thermogravimetry and Raman Spectroscopy', Journal of Thermal Analysis and Calorimetry 64,
2001, pp. 987-1000.
82 Ciomartan, D.A. and Clark, R.J.H., 'Raman Microscopy Applied to the Analysis of the
Pigments Used in Two Persian Manuscripts', Journal of the Brazilian Chemical Society 7, 1996, pp.
395-402.
83 Coupry, C, 'Application of Raman Microspectrometry to Art Objects', Analusis 28, 2000, pp.
39-46.
84 Edwards, H.G.M., Farwell, D.W. and Quye, A., "Dragon's Blood' I - Characterization of an
Ancient Resin Using Fourier Transform Raman Spectroscopy', Journal of Raman Spectroscopy 28,
1997, pp. 243-9.
85 Edwards, H.G.M., Farwell, D.W., Perez, F.R. and Villar, S.J., 'Spanish Mediaeval Frescoes at
Basconcillos del Tozo: A Fourier Transform Raman Spectroscopic Study', Journal of Raman
Spectroscopy 30, 1999, pp. 307-11.
86 Coma, L., Breitman, M. and Ruiz-Moreno, S., 'Soft and Hard Modelling Methods for
Deconvolution of Mixtures of Raman Spectra for Pigment Analysis. A Qualitative and Quantitative
Approach', Journal of Cultural Heritage 1, 2000, pp. S273-6.
87 Bernard, M.-C, Goff, A.H.-L., Thi, B.V. and Torresi, S.C.de, 'Electrochromic Reactions in
Manganese Oxides, I. Raman Analysis', Journal of the Electrochemical Society 140, 1993, pp.
3065-70.
88 Burgio, L., Clark, R.J.H. and Firth, S., 'Raman Spectroscopy as a Means for the Identification
of Plattnerite (PbO2), of Lead Pigments and of Their Degradation Products', The Analyst 126, 2001,
pp. 222-7.
89 Smith, G.D., Derbyshire, A. and Clark, R.J.H., 'In Situ Spectroscopic Detection of PbS on a
Blackened Manuscript Illumination by Raman Microscopy', Studies in Conservation, in press.
90 Smith, G.D., Burgio, L., Firth, S. and Clark, R.J.H., 'Laser-Induced Degradation of Lead
Pigments with Reference to Botticelli's Trionfo d'Amore', Analytica Chimica Ada 440, 2001, pp.
185-8.
91 Edwards, H.G.M., Drummond, L. and Russ, J., 'Fourier-Transform Raman Spectroscopic
Study of Pigments in Native American Indian Rock Art: Seminole Canyon', Spectrochimica Ada A
54, 1998, pp. 1849-56.
92 Brooke, C.J., Edwards, H.G.M. and Tait, J.K.F., 'The Bottesford Blue Mystery: a Raman
Spectroscopic Study of Post-Mediaeval Glazed Tiles', Journal of Raman Spectroscopy 30, 1999, pp.
429-34.
93 Edwards, H.G.M., Drummond, L. and Russ, J., 'Fourier Transform Raman Spectroscopic
Study of Prehistoric Rock Paintings from the Big Bend Region, Texas', Journal of Raman
Spectroscopy 30, 1999, pp. 421-8.
94 Edwards, H.G.M., Newton, E.M. and Russ, J., 'Raman Spectroscopic Analysis of Pigments
and Substrata in Prehistoric Rock Art', Journal of Molecular Structure 550-551, 2000, pp. 245-56.
Vandenabeele, P., von Bohlen, A., Moens, I.., Klockenkamper, R., Joukes, F. and Dewispelaere, G.,
'Spectroscopic Examination of Two Egyptian Masks: A Combined Method Approach', Analytical
Letters 33, 2000, pp. 3315-32.
Moreno, J., Stodulski, L., Trentelman, K., Jourdan, J. and McCann, L.I., 'An Examination of the
Materials and Methods Used in the Creation of Tintoretto's The Dreams of Men', in Goupy, J. and
Mohen, J.P., eds, Art et Chimie, la Couleur, CNRS Editions, 2000, pp. 60-3.
Castillejo, M., Martin, M., Silva, D., Stratoudaki, T, Anglos, D., Burgio, L. and Clark, R.J.H.,
'Analysis of Pigments in Polychromes by Use of Laser Induced Breakdown Spectroscopy and
Raman Microscopy', Journal of Molecular Structure 550-551, 2000, pp. 191-8.
Burgio, L., Melessanaki, K., Doulgeridis, M., Clark, R.J.H. and Anglos, D., 'Pigment Identification
in Paintings Employing Laser Induced Breakdown Spectroscopy and Raman Microscopy',
Spectrochimica Ada B 56, 2001, pp. 905-13.
Marquardt, B.J., Stratis, D.N., Cremers, D.A. and Angel, S.M., 'Novel Probe for Laser-Induced
Breakdown Spectroscopy and Raman Measurements Using an Imaging Optical Fiber', Applied
Spectroscopy 52, 1998, pp. 1148-53.
Bruni, S., Cariati, F., Bianchi, C.L., Zanardini, E. and Sorlini, C, 'Spectroscopic Investigation of
Red Stains Affecting the Carrara Marble Facade of the Certosa of Pavia', Archaeometry 37, 1995,
pp. 249-55.
Toniolo, L., Colombo, S., Bruni, S., Fermo, P., Casoli, A., Palla, G. and Bianchi, C.L., 'Gilded
Stuccoes of the Italian Baroque', Studies in Conservation 43, 1998, pp. 201-8.
102 Edwards, H.G.M., Farwell, D.W., Seaward, M.R.D. and Giacobini, C, 'Preliminary Raman
Microscopic Analyses of a Lichen Encrustation Involved in the Biodeterioration of Renaissance
Frescoes in Central Italy', International Biodeterioration 27, 1991, pp. 1-9.
103 Edwards, H.G.M., Farwell, D.W. and Seaward, M.R.D., 'Raman Spectra of Oxalates in Lichen
Encrustations on Renaissance Frescoes', Spectrochimica Ada A 47, 1991, pp. 1531-9.
104 Edwards, H.G.M., Farwell, D.W., Jenkins, R. and Seaward, M.R.D., 'Vibrational Raman
Spectroscopic Studies of Calcium Oxalate Monohydrate and Dihydrate in Lichen Encrustations on
Renaissance Frescoes', Journal of Raman Spectroscopy 23, 1992, pp. 185-9.
105 Seaward, M.R.D. and F2dwards, H.G.M., 'Biological Origin of Major Chemical Disturbances
on Ecclesiastical Architecture Studied by Fourier Transform Raman Spectroscopy', Journal of
Raman Spectroscopy 28, 1997, pp. 691-6.
106 Edwards, H.G.M., Gwyer, E.R. and Tait, J.K.F., 'Fourier Transform Raman Analysis of
Paint Fragments from Biodeteriorated Renaissance Frescoes', Journal of Raman Spectroscopy 28,
1997, pp. 677-84.
107 Edwards, H.G.M., Farwell, D.W. and Rozenberg, S., 'Raman Spectroscopic Study of Red
Pigment and Fresco Fragments from King Herod's Palace at Jericho', Journal of Raman
Spectroscopy 30, 1999, pp. 361-6.
108 Sister Daniilia, Sotiropoulou, S., Bikiaris, D., Salpistis, C, Karagiannis, G., Chryssoulakis, Y,
Price, B.A. and Carlson, J.H., 'Panselinos' Byzantine Wall Paintings in the Protaton Church, Mount
Athos, Greece: A Technical Examination', Journal of Cultural Heritage 1, 2000, pp. 91-110.
109 Edwards, H.G.M., Brooke, C.J. and Tait, J.K.F., 'Fourier Transform Raman Spectroscopic
Study of Pigments from English Mediaeval Wall Paintings', Journal of Raman Spectroscopy 28,
1997, pp. 95-8.
110 Perez, F.R., Edwards, H.G.M., Rivas, A. and Drummond, L., 'Fourier Transform Raman
Spectroscopic Characterization of Pigments in the Mediaeval Frescoes at Convento de la Peregrina,
Sahagun, Leon, Spain. Part 1 - Preliminary Study', Journal of Raman Spectroscopy 30, 1999, pp.
301-5.
111 Edwards, H.G.M., Rull, R, Vandenabeele, P., Newton, E.M., Moens, L., Medina, J. and
Garcia, C, 'Mediaeval Pigments in the Monastery of San Baudelio, Spain: A Raman Spectroscopic
Analysis', Applied Spectroscopy 55, 2001, pp. 71-6.
112 Edwards, H.G.M., Ellis, E., Farwell, D.W. and Janaway, R.C., 'Preliminary Study of the
Application of Fourier Transform Raman Spectroscopy to the Analysis of Degraded Archaeological
Linen Textiles', journal of Raman Spectroscopy 27, 1996, pp. 663-9.
113 Edwards, H.G.M. and Falk, M.J., investigation of the Degradation Products of Archaeological
Linens by Raman Spectroscopy', Applied Spectroscopy 51, 1997, pp. 1134-8.
114 Coupry, C, Sagon, G. and Gorguet-Ballesteros, P., 'Raman Spectroscopic Investigation of
Blue Contemporary Textiles', Journal of Raman Spectroscopy 28, 1997, pp. 85-9.
115 Guineau, B., 'Experiments in the Identification of Colorants In Situ: Possibilities and
Limitations', Dyes in History and Archaeology 10, 1992, pp. 55-9.
116 Bourgeois, D. and Church, S.P., 'Studies of Dyestuffs in Fibres by Fourier Transform Raman
Spectroscopy', Spectrochimica Ada A 46, 1990, pp. 295-301.
117 Ciomartan, D.A., Clark, R.J.H., McDonald, L.J. and Odlyha, M., 'Studies on the Thermal
Decomposition of Basic Lead(II) Carbonate by Fourier-Transform Raman Spectroscopy, X-ray
Diffraction and Thermal Analysis', Journal of the Chemical Society, Dalton Transactions 1996, pp.
3639-45.
118 Edwards, H.G.M., Farwell, D.W., Newton, E.M., Perez, F.R. and Villar, S.J., 'Application of
FT-Raman Spectroscopy to the Characterisation of Parchment and Vellum, I; Novel Information for
Paleographic and Historiated Manuscript Studies', Spectrochimica Ada A 57, 2001, pp. 1223-34.
119 Mannucci, E., Pastorelli, R., Zerbi, G., Bottani, C.E. and Facchini, A., 'Recovery of Ancient
Parchment: Characterization by Vibrational Spectroscopy', journal of Raman Spectroscopy 31,
2000, pp. 1089-97.
120 Grosso, V, 'Fourier Transform Infra-red and Raman Spectroscopy of Ancient Paper', in
Burgess, H.D., ed., Symposium 88, Conservation of Historic and Artistic Works on Paper, Canadian
Conservation Institute, Ottawa, 1988, pp. 245-50.
121 Otieno-Alego, V., Hodgeman, J. and Creagh, D.C., 'Micro-Raman Identification of Bloom
Formed on a Historical Print Artifact', journal of the American Institute of Conservation 40, 2001,
pp. 35-41.
122 Golovlev, V.V., Gresalfi, M.J., Miller, J.C., Romer, G. and Messier, P., 'Laser Characterization
and Cleaning of Nineteenth Century Daguerreotypes', Journal of Cultural Heritage 1, Suppl. 1,
2000, pp. S139-44.
123 Athanassiou, A., Hill, A.E., Fourrier, T., Burgio, L. and Clark, R.J.H., 'The Effects of UV
Laser Light Radiation on Artists' Pigments', Journal of Cultural Heritage 1, 2000, pp. S209-13.
124 Gobernado-Mitre, I., Medina, J., Calvo, B., Prieto, A.C., Leal, L.A., Perez, B., Marcos, F. and
Frutos, A.M.d., 'Laser Cleaning in Art Restoration', Applied Surface Science 96-8, 1996, pp. 474-8.
125 Kontoyannis, C.G., Orkoula, M.G. and Koutsoukos, P.G., 'Quantitative Analysis of Sulfated
Calcium Carbonates Using Raman Spectroscopy and X-ray Powder Diffraction', The Analyst 122,
1997, pp. 33-8.
126 Higuchi, S., Hamada, T. and Gohshi, Y., 'Examination of the Photochemical Curing and
Degradation of Oil Paints by Laser Raman Spectroscopy', Applied Spectroscopy 51, 1997, pp. 121823.
127 Daniels, V., 'Discoloration of Paper Induced by Pigments Containing Zinc', Restaurator 11,
1990, pp. 236-43.
Authors
Gregory D. Smith, PhD, obtained an undergraduate education at Centre College of Kentucky in
chemistry and anthropology and continued his studies in the sciences and archaeology at Duke
University, Durham, North Carolina, before earning his doctorate in analytical chemistry in 2000.
He is currently a Marshall Sherfield Postdoctoral Fellow at University College London. His
research interests include the applications of chemistry in art and archaeology, and he currently
serves as an excavation supervisor at two archaeological sites in Israel.
Robin J.H. Clark, D.Sc, FRS, FRSA, FRSC, Hon. FRSNZ, graduated in chemistry at the University
of Canterbury, Christchurch, New Zealand, and joined the faculty of University College London in
1962. He has been the Sir William Ramsay professor of chemistry since 1989. His research interests
include most aspects of physical inorganic chemistry and spectroscopy, most recently the use of
Raman microscopy in the identification and study of pigments on manuscripts and other artefacts.