Materials science and cultural heritage
Marco Martini, Anna Galli
1 Introduction
In these short review we will introduce a few applications of Materials Science to two types of human artefacts, i.e. the use of luminescence to the study of ceramics, mainly with the aim of determining their chronology, and the use of X-ray spectroscopy, XRF in particular, in the study of paintings’ pigments. Even if a huge number of other techniques are currently used, the two examples here reviewed consider one of the most diffused material all along men’ history, i.e. ceramics, and one of the nowadays most diffused and powerful technique, i.e. XRF, that in the case of paintings has recently acquired the possibility of being portable with accuracy and sensitivity comparable to the great facilities, allowing the intervention without moving the work of art.
2 Ceramics and luminescence
One of the most cited scientific
applications to the Cultural Heritage
is surely luminescence dating and
in particular thermoluminescence.
Indeed, the luminescence properties
of ceramics are exploited not only as
regards thermoluminescence, but also
in other applications that will be shortly
reviewed here.
It is rather well known that the
luminescence properties of the crystals
contained in the ceramic material
can be fruitfully exploited in order to
date them. This is mainly carried out
studying the thermoluminescence
(TL) of the ceramics. This technique
was proposed in the sixties of the
last century and some well assessed
methods are now currently used to
support archaeological excavations
and in determining the chronological
phases of historical buildings.
TL has been giving useful results also
in the study of clay-cores in bronze
statues. It must be noted that claycores
are not always made only of
ceramic material and this can give high
uncertainties in this application.
A short review of archaeological and
building dating, together with a few
examples of clay-core studies will be
given in sect. 2.1.
Besides TL, a further luminescence
property that is increasingly exploited
is Optically Stimulated Luminescence,
OSL, which is based on the de-trapping
of charges due to light exposure rather
than heating, as is the case of TL. The
physical basis of this phenomenon
is rather complex and its application
must be specifically carried out on
each mineral extracted from the
ceramic: as an example, the stimulating
light for quartz inclusions should be
at 470 nm, while feldspars can be
stimulated both in blue and in the
infrared. OSL is currently widely applied
in sediment dating, but an interesting
application is under study in the field
of Cultural Heritage: it is the so called
“surface dating”, that can in principle
determine when a surface was put
in the dark and then accumulate
charges in the traps due to the natural
radiation environment, similarly to the
TL application. Surface dating will be
introduced in sect. 2.2, together with an
example of application.
Apart from ceramics, the
luminescence properties of other
materials should in principle be used
to study the chronology of historic and
artistic artefacts. The first material to be
considered is glass whose amorphous
structure however prevents a correct
dating procedure in most cases. Some
exceptions will be treated in sect. 2.3,
where the feasibility of chronological
studies on mosaic glass tesserae will be
shown, when some microcrystals are
present in the glass matrix.
In a further sect. 2.4, a different kind
of luminescence will be exploited to
contribute to the study of specific
artefacts, i.e. lustre ceramics. In this
case the radio-luminescence spectra
demonstrate to be a kind of fingerprint
in telling apart the provenance of the
objects.
2.1 Luminescence dating in archaeology and in history of architecture
Thermoluminescence (TL) dating
is a powerful tool in Archaeology
and its reliability has been confirmed
since the eighties of the XX century.
TL is a phenomenon typical of many
insulating materials, which emit a weak
light amount when heated up to some
hundreds degrees (°C), provided that
they had been previously irradiated
by ionizing radiation. It is a typical
dosimetric property and is exploited
in health physics and radioprotection,
because it records the amount of
energy absorbed by a material exposed
to ionizing radiation. Dating by TL is a
specific application of TL dosimetry in
which there is a source of practically
constant irradiation, the natural
radioactivity of the ceramic itself and
the surrounding environment.
The dosimeters are the small
inclusions always present in ceramics,
typically quartz and feldspar crystals,
which are constantly exposed to the
mentioned radiation field. Of course,
micro-dosimetry is the basis for
determining the amount of each type
of radiation, alpha, beta and gamma,
absorbed by the inclusions of different
dimensions.
The precise measurements of the
dose rate, which is currently reported
in unit of Gray per year (Gy/y), i.e.
the amount of dose absorbed by the
ceramic inclusions per unit time, is
a great source of uncertainty. As a
matter of fact, what is currently done is
the measurement of the radioactivity
content of the ceramic itself and the
surrounding environment, assuming
that all the emitted radiation is
absorbed by the ceramic, in a radiation
equilibrium. The main source of
uncertainty is the water content of
both the ceramic and the environment,
whose presence reduces the absorbed
dose by the ceramic, due to the
dose absorbed by the water itself.
Corrections can be taken into account
and the precision of the method can
by rather good, specifically when the
amount of water in the past can be
precisely known: this is the case of
very humid or very dry environments
or when the ceramic is particularly
compact and reduces to extremely low
level the water that can be present in
the ceramic.
Specific methodologies have been
developed to apply TL to chronological
studies in the field of archaeological
excavations and buildings. We refer
to a wide literature for details in
these applications, specifically to
the Proceeding of the series of LED
Conferences. A few notes are
however to be mentioned: 1) it is
necessary to take samples from the
ceramic to be dated, which means that
the application is destructive, even if
the amount to be taken can very very
small. 2) To get a sufficient precision
in the date, an accurate knowledge
of the radiation field of the ceramic is
necessary, i.e. a precise measurement of
the radioactivity content of the ceramic
and the environment must be carried
out. 3) As mentioned above, also the
water content of the ceramic and the
environment should be known.
To summarize, it is necessary to take
as much sample as possible, even if
we are dealing with few grams, and
the surrounding environment should
be very well known. This is why the
most precise applications of TL dating
are relative to the studies of building
chronologies, because it is possible to
obtain large amounts of samples and
the radiation environment and climate
can be known with high precision.
In such cases the overall precision
can be better than 4−5 % of the age.
A little lower precision is typical of
archaeological excavations, even if in
some cases very high precision can
be reached, depending on the type of
ceramics, which sometimes can present
very good dosimetric properties, with
high reproducibility and sensitivity.
An interesting case study is given
by the Basilica di S. Lorenzo Maggiore
in Milan, whose date of construction
is uncertain, but it is known to be
already built in the VI century AD.
Various destructions, restorations and
re-building are documented, but the
complete chronology of the monument
is still under study. In a wide three-year
restoration project, collaborating with
art historians and architects, a huge
number of samples, more than one
hundred, were taken and their TL dates
were obtained. Their TL response was
not always accurate and repeatable. In
table 1 the results relative to 24 out of
42 well dated samples are reported.
It can be noted that many samples
are surely older than the Basilica itself,
documenting the frequent habit of
re-using older bricks taken from other
structures. In this case, not far from the
San Lorenzo Basilica there was a huge
Arena of the Imperial Age, that was
abandoned in the period of the Basilica
construction. Various construction
phases are documented by the results
reported in table 1. Samples D124 and
D125 correspond to a documented
big fire in that part of the building.
Of course a high-temperature heating
of the sample erases the TL signal
starting again the accumulation of
charges in the traps.
Similarly to archaeological and
monument dating, also bronze
statues can be indirectly dated by
luminescence. The principle is always
the same, but the material to be
considered changes, in the sense that
clay core has to be measured, in the
hope that it was affected by the high
temperature needed to melt the bronze
and that no contamination occurred if
the statue was buried in the earth.
Examples of dating of bronze statues
are reported: the statue of St. Peter
in the Basilica Vaticana in Rome was
precisely dated to the XIII century, while
many attempts were made in order to
date the she-wolf, symbol of the city of
Rome (Lupa Capitolina, fig. 1). In this
case the material was not as dosimetric
as necessary and the uncertainty was
rather high. However, it was possible to
exclude an Etruscan attribution. This
result was confirmed by radiocarbon
dating of many small pieces of carbon
found in the clay-core.
2.2 Luminescence dating of surfaces
An interesting new type of
luminescence dating comes from the
application of OSL to the surfaces of
artifacts that were put in the dark in
the past. In fact surfaces of stones or
marbles or bricks that were exposed
to sunlight for long periods had the
traps in the surface layers kept empty
by light stimulation. Once covered
by other stones or objects, also the
surfaces started to accumulate charges
in the traps. A subsequent stimulation
by light of appropriate wavelength
gives a luminescence emission (OSL)
proportional to the dose absorbed since
the darkening, in a similar way like TL.
Looking at bricks, the luminescence
signal from the bulk is higher than
the surface one, because the former is
relative to the time elapsed since the
heating in a kiln, while the latter to
the period when the brick was in the
darkness.
This advanced application has been
successfully attempted on rocks,
marble and stone artifacts but not yet
on bricks. A recent conservation
campaign at the Certosa di Pavia gave
the opportunity to sample some bricks
belonging to a XVII century collapsed
wall, still tied to their mortars.
This was a good condition to test this
technique, comparing the dating results
with precise historical data. The attempt
gave satisfactory results, allowing to
identify bricks surely reused and to
fully confirm that the edification of the
perimeter wall occurred at the end of
XVII century.
In fig. 2 are summarized the results
relative to six samples that showed good
dosimetric behavior, being the signal
proportional to the absorbed dose.
It must be reminded that the
luminescence emission of ceramics
and bricks is mainly due to quartz and
feldspars: while blue light stimulates
the emission of both quartz and
feldspars, IR light only stimulates that
of feldspars. In this study, however,
a different approach, using the poly
mineral (4−10 μm)-grained fraction
of the material, has been chosen. The
main reason of this choice, despite
the disadvantage of a luminescence
emission due to the superimposition
of different contributions, which
forces the use of different stimulation
wavelengths, is the limited amount of
material required, the sample needing
no chemical treatments.
Looking at fig. 2 relatively low
precision is evident, but the possibility
of discriminating the re-used bricks
from the ones contemporary to the
construction of the wall is also evident.
The period 1530−1650 AD comes
out from the surface data, in good
agreement with historical records
and archaeological evidences. Two of
the bricks were surely reused, their TL
date indicating the late middle age for
firing in kiln (PV18 and PV23 samples).
A limited number of papers have
been published on “surface dating”,
evidencing the troubles that derive
from one of the most critical items, i.e.
the erasure by light of the OSL signal.
In fact, a non- rapid decrease of the
surface signal as a consequence of a
short exposure to light results in a limit
of the applicability of surface dating
technique.
2.3 Dating mosaic glass tesserae
The amorphous structure of glasses
evidently limits the possibility of dating
them. In fact, many recombination
channels are possible in a vitreous
material, which give as a consequence
instability in the number of the trapped
charges and fading of the luminescence
signal. Various studies demonstrated
that the presence of micro- and nano-crystals
circumvent this limitation
and allow to use glass as a dosimetric
material as firstly reported by Mueller
and Schvoerer in 1993.
In a study of 2003 it was put in
evidence that the presence of antimony
enhances the TL signal of the vitreous
matrix. This opacifier possibly forms
new TL centres, when producing
calcium antimonate crystals (CaSb2O6).
The simple addition of antimony to a
modern glass sample did not produce
the stable TL signal as is the case of
historic glass tesserae, where calcium
antimonate is present. A comparison of
the TL signal obtained from an ancient
mosaic tessera from Faragola and of a
CaSb2O6 standard is reported in fig. 3a
and fig. 3b respectively, where the
reported spectra refer to spectrally
resolved measurements, using a
specifically developed system.
Similarly to the TL signal produced
by the presence of calcium antimonate
crystals, it was observed that mosaic
tesserae from the vault of the Basilica
of St. Peter in Rome, opacified with
tin-based additives (in particular,
cassiterite, tin dioxide) showed higher
luminescence sensitivity and thermal
stability, being therefore more suitable
for dating applications.
2.4 Radioluminescence of lustred ceramics
Lustre is a decorative metal-like film
applied to a white opacified surface
of glazed pottery; the tradition of
decoration with this technology began
in the early Islamic period and has
continued in the Mediterranean area
until the present days. In central Italy
the production of lustred majolica
achieved its maximum magnificence
during the XV and the XVI century. This
type of pottery consists of a ceramic
body, made of fired clay, covered with
a decorative glaze on which a lustre
layer is applied. The glaze is a glass-like
material, usually 100–300 μm thick,
which contains a mixture of silica,
soda and lead-alkali. The lustre film
(0.1–2.0 μm thick) is the external layer,
characterized by the presence of copper
and silver nanocrystals embedded in a
glassy matrix . The lustred ceramic
is than submitted to three subsequent
heating processes.
In a study of 22 samples found
during excavations made in central
Italy, in Egypt, or provided by private
collectors their luminescence properties
have been studied. The TL of the
ceramic body was used to obtain the
date of production. The luminescence
emissions under irradiation, the so-called
radio-luminescence (RL), of the
different parts of the lustred ceramics
were studied.
The main results can be summarized
as follows:
TL dating of lustred ceramics
was successfully performed on the
22 samples of different age and
provenance. In most cases the
results confirmed most scholars’
attribution but definitely excluded the
Hispano-Moresque dating of a few of
them. Two samples turned out to be
Fatimid, and this result is particularly
important, due to the uncommon
finding of Fatimid lustred ceramic
The application of RL gave interesting
results for the characterization of
ceramic bodies, and the preliminary
results obtained on lustred ceramics are
encouraging, even for the identification
of technological features. Characteristic
RL emissions have been in fact
successfully associated to the particular
manufacturing technology of the lustre,
being independent of the type of clay.
For the emissions relative to the glaze
and the lustred glaze the presence of
cassiterite is confirmed, associated with
the glaze as proposed in and that
the metallic silver clusters are typical of
the Italian gold luster.
3 Paintings and Spectroscopy
Understanding how painted works of
art were realized layer by layer, requires
a wide range of spectroscopic methods.
The main challenge in this research
topic is the wide variety of materials to
be considered, which include: inorganic
or organic pigments, either natural or
synthetic; bio-organic materials such as
wood, paper, parchment; oil, protein,
sugar-based binding media, glues,
synthetic or natural varnish coatings
and associated by-products arising
during aging. Furthermore, given the
value and fragility of artifacts, analyses
need to be carried out in as non-invasive
a manner as possible, often
directly on the object itself or, in certain
cases, on small samples.
In addressing these types of
demanding analytical challenges,
it has been necessary to develop and
use powerful and versatile analytical
instruments and methodologies.
The micro-invasive approach
is particularly challenging, due
to the fact that artwork materials
are complex mixtures intrinsically
heterogeneous, composed of a wide
range of compounds, both organic and
inorganic, as mentioned. The studies
include a wide range of size scale, which
goes from the chemical identification
of compounds to the mapping of trace
elements, alteration or restored phases.
It is from this intrinsic complexity that
comes the need for multi-analytical
approaches, to overcome the
limitation of individual spectroscopic
methods. The systematic application
of spectroscopy techniques in the
conservation science dates back to the
the fifties of the XX century.
In sect. 3.1 to 3.4 the attention is
focused on the analysis of painting
materials due to their importance
in conservation science. The precise
identification of pigments can help in
dating and authentication studies, since
each pigment has its own and well-known
period of use. Furthermore, it
can help in understanding the painter
technique and in tracing historic
trade routes. The deep knowledge of
the material and of the degradation
processes are crucial for the fine tuning
of the restoration and conservation
protocol, since not all the pigments
react in the same way to the exposure
to light or environmental conditions.
Several analytical techniques for the
identification of pigments have been
in use for many years; they include
microscopy, X-ray−based techniques,
UV-Vis and IR spectroscopy, laser-based
techniques, such as Raman and Laser-Induced breakdown spectroscopy, and
spectrometry techniques coupled with
gas or liquid chromatography.
We will focus on X-ray Fluorescence
(XRF), the most diffused and powerful
technique, whose application is still
giving extremely useful results, also as
a consequence of the development of
new instrumentation, more and more
sensitive and reliable, mainly when
coupled with other spectroscopic
techniques.
A short introduction of the EDXRF
analysis applied to painting study,
together with a few examples will be
given in sect. 3.1.
In the following, we will
discuss the use of spectroscopic
analytical techniques with portable
instruments (sects. 3.2 and 3.3),
because archaeometric analyses
took great advantages from this
class of spectrometers and some
dedicated solutions are now available.
Unambiguous identification is usually
obtained with a multi-analytical
combination of techniques, so in
sect. 3.4 an example of FORS (Fiber
Optics Reflectance Spectroscopy)
coupled with EDXRF will be briefly
described. Working on unique and
not standardized objects requires to
pay attention on details and to know
how to choose correct parameters and
calculation algorithms to obtain reliable
results. The opportunities to deal with
these objects are very limited and it
must be noticed that the development
of very sensitive specific methodologies
and instrumentations to be applied to
the Cultural Heritage has been giving
useful examples for their applications to
other fields of research.
3.1 EDXRF (Energy Dispersive X-ray Fluorescence)
The property of all the elements of
emitting characteristic X-rays when
stimulated by various types of radiation
is widely exploited in the analysis of
materials.
Specifically in the field of Cultural
Heritage, Energy Dispersive X-ray
Fluorescence (EDXRF) is surely the
most diffused type of study of the
composition of the materials. To excite
X-ray emission various kinds of
radiation are used, the main relative
techniques being: PIXE (particle
induced X-emission), SEM-EDS
(scanning electron microscope energy
dispersive emission) and synchrotron
radiation in the SRXRF analysis. All
of them require the work of art to be
taken to the measuring facility. More
recently portable XRF instruments have
been developed and will be treated in
sect. 3.2.
In accordance to Lahanier et al.,
the XRF analysis could be considered
the ideal procedure for analyzing
Cultural Heritage materials because is
non-invasive (respecting the physical
integrity of the object), but also fast (to
investigate the object at various points),
universal (to analyze many materials),
versatile (allowing to obtain average
compositional information but also
permitting local analysis of small areas),
sensitive (allowing to detect trace
element as fingerprints) and multielemental
(to obtain simultaneously
information on many elements in a
single measurement).
The identification of almost all
the elements is possible through
a qualitative analysis. It must be
noted that the low energy of X-ray
emitted by light elements precludes
their detection: as a general rule
only elements with $Z > 15$ can be
detected. Much more difficult is the
quantitative analysis in archaeometric
applications. The irregular shape or
the non-homogeneous composition
of the sample have generated a
widespread opinion that only semiquantitative
analyses are possible in
XRF applications to archaeometry.
In fact, this is always true for nonhomogeneous
samples like, typically,
metallic corroded samples or many
pigments and binder mixtures. In some
cases the just mentioned limitations can
be overcome and quantitative analyses
have been successfully obtained.
Even if detailed quantitative analysis
is very useful, in the field of Cultural
Heritage it is not always necessary, as
also qualitative information by XRF
spectra increase the knowledge of
artifact.
As an example Dik and colleagues
used high-energy X-rays at the
L beam line of the DORIS III synchrotron
facility (DESY, Hamburg, Germany) to
record XRF spectra from a $17.5\times 17.5$
cm2 area of the Van Gogh painting Patch
of Grass. Studying the acquired data,
the authors have visualized the portrait
below the visible landscape: the flesh
tones of the hidden head of a woman
were reconstructed by combining the
antimony and mercury distributions.
By the use of a proton beam, PIXE
spectra of the inks of the Galileo
manuscripts at the LABEC laboratory
in Firenze allowed to reconstruct the
chronological history of Galileo studies.
For the purpose of examining artists’
materials, as this method detects
only the elements, not their chemical
composition, some pigment can only
be inferred. Typical examples are
green pigments such as malachite and
verdigris that have in common the
copper, detected by XRF, but different
combination of light element, as H, C,
O, whose detection and quantification
is not possible, as already noticed . For
some pigments, such as zinc white
(ZnO) or lead white ((PbCO3)2Pb(OH)2),
pigment identification can be a
straightforward task. On the contrary
the discrimination between type I and
type II of lead-tin yellow cannot be done
only on the basis of their elemental
composition: the Pb/Sn ratio is not
always specific to the actual type of this
kind of pigments. Other compounds
may have different crystallographic
phases, e.g. titanium oxide, which can
appear as either anatase (produced
since 1920) or rutile (since 1938).
3.2 p-XRF (Portable Energy Dispersive X-ray Fluorescence)
Recent advances in X-ray tube and
detector technologies have allowed
the development of portable X-ray
Fluorescence devices (p-XRF). The
combination of conveniently portable,
high-resolution instrumentation
capable of non-destructive, multielement
analysis has underpinned the
rapid growth in the application of p-XRF
in the field of Cultural Heritage. This
is because portable instrumentation
capable of non-invasive elemental
characterization has obvious benefits in
the access to museum collections and
archaeological sites.
However, the advantages of p-XRF are offset by limitations of the instrumentation itself (for
instance lower sensitivity than lab-instruments) and analytical constraints already discussed in
the previous paragraph due to the nature of the sample surface and matrix.
Like in the other kinds of XRF also in p-XRF, one must consider that the thickness of the
investigated layer depends on the primary X-ray beam energy, on the analyzed elements and
on the sample (matrix) mean composition. When the analyzed materials are inhomogeneous
in depth, the problem of discriminating the composition of the surface layers from that of the
underlying layers arises. This is the typical case of layer structured Renaissance paintings on
canvas or wood: one or more paint layers are laid on a properly prepared substrate (typically
calcium sulphate, plaster, or calcium carbonate, sometimes covered with a lead white or
colored layer, the so-called priming or imprimatur).
To solve the problem of discriminating the composition of the painted surface layers from
that of the underlying ones, in 2007 Bonizzoni et al. suggested an experimental method, that,
exploiting the fundamental parameter method (FPM), allows to recognize a multilayer
structure assigning to each layer the proper elements detected and the proper thickness. This
assignment, cannot be given by XRF alone, being impossible to understand which layer the
signals come from: for this reason a tool to recognize the composition of the external layer
(surface pigments) is needed, like for instance visible reflectance spectroscopy (vis-RS).
If, as usually occurs, the sample is a sequence of layers, the characteristic line intensity Ii of
element i from a layer depth t excited by monochromatic exciting radiation is detected as:
$I_{i}= \frac{I_{0} exp (- \sum\limits_{k>x} \mu_{k}^{M} (E_{0}) \rho_{k} t_{k}+ \sum\limits_{k>x} \mu_{k}^{M} (E_{i}) \rho_{k} t_{k})P_{i} \epsilon_{i} Gw_{i}\mu_{i}^{ph} (E_{0})}{\frac{\mu_{x}^{M}(E_{0})}{sin \varphi_{1}} + \frac{\mu_{x}^{M}(E_{i})}{sin \varphi_{2}}}[ 1 - exp (- (\frac{\mu_{x}^{M}(E_{0})}{sin \varphi_{1}} + \frac{\mu_{x}^{M}(E_{i})}{sin \varphi_{2}}) \rho_{x} t_{x})]$
where:
$I_{0}$ = intensity of incident monochromatic radiation,
$E_{0}$ = energy of incident monochromatic radiation,
$P_{i}$ = atomic factors,
$w_{i}$ = relative content of element i in the sample,
$m_{i}^{0}(E_{0})$ = photoelectric absorption coefficient of element i at energy $E_{0}$,
$\mu_{x}^{M}(E_{0})$ = mass absorption coefficient of matrix of x layer at energy $E_{0}$,
$\mu_{x}^{M}(E_{i})$ = mass absorption coefficient of matrix of x layer at energy $E_{0i}$,
$\varphi_{1}$ = incident angle,
$\varphi_{2}$ = exit angle,
$\rho$ = density of layer,
$k$ = sequence of the layers immediately over the x layer characterized by element i.
If the element i of eq. (1) characterizes the pigment of the surface layer, which can be
identified by reflectance spectroscopy measurements, the only unknown value is the layer
depth, as $w_{i}$ is known by literature. Absorption coefficients are known and the
geometry factors can be obtained measuring a few standards under the same conditions as
the samples. As regards the density $\rho$ and the composition of matrix of the layer, a mixture of
pure pigment and binder (linoleic oil) in different proportions for the pigment drawing up has
been assumed in the calculation.
A large set of p-XRF (fig. 4) were performed on a famous wood painting by Andrea
Mantegna - “Madonna col Bambino e un Coro di Cherubini”, Pinacoteca di Brera, Milan, in
the occasion of its restoration directly in the museum room. One of the main results was the
determination of the stratigraphic sequence of the Virgin’s cloak, where the azurite is covered
by a velatura of lapis lazuli, as frequently found in Mantegna, and a lead white priming
under the Madonna and child is present. As an example for the Virgin’s blue mantle the
results obtained from the areas tagged 8, 9, 14 and 15 suggest two possibilities for painting
sequence: one consists of a lapis lazuli layer over azurite mixed with lead white (2 layers); the
other consists of a lapis lazuli layer over azurite laid on lead white (3 layers). The reflectance
spectroscopy in the visible (vis-RS) measurements acquired where lapis lazuli layer was lost
because of conservation problems, confirms that the most probable sequence is the second
one, with almost no evidence of lead white in the azurite layer.
The portable scanning system developed at LABEC gave results comparable with
sophisticated (SR-based) methods as described in the first example of sect. 3.1. In fact,
taking advantage of the elemental imaging, it has been possible to determine not only the
general composition of the painting, but also to suggest a sequence of paint layers and to
find “hidden” features such as, for example, pentimenti (underlying paintings in the artwork,
evidence of revision by the artist). Thanks to the collaboration of the authors with the “Opificio
delle Pietre Dure” (OPD) in Florence, primary results from measurements performed with
the XRF scanner on “The portrait of a young woman” by Raffaello Sanzio have been achieved.
An example of the efficacy and versatility of the new XRF system is in the revealed hidden
bow in the shoulder of the lady’s dress: the acquired elemental maps of Pb M series, Ca K
series and the Hg L series show the presence of two bows, one evident to unaided eye and
one hidden by a black pigment.
Both the areas are characterized by a strong presence of Hg, due to the hypothesized use
of cinnabar by Raffaello, and where the two bowls overlap the intensity of Hg increases. It is
interesting also to note a very high intensity of K lines of Ca in the hidden bow. This is mainly
due to the upper layer of bone black, painted by Raffaello to hide the bow.
3.3 FORS (Fiber Optics Reflectance Spectroscopy)
When a radiation beam impinges onto an object part of the radiation is reflected at the
interface air/object, maintaining the same spectral composition of the primary beam, and
part penetrates inside the object, where it can be selectively absorbed depending on the
chemical and physical nature of the object itself, and then isotropically scattered. When
reflectance measurements are performed, both contributions are measured and only the
partly absorbed radiation is useful for identification purpose because provides information
about the material composition.
Some grouping of the different kinds of electronic transitions is advisable as it can also
reflect the electronic structure of the materials under examination. The color of indigo, or
in general of the organic pigments, is due to the transitions between delocalized molecular
orbitals (DMO), while the charge transfer transitions are responsible for the yellow color
of lead chromate or the blue color of Prussian blue. The color of many inorganic pigments
as azurite, smalt or verdigris is determined by ligand field transition and the energy band
transitions are responsible for the color of a lot of “semiconductor” pigments as cinnabar,
litharge, cadmium red. The identification of a pigment is based on having some specific
features in the spectrum (the maximum, minimum, inflection points ,shoulders), even if it
strongly depends on the presence of mixtures, on the pigment grain size and on painting
surface conditions (varnish yellowing, dust darkening and so on).
Progress in fiber optics technology in recent decades has increased the use of in situ
instruments to perform reflectance spectroscopy measurements. Diffuse reflectance
spectroscopy has mainly three areas of application: the identification of pigments (single or
mixture), the measurement of color changes and color-matching. An important contribution
to the application of fiber optics to reflectance spectroscopy was given by the researchers
at IFAC-CNR in Firenze (Italy). Creating a spectral database, that includes both mixtures
and single pigments, they helped in the identification of unknown spectra by means of
comparison with those of the reference materials.
An interesting application of FORS is that reported by Poldi and Caglio; in this work
they discussed the importance of identifying pigments synthesized and marketed during
XX century, by studying phthalocyanine blue and green pigments (i.e. organic polycyclic
compounds). This research evidenced that reflectance spectroscopy, performed in the range
200-1000 nm, allows to detect blue and green phthalocyanines, whose spectroscopic
features are detected in the wide range 400–850 nm. As a consequence standard portable
spectrometers—normally used as colorimeters—operating within the 360–740 nm range can
be considered only partially adequate to detect phthalocyanine dyes and pigments.
3.4 Synergy between EDXRF and FORS
Recently non-invasive in situ analyses were performed on seven books of the collection
of choir-books with lyrics and scores stored in the old library of the Certosa di Pavia (Italy),
considering fourteen illuminated pages in total, painted by four authors. They showed
different technical features, but the same materials. Pigments and binders were examined
exploiting the synergy between four complementary techniques performed by portable
instruments, namely EDXRF (Energy Dispersive X-ray fluorescence), FORS (Fiber Optics
Reflectance Spectroscopy), FTIR (Fourier transform infrared spectroscopy) and Micro-Raman
spectroscopy.
Intriguing is the case of the use of madder lake for red details, the responsible for the bright
fuchsia or magenta color. This pigment was largely used but it is photosensitive, so it usually
fades. Instead the choir-books were kept closed most of the time, and so they still hold their
original strong shade. It is easily recognizable, for instance by FORS from the band at about
530−540 nm.
The combination of natural ultramarine blue and azurite offers a good example to better
understand how EDXRF and FORS can work together. Four blue areas were considered (point
25 on folio 1R from book 822, point 65 on folio 25R from book 821 and point 73 from folio 5V
and point 93 on folio 16R from book 814, see fig. 5).
From EDXRF analysis, no chromophore elements were detected for point 65, indicating
that a blue pigment based on light element was used. Consistently, FORS spectrum showed
the trend of natural ultramarine blue that is mainly composed of Na, Al, Si, O, S. In this case, a
monolayer of natural ultramarine can be inferred. The three other points (25, 73 and 93) are
characterized by the significant presence of Cu in EDXRF spectra, indicating a probable use
of azurite; FORS spectra showed instead different trends, that can be ascribed, respectively,
to azurite, natural ultramarine, both pure and combined with azurite. In the first case, the
blue color can be supposed to be due to a monolayer of azurite. Results for point 73 reveal
instead the presence of a multilayered structure, with ultramarine blue thick layer over azurite.
For point 93, the FORS spectrum showed in the infrared region features of both pigments: in
fact, the relative reflectance grows up after 700 nm as ultramarine and after 900 nm starts to
increase like azurite spectrum. This trend indicates that the spectrum is the combination of
ultramarine and azurite: this can be interpreted as a mixture of the two pigments, or, more
probably from art historian point of view, a very thin layer (velatura) of lapis over azurite.