μ-XRF/μ-RS vs. SR μ-XRD for pigment identification in illuminated manuscripts

Abstract

Journal article

Full text

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DOI:10.1007/s00339-008-4447-9
Appl. Phys.A 92, 59–68 (2008)
Materials Science & Processing
AppliedPhysicsA
g.vandersnicktu
w.denolf
b.vekemans
k.janssens
µ-XRF/µ-RS vs. SR µ-XRD for pigment
identification in illuminated manuscripts
Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium
Received: 14 September 2007/Accepted: 15 January2008
Published online: 20 March 2008• © Springer-Verlag 2008
ABSTRACT For the non-destructive identification of pigments
and colorants in works of art, in archaeological and in foren-
sic materials, a wide range of analytical techniques can be used.
Bearing in mind that every method holds particular limitations,
two complementary spectroscopic techniques, namely confo-
cal µ-Raman spectroscopy (µ-RS) and µ-X-ray fluorescence
spectroscopy (µ-XRF), were joined in one instrument. The
combined µ-XRF and µ-RS device, called PRAXIS unites both
complementary techniques in one mobile setup, which allows
µ- and in situ analysis. µ-XRF allows one to collect elemen-
tal and spatially-resolved information in a non-destructive way
on major and minor constituents of a variety of materials. How-
ever,themaindisadvantages ofµ-XRFarethepenetrationdepth
of the X-rays and the fact that only elements and not spe-
cific molecular combinations of elements can be detected. As
a result µ-XRF is often not specific enough to identify the pig-
ments within complex mixtures. Confocal Raman microscopy
(µ-RS) can offer a surplus as molecular information can be ob-
tained from single pigment grains. However, in some cases the
presence of a strong fluorescence background limits the appli-
cability. In this paper, the concrete analytical possibilities of the
combined PRAXIS device are evaluated by comparing the re-
sults on an illuminated sheet of parchment with the analytical
information supplied by synchrotron radiation µ-X-ray diffrac-
tion (SR µ-XRD), a highly specific technique.
PACS 33.20.Fb; 61.05.cp; 33.20.Rm; 07.85.Qe; 91.65.An
1Introduction
During the last decade, diagnostic techniques have
become an essential tool for the conservation of works of art
as they can provide decisive information for the selection of
the appropriate conservation treatment. As a result, numer-
ous analytical techniques have found a valuable application
in the conservation of art. However, every single method has
particularlimitations.Forthisreason,inmostcasesitisneces-
sarytoemployacombinationofseveraltechniquesinorderto
obtain a complete overview of the composition of a layer of
paint.
XRF and µ-XRF are among the most cited analytical
methods in literature dealing with the investigation of inor-
u Fax: +32-3-820-2376, E-mail: geert.vandersnickt@ua.ac.be
ganic materials such as pigments, glasses, ceramics, etc. [1].
The popularity of XRF in art conservation can be explained
by the fact that no other analytical technique can identify so
many different elements in an efficient and non-destructive
way. On the other hand, some drawbacks limit the applica-
bility of XRF. For example, unless a confocal setup is being
used, the identification might be hampered in case of a multi-
layered paint sample. When cross-sections are analysed, the
spatial resolution is often too low to investigate the composi-
tionofseparatelayers,whichsometimesareonlyafewµmof
thickness. Alternatively, the incident beam can penetrate sev-
eral layers of paint if the X-rays are directed perpendicular
on to the surface of the paint. In this situation, it is often not
clear to which layer the detected elements can be attributed.
Also,beinganelement-specifictechniquethatisinsensitiveto
thechemicalstate,crystalphaseorthemolecularenvironment
in which the elements are present, XRF is often not specific
enough to identify the pigments with certainty. For instance,
the detection of copper can indicate the presence of several
pigments (azurite, malachite, etc.). In that case, other criteria,
forinstancethecolourofthepaintlayerortheageofthepaint-
ing, have to be considered in order to identify the pigment
by deduction. Also the characterisation of mixtures of sev-
eral pigments can be problematic. Moreover, light elements
canbedifficulttotrace, dependingontheinstrumentation and
measurement conditions. In this way the identification of or-
ganicpigmentsand dyestuffsandpigments composedofonly
lightelements(e.g.,thecarbon-basedpigmentlampblack)are
excluded.
µ-RS is a powerful and more specific technique for the
identification of painting materials [2,3]. It is a vibrational
technique based on the fact that a small part of incident ra-
diation is scattered inelastically by a material. The differ-
ence in energy between the incident beam (laser light) and
the scattered light provides a unique spectral finger print
by means of which the molecule can be identified. This
method is in many ways complementary to µ-XRF, as it
has already been demonstrated by Ricci, et al. [4]. Con-
focal Raman microscopes can achieve a high spatial and
depth resolution, allowing the identification of single pig-
ment grains without interference of surrounding materials.
Nevertheless, a strong fluorescence background often caused
by the organic medium can limit the applicability of µ-RS
spectroscopy. Bearing in mind the complementarity of both
techniques, a combined µ-XRF and µ-RS instrument, called
PRAXIS, was built at the University of Antwerp, together

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60 Applied Physics A – Materials Science & Processing
FIGURE 1
view of the analysed sheet of parch-
ment
Front and back side
with several other partners: European Laboratory for Non-
Linear Physics, Firenze (I); Institut für Gerätebau, Berlin
(D); Jobin-Yvon S.A., Longjumeau (F); Ormylia Arts Di-
agnostic Centre, Chalkidiki (Gr) [5]. In this way, PRAXIS
links the elemental, ‘bulk’ information obtained by XRF to
the molecular, surface-related data supplied by Raman. Both
techniques were joined in one setup, allowing in situ and
micro-analysis.
To evaluate the combined analysis possibilities of
PRAXIS, an excerpt from an illuminated, fifteenth or six-
teenth century book of tides was analysed (see Fig. 1). The
sheet of parchment was written on and painted on both sides.
A Latin text is present in black ink, accompanied by a French
translation. Red-pink rulers were drawn on the parchment in
ordertofacilitate thewriting.Intherightmargin, variousveg-
FIGURE 2
PRAXIS, the combined µ-XRF and µ-RS instru-
ment. On the left a detail of the remote measuring
head. Dimensions of the transportable head are
ca. 25×25×40 cm3
Schematic representationof
etal motives such as strawberries and flowers were applied
in gilded cartouches with black borders. The cartouches are
separated by decorative scrollwork in blue and gold. The fine
trace lines of the figurative depiction were executed in a pink
fluid material, which looks verysimilar to that of the rulers in
thetext.
2Experimental
The PRAXIS device consists of an easily trans-
portable base unit which holds the spectrometer, the laser
sourceand acold lightsource.A remotemeasuring head con-
tains a computer controlled x–y–z sample stage, a µ-focus
X-ray tube, a polycapillary X-ray lens, an X-ray detector and
aRaman probe, asillustrated by Fig. 2.

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VAN DER SNICKT et al. µ-XRF/µ-RS vs. SR µ-XRD for pigment identification in illuminated manuscripts61
FIGURE 3
the PRAXIS instrument
The sheet of parchment on the motorised x–y–z sample stage of
2.1
µ-RS spectroscopy
The Raman measurements were carried out di-
rectly on the sample which was placed on the translation
stage. The analysed pigment particles were selected by mov-
ing the translation stage and viewing the image of the sam-
ple using the on-axis TV-camera that is incorporated in the
Raman probe. Due to the current set-up, the sample size is
limited to ca.30×30cm2.
The laser excitation source was a diode laser (Sacher
Lasertechnik, Lynx tunable Littrow external cavity diode
Laser) consisting in a laser head and a laser driver (Pilot) to
controlthelaseroutputpower.Thelaseremitted at785nm(to
minimizefluorescenceoftheorganicmedium).Thelaserlight
FIGURE 4
lected on the unpainted parchment,
from the spot of analysis shown in
Fig. 6
µ-XRFspectrum col-
was transmitted to the Raman probe (Jobin Yvon Superhead)
viaasinglemodeopticalfibre.Theprobecontainedanonaxis
TV-camera, an interference filter to eliminate the scattered
Raman radiation caused by theoptics andan extralong work-
ing distance (ELWD) objective (Mitutoyo, 50×/0.42 with
aworkingdistance of 20.5mm), which permitted to focus the
laserbeamonaspotwithadiameter of∼ 5µm.Thescattered
light was collected through the same objective and directed
to the spectrometer via a separate multimode optical fibre.
The entrance to this fiber functions as a confocal pinhole, re-
sulting in a depth resolution of ∼ 8µm. A holographic notch
filter was employed to separate the inelastic scattering from
theRayleigh scattering (seeFig. 3).
µ-Raman spectra were obtained using an Induram scan-
ning spectrometer equipped with a cooled charge-coupled
devicedetector (CCD,Andor,1024pixels×256pixels),ther-
moelectrically cooled at −60◦C. The spectrometer was
equipped with two dispersive gratings mounted on the same
axis (1200lines/mm and 950lines/mm). All Raman spec-
tra were collected using the 950l/mm grating. The spec-
tral resolution achieved with this system is on average
1.5cm−1/pixel with a 1200l/mm grating and 1.3cm−1/pixel
witha950l/mmgrating.
Calibration of the spectrometer was accomplished by
using the Raman lines of a slab of metallic silicon. Meas-
urements were carried out for each type of material. The
pigments were identified by comparison of their raman spec-
tra with those in a home made database of reference spectra.
This extensive database consists of spectra collected from
powderedpigments, suppliedby severalsources.
2.2
µ-XRF (EDS)
X-rays were generated with an air cooled, low-
power Mo-tube with an acceleration voltage of maximal
50kV.TheprimaryX-rayswerefocussedbymeansofapoly-
capillary X-ray lens, resulting in a measuring spot size with
a diameter of ca. 50µm. All µ-XRF-spectra were collected
using an electro-thermally cooled (−15◦C) silicon drift de-
tector (Röntec Xflashdetector Type 1207) with an active area

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62 Applied Physics A – Materials Science & Processing
FIGURE 5
trum of calcite (black) vs. the µ-RS
spectrum collected on the unpainted
parchment (red), from the spot of an-
alysis shown in Fig. 6
Thereference µ-RSspec-
of 5 mm2, an 8 µm thick beryllium window and a zirconium
collimator with an aperture of 2.4mm. Working under a He-
lium atmosphere is possible but this option was not exploited
during these experiments. All measurements were performed
at 35kV, 500µA, and with an acquisition time of 200s (live
time). The sample positioning was identical as for Raman
spectroscopy.
2.3
µ-XRD
µ-XRD experiments were carried out at HASY-
LAB, Beam Line L. Monochromatic X-ray microbeams of
10–15µm diameter of low divergence (< 4 mrad) were gen-
erated by means of a Si?111? monochromator and a single-
bounce elliptical capillary [6]. A Bruker 1000 X-ray diffrac-
tion camera, positioned in transmission geometry, together
with a Si(Li) detector at 90◦, allows one to simultaneous col-
lectµ-XRFandXRDdataontheirradiatedpartofthesample.
3Results and discussion
Prior to the analysis on the parchment, a µ-RS and
µ-XRF reference database was established by measuring raw
pigments from several sources (Kremer, Winsor & Newton,
etc.). The database also includes optimum measuring param-
eters and the damage thresholds of the pigments under the
785nmlaserlight. Thisinformation facilitated theRamanan-
alysis and also allowed to avoid damage or degradation to the
paintlayer bylocalized overheating.
As Figs. 6, 8 and 11 illustrate, several spots on the parch-
ment, representing all present materials, were selected for
analysis. It concerned measurement positions inside the red,
green, blue, and pink painted areas and positions on the un-
painted parchment, the gilding and the black ink. All spots
wereanalysed both bymeans ofµ-RSand µ-XRF.
3.1
Parchment
The µ-XRF spectrum shown in Fig. 4 revealed
a substantial amount of Ca in the parchment. This was ex-
FIGURE 6
painted parchment and the ink were collected. Data shown in Figs. 4 and 5
respectively
Detail of the area where the µ-XRF and µ-RS spectra of the un-
pected, as chalk was abundantly used during the preparation
ofparchment. Atthesamespot,Raman spectrawereobtained
whichconfirmedtheseresults(seeFig.5).Thecollectedspec-
tramatched thereferencespectrumofcalcite (CaCO3),show-
ingthecharacteristic carbonatestretch around 1097cm−1.
3.2
Black ink
Four types of black ink were known in the 15th–
16th century: soot ink, bister, sepia and iron gall ink. Sootink
and bister are two carbon-based inks, prepared from wood-
ashes, while sepia is an animal product obtained from squids.
The main ingredients of the fourth type, namely iron gall ink,
werevitriol(FeSO4),tannicacid(fromgallnuts),Arabicgum
andwater[7].Bisterandsepiahaveabrownishinkcolourand
werepredominantly used for drawings, so it is not very likely
that they were used as a writing material in this case. The fact
thattheµ-XRF-measurementsdidnotrevealanyFeintheink,
allowed to exclude iron gall ink, leaving soot ink as the only
ink compatible with these indications. Unfortunately, it was

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VAN DER SNICKT et al. µ-XRF/µ-RS vs. SR µ-XRD for pigment identification in illuminated manuscripts 63
FIGURE 7
lected on the black ink
µ-XRF spectrum col-
not possible to produce a clear Raman spectrum demonstrat-
ing the carbon vibration stretch to confirm this hypothesis.
Thecharacteristic broad bands atabout 1350–1380cm−1and
1580–1600cm−1werenotobserved.Theµ-XRFspectrumof
theblackink(seeFig. 7)onlyshowedthecharacteristic X-ray
emissionlinesofCa,whichcouldbeattributedtotheunderly-
ingparchment.
3.3
Paint and gilding
The µ-XRF spectrum of the red paint (Fig. 9)
showed intense Hg and S peaks, two elements which can
be related to the red pigment vermilion (HgS). This mer-
cury(II)sulphide is a red pigment that was frequently used by
artists from antiquity until the 20th century. Both vermilion
from mining sources (also called cinnabar) and from artifi-
cialproductionwasused;however,neitherX-rayfluorescence
nor Raman spectroscopy allow to make a distinction between
the natural and synthetic variants [8]. The presence of a small
peak of Ca in the X-ray spectrum can be due either to chalk,
which was often used as a filler in paint, or to the parchment
itself (as explained above). The identification of the red pig-
ment was confirmed by the result of the µ-RS analysis, as the
collected spectrum correlated with the reference spectrum of
cinnabar (Fig. 10). The Raman spectrum collected in the red
paint yielded strong bands at 253 and 343cm−1and a weaker
bandat282cm−1.
In Fig. 12, the µ-XRF spectrum of the gilded area shows,
apart from the expected Au peaks, also significant Cu peaks.
The copper is not related to the gilding but to the blue paint
on the back of the parchment. This is an example of how
the penetration depth of the X-rays can lead to interpreta-
tion problems, a problem which has been reported by other
authors [1]. µ-RS analysis of the blue paint indicated that the
copper containing pigment was azurite (2CuCO3·Cu(OH)2)
withcharacteristicbandsat398,763,834,931and1094cm−1
(Fig. 13). Azurite is a basic copper(II) carbonate which was
one of the important blue pigments in European painting
until ca. the eighteenth century when it was largely re-
placed by Prussian blue, next to the more expensive lazurite
(Na8−10Al6Si6O24S2−4).
Inthiscase, theinterpretation ofRaman datawassubstan-
tially simplified by the results of µ-XRF, as this technique
FIGURE 8
µ-RS spectra were collected. Data shown in Figs. 9 and 10 respectively. The
XRD data presented in Figs. 18 and 19 were recorded in the green paint area
Detail of the green, red and pink areas where the µ-XRF and

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64 Applied Physics A – Materials Science & Processing
FIGURE 9
from the spot of analysis shown in Fig. 8
µ-XRF spectrum collected in the red paint,
FIGURE 10 The reference µ-RS spectrum of vermilion
(black line) matches the µ-RS spectrum collected in the
red paint (red line), from the spot of analysis shown in
Fig. 8
allowedto narrowdownthenumberofpossiblepigmentsthat
are present. The identification by µ-RS of a completely un-
knownpigmentcantakeasubstantialamountoftime,sinceall
pigmentsnotonlyrequirevariousoptimalparametersbutalso
show different damage thresholds toward laser light. As only
one copper containing blue pigment was known at that time,
the proper parameters could be selected quickly, by means of
the reference database. The gold could not be identified by
meansofRamanspectroscopy,asmostmetals donotproduce
characteristic Raman bands[8].
According to the µ-XRF measurements (see Fig. 15),
the green paint contains a substantial amount of both cop-
per and lead. This last element could be ascribed to the
presence of lead-tin yellow (Type I: Pb2SnO4 or type II:
PbSn2·SiO7) and/or lead white (2PbCO3·Pb(OH2)). With
regards to Cu, three green copper-based pigments were in
use at the time, namely malachite (CuCO3·Cu(OH)2), verdi-
gris (Cu(CH3COO)2·nCu(OH)2) and copper resinate. Cop-
per resinate is the general name for a group of organo-
copper complexes and is in fact verdigris dissolved in oil
and/or resin [10]. The image in Fig. 14 demonstrates that
the green paint contains yellowish, greenish and blue grains.
The Raman spectra of this area, shown in Fig. 17, revealed
that the green grains are malachite. The characteristic peaks
FIGURE 11 Left: a detail of the gilded area where a µ-XRF spectrum was
collected (data in Fig. 12). Right: the same area on the back side of the parch-
ment with blue paint. Signals from the gilding and the blue pain appear in the
µ-XRF spectrum

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VAN DER SNICKT et al. µ-XRF/µ-RS vs. SR µ-XRD for pigment identification in illuminated manuscripts 65
FIGURE 12 µ-XRF spectrum collected in
the gilded area, from the spot of analysis in
Fig. 11
FIGURE 13 Reference µ-RS spectrum of
azurite (upper line) vs. the µ-RS spectrum
collected in the blue paint (lower line), from
the spot of analysis shown in Fig. 11
around 217, 265, 350, 430, 531 and the carbonate stretch vi-
bration around 1050–1110cm−1are present. Most Raman
spectra only yielded a weak signal while the collection of
a clear malachite spectrum required a considerable amount
of time, probably due to the fact that the red laser light was
stronglyabsorbedbythegreenpigment. Thebluegrainswere
identified as azurite (2CuCO3·Cu(OH)2), as demonstrated
by Fig. 13. The distribution of the blue grains is not homo-
geneous and it is not clear whether the azurite was added
intentionally by the artist or if it concerns a contamination
of the green, copper containing pigment. Such a contamina-
tion is plausible since the azurite mineral is usually found
in association with malachite [9]. Raman spectroscopy indi-
cated that the yellowish grains are lead-tin yellow (Type I:
Pb2SnO4)withbandsat198,275,293,338and457cm−1(see
Fig. 16). Burgio et al. also identified lead-tin yellow mixed
with malachite to produce a green colour on a 15th century
Latin manuscript. A combination of a green with a yellow
pigment, was common practice at that time, according to the
FIGURE 14 Detail of the green paint, shown in Fig. 8. Blue and yellow
grains are present in the green paint. Optical microscope Olympus BC40,
X50

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66 Applied Physics A – Materials Science & Processing
FIGURE 15 µ-XRF
lected in the green painted area, from
the spot of analysis shown in Fig. 8
spectrum col-
FIGURE 16 Reference µ-RS spectrum
of lead-tin yellow type I (black line) vs.
the µ-RS spectrum of the yellow grains
in the green paint (red line), from the
spot of analysis shown in Fig. 8
same authors[11,12]. No lead white was found by means
of µ-RS. This case highlights that µ-RS is a valuable tool
for the characterisation of mixed pigments and contamina-
tions. While µ-XRF gave an indication about the nature of
the main green pigment (Cu), µ-RS identified it as malachite
and established the presence of two other pigments (azurite
and lead-tin yellow) which could not be identified by µ-XRF
alone.
The pink pigment, used to draw the rules, could not be
identified, as it was notpossibleto detect by means of µ-XRF
any elements related to a red pigment. Furthermore, the col-
lectedRamanspectrashowedonlyafluorescencebackground
andnobandsofanyknownredpigment.Thisresultcould im-
ply thatsomekind of organicdyewas used,for example ared
lake. Dyes and lakes are known to fluorescence in the visi-
ble region and the signal from this fluorescence can hide the
characteristic Raman peaks oftheorganicmaterial [13].
3.4
SRµ-XRD
In order to estimate the results of the above-
discussed pigment identification by means of the PRAXIS
instrument, measurements with SR µ-XRD, a highly specific
method of phase identification, were carried out at the Syn-
chrotron facility in Hamburg (Hasylab, Beam Line L). These
analyses confirmed the presence of calcite in the parchment,
azurite in the blue coloured area, malachite and lead tin yel-
low in the green paint, vermilion in the red paint and gold in
the gilded area. The XRD pattern in Fig. 18, recorded in an
area with green paint and gilding, also demonstrates the pres-
ence of hydrocerussite (lead white), a material which was not
traced bymeansofµ-XRF/µ-Raman.Foreachphaseareflec-
tion was chosen, which did not overlap with other lines: gold
(111), calcite (104), lead-tin yellow (121), lead-white (110)
andmalachite (120).

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VAN DER SNICKT et al. µ-XRF/µ-RS vs. SR µ-XRD for pigment identification in illuminated manuscripts 67
FIGURE 17 Reference µ-RS spectrum
of malachite (black line) vs. the µ-RS
spectrum of the green grains in the
green paint (red line), from the spot of
analysis shown in Fig. 8
FIGURE 18 XRD patern recorded in the green and gilded area (shown in Fig. 8) confirming the presence of calcite and gold; also the presence of malachite
and hydrocerussite (lead white) could be established
In addition, enough crystals were present in the beam to
giveapowderdiffractogram,e.g.,fullyoccupiedDebyerings.
Therefore the reflection intensity distributions represent the
pigment composition in the area. Figure 19 clearly demon-
strates that the presence of hydrocerussite is limited to the
green paint. Indeed, based on the outcome of the PRAXIS
µ-XRF analyses, the presence of lead white in the green area
could not be excluded (see Fig. 15). The fact that µ-RS did
not detect any lead white could be related to the location of
the pigment. Since it is not certain that the lead white was
mixed with the green paint, it cannot be excluded that the
green paint was superimposed on a locally applied layer of
lead white. This hypothesiscould notbeconfirmed,but could
explainwhythepenetratingsynchrotronradiationtracedcrys-
tals of lead white, while the laser beam of µ-RS did not en-
counteranyleadwhiteatthesurface.Ontheotherhand,µ-RS
clearly identified the blue grains in the green paint as being
azurite, this pigment was not detected by SR µ-XRD. This

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68 Applied Physics A – Materials Science & Processing
FIGURE 19 Phase composition and corresponding elemental distribution
maps obtained by combined µm-XRD-XRF, are shown for the ‘green paint’-
area displayed in Fig. 8. This 2×2 mm2region was mapped with 40×40 µm
steps
might be due to the inhomogeneous distribution of the grains
(see above) and the fact that probably only a few grains were
presentin theanalysed area.
4 Conclusion
The combination of µ-XRF and µ-RS in the
PRAXIS-device has proven itself to be very useful for the
identification of pigments. A more complete and definite
image of the composition of the samples could be estab-
lished by linking the bulk-representative, elemental infor-
mation supplied by µ-XRF to the surface-related, molecular
data offered by µ-RS. The complementarity of both analyt-
ical techniques became in particular clear during the study
of the copper containing green and blue paint layers. It has
been demonstrated that µ-XRF can narrow down the num-
ber of probable pigments and in that way can facilitate and
render the pigment identification more rapid with µ-RS an-
alysis. Also, it was shown how µ-XRF alone, in some cases
onlyallowsonetoestablishuncertain hypotheses.(e.g.,about
the green pigment used for the vegetal leafs). Nevertheless,
these could be ascertained by µ-RS spectroscopy. The spa-
tial resolution (∼ 50µm) of µ-XRF was beneficial for the
analysis of the monochrome areas on the parchment, but ap-
peared insufficient to investigate mixtures or multilayered
paint layers, whereas µ-RS offered the possibility to identify
single pigment grains. In this way, the problems with µ-XRF
thatoriginatedfromthepenetrationdepthoftheX-rays,could
be counteracted by the depth resolution (∼ 8µm) of the con-
focal µ-RS setup. Also, the evaluation of these results by
means of SR µ-XRD highlighted the fact that this additional
technique was necessary to show the presence of lead white
with certainty. In spite of this culmination of analytical tech-
niques still a number of organic or non-crystalline pigments
(e.g.,blackand pinkink)could notbeidentified. Thisleads to
the conclusion that a supplementary method like for example
FT-Raman (Nd:YAG laser emitting at 1064nm) or FT-IR is
desirableto enable the identification of organicsubstances, in
order to obtain a complete overview of the composition of all
applied materials.
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