Spectrochimica Acta Part B 56 Ž2001. 2337᎐2346
Laser induced breakdown spectroscopy and
hyper-spectral imaging analysis of pigments on an
illuminated manuscript
K. Melessanaki, V. Papadakis, C. Balas, D. AnglosU
Foundation for Research and Technology-Hellas (FO.R.T.H.), Institute of Electronic Structure and Laser, P.O. Box 1527,
GR 71110 Heraklion, Crete, Greece
Received 12 October 2000; accepted 12 July 2001
Abstract
Laser induced breakdown spectroscopy ŽLIBS. was used for the first time in the in-situ identification of pigments
in an illuminated manuscript dated from the 12th-13th century AD. Spectral data are presented from the analysis
performed on the illumination of an initial letter ‘T’ and on the gold paint used in several parts of the writing.
Identification of most pigments, in a nearly non-destructive way, was achieved. In parallel to LIBS, hyper-spectral
imaging analysis was performed, which enabled the mapping of the pigments’ spatial distribution on the basis of their
characteristic, visible and near infrared absorption spectral features. The identification of the red pigment based on
hyper-spectral imaging analysis is demonstrated. Identification of pigments and inks is of great importance for the
dating and systematic characterization of illuminated manuscripts and, as shown in this work, a combined analytical
approach can provide important and useful information. 䊚 2001 Elsevier Science B.V. All rights reserved.
Keywords: Laser induced breakdown spectroscopy; LIBS; Hyper-spectral imaging; Pigment analysis; Illuminated
manuscripts
1. Introduction
Identification of pigments used in painted works
of art is important in understanding the artistic
and technological content of such objects w1᎐4x.
U
Corresponding author.
E-mail address: [email protected] ŽD. Anglos..
Illuminated manuscripts are a distinct class of
painted artefacts where beautiful painting, of particularly elaborate and fine structure, is used to
decorate the text. Analysis of pigments, inks and
binders, used by the artist, is obviously of great
importance to art historians, paleographers and
conservators. Given the value of the manuscript,
the fine structure of the illumination and the
sensitivity and fragility of both paint and sub-
0584-8547r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 3 0 2 - 0
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K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
strate Žpaper, parchment., especially in the case
of old manuscripts, restrictions are imposed on
sampling, often necessary for different types of
analysis. In this respect, non-destructive techniques, applicable in-situ are preferred for the
examination of illuminated manuscripts. Several
types of techniques have been used for pigments
analysis in manuscripts, including X-ray based
techniques, optical microscopy or spectroscopic
techniques w5,6x. Among these techniques Raman
microscopy is considered from many respects the
best as it combines microscopic resolution with
direct detection of the pigments present based on
their distinct spectral signature, while it is nondestructive and is applied in situ w7᎐11x.
In this work we present spectral data from
analysis performed on an illuminated manuscript
employing laser induced breakdown spectroscopy
ŽLIBS.. Although the application of LIBS for
identifying pigments in paintings, icons and objects of archaeological importance has been previously shown w12᎐16x, this is the first time the
technique is used in the examination of an illuminated manuscript. The aim was to test the applicability of LIBS for pigment analysis on sensitive
substrates, having very fine paint structure such
as the illuminated letter examined. The
manuscript ŽFig. 1a. is part of an old Ž12th᎐13th
century AD. ecclesiastical book made on parchment. The analysis focused mainly on the pigments on an illuminated initial letter ‘T’ ŽFig. 1b.
and on the gold paint used in several parts of the
writing.
In addition to the LIBS measurements, a systematic hyper-spectral imaging analysis was carried
out, which enabled the mapping of the pigments’
spatial distribution on the illuminated manuscript
on the basis of their characteristic visible and
near infrared absorption spectral features as these
are manifested in the diffuse reflectance of the
surface examined. Imaging provides direct information on the two-dimensional distribution of
paint features in a strictly non-destructive way.
Combined with spectroscopy, it can provide improved information for the discrimination and
identification of pigments having similar colour
appearance but different chemical structure. In
fact this combination is the essence of hyper-
spectral imaging, which produces data in the form
of a large number of images corresponding to
different wavelengths across the visible and near
infrared part of the spectrum, that in turn can
provide a full diffuse reflectance spectrum at each
image point.
2. Experimental
The experimental arrangement used for the
LIBS measurements has been described in detail
elsewhere w12,13x. Briefly, the third harmonic of a
nanosecond Q-Switched Nd:YAG laser Žpulse
duration approx. 5 ns. is focused on the sample
surface by means of a UV grade fused silica plano
convex lens Ž f s q50 mm. and the emitted light
is collected through an optical fibber into a 20-cm
focal length spectrograph ŽPTI model 01-002AD.
equipped with two holographic gratings of 1200
and 300 linesrmm. All spectra shown in this work
have been recorded under the higher-resolution
Ž0.5 nm. configuration, which covers a spectral
range of approximately 70 nm. The detector is an
Optical Multichannel Analyzer ŽOMA III system,
EG &G PARC model 1406 with an intensified
photodiode array detector, EG & G PARC model
1420UV. operated in the gate mode. A single
laser pulse of energy in the range of 1᎐2 mJrpulse
is used for each measurement.
In view of the manuscript’s fragility and sensitivity, before performing LIBS measurements on
it, we worked on optimising our experimental
conditions paying attention to focusing, energy
density and efficiency in emission collection. For
this purpose model samples of paint on parchment were made and analysed beforehand. The
object’s location with respect to the focal point
was optimised and a relatively low energy density
was employed Žapprox. 10 Jrcm2 .. Likewise, the
orientation of the optical fiber was finely adjusted
Žapprox. 45⬚ from the normal to the object at a
distance of 1 cm from its surface . in order to
maximise the collection efficiency. Because of the
low energy density the signal was rather weak and
for this reason the delay time employed was considerably short Ž200᎐300 ns. while the gate time
was 500 ns.
K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
Fig. 1. Ža. The full page of the manuscript examined, Žb. a close-up of the illuminated letter ‘T’, Žc,d. details of the blue paint on the left and right part of the
horizontal line of ‘T’ respectively, showing the nature of the paint, and area affected in the LIBS analysis Žbars indicate a length of 100 ␮m..
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K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
Reflectance spectra were collected using a fiber
optic spectrometer based on a 1r8 m spectrograph and 1024-channel diode array detector. Illumination is provided by two quartz-tungsten
halogen lamps located approximately 20 cm above
the surface of the object at "45⬚ angle with
respect to the normal to the point of observation.
The diffuse reflectance is collected by the optical
fiber oriented perpendicularly to the surface at a
distance of 1᎐2 mm from it. Spectra are referenced to an ‘ideal white’ sample Žbarium sulfate
pellet. and corrected for the dark count response
of the detector.
Imaging of the manuscript was performed with
the aid of a computer controlled Hyper-Spectral
imaging apparatus, developed by one of the authors ŽC. Balas. at FORTH-Instruments. The system is capable of acquiring narrow band ŽFWHM
s 5 nm. spectral images that cover the spectral
range from 400 to 1000 nm with a 2-nm tuning
step. The critical component of the apparatus is
an innovative Žpatent pending. imaging monochromator, which enables the tuning of the imaging wavelength. This module is coupled with a
two-dimensional detector array yielding a tunable
wavelength camera system. Electronic controllers
are employed for detector and monochromator
synchronisation and driving, while the system’s
calibration, image processing and analysis are
performed with the aid of specially developed
software. The system records light intensity as a
function of both wavelength and location. In the
image domain, the data set includes a full image
at each individual wavelength. In the spectroscopy domain, a fully resolved diffuse reflectance
andror fluorescence spectrum at each individual
pixel can be recorded. As a result, more than one
million spectra are collected after the image spectral scan, which lasts approximately 2 min.
3. Results and discussion
3.1. Laser induced breakdown spectroscopy
LIBS spectra were collected from the differently painted areas of the letter ‘T’ by aiming the
focused laser beam at the point of interest. Given
the value of the object and the fragile nature of
the old illumination we limited our analysis to six
different points on the letter ‘T’ with no more
than two or three laser pulses delivered at each
point.
The white line along the horizontal part of the
letter gave rise to a spectrum with clean emission
lines due to lead proving the presence of lead
white Ž2PbŽOH. 2 ⭈ 2PbCO 3 , basic lead carbonate.,
a pigment very widely used in many types of
painting from antiquity until the middle of the
20th century ŽFig. 2a.. In the spectrum obtained
Table 1
Emission lines a used in the identification of elements
Element
Wavelength Žnm.
Al
Ag
Au
Ca
Cu
Hg
Mg
Na
Pb
Si
Sn
308.22 ŽI., 309.27 ŽI.
328.07 ŽI., 338.29 ŽI.
267.60 ŽI., 274.82 ŽI., 302.92 ŽI., 312.27 ŽI.
315.89 ŽII., 317.93 ŽII.
324.75 ŽI., 327.40 ŽI.
296.73 ŽI., 302.15 ŽI., 312.57 ŽI., 313.16 ŽI.
279.55 ŽII., 280.27 ŽII., 285.21 ŽI.
330.14 ŽI.
262.83 ŽI., 266.32 ŽI., 280.20 ŽI., 282.32 ŽI., 283.30 ŽI., 287.33ŽI.
288.16 ŽI.
270.65 ŽI., 284.00 ŽI., 286.33 ŽI., 300.91 ŽI., 303.41 ŽI., 317.50 ŽI.,
326.23 ŽI., 333.06 ŽI.
319.99 ŽI., 334.19 ŽI., 334.94 ŽII., 335.46 ŽII., 336.12 ŽII., 337.280 ŽII.
Ti
a
Emission from neutral species and singly charged ions is indicated by ŽI. and ŽII., respectively.
K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
2341
The gold paint, used in the drawing lines of the
illuminated letter contains gold, silver and copper
as shown by the characteristic emission lines indicated on the LIBS spectrum ŽFig. 3a.. The presence of copper and silver emission in the spectrum of the red paint suggests an overlap of red
with adjacent gold paint, which is verified by
optical microscopic examination of the
manuscript.
The green pigment in the bottom section of the
vertical line of ‘T’ showed an emission spectrum,
which consisted primarily of Pb and Sn lines ŽFig.
3b.. The only known pigment containing Sn is
lead tin yellow Žtype I: Pb 2 SnO4 , type II:
PbSn 1y x Si xO 3 ., which has been found in other
illuminated manuscripts as well w5,9x. In fact lead
tin yellow type II has been used as an artists’
Fig. 2. LIBS spectra of Ža. white paint on the manuscript, Žb.
red paint on the manuscript and Žc. a reference sample of
vermilion Žpure pigment..
from the red paint, found in the lower section of
the vertical part of the letter ‘T’, mercury lines
are observed proving the use of vermilion ŽHgS,
mercury sulfide. again a well-known red pigment
ŽFig. 2b.. For comparison, the LIBS spectrum
obtained from a pure sample of vermilion is shown
as well ŽFig. 2c.. In both red and white paint
spectra, emission lines due to other elements
such as Mg, Si, Al and Ca are evident. These
suggest the presence of other components in variable amounts relatively high in the case of the red
paint, which might originate from impurities in
the pigments, from the binding medium used for
painting or from environmental deposits. Table 1
lists characteristic emission lines used in the identification of different elements, which are also
labelled in the corresponding spectra.
Fig. 3. LIBS spectra of Ža. gold paint on the manuscript, Žb.
green paint on the manuscript and Žc. a reference sample of
lead tin yellow Žpure pigment..
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K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
Fig. 4. LIBS spectra of Ža. the blue paint on the manuscript,
Žb. ultramarine blue Žpure pigment. and Žc. non-illuminated
part of the parchment.
pigment since around 1300 AD w2,9x. This places
the date of production of this manuscript not
earlier than late 13th century AD, which is in
reasonable agreement with the historical information available. The spectrum of pure lead tin
yellow type I is given for reference ŽFig. 3c.. This
result suggests that a mixture of lead tin yellow
and a blue pigment has been used to produce the
green paint on the illuminated letter. The presence of distinct emission from Al, Si and Na
suggests the possible presence of lazurite
ŽNa 8 Al 6 Si 6 O 24 S n .. However, given that these elements come from quite common materials often
accompanying pigments or accumulated on objects, no unambiguous identification of the blue
component of the paint can be done on the basis
of the LIBS spectrum. The pure blue paint on the
manuscript ŽFig. 1c,d. was examined and the
spectrum obtained ŽFig. 4a. does contain emission
lines due to Si, Na and Al, such as the ones
distinctly seen in the spectrum of a sample of
pure ultramarine blue Žthe synthetic form of lazurite. used as a reference ŽFig. 4b.. However, despite the similarity of the spectra, some doubts
still remain for the identity of the blue pigment
used in the manuscript. This is because of the
presence of contaminants or impurities across the
surface of the manuscript, which also give rise to
Si, Na and Al emission, as shown for example in a
spectrum obtained on a non-illuminated spot on
the parchment ŽFig. 4c..
Furthermore, to characterise the effects of laser
irradiation on the paint, we examined with an
optical microscope the areas of the illuminated
manuscript subjected to LIBS analysis. In most
cases the effect of the first pulse on the surface
was minimal, however, partial or total paint removal was observed, after irradiation with two or
three laser pulses, across an area of diameter
ranging from 100 to 200 ␮m. Indicative examples
of such craters, formed during the examination of
the blue paint at two different locations on the
manuscript, are shown in Fig. 1c,d. The crater
size was comparatively larger than that observed
in test measurements, during the optimisation of
experimental conditions, carried out using model
paint samples. This observation is, however, not
unexpected given the poor adherence of the paint
on the parchment and the rather low surface
concentration of pigment grains on the illumination, particularly evident in Fig. 1d. Despite the
relatively low energy density employed in our
measurements it seems that the shock wave accompanying the ablation process was quite effective in removing material from around the irradiated area thus leading to larger than expected
crater diameters.
We believe that the effects observed can be
further minimised if critical experimental
parameters are improved including laser beam
quality and focusing through a proper optical
set-up such as a microscope w17x. Such improvements will be necessary for the effective implementation of LIBS in a practically non-destructive way, in demanding cases such as illuminated
K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
2343
manuscripts or miniatures. Overall, material, laser
irradiation and detection parameters are critical
in achieving the optimum analytical result, namely
reliable identification of elements present, with a
minimum effect on the substrate.
Finally, an attractive feature of LIBS is the
ability for in situ stratigraphic analysis by recording individual LIBS spectra produced by successive laser pulses on a single point, especially important in cases of multi-layered paint structures
w12,13x. In addition quantitative LIBS elemental
analysis is possible, as reported in the literature
w15,18x, which in certain cases can provide with
additional valuable information on the elemental
composition of the investigated paint layers.
3.2. Hyper-spectral imaging
Complementary to the LIBS measurements, a
hyper-spectral imaging analysis was performed the
aim being to map and possibly identify the pigments present on the illuminated manuscript. As
outlined in the introduction, the analysis was carried out by recording narrow band images of the
manuscript across the visible and near infrared
part of the spectrum and comparing the reflectance behaviour of the real paint to that of
reference samples of pure pigments or mixtures
of pigments. To indicate the basic principle of this
analysis, diffuse reflectance spectra of several
model samples made of different pigments Žred
and blue. on parchment were recorded on a regular diffuse reflectance spectrometer, described in
the experimental section, and are shown in Fig. 5.
These spectra reveal whether different pigments
of similar colour appearance can be discriminated
based on their spectral differences in the visible
or near infrared part of the spectrum. For example in the case of the blue pigments ŽFig. 5b. it
can be seen that significant differences exist in
the spectra leading to discrimination of Prussian
blue against ultramarine and cobalt blue in the
700᎐800 nm range and against azurite in the
450᎐500 nm range.
The red paint used in the upper part of the
bottom section of the vertical line of ‘T’ ŽFig. 1b.
was examined in detail with the hyper-spectral
imaging camera system. A full series of images of
Fig. 5. Diffuse reflectance spectra from model paint samples
on parchment. Ža. Red pigments Žcadmium red, mars red,
vermilion., Žb. blue pigments Žcobalt blue, azurite, ultramarine
blue, Prussian blue..
the area under investigation were collected in
direct comparison with model samples of vermilion, mars red wred ironŽIII. oxide, Fe 2 O 3 x and
cadmium red Ž cadmium sulfide selenide,
CdSe1y x S x . to assure identical conditions of examination for both the manuscript and the reference samples. Selected images at several different
wavelengths are shown in Fig. 6. The reflectance
behaviour of the red paint on the manuscript
matches quite well that of vermilion in the reference sample while it is in clear contrast with that
of mars red. More specifically the reflectance of
the red paint and of the vermilion sample appear
both to increase similarly above 580 nm as indicated in the images shown. The dark shade due to
the red paint faints as the observation wavelength
shifts from 570 to 600 nm. On the contrary the
mars red sample remains highly non-reflective up
to 620 nm as clearly seen on the corresponding
image. Cadmium red on the other hand, has an
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K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
Fig. 6. Images recorded on a detail of the illuminated letter ‘T’ Žlower part of the vertical line indicated by a rectangle in Fig. 1b in
the range of 570᎐620 nm. Next to the area examined reference samples of red paint, cadmium red Ž1., mars red Ž2. and vermilion
Ž3., are placed for direct comparison. The red paint appears as a dark area in the center of the image at 570 nm.
intermediate behaviour showing an increase in
reflectance above 610 nm as seen in the 610 and
620 nm images, which is in perfect agreement
with the diffuse reflectance spectral data obtained
independently ŽFig. 5..
Similar analysis was done on the blue paint on
the upper horizontal line of the letter ‘T’ in
comparison to model samples of cobalt blue
wcobaltŽII. doped alumina glass, CoO⭈ Al 2 O 3 .,
ultramarine blue, Prussian blue wironŽIII. hexacyanoferrate ŽII., Fe 4 wFeŽCN. 6 x 3 ⭈ nH 2 Ox and azurite wbasic copper ŽII. carbonate, 2CuCO 3 ⭈
CuŽOH. 2 x. The paint’s reflectivity, as seen through
a series of images collected in the range of
550᎐700 nm, appears to match well that of ultramarine blue as shown in Fig. 5b. Prussian blue
and azurite remain highly absorbing through this
spectral range while cobalt blue appears very
bright when observed in the range of wavelengths
longer than 660 nm. On the other hand, in the
range of 450᎐550 nm, the expected increased
reflectivity seen in the diffuse reflectance spectrum of ultramarine blue is not observed for the
blue paint on the manuscript, leaving uncertain
the identity of the blue pigment. An obvious
explanation of this result is the possible use of a
pigment other than ultramarine blue or the presence of a minor component in the paint with
increased absorption in the blue part of the spectrum. The latter is particularly important because
pigment mixtures have been commonly used in
painting; a systematic examination of such systems is the subject of current studies.
Through the examples illustrated above it becomes evident that with a carefully planned approach the hyper-spectral imaging analysis can
K. Melessanaki et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 2337᎐2346
provide not only mapping but also identification
of pigments. In this respect it is important to have
a good database of reflectance spectra from a
variety of pigments and mixtures of pigments in
appropriate media and substrates, which closely
simulate realistic objects examined.
4. Conclusions
LIBS analysis has been employed for the first
time in the identification of pigments in a 12-13th
century AD illuminated manuscript. Analysis of
emission spectra collected from selected points
on the illumination of an initial letter ‘T’ and the
gold paint in several parts of the writing led to
the identification of most pigments. Indirect dating of the illuminated manuscript became possible on the basis of identifying lead tin yellow as
one of the pigments used, which is known to have
been employed by artists since around 1300 AD.
The effects of laser radiation on the spots examined were evaluated and appear to depend on
the nature of the paint layer Žthickness, surface
density and adherence to the parchment. as well
as on certain experimental parameters, such as
energy density and beam focusing. Hyper-spectral
imaging analysis complemented the LIBS results
by providing mapping information regarding pigments’ spatial distribution. It was also shown to
be capable of identifying pigments based on direct comparison with reference samples and as
demonstrated in the case of the red pigment
vermilion used in the manuscript. In conclusion,
new techniques and their combined use can lead
to important analytical information useful in the
characterization and analysis of illuminated
manuscripts and in general a wide variety of
works of art.
Acknowledgements
The authors would like to thank Mr A. Tselikas
from the Center of Paleography in the Cultural
Foundation of the National Bank of Greece
ŽAthens, Greece. for providing the manuscript
and relevant information. Funding from the Gen-
2345
eral Secretariat for Research and Technology
through the ⌸ENE⌬ Program Žcontract No.
99E⌬6. is acknowledged for supporting one of
the authors ŽMK. with a graduate fellowship.
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