Removing foxing stains from old paper at 157 nm
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E. Sarantopouloua,*, Z. Samardzijb, S. Kobeb,
Z. Kolliaa, A.C. Cefalasa
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Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation,
48 Vassileos Constantinou Avenue, Athens 11635, Greece
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Jozef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia
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Abstract
Using a molecular fluorine laser at 157 nm foxing stains were removed successfully from a 16th century old paper. Laser
cleaning of stains and foxing from old paper manuscripts is far more effective at 157 nm in comparison to different wavelengths
without leaving any yellowish after-effect on the paper. This is because at 157 nm illumination of old paper, complete bond
breaking of all the organic molecules of the paper is taking place. Mass spectroscopy at 157 nm and for moderate laser intensities
up to 1 mJ/cm2 of old paper suffering from foxing indicate organic matter disintegration to small photofragments atomic,
diatomic or triatomic, which are flying apart with supersonic speed. In addition high spatial resolution energy dispersive X-ray
system (EDXS) analysis over the effected areas indicate the presence of iron, suggesting that biological activity is taking place
preferentially in paper areas containing iron.
# 2002 Published by Elsevier Science B.V.
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Keywords: Energy dispersive X-ray system; Foxing stains; Mass spectroscopy
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1. Introduction
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The rusty red irregular shaped areas in old manuscripts dated from the 16th to the 19th century are known
as foxing. They may vary in size from just visible spots
to large areas covering most of the page. Foxing is a
serious problem because the stains might migrate
through successive pages and causing thus irreversible
damage of old paper and manuscripts. It was long
thought to have been the result of rust, from tiny
fragments of iron metal, which were worn off papermaking machinery. However it is now known that initial
cause of foxing was a group of conidia, which had been
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Corresponding author. Tel.: þ30-10-7273840;
fax: þ30-10-7273842.
E-mail address: [email protected] (E. Sarantopoulou).
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deposited on the surface of the paper and germinated in
situ [1]. Possible cause of foxing is the contamination by
the auto-oxidization of lipids from conidia. The color in
the foxed areas is due to an alkaline-soluble rusty red
material and an insoluble straw-colored stain in the
paper fibers. Therefore foxing could be spread all over
the paper volume, devaluating manuscript appearance
and destroying unique evidences of human history.
Preventing action therefore should be taken for foxing,
and besides treatment by conventional dry and yet
methods laser techniques have been applied before at
various wavelengths on paper parchment and paintings
[2–5]. Immediate and long-term effects on paper treatment using lasers were assessed by Kolar et al. [6], by
determining the degree of polymerization of cellulose.
They found a strong and immediate cellulose degradation after laser treatment at 308 nm while laser irradia-
0169-4332/02/$ – see front matter # 2002 Published by Elsevier Science B.V.
doi:10.1016/S0169-4332(02)01379-X
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2. Experimental
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The experimental apparatus consists mainly of the
F2 laser source and the vacuum chamber, where the
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quadrupole mass spectrometer and the Ptolemaic map
were placed. The map was placed 1 mm apart from the
quadrupole mass spectrometer (Baltzers QMG 311) at
right angle to its axis (Fig. 1). The all stainless steel
316 vacuum chamber was evacuated to 106 mbar
using a turbomolecular pump. The laser head at
157 nm delivers 10 1 mJ per pulse, and the pulse
duration was 12 ns at FWHM. The laser beam was
focused on the old paper map, using a quartz lens of
40 cm focal length. Following photodissociation of
the parent molecule, the molecular photofragments
were ionized using an electron gun inside an isopotential chamber (Welnet). The molecular ions were
focused with an Eizen lens, and eventually they were
directed alongside the quadrupole mass filter. The
upper detection limit of this filter was 300 amu.
The ions after entering in the mass filter, were
deflected at right angle, and they were detected using
a high gain 108, secondary electron multiplier
(SEM). The signal was then amplified and it was
registered using a boxcar integrator and a computer.
Regarding the photoablation experiments, the etch
depth at 157 nm on the manuscript’s paper, corresponding to a particular fluence, was measured at
room temperature in nitrogen buffer gas, by counting
the number of pulses required to penetrate the paper.
The energy of the laser pulse falling on the sample was
attenuated using lithium fluoride plates, and by placing them between the sample and the laser. The laser
operating frequency was kept below 1 Hz to avoid
cumulative heating effects on the sample. The laser
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tion at 1064 nm resulted in an increase of the degree of
polymerization due to the formation of inter and intramolecular ether bonds. In this communication we have
used the molecular fluorine laser at 157 nm to remove
foxing marks from a medieval Ptolemaic map (21 cm
30 cm) from the book of S. Münster, ‘‘Nova Graecia
secundum omnes eius regions et provincias citra ultra
et Hellespontum’’, published by H. Petri in 1585 A.D.
Foxing was successfully removed after few laser shots.
By applying laser ablation techniques the absorption
coefficients and the threshold energy fluences of the
paper of the manuscript were determined at 248 nm
besides 157 nm. The absorption coefficient at 157 nm
was found to be one order of magnitude higher than at
248 nm, suggesting complete photochemical dissociation of the paper fibers without any evidence of heating
of the illuminated area (yellowish color), and in agreement with previous results for various organic polymers
[7–10]. In order to investigate the basic photochemical
mechanism of the photodissociation dynamics of the
paper fibers under vacuum ultraviolet (VUV) irradiation, we apply mass spectroscopic techniques. Mass
spectroscopy reveals that even at moderate laser energy
there was a complete breaking of the molecular bonds.
There were no photofragments observed for m/e larger
than 30 amu (atomic mass units of m/e). Photofragments with two carbon atoms have a relatively higher
probability to be dissociated from the parent cellulose
molecule, than heavier photofragments with four carbon atoms. These experimental findings suggest that the
bound potential energy surfaces of the excited electronic states of the parent molecule correlate with the
dissociative potential energy surfaces of the excited
states of the molecular photofragments over a wide
energy range above 5 eV. Cellulose is disintegrated to
small photofragments atomic, diatomic or triatomic,
which are flying apart with supersonic speed. In addition
high spatial resolution energy dispersive X-ray system
(EDXS) analysis over the effected areas indicate the
presence of iron, suggesting that biological activity is
taking place preferentially in paper areas containing
iron.
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Fig. 1. Experimental set-up for recording the mass spectrum from
small pieces of the old paper, remains of conventional preservation.
LB, laser beam; L, lens; VC, vacuum chamber; TM, turbo
molecular pump; W, window; S, sample; P, photofragments; QMS,
quadrupole mass spectrometer.
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Fig. 2. When an organic molecule is illuminated with laser light at
157 nm (blue arrow) molecular disintegration is taking place.
Illumination at longer wavelentgths (red arrow) is usually
accompanied by photon emission and heating.
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3. Results and discussion
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3.1. Photo ablation processes
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In the case of light excitation, mainly two kind of
processes are induced on molecules: the first being
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pulse was monitored with a Tektronix 7104 fast
oscilloscope and a pyroelectric detector, which was
placed behind the sample. In addition high spatial
resolution EDXS analysis over the effected areas was
used to analyze foxing areas.
excitation followed by relaxation to the ground state,
and the second being excitation followed by molecular
disintegration (Fig. 2).
Regarding the first process, the electronic excitations
of the molecular specimens could relax by internal
conversion to thevibrational excitationsof themolecule.
Alternatively, UV excitation light could dissociate
the molecule. In this case, the laser energy is expended
in breaking the chemical bonds, and forming molecules
with smaller number of atoms. The excess energy is
converted to translational energy of the photofragments. The energy transfer in this case is considerably
faster than vibrational relaxation mentioned previously.
Investigation of dissociation dynamics at 157 nm could
be assisted from the absorption spectra providing thus
information on the excited electronic states of the
molecules. However in this case direct absorption using
a VUVabsorption spectrometer is impossible and hence
an alternative method has to be used. The ablative and
etch rate method fits well its results to the Beer’s law
and in the case of polymeric materials, the absorption
coefficient measurements are in excellent agreement
with direct absorption measurements using a VUV
absorption spectrometer [8]. The etch depth per pulse,
as a function of the incident laser fluence in the case of
the old paper is shown in Fig. 3 for 157 nm.
The dispersion of the experimental data at high
fluence is due to the fluctuations of the energy of the
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Fig. 3. Etch rate at 157 nm as a function of fluence.
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Table 1
Absorption coefficient a and threshold fluence FT at different
wavelengths
a (per cm) 104
FT (mJ/cm2)
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157 nm
1.2 0.4
40 8
4 0.5
12 3
16 3
0.3 0.1
laser beam. As it is generally accepted, the etch depth
per pulse, d, is given by the Beer’s law
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d ¼ a ln
FT
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3.2. Mass spectroscopy
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Mass spectra of paper pieces left out after conventional preservation, were recorded at 157 nm and with
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where a is the absorption coefficient (per cm) of the
material at a given wavelength, F the fluence of the
laser pulse and FT is the threshold fluence which
describes the minimum fluence of light where the
photodissociation process starts to be competitive to
the thermal relaxation processes, such as intersystem
crossing or collision quenching. These processes
merely degrade the energy without engendering any
photodissociation. The values of the threshold fluence
FT, and the absorption coefficient a at different wavelengths are tabulated in Table 1, following least square
fitting of the Beer’s law to the experimental results.
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193 nm
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248 nm
laser energy around 10 mJ and laser intensity 15 MW/
cm2 in order to produce significant etching of the
paper sample due to the fact that only few pieces of
paper were available. The background pressure of
2 107 mbar sets the detection limit for the minimum energy of the laser pulse for detecting photofragments at 0.1 mJ.
First, the mass spectrum with the old paper sample
inside the chamber, was recorded with the F2 laser off.
The mass spectrum in this case indicates that there is
not any significant paper degassing at the background
pressure of 106 mbar. The peaks at 14, 18, 28 and
32 amu, correspond to the presence of N, H2O, N2 and
O2, respectively. The mass spectrum of the old paper
sample under moderate laser intensity up to 15 MW/
cm2, is indicated in Fig. 4.
No fragments were observed for m/e higher than 32,
contrary to previous recorded mass spectra at 193 and
248 nm, suggesting that photodissociation of organic
molecules at 157 nm is one photon process [8]. Hence,
taking into consideration that the molecular weight of
the cellulose monomer is 178; we come to the conclusion that there is a complete bond breaking of
the polymeric paper fiber chain under irradiation at
157 nm. The photofragments are the products of either
direct dissociation of the parent molecule, or recombination of the various excited fragments. It is worthwhile to mention that under these experimental
conditions, no fragments with m/e higher than 32
were observed.
Fig. 4. Mass spectrum of old paper at 157 nm.
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Fig. 5. Mass spectrum of foxing areas at 157 nm.
Fig. 6. EDXS analysis over the effected areas, indicate the presence of iron.
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4. Conclusions
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Foxing marks have been efficiently removed from a
medieval Ptolemaic map edited in 1580 A.D. using a
molecular fluorine laser at 157 nm. The mass spectrum of foxing areas indicates the presence of excess
amount of H, OH and H2O, a fact which suggests that
the foxing areas consists of organic moieties of a
simpler chemical structure than cellulose. High spatial
resolution EDXS analysis over the effected areas,
indicate the presence of iron, suggesting that biological activity is taking place preferentially in paper
areas containing iron.
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References
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The mass spectrum of the foxing areas of the old
paper sample following illumination with laser light of
moderate intensity, up to 15 MW/cm2, is indicated in
Fig. 5. The mass spectrum indicates the presence of an
excess amount of H, OH and H2O, and the radical
group around 28 amu. From the mass spectrum it is
evident that the foxing areas consists of organic
moieties of simpler chemical structure than cellulose.
In addition high spatial resolution EDXS analysis
over the effected areas (Fig. 6) indicate the presence of
iron, suggesting that biological activity is taking place
preferentially in paper areas containing iron.
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