FTIR - Deconvolution Spectra of Paper Documents
by P. CALVINI & A. GORASSINI
INTRODUCTION
Infrared spectroscopy is widely applied to the analysis of cellulose and its derivatives1-3, owing to
their industrial importance in textile and papermaking technology.
With a few exceptions4-7, much less work has been published about the characterisation of ancient
paper documents, although this microdestructive technique seems to be very attractive in the field
of paper conservation science, particularly today, when the development of Fourier transform
spectrometers (FTTR) provide a reliable improvement of instrumental sensitivity. Moreover, digital
computing of FTTR data allows the deconvolution7 and subtraction of overlapping bands, resulting
in a more significant attribution of recorded peaks.
The intention of this paper is to show how the numerical elaboration of FTIR bands can shed light
on the composition and degradation of paper, of laboratory-made papers as well as of historic items.
EXPERIMENTAL
Composition and preparation of samples.
The standard paper was a Whatman no.l sample, chromatographic grade. It was
selected from a recent stock, and showed a very good FTlR spectrum, where only the cellulose
bands and the adsorbed water are present.
The laboratory-made sized papers (Table 1) were prepared with another sheet of Whatman no.l
paper, stored in the laboratory without particular precautions. This caused a faint oxidation,
revealed by the FTIR spectrum, so that the "untreated" and/or "unaged" specimens of this work
were, indeed, slightly oxidized. Details of samples preparation and properties as well as their FTIR
spectra have been published elsewhere6.
Accelerated ageing
Accelerated ageing was performed in 13 ml glass tubes at 90±10°C using 0.6 to 0.7 g. of the
samples. Tubes were sealed in ambient air with and without adding a drop of concentrated HCl in
order to create an acidic and a normal atmosphere respectively6. The contact between paper samples
and acid was avoided by introducing some small glass balls at the bottom of the tube. A rough
evaluation8 quotes the relative humidity inside the tube to be near to 70%. Ageing was performed
until a visible discoloration took place (see Table 1).
Degree of polymerisation (DPv)
The DPv was measured at 20.0°C, according to the French Standard AFNOR T12-005.
Colour changes
The brightness of the samples was determined by scanning a small area of samples (2.5x2.5 mm) at
1200 dpi resolution and 256 grey level. The image was averaged in order to obtain a single value of
grey, varying from 0 (black) to 255 (white).
FTIR analysis
A Perkin-Elmer 1710 FTIR was used to obtain a 4 cm-1 resolution spectra in the 500-4000 cm-1
region, scanned ten times (KBr pellets technique). In order to minimize the amount of adsorbed
water, both KBr and ground samples were heated at 100°C for at least 1 hour, and the halide pellets
were prepared using a warm evacuable die under vacuum.
All the calculations were made with the absorbance units, in order to obtain a semi-quantitative
analysis, the absorbance being proportional to the concentration through the well-known9 LambertBeer law. Lacking an internal reference, the absorbance of all the spectra was normalized between 0
(at the minimum of absorbance near 1850 cm-1) and 1 (peak at ~1430 cm-1). The latter peak, due to
CH2 symmetrical bending, is slightly modified by changes in the cellulose composition2. Since the
forced zero absorbance near 1850 cm-1 can bias the de-convolution procedure, a factor of 0.1 has
been added to the whole set of normalised absorbance data.
Digital deconvolution of spectra
Just a few years ago, when the computational performances of personal computers were far lower
than today, the second derivative technique4,5,10 allowed only a rough identification of overlapping
bands. Nowadays, more sophisticated software of non-linear curve fittings are commercially
available and overlapped peaks can be resolved by a sum of Gaussian (or Lorentzian) curves, which
gives a very good approximation of the original spectrum. The deconvolution procedure reported in
this paper follows the indications of Choisy et al. , e.g.:
1. A good fit is one for which the convergence has been reached with a low value of Chi square.
Rather than the value of Chi square, we have preferred to evaluate the difference between the
original spectrum and the sum of decon-voluted peaks, in order to verify if a straight line is
obtained, parallel to the abscissa and with a low random noise.
2. Among several solutions, the better fitting curve is the one which passes the first rule and which
has the least number of peaks.
In order to obtain a carefully deconvoluted spectrum, further rules ought to be added:
3. The centre of deconvoluted peaks should correspond to known IR assignments.
4. The bandwidth (e.g., the width at half height of deconvoluted peaks ought to have the same order
of magnitude when the peaks are found in samples of different origin or composition.
5. When these last two rules do not satisfy the second rule , this latter can be partly neglected, so
that more than the least number of peaks can be accepted. Nevertheless, the first rule must be
always fulfilled.
There is yet another problem, related to the lower and higher extremities of
the deconvolution range of the spectrum. Since we (or the computer) do not know
the FTIR absorbance beyond these limits there is the possibility of an error (the
so-called finite-size effect), due to the tails of external, unknown bands, which can
penetrate into the examined area. To minimize this effect, a sixth rule needs to be observed:
6. The range of frequency selected for deconvolution must fall between two
extremities where the sample shows a nearly zero IR absorbance. When these
zones are missing, only the central part of the deconvolution area is significant (where the tails of
side bands are negligible.
Fitting was performed by means of the Lorentzian curve:
where H is the peak height, w the full width at half height, x0 the centre of the peak and x the
frequency in cm-1 units).
Calculations were made with commercial software11 which allowed deconvolution by a non-linear
curve fitting method. Some simple macros were written in a spreadsheet (Microsoft Excel) to
perform the normalisation of the experimental IR data as well as the subtraction of the peaks
obtained from the original spectrum.
Some preliminary deconvolutions between the full range of FTIR frequencies (500 to 4000 cm-1) of
complex samples (modern discoloured papers from wood-pulp, rosin sized, with high lignin and
fillers content) showed that:
• higher frequencies (between 1900 and 4000 cm-1) only show a few broad bands due to OH
(3000 to 3770 cm-1), the deconvolution of which is rather difficult, and to CH2 and CH3 (2790 to
2990 cm-1). Sometimes other characteristic bands can be seen even without deconvolution, such as
carbonates (~2500, ~1800, ~1450 and ~870 cm-1), OH-containing fillers (3690 and 3600 cm-1) and
environmental CO2 (near 2300 cm-1);
• a number of significant differences appear in the region 1400-1900 cm-1; and this range will be
discussed later:
• the so called fingerprint region between 1000 and 1400 cm-1, where the C-OH and C—H
absorbance is strong2 masks other significant bands even in de-convolution (if there are any);
• lower frequencies, between 500 and 1000 cm-1 (mainly C-H and C-OH absorbance), are difficult
to deconvolute, because of the finite-size effect, but the absorbance of some functional groups can
be easily detected without deconvolution. Particularly interesting is a ~820 cm-1 absorbance, which
can vary from a faint shoulder to a small (but clearly visible) peak. This might be due to a substituted aromatic ring, to a substituted C=C aliphatic group or to a substituted furan. Lignin,
hemicelluloses and rosin show this peak, which appears sometimes in oxidised and aged cellulose.
Since most of the other functional groups crowd together between 1400 and 1900 cm-1, the
deconvolution has been restricted to this range. Because of rule 6., calculations were done between
1250 and 1900 cm-1, and the preliminary de-convolutions of modern papers previously depicted
showed that the tails of IR bands beyond these limits give a negligible contribution.
Table 1: Physical and chemical properties of laboratory-sized paper samples.
* Sized samples showed a high content of undissolve d residue, so that their degree of polymerisation was not determined.
** Gradient of colour change due to non-uniform degradation of the sample (that showed a darker
area near the source of HCl).
Although restricted in the range, deconvolution ought to be performed carefully. A suitable method
starts with a coarse fitting of the 1250-1900 cm-1 region. After that, the peak (or the peaks)
underlying the broad band at around 1630 cm-1 should be subtracted from the spectrum and the
remaining pattern carefully de-convoluted. After removing these latter bands from the original
spectrum, the fine deconvolution of the region around 1630 cm-1 can be done.
FTIR SPECTRA
Table 1 gives the main physical and chemical properties of the prepared samples. Tables 2 to 4
show the FTIR-deconvolution bands of the unsized and sized samples subjected to accelerated
ageing. When aged in an HCl atmosphere, all the samples showed characteristic acidic degradation
with a more resolved structure in the 1000-1200 cm-1 zone (fingerprint regionl when compared with
the unaged samples6. This feature can be attributed to the acidic attack of the amorphous regions,
since a similar behaviour is generally shown by the IR spectra of crystalline organic compounds
compared to their amorphous state.
Another band, often associated with acidic degradation, was an unassigned "tooth" centred at 3340
cm-1 in the OH region, and was possibly due to a strong intermolecular hydrogen bonding 4, 6.
Table 2: Deconvolution peaks of unsized paper in the range of 1400-1900 cm-1.
The samples aged in sealed tube with H+ show a 3340 tooth above the broad OH absorption.
Unsized paper
Table 2 shows the results of deconvolution applied to the standard and reference paper in the 14001900 cm-1 region.
Peaks 1 to 5 of Table 2 can be interpreted with the following assignments:
1 1429-1430 cm-1 is generally assigned to CH2 bending2 (C6 position), and often used for
quantitative measurements.
2 1457-1462 cm-1 corresponds to OH in-plane bending2. Together with the previous peak it is a
characteristic of all the samples examined, and does not change with accelerated ageing.
3 1523-1537 cm-1, unassigned, should be due to a deconvolution artefact, since it is shown also by
the standard paper and does not correspond to a known chemical group of cellulose. In fact, the
Lorentzian function might not pattern fully an IR band (nor on the other hand does the Gaussian
curve, used by other authors'). These interpolating curves appear to work fairly well where a
multiplet of bands, visible as peaks or shoulders, crowd together in a spectrum. Instead, when two
strong bands are separated by 100-200 cm-1, the Lorentzian or Gaussian curves might not follow
exactly their intensity in the lower part of the wings, so that a ghost peak is necessary to fit the
absorption spectrum.
4 1629-1638 cm-1: generally assigned to bound water2.
5 1658-1660 cm-1: a,(3 unsaturated carbonyl groups (C=C-C=O) vibrations show an IR absorbance
in this range12. The corresponding weak C=C stretching absorbs near 1640 cm-1, and should be
masked by the water band. Another kind of unsaturation can arise from a-diketones (enol tautomer
C=C(OH)-C=O), such as 1,2 cyclohexanedione, which shows as only a signal in this range13, owing
to being in nearly 100% enol form.
The following peaks 6 to 8 are due to C=O vibrations (CO, CHO, and COOH). The differentiation
between aldehydes, ketones and acids cannot be made easily, since conjugations and hydrogen
bonding cause a shift toward lower frequencies of the standard CO vibrations. Further chemical
treatments of oxidation and reduction can shed light on this problem9,14, and will be discussed in a
forthcoming paper. The anomalous behaviour of the COOH signal should be stressed, however.
This group absorbs at the normal position (~ 1705 cm-1) in rosin, as shown in Table 4, while it
shows a high-frequency shift (~ 1740 cm-1) in unsized paper samples. Studies on mono and
dicarboxycellulose confirm this finding, the origin of which is rather uncertain1 .
Putting aside the CO peaks assignation, some considerations can be drawn from Table 2 data.
Above 1600 cm-1, the standard paper showed only the water signal at 1638 cm1, while the reference
(unaged) paper also showed a faint trace of oxidation (1713 and 1745 cm-1). Some chemical
analyses done by other authors with similar samples confirm this finding, generally ascribed to an
iron- or copper-induced auto-oxidation15.
After ageing, a new FTIR signal appeared at ~ 1660 cm-1. and this finding requires a short
explanation. In the discussion of Table 2 data (peak 5) this band was attributed to conjugated
structures such as α-diketones or αβ-unsaturated carbonyl groups. Besides hydrolysis, the
degradation of cellulose samples is a balance between two other mechanisms: oxidation and internal
dehydration6,16, shown in Fig.l. In mild conditions of ageing the first mechanism gave rise to a C6
aldehyde group and/or to a C2-C3 α-diketone (Fig.la) . with or without hydrolysis of the C4-bonded
fragment of cellulose chain. In an acidic environment the second mechanism also produced some
C=C groups in the pvranose ring. This internal dehydration generally requires drastic conditions
ncentrated sulphuric acid and ~ 100 °C), but occurs in mild conditions with p-bydroxy carbonyls.
Because of the acidity, both hydrolysis and internal deydration occur, so that structures like those
of Fig.lc and e are highly probable.
Referring to the C2-C3 position, the a-diketone structures (Fig. la and d) are responsible for
yellowing, because of a forbidden n-π* electronic transition17 in the near-UV/Visible region at
340-440 nm. These structures show a FTIR absorbance at frequencies higher than 1700 cm-1.
Nevertheless, diketones are subjected
Fig.1: Conjugated groups in cellulose subjected to mild oxidation and internal dehydration. I)
oxidation; II) oxidation, hydrolysis and internal dehydration; a, b, c and d: a-diketones and their
enol tautomers; e: αβ-unsaturated carbonyl group.
to the keto-enol tautomery when a neighbour C-H is available (Fig.lb and c): in this case the
forbidden n-π* electronic transition shifts towards the near-UV region and the FTIR spectrum
shows a ~ 1660 cm-1 signal.
Further oxidation of the C2-C3 groups causes the breaking of the ring structure, so that many
conjugated unsaturations are lost. This reaction is the basis for the hypochlorite bleaching, as
well as for the sunlight bleaching (with a free radicals mechanism). Referring to the C6
oxidation, the internal dehydration gives rise to an αβ-unsaturated aldehyde (Fig.le), and a n-π*
transition is observed in the near-UV region, while the FTIR signal should be centred at ~ 1670
cm-1 (C=O), with a weak ~ 1640 cm-1 band (C=C) masked by the water absorption at the same
frequency.
Bearing in mind all these possibilities, it is easy to understand the difficulty of interpretation of the
FTIR spectra of yellow-brown samples of aged, unsized paper.
Referring to the Table 2 data, it appears that ageing in a "normal" atmosphere caused an increase of
both the unconjugated CO absorption at ~ 1713 cm-1 and the unconjugated COOH absorption at ~
1745 cm-1, as well as a strong signal at 1658 cm-1. This latter band ought to be due to the enol
tautomeric form of α-diketones (Fig.lb and c) or to an αβ-unstaurated aldehyde (Fig.le). The nearUV absorption of these structures, which can extend to visible when an auxochrome (such as an OH or -OR group) is present, can explain the dirty-white colour of the sample.
The sample aged in an acidic atmosphere showed only a few traces of conjugated C=C-CO
structures, the disappearance of the COOH signal (possibly due to decarboxylation) and a strong
doublet in the region of unconjugated ke-tones and aldehydes. On the whole, its FTIR spectrum
revealed those mechanisms that lead to dark coloured products. The DP dropped to a very low
value, and the more resolved structure in the 1000-1200 cm-1 zone, together with the tooth at ~3300
cm-1, indicated the destruction of almost all the amorphous region.
Gelatine sized paper
Table 3 shows the results of deconvolution applied to the gelatine sized paper in the 1400-1900 cm-1
region. Peaks 1 to 7 can be interpreted with the following assignments:
1 1429-1430 cm-1 and
2 1458-1461 cm-1 are the CH2 and OH frequencies already discussed.
3 1517-1529 cm-1 and
4 1546-1559 cm-1: are the amide II band (N-H bending) of the peptide bond12, 18 of gelatine.
5 1631-1644 cm-1: assigned to bound water, as already discussed.
6 1657 cm-1: as for unsized papgr, this band can be attributed to conjugated C=C-CO, arising from
oc-diketones (enol form) or αβ-unsaturated carbonyls.
7 1673-1681 cm-1: typical of peptidic CO (amide I band/ of gelatine12, 18. For analytical
purposes, it should be stressed that the amide II band is lower than
Table 3: Deconvolution peaks of gelatine sized reference paper in the range of 1400-1900
cm-1.
The samples aged in sealed tube with H show a 3340 tooth above the broad OH absorption *
brown-yellowish area of intermediate degradation ** dark-brown area
this correlated amide I, so that the first band might not appear in authentic items, degraded or poorly
sized with gelatine19.
The following peaks 8 to 10 are due to C=O vibrations (-CO, -CHO, and -COOH), as already seen.
Gelatine-sized paper behaved quite differently from the unsized samples. The unaged sample
already showed a strong 1736 cm-1 band of C=O vibrations, and after ageing in a normal
atmosphere, another band of oxidation appeared at 1723 cm-1. In an acidic atmosphere these two
bands joined together, with a high-frequency shift, in the brown-yellowish intermediate area of
degraded sample and fully disappeared in the very brown area. As far as the discoloration increases,
it appears that the amide II bands (N-H bending) decreased, while a new band of the C=C-CO
group appeared. A fragmentation of both the peptide bond and side aminoacids is highly probable,
with the formation of volatile amines and acetaldehyde.
On the whole, the discoloration mechanism involves the reaction between cellulose and amino
groups, that is known to produce various red-brown and dark brown compounds (Maillard
reaction)20.
Table 4: Deconvolution peaks of alum-rosin sized reference paper in the range of 1400-1900
cm-1.
The sized unaged samples show a 820 low shoulder
The sized and sealed tube aged samples show a 820 shoulder above the broad OH absorption
The sized samples which were aged in a sealed tube with H+ show a 3340 tooth above the broad
OH absorption and a very small, but clearly visible 820 peak
* The 1696-1710 cm-1. signal of rosin-sized samples is due to the COOH absorption band of rosin,
while carboxyl group in cellulose (Table 2, peak 8) absorbs at higher frequencies1.
Rosin/alum sized paper
Table 4 shows the results of deconvolution applied to the rosin/alum sized paper in the 1400-1900
cm-1 region. Peaks 1 to 6 can be interpreted with the following assignments:
1 1429-1430 cm-1 and
2 1457-1462 cm-1 are the CH2 and OH frequencies already discussed,
3 1563-1581 cm-1 and
4 1606-1612 cm-1: a doublet in this range is typical of C=C stretching12 in conjugated structures
included in the ring systems of rosin.
5 1631-1637 cm-1: assigned to bound water2 as already seen.
6 1651-1662 cm-1: αβ unsaturated carbonyl groups (C=C-C=O) already discussed.
Fig.2: Abietic acid in rosin (a) and its oxidation product (b).
The following peaks 7 to 10 are due to C=O vibrations (-CO, -CHO, and -COOH).
The rosin/alum-sized samples appeared more resistant towards thermal oxidation. The COOH
vibration of abietic acid at 1703-1710 cm-1 (Fig. 2a and notes to Table 4) remained quite unaltered,
both in the unaged and sealed-tube aged (normal atmosphere) specimens. Similarly, the doublet of
conjugated C=C did not change (Table 4 peaks 3 and 4), nor the absorbance of the C=C-CO group
(peak 6). The latter band is due to the already partially oxidised rosin (commercial sample). The
faint traces of C=O and COOH of the reference paper merge together (possibly owing to the
preparation of sized sample), but did not increase substantially.
When aged in an acidic atmosphere, a further dehydration and oxidation occurred, with a partial
aromatization of the cycloalyphatic groups. The resulting 7-oxo dehydroabietic acid (Fig. 2b) was
responsible both for the shift toward lower frequencies of the CO bands and the behaviour of the
C=C doublet, that merged itself into a single band (Table 4 peaks 3 and 4). The increase of the 820
cm-1 band (from a very low shoulder to a small peak) also indicated the increasing aromatization of
the rosin/alum size.
FTIR SPECTRA OF SOME AUTHENTIC ITEMS
Fig. 3 shows the infrared spectra of an oriental paper, the paper of a book printed 1620, discoloured
and poorly sized, and an archive paper belonging to the 1930s, slightly discoloured but in good
conditions.
All these items showed some unconjugated carbonyl groups in the 1699-1736 cm-1 region, but
none of them appeared to be strongly degraded by acidity, since the ~3300 cm-1 tooth was
absent.
Fig.3: Infrared spectra of the three historic papers.
Table 5a: Deconvolution peaks of the oriental paper sample ( Fig. 3) in the range of 1400-1900
cm-1.
Fig. 4: Infrared spectra of modern papers, partly aged according to different methods
The oriental paper contained a large amount of CaCO3, as shown21 by the bands at 1497 and 1800
cm-1, confirmed also by the peaks at 2500 and 876 cm1, not reported in Table 5. This sample did not
show the peaks of gelatine or rosin nor of hemicelluloses, so that the most probable sizing material
is starch, which shows the same FTIR bands of cellulose and cannot be identified with this technique. The 1667 cm-1 band can be attributed to conjugated CO αβ-unsaturated or in enol
tautomery), since the parent peaks of gelatine were absent.
The 1620s paper seemed to be slightly sized with gelatine, as shown by the 1520, 1548 and 1669
cm-1 bands.
The archive paper was obviously rosin-sized, as shown by the 1567-1596 cm-1 doublet and the 1652
cm-1 signal. Without deconvolution, it is rather easy to identify in this sample two small, sharp
peaks at 3690 and 3600 cm-1 (the latter partly hidden by the broad OH absorption at nearly 3350
cm-1), due to OH-containing fillers (Fig.3).
THE LIGNIN-CONTAINING PAPERS
In order to evaluate the influence of lignin on the FTIR spectra, a modern newspaper was
subjected both to sunlight yellowing and to artificial dark ageing (45 days, 90°C and 70% R.H.).
The results were compared with an authentic item, a book from 1960, where some pages are
very brown, while other consecutive pages appear white and in good condition. Fig.4 and Table
6 report the results.
Table 6: Deconvolution peaks of the Fig.4 samples in the range of 1400-1900cm-1.
* All samples, except the white page of the book from 1960, show a very small but clearly visible
820 peak
Lignin in paper is characterised by its aromatic structure, which can be easily seen with the FTIR
analysis. It is known12 that the aromatic ing absorbs at ~ 1510 and ~ 1605 cm-1. Among these bands,
the signal at 1510 cm-1 is a clear marker of the presence of lignin in paper3. When an aromatic ring
is conjugated with another unsaturated group, a third band appears at ~1580 cm-1. From Table 6
data it appeared that the further conjugated unsaturated group was a carbonyl, which
absorb at ~ 1660 cm-1. The rosin bands were masked by the more complex spectrum of lignin, so
that the presence of rosin size could not inferred from the FTIR analysis.
The sunlight degradation that arises from a free radical mechanism, causes a
decrease of the unconjugated aromatic bands, an increase oft the conjugated
bands and higher signals in the range of unconjugated CO. It appears then that
yellowing is mainly caused by non-enolizable diketones as well as by double
bonds and CO groups conjugated with the aryl residue22, 23. The increase of the
~ 1590 cm"1 signal and the enlarged "water" band at 1624 cm-1 agreeed with the
formation of lignin-derived degradation products (vanillin and vanillic acid),
found in the light treatment of wood24. Conversely, the oven aged brown sample
showed a decrease of the unconjugated CO groups and an increase of the aryl-
conjugated C=C groups, thus confirming that the brown colour arises mainly from dehydration.
Referring to the book of 1960, the brown page showed a decrease of un-conjugated CO groups and
the presence of a small peak near 820 cm-1, while the 3690 cm-1 peak was absent. It appears then
that the brown colour of this sample is mainly due to an internal dehydration and perhaps to
decarboxylation rather than to an oxidation mechanism.
The white page showed the presence of OH containing fillers (3690 cm-1 small peak), that are
absent in the brown item. Since the signal at 3690 cm-1 remained in the newspaper samples, which
were both light and heat degraded, it appears that the OH containing fillers stabilize paper toward
dehydration.
CONCLUSIONS
In a paper published elsewhere6 it has been shown, by thermal analyses, that two main mechanisms
- oxidation and internal dehydration - are responsible for the yellowing and browning of paper
samples. The deconvolution of FTIR spectra allows for a rough differentiation between these
mechanisms, although they generally act together.
Referring to the unsized paper, most of the previous work focused on the increase of unconjugated
CO groups, because at first sight only the region above 1700 cm-1 shows some distinct changes as
discoloration proceeds. Nevertheless, unconjugated COs alone do no cause discoloration, although
they are the necessary chromophores22. The simplest chromogenic group with FTIR signals above
1700 cm-1 is the ot-diketone system, which absorbs in the blue region of the electromagnetic
spectrum17 and is therefore responsible for yellowing, provided that tautomeric enolization is
hindered. Other conjugated-CO systems, such as β-diketones, β-ketoaldehydes and αβ-unsaturated
carbonyls cannot be seen easily in the FTIR spectrum, except with the deconvolution process
depicted in this paper. They generally absorb in the near-UV region, so that their contribution to the
yellowing of paper is relatively small.
In the presence of metallic impurities (Fe+++), both enolizable α-diketones and β-diketones give
rise to a coordination complex, the dark-red colour of which might account for foxing stains of
chemical origin. In these cases, the CO signal should shift to the 1525-1610 cm-1 region,
depending on the stability of the metallic chelate12. The dehydration mechanism gives rise to
isolated C=C groups, which can be barely seen in the FTIR spectra even with deconvolution,
and to conjugated cyclo-alkenes. These groups hardly contribute to discoloration, with the
exception of extended conjugations in very large systems (lignin) or in condensed rings
(rosin)22.
Nevertheless, the dehydration mechanism occurring in an acid environment and in the presence of
oxidized groups subjects the pyranose ring of cellulose to a complex path of reactions, which
combine internal dehydration with decarboxyla-tion, internal transpositions and C-C cleavage,
finally resulting in a carbonaceous structure.
In sized paper, the discoloration path can be different It has been shown by thermal analysis6 that
gelatine, gelatine-alum and rosin-alum sizing decrease the stability of paper samples toward thermal
oxidation and dehydration. In fact, it appears from FTIR deconvolution that both gelatine and rosin
mainly degrade themselves, perhaps preserving the underlying cellulose fibre25. Generally, yellowbrown compounds appear in sized paper samples after a less prolonged ageing than in unsized
samples.
Woodpulp papers, with a high lignin content, can be easily detected by FTIR, and the deconvolution
of their spectra shows that the discoloration path can follow different mechanisms, depending on
the degrading agent light or acidity).
ACKNOWLEDGEMENTS
This work has been supported by the Italian C. N.R., "Progetto Finalizzato Beni
Culturali 1997-2001" - Operative Unity Antonio Zappala.
SUMMARIES
FTIR - Deconvolution Spectra of Paper Documents
Infrared spectra of paper samples often show many overlapped bands in the most significant region
of 1400-1900 cm"1. The digital deconvolution reveals the underlying information, as shown by
some examples of both authentic items and reference samples, appositely preparated.
The IR absorbance of oxidized groups (both unconjugated) in cellulose, as
well as gelatine, rosin and lignin can be detected with the deconvolution technique even in the
presence of the broad band at -1630 cm"1 due to adsorbed water. The deconvolution technique
allows a rough identification of the mechanism of degradation which lead to yellowing and
browning of the paper samples.
Deconvolution des spectres FTIR de differents documents en papier
Les spectres infrarouges des echantillons de papier montrent souvent de nombreuses bandes qui
se chevauchent de facon tres significative aux alentours de 1400-1900 cm-1 La deconvolution
numerique revele les informations sous-jacentes comme l΄ont demontre certains exemples de
de-
convolution effectuee sur des echantillons anciens de papier original ainsi que sur des echantil-lons
de papier specialement prepares.
Grace a la technique de deconvolution numerique il est possible de detecter l'absorbtion
d'infrarouge par les groupes oxydes (conjugues ou non conjugues) dans la molecule de cellulose,
ainsi que dans la gelatine, la colophane et la lignine et ceci meme en presence de larges bandes
provoquees par absorbtion d'eau a -1630 cm-1.
La technique de deconvolution permet d'identifier approximativement le mecanisme de degradation
qui entraine le jaunissement et le brunissement des echantillons de papier.
Entflechtung von FTIR - Spektren verschiedener Papiere
Die Infrarot-Spektren von Papieren zeigen häufig zahlreiche überlappende Banden, gerade im
aussagekraftigsten Bereich bei 1400-1900 cm-1 . Eine rechnerische Entflechtung kann die darin
enthaltenen Informationen enthüllen, wie an einigen Beispielen, sowohl entsprechend hergestell-ten
Laborblättern als auch originalen historischen Papieren gezeigt wird.
Die Absorption von Infrarot durch konjugierte oder nicht konjugierte oxidierte Gruppen im
Cellulosemolekül, in Gelatine, Harz, Lignin können mit der Entflechtungstechnik auch beim
Vorhandensein der von absorbiertem Wasser verursachten breiten Banden bei ~1630 cm-1 er-kannt
werden. Die Entflechtungstechnik ermöglicht eine annahernde Identifizierung der Abbaumechanismen, die zum Vergilben und Verbräunen von Papier führen.
REFERENCES
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Paolo Calvini*
Ministero per i Beni e le Attivita Culturali
Soprintendenza per i Beni Architettonici ed il Paesaggio
ViaBalbi 10
16126 Genova-Italy
E-mail paolo.calvini@tin.it
Andrea Gorassini
Laboratorio di Restauro del Libro, with C.N.R and University
Dipartimento di Storia e Tutela dei Beni Culturali, Universita <k
Via Antonini 8
33100 Udine- Italy
* Author to whom correspondence should addressed.