The Degrading Action of Iron and Copper on Paper
A FTIR-Deconvolution Analysis
by P. CALVINI & A. GORASSINI
INTRODUCTION
It is well known that iron and copper salts cause damage to paper, depolymeriz-ing cellulose chains
and increasing discoloration of paper containing cellulose1-5. Generally, depolymerization is
ascribed to the acidity, and colour formation depends on the oxidation and dehydration of the
glucopyranose ring.
While the degradation of unoxidized paper can be monitored by the decrease of intrinsic viscosity6 '
in Cupriethylendiamine (CUEN), the presence of aldehydes or ketones in oxidized paper causes an
anomalous decay of intrinsic viscosity, because the strong alkalinity of the CUEN solution causes a
p-alkoxy fragmentation8. The determination of carbonyl and carboxyl groups is therefore of primary
importance in the field of paper conservation science. The chemical determination of CO groups is
generally performed by the hydrazine9 or triphenylformazan8 methods, while the amount of COOH
groups can be determined by means of the Methylene Blue procedure10.
A quick and easy method of determining both the hydrolytic scissions and the amount of carbonyls
along the chains, recently proposed in this Journal8, relies on the viscometric analysis of oxidized
paper before and after reduction of the CO groups, to restore the initial alcoholic function. These
analyses detect the overall amount of oxidized groups, but do not tell us anything of their positions
in the pyranose ring or of the presence of conjugated structures, responsible of discoloration11- 12.
Some efforts have been made to detect, at least qualitatively, the oxidized functions by means of
Infrared Spectroscopy13' 14, but results are generally unsatisfactory. The main problem is that
aldehydes show a reduced IR absorbance in the normal position of CO vibration (>1700 cm"1),
possibly owing to their hy-dration to gemdiols or to the formation of other structures11' !(i. COOH
groups generally absorb at ~ 1740 cm"1, but an alkaline pH shifts the absorbance of car-boxylates10'
!/
to ~ 1600 cm"1. Even if the pH is accurately buffered to a low acidic
The Degrading Action of Iron and Copper on Paper
range, so that COOH groups absorb in the normal region, and the sample is heated enough to show
the CHO absorbance, IR analysis reveals, at first sight, only the unconjugated groups, which are
rarely responsible for discoloration. The conjugation of these carbonyl groups causes a lowfrequency shift, in a region of the spectrum where the broad IR band of the adsorbed water
generally masks the underlying information12' 18' 19.
It was shown in a previous paper12 that the FTIR-deconvolution technique can detect some of these
conjugated structures. We present here the results of this technique applied to the analysis of the
iron- and copper-induced degradation of paper.
EXPERIMENTAL
Preparation of samples
0.1M aqueous solutions of analytical grade FeCl3 and CuCl2 respectively, were
added to 2 g of Whatman filter paper no.l (chromatographic grade), in a volume (1 ml) sufficient to
wet the paper samples completely. Application was done slowly with the aid of a hair dryer on a
warm setting, so that the solids remained deposited in the paper and the water evaporated. This
procedure resulted in an average of 1 metal ion for every 124 glucose units.
Ageing
Having been conditioned for 7 days at 23°C and 50% RH untreated and treated samples (0.6 to 0.7
g) were aged in 25 ml sealed glass tubes at 90±10°C.. According to a rough evaluation20, relative
humidity inside the tube was ca. 60%. Ageing was performed until discoloration became visible
(Table 1).
Measurements
Degree of polymerization (DPv) was measured at 20.°C, according to the French Standard AFNOR
T12-005.
pH was measured according to TAPPI standard method T435m-52 (cold extraction).
For infrared analysis a Perkin-Elmer 1710 FTIR was used to obtain 4 cm ! resolution spectra
(absorbance mode) in the 4200-450 cm ~' region, scanned ten times.
Table 1: Properties of the samples before and after sealed tube ageing
* No colour change caused by subsequent alkaline washing (pH 10)
Details about sample preparation and the deconvolution procedure have been published elsewhere12.
Comparison of absorbance data was performed12 by means of a sum of peaks shaped as Lorentzian
curves (Y):
where H is the peak height in absorbance units, w the full width at half height, xo the centre of the
peak and x the frequency; the unit of w, xo and x are cm-1 units.
CHARACTERISTICS OF THE SAMPLES
Table 1 gives some chemical and physical data of unaged and aged samples. From DPv of the
unaged Fe(III) and Cu(II) treated samples it can be seen that these metal ions promoted an
immediate decrease of the cellulose chain length. The initial degradation of Fe(III) was higher than
that of Cu(II), as already found by other authors1. After ageing, both Fe(III) and Cu(II) treated
samples showed a very strong depolymerization and little pH variation. All the aged samples
showed a more or less pronounced brown discoloration, which does not disappeared after alkaline
washing (pH 10).
INFRARED ANALYSIS
For a better understanding of the deconvolution bands in the 1600-1700 cm-1 region, Table 2 gives
the FTIR bands of some model compounds21, whose structural formula are shown in Fig.l.
The FTIR absorption at ~ 1670 cm"' of the cx-diketone in Fig.la indicates that this compound is
fully enolized, since un-enolized diketones absorb in the normal position1' of CO groups (>1700
cm"1). Similarly, ap-unsaturated groups (Figs.lb
The Degrading Action of Iron and Copper on Paper
Table 2: FTIR bands of some conjugated carbonyl compounds in the range of 1600-1700 cm '
Fig.1. Structural formula of some models of oxidized groups in cellulose, a) 1,2-Cyclohexanedione
(enol form); b) 1-Acetyl-1-cyclohexene; c) Shikimic acid; d) keto-enol tautomery and chelate
resonance form of 2-Acetyl-cyclohexanone.
and Ic) absorb in the 1670-1680 cm1 region, with a weak band at -1640 cm1, generally masked by
adsorbed water in oxidized cellulose. A particular ab-sorbance in the 1600-1620 cm ' region is
shown by p-diketones when their steri-cal conformation allows for a resonance-stabilised hydrogen
bond in the enol structure (chelate form) as in Fig.Id.
From the FTIR-deconvolution data of metal-treated cellulose it appears that copper and iron have
different mechanisms of degradation; so their properties will be discussed separately, together with
the behaviour of the reference samples.
Table 3: FTIR-deconvolution bands of Whatman no.l reference paper

For peak identification see text and Ref.12.
Fig. 2: FTIR absorbance of unaged (a) and sealed tube aged (b) Whatman no.1 reference paper in
the range of 1250 to 1850 cm-1.
The degradation of reference samples
Fig. 2 shows the FTIR spectra of unaged and aged reference samples, Table 3 the results of
deconvolution.
The unaged cellulose was the same as used for the experiment reported in our previous paper12; it
shows a faint trace of oxidation due to its storage without particular precautions in the laboratory.
15 days ageing caused a decrease of DPv, with a clear increase of unconjugated CO groups, and
some unconjugated COOH. A rather strong deconvolution band at 1658 cm"1 revealed the
formation of a-diketones (enol tautomer structure, as in Fig.la) or of some ap-unsaturated car-bonyls
(aldehyde or carboxyl groups as in Fig.lc). A clear cut association to the 1658 cm ! band to these
groups cannot be done without further chemical analyses.
The Degrading Action of Iron and Copper on Paper
Fig. 3: Accelerated ageing of cellulose paper (sealed tube technique). Oxycelluloses a, c and e
account for the increase of unconjugated CO groups (1716 and 1742 cm-1); oxycelluloses b, d and f
account for the formation of the 1658 cm-1 signal. Structures a and b are in the keto and enol form
respectively.
Taking into account the FTIR-deconvolution data, a reasonable model for the accelerated ageing of
the reference paper involves (Fig. 3)
• some oxidation at C2 and C3 position, as well as a C6 oxidation to CHO and COOH;
• a slow acidic hydrolysis with splitting of the C4-bonded cellulose side chain (which accounts for
the decrease of DPv) and formation of a C4-OH group;
• dehydration at the C4-C5 position, with formation of a double bond; together with the second
mechanism, this step leads to the formation of an α-un-saturated aldehyde or acid, like that
illustrated in Fig.3 d and f.
The first mechanism accounts for the oxidation bands at 1716 and 1742 cm"1, and the second and
the third for the FTIR-deconvolution band at -1658 cm"1.
Table 4: FTIR-deconvolution bands of the iron-treated sample
For peak identification see text and Ref. 12.
tooth = small and sharp signal above the large OH absorption; mrs= more resolved structure (both
characteristic of acidic degradation22; spk = small, but clearly visible peak, characteristic of C=C
double bond12.
Fig. 4: FTIR absorbance of unaged (a) and aged (b) Fe-treated paper in the range of 1250 to 1850
cm-1. Curve c was obtained after pH10 washing of sample (b).
The degradation of iron-treated sample
Fig.4 shows the FTIR spectra of unaged and aged iron-treated samples, and Table 4 the results of
deconvolution. The FTIR-deconvolution spectra of this sample
The Degrading Action of Iron and Copper on Paper
strongly follows a similar pattern to that of the accelerated degradation of the reference paper, with
an important difference: the degradation processes that require 15 days for the reference sample
occur after 1 day or even less with the Fe-treated samples. The DPv of the unaged sample is
approximately three times lower than that of the unaged reference, although the FTIR oxidation
bands are largely the same. This means that the initial reduction of DPv is due to the acidity of the
iron solution rather than to a (3-alcoxy fragmentation in CUEN. After ageing, the 1660 cm' band
reveals the formation of some α conjugations, while unconjugated CO groups at >1700 cm ' do not
increase substantially.
The full FTIR spectrum of the aged sample (not given in Fig.4) showed a more resolved structure in
the 1000-1200 cm~l range, which indicated, together with a "tooth" at 3340 cm"1, a strong acidic
degradation12- 22. A small, but clearly visible peak at 800 cm"1 revealed some kind of unsaturated
C=C group12.
The colour changes from yellowish to brown during ageing and is not reversed after alkaline
washing. This means that the discoloration was not due to low-molecular weight compounds, if
there were any. It might have been due to an iron-cellulose complex, as the deconvolution"'band in
the 1533-1576 cm~l region, which did not disappear during ageing and alkaline washing, seems to
indicate17.
It maybe mentioned that alkaline washing caused a partial decrease of the unconjugated COOH
band, and the height of the conjugated CO band at - 1660 cm4 was reduced to approximately one
third of the original value. The formation of carboxylate ion along the cellulose chains was
responsible both for these band reductions and for the new band1' at 1594 cm"1. A C6 oxidation
(partly to CHO and partly to COOH) is then highly probable.
The degradation of copper-treated sample
Fig. 5 shows the FTIR spectra of unaged and aged copper-treated samples, and Table 5 the results
of deconvolution. After ageing the FTIR-deconvolution spectrum showed an increase of the COOH
groups with a small, low-frequency shift: from 1745 cm~l of the reference paper to 1734 cm"1; this
was possibly due to intramolecular hydrogen-bonding. Another band at 1622 cm"1 suggested the
presence of a fi-conjugated diketone (enol tautomer) stabilised by internal resonance1', like that of
Fig.Id.
The full FTIR spectrum of the aged sample (not shown in Fig.5) appeared more resolved in the
1000-1200 cm ; range, which indicated, together with a "tooth" at 3340 cm"1, an acidic
degradation22. Nevertheless, the C=C signal at -800 cm"1 was absent. As in the case of iron, a signal
in the 1541-1547 cm"1 region seemed to indicate a copper-cellulose complex, responsible for
discolora-
Table 5: FTIR-deconvolution bands of the copper-treated sample
For peak identification see text and Ref. 12.
tooth = small and sharp signal above the large OH absorption; mrs= more resolved structure; both
characteristic of acidic degradation22.
Fig. 5: FTIR absorbance of imaged (a) and aged (b) Cu-treated paper in the range of 1250 to 1850
cm-1. Curve (c) was obtained after pH10 washing of sample (b)
tion1'. This brown discoloration did not change with alkaline washing, so that low-molecular-weight
compounds seemed to be absent. Nevertheless, the pH 10 washing gave rise to the carboxylate form
of COOH, inferred by the presence of the 1601 cm"1 band, with partial and full disappearance of the
1734 and 1622 cm' bands respectively.
The Degrading Action of Iron and Copper on Paper
Fig. 6: FTIR absorbance of Fe (a) and Cu (b) treated paper in the range of 1250 to 1850 cm-1. Both
samples have been treated with a concentration of metals ten times higher than those of Figs.4 and
5, and aged 1 day at 90°C (sealed tube technique).
THE MECHANISM OF IRON AND COPPER DEGRADATION OF PAPER
In a paper published previously in this journal, related to the dry ageing of iron-and copper-treated
paper at 80°C and 65% RH, M. Bicchieri & S. Pepa1 monitored the DPv and the formation of
COOH groups at these conditions. In that experiment both the deionised paper and the iron(III)treated sample showed an increase of COOH after ageing, with an S-shaped kinetic, while the
copper(II)-treated sample showed an exponential-like kinetic. In order to evaluate the rate of
formation of COOH groups, the authors interpolated their experimental data with an arbitrarily
chosen three-parameters non-linear function, which is a good empirical method, but does not clarify
the underlying chemical mechanism.
Bearing in mind that the formation of carboxyl groups in cellulose is a two-stage consecutive
process (C-OH • CHO • COOH), we can define two constants: k1, i.e. formation of aldehyde groups
from -OH or from >CO, and k2, i.e. formation of COOH. The actual shape of the kinetic plot
depends on the relative value of ki and k2. The fully-developed kinetic law is shown in the
Appendix. By means of a non-linear curve fitting procedure, also illustrated in the Appendix, the
actual values of ki and ka can be obtained, as shown in Table 6.
Referring both to the deionised and iron-treated data, the rate of carbonyl formation approximately
equalled the rate of carboxyl production, so that it is likely that only the C6 position of cellulose
was involved in the process.
Referring to the copper-treated data, ki was lower than k2, so that C2 and C3 oxidation is highly
probable, where the opening of the pyranose ring, with formation of CHO groups, is the ratedetermining step.
Table 6: Non-linear fit of Bicchieri & Pepa1 COOH data
k1 rate constant of CHO formation
k2 rate constant of COOH formation
For the meaning of Z, M and N parameters see Eq. 1 in the Appendix.
A further confirmation of the different mechanism of iron and copper degradation is shown in Fig.
6, where the concentration of metal ions deposited on the fibres was ten time higher and the treated
samples were subjected to 1 day's ageing at 90°C using the sealed tube technique.
Both iron- and copper-treated items showed a broad, strong band in the 1500-1750 cm' region, thus
revealing a large amount of oxidation and conjugations. Nevertheless, the iron-treated sample (Fig.
6a) showed higher signals in the region of conjugations (1500 to 1700 cm"1), while the coppertreated sample (Fig. 6b) showed a higher signal in the region of unconjugated CO groups (>1700
cm~1). Since the fragmentation of the pyranose ring reduced the possibilities of conjugation, it
appeared that iron gave rise mainly to a C6 oxidation, while copper preferred to attack at the C2 and
C3 positions, opening the glucose monomers of cellulose, in accordance with the Bicchieri & Pepa1
kinetic data of COOH formation.
CONCLUSIONS
The FTIR-deconvolution analysis shown in this paper highlights the different pathway of iron and
copper degradation.
Iron(III) treatment in an acidic medium caused, after artificial ageing, the following reactions (Fig.
7):
• Strong hydrolysis of the cellulose chains induced by acidity, with a random attack (decrease of
the DPv and absence of low-molecular-weight compounds);
• dehydration of the C4-C5 bond, with formation of a C=C group (small peak at 800 cm1);
• oxidation of the C6 hydroxyl to CHO and then to COOH (Fig. 7a,b). Together with the second
step, this mechanism resulted in the formation of α-
The Degrading Action of Iron and Copper on Paper
Fig.7. Accelerated ageing (sealed tube technique).of iron-treated paper. Oxycellulose a accounts for
the relatively high direct hydrolysis and, together with oxycellulose b, for the formation of the 1660
cm-1 signal; oxycellulose c causes the -alcoxy fragmentation in CUEN found in Ref.1
unsaturated aldehydes C=C-CHO or acids C=C-COOH, inferred by the presence of the 1662 cm'
band. As indicated by the FTIR-deconvolution analysis of the pH 10 washed sample and by the ki
and ka values of Table 6, both unsaturated aldehydes and acids should have been present at the
same time, and only the latter were ionised by alkalinity, with a low-frequency shift to 1594 cm-1;
• oxidation of the C6 hydroxyl without hydrolysis (Fig. 7,11).
Copper(II) treatment in an acidic medium caused, after accelerated ageing, the following reactions,
where oxidation prevails over hydrolysis, as illustrated in Fig.8:
• oxidation to aldehyde of the pyranose ring, with breaking of the C2-C3 bond (although this was
the rate-determining step, the ki value was higher than that of the iron-treated sample, indicating a
stronger oxidising power of copper), quickly followed by the formation of COOH groups (Fig. 8,1);
• random hydrolysis of the cellulose chains (induced by acidity), with an immediate oxidation of
the new OH group in the C4 position;
• oxidation to COOH in the C6 position. Together with the previous step, this mechanism
accounted for the 1622 cm • band by the formation of a -keto-
acid, enol form C(OH)=C-COOH (Fig.8,11), the internal resonance of which disappeared with
alkaline washing, when the carboxyl group became a car-boxylate.
The relative importance of mechanisms I and II illustrated in Figs. 7 and 8 cannot be inferred from
FTIR analysis alone, since the relationship between the actual concentration and the infrared
absorbance of CO, CHO and COOH is unknown. Moreover, the absence of a well defined signal of
unconjugated aldehydes11' I6 makes it difficult to detect such an important group, therefore, further
studies are in progress to better determine the characteristics of oxidized groups.
ACKNOWLEDGEMENTS
This work has been supported by the Italian C.N.R., "Progetto Finalizzato Beni Culturali" Operative Unity Antonio Zappala.
APPENDIX
The formation of COOH groups in cellulose can be seen as a multistep process:
The Degrading Action of Iron and Copper on Paper
where A indicates the OH groups of cellulose, B the aldehyde and C the final carboxyl content.
The rate equations are given by
where ki and lea are the rate constants of steps 1) and 2) respectively.
The solution of this set of differential equations can be found making the Laplace transform23 of all
the involved variables:
where ao, bo and co are the initial concentrations of A, B and C. A, B and C are the Laplace
transforms of A, B and C; s is a complex variable (s = a+i(i).
With the aid of Laplace transforms, our system of differential equations becomes a system of
algebraic equations, which can be easily solved in the complex domain, giving:
from which we obtain after some algebra:
The inverse Laplace transform of A, B and C gives the kinetic laws of A, B, and C respectively.
Since we are interested in the formation of carboxyl groups C, we obtain:
When ki = ka an S-shaped kinetic plot of C vs. t is obtained, and the maximum rate of COOH
production (dC/dt = max) is peaked near t =• 1/ki = l/fcz.
For ki > ka (or ki < ka) the kinetics is dominated by the rate-determining step (lower value of k),
and an exponential-like plot is obtained, similar to first-order kinetics. In what follows, we obtained
the kinetic constants from Bicchieri & Pepa data on the artificial ageing of paper1 by means of a
non-linear curve-fitting procedure, starting from a reasonable guess of the unknown parameters of
Eq.l. The initial values have been processed by means of the iterative Davidon's algorithm24' 25, and
a MathCAD program26 has been written to obtain the values shown in Table 6.
SUMMARY
The Degrading Action of Iron and Copper on Paper: a FTIR-deconvolution Analysis
Samples of FeCb and CuCla treated cellulose paper were analysed by FTIR before and after accelerated ageing at 90±10°C (sealed tube technique). The main mechanism of Fe(III) degradation
appeared to be a strong hydrolysis and dehydration of cellulose chains, with formation of a C4-C5
double bond, and C6 oxidation. Cu(II) oxidized the C2- and C3-OH groups to aldehydes and
carboxyls, thus opening the pyranose ring, and hydrolyses and oxidized the C4-OR group with the
formation of a C4-C6 (5-ketoaldehyde or (3-ketoacid. Among the FTIR-deconvolution bands in the
1250-1850 cnv1 region, the -1660 cm ' signal is attributed to the unsaturated C=C-CO, and the
1612-1622 cnv1 signal to CO-C-CO group (chelate form of the enol tautomer). These bands, which
can only be detected by the deconvolution technique, appear more significant than those of
unconjugated CO groups above 1700 cnr1.
L'effet de degradation dufer et du cuivre sur le papier : une analyse FTIR de deconvolution
Des echantillons de papier de cellulose contenant du FeCk et du CuCb ont ete soumis a une analyse
FTIR avant et apres le vieillissement accelere a 90+/-10°C (technique des tubes scelles). On a pu
constater qu'une forte hydrolyse et une deshydratation des chaines cellulosiques avec formation de
liaison double de C4-C5 et d'une oxydation de C6 sont les phenomenes essentiels observes lors de
la degradation de FE(III). Cu(II) oxyde les groupes C2- et C3-OH en aldehydes et carboxyles en
entrainant une cassure des composes cycliques qui hydrolyse et oxyde le groupe C4 OR avec
formation de C4-C6 p-ceto-aldehyde ou p-ceto-acide. Parmi les bandes de de-convolution FTIR du
domaine situe entre 1250 et 1850 cnr1 on attribue le signal 1660 cnr1 au groupe insature de C=C-cO
et le signal 1612-1622 cm-' au groupe CO-C-CO (forme chelatee de tautomerie ceto-enolique). Ces
bandes qui ne peuvent etre detectees qu'a 1'aide de la technique de deconvolution apparaissent plus
distinctement que celles de groupes libres (non-lies) au-dela de 1700cm'.
Die abbaufordernde Wirkung van Risen und Kupfer: eine The Degrading Action of Iron and
Copper on Paper: a FTIR-Entflechtungs-Analyse
Proben von Cellulose, die mil FeCla und CuCL> dotiert waren, wurden vor und nach einer beschleunigtenAlterung (90±10°C, in verschlossenem Glasbehalter) einer FTIR-Analyse unterworfen.
Es zeigte sich, daB starke Hydrolyse und Wasserstoffentzug mil der Bildung von Doppelbindungen an C4-C5 und Oxidation an C6 des Cellulosemolekiils die wesentlichen Vorgange des von
Fe(III) bewirkten Abbaus sind. Cu(II) oxidiert unter Ringoffhung die Hydroxylgruppen an C2 und
C3 zu Aldehyden und Carboxylen, und die Gruppe C4-OR unter Bildung von C4-C6 (5Ketoaldehyd oder einer sauren (5-Ketoguppe. Von den FTIR-Entflechtungsbanden im Bereich von
1250-1850 cm ' wird die Bewegung bei -1660 cnr1 der ungesattigten Gruppe C=C-CO, die
Bewegung bei 1612-1622 cnr1 der CO-C-CO Gruppe zugewiesen: die Chelatform des Enol-
The Degrading Action of Inn and Copper on Paper
Tautomers. Diese Banden, die nur mit Hilfe der Entflechtungstechnik erkannt werden konnen,
zeigen sich deudicher als diejenigen von unverbundenen CO Gruppen oberhalb von 1700 cm1.
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Paolo Calvini
Ministero per i Beni e le Attivita Cultural!
Soprintendenza per i Beni Architettonici ed il Paesaggio
Via Balbi 10
16126 Geneva
Italy
E-mail: paolo.calvini@tin.it
Andrea Gorassini
Laboratorio di Restauro del Libro, with C.N.R and University of Udine grant
Dipartimento di Storia e Tutela dei Beni Culturali, Universita di Udine
Via Antonini 8
33100 Udine
Italy
Author to whom correspondence should addressed