Characterisation by FTIR Spectroscopy of Ink Components in Ancient Manuscripts
by NÚRIA FERRER & M. CARME SISTACH
INTRODUCTION
Degradation of paper and parchment is studied because of its destructive effects on libraries and
archive materials. Serious damage is caused by iron-gall inks1-4.
The destructive mechanism of the inks can be influenced and exacerbated by various characteristics
and processes. These include the characteristics of the paper, the composition of the ink,
environmental conditions and chemical reactions with other molecules5-12. An understanding of the
reaction mechanisms of the components of the original ink is crucial for an understanding of the
damage caused to paper and parchment13-17. Characterisation of these components can help to
describe these mechanisms and consequently avoid degradation18-21.
One of the biggest problems with this kind of research is that only a small amount of ink can be
removed from original manuscripts showing such damage, which means that microscopic methods
are required. FTIR spectroscopy has been used to characterise ink components in ancient
manuscripts.
In this study two methods were used to obtain good infrared spectra. Reflection techniques, such as
attenuated total reflectance (ATR), and transmission tech-niques. They were used in conjunction
with an infrared microscope, because of the tiny amount of sample involved. Transmission
techniques, using the diamond cell, require the removal of some particles from the surface of the
ink, whilst avoiding damage to the paper or parchment. ATR techniques are not destructive at all,
but they do require there to be a perfect contact between the ZnSe crystal and sample.
16 inks were analysed and different salts of oxalic acid, i.e. the end product of cellulose
degradation, were characterised. A close correlation between the acidity of ink and oxalate salts
with different cations was found. Iron (II) sulfate was also detected in some samples.
Comparison with other techniques, such as scanning microscopy and gas chromatography combined
with mass spectrometry (GC-MS), gave good results
Table 1 : The samples.
and corroborated infrared spectra findings22-25. The amount to which the ink contributes to the
degradation process of manuscripts is also linked to the composition of the paper: thickness of
paper, alkaline buffering and the type of sizing are important factors. Identification of inorganic
elements in ink (SEM-EDX, PIXE) and of acidic organic molecules from carbohydrate degradation
(GC-MS and FTIR) corroborates these findings.
EXPERIMENTAL
Manuscripts tested were chosen according to their age, acidity and paper composition. Table 1
shows the relevant data.
Apparatus
A Bomem MB-120 infrared spectrometer was used. The instrument has a Glowbar source, a KBr
beamsplitter and a triglycine sulphate (TGS) detector. A Spectra-tech IR Plan Microscope, which
has an MCT detector refrigerated with liquid nitrogen and an ATR objective of ZnSe, is attached to
the spectrometer.
Infrared spectra, were measured by accumulating 100 scans at a resolution of 4 cm-1 when using the
transmission mode of the microscope. Accumulations of 200 scans and a resolution of 8 cm-1 were
used when measuring with the ATR objective.
Spectral range was from 4,000 to 720 cm-1 . The spectral data were processed with the GRAM/32
program.
Sample handling
Two different methods were used for the analysis of paper and inks: diamond cell with the
microscope in transmission mode and attenuated total reflectance (ATR), with a ZnSe crystal fitted
to the microscope.
The diamond cell method involves the placing of a small particle of the sample, whether pure ink,
fibre with ink or just clean fibre, in the middle of a diamond window and pressing it against another
window. This increases the surface area of the sample and decreases its thickness, in such a way
that a beam of light can pass through it and the spectra of the solid can be measured by
transmission. This procedure allows the analysis of very small particles, as small as 10 microns .
When analysing fibres that contain ink, it is possible to select and focus on the desired area, with or
without ink. Tungsten needles are normally used to transport the particle or fibre from the
manuscript to the diamond cell.
Inks on parchment samples are normally easier to remove, whilst cellulose fibres tend to absorb ink,
which makes separation difficult.
The ATR method is the second option. This is especially useful when the sample is either too thick
or cannot be destroyed, separated or manipulated. The ZnSe crystal of the objective is pressed
against the sample and a single reflection penetrates the sample slightly.
As commented above, inks on parchment are easier to analyse because of the greater homogeneity
of the parchment's surface. In the case of ink on paper it is sometimes necessary to press the crystal
several times on different places, in order to find an area with a smooth surface.
RESULTS AND DISCUSSION
After having analysed a large number of dark and light, acidic and non-acidic, corrosive or noncorrosive ink samples, both on parchment and paper, we found that, in the samples where a good
spectrum was obtained, some characteristic peaks appeared. Many of the analysed samples did not
show a neat spectrum, probably because of a lack of ink or the impossibility of extracting the information from the fibre. In fact, the ink was often so deeply impregnated into the fibres that it was
very difficult to see any signal apart from those of the paper support.
The common peaks that appeared in the best spectra showed bands that were similar to oxalate
bands. This was something we had found previously in samples from very different sources, such as
paintings and architectural facades and is obviously, related to some kind of degradation. In some
samples, oxalates fitted perfectly with the bands obtained when analysing calcium oxalate, but in
others
Fig. 1 : Two spectra of the same sample obtained using ATR and transmission.
they showed different absorption bands or some shift of the most intense peaks, apart from
differences in relative intensity. The study of the same inks using Scanning Electron Microscopy
showed other elements, apart from iron and calcium. In some cases, a strong band of potassium
could be observed. The samples analysed by GC-MS showed the presence of oxalic acid. According
to these results, which showed oxalate bands different from calcium oxalate, we started the search
for other oxalates containing the elements found by SEM.
After searching the spectra of some oxalate salts, we found not only calcium oxalate hydrate
(CaC2O4.H2O) bands in some spectra, but also iron (II) oxalate dihy-drate (FeC2O4.2H2O) and iron
(III) potassium oxalate trihydrate (FeK3C6O12.3H2O). This has not been reported before in any
article on the composition of ink of ancient manuscripts.
Comparison between transmission and reflection techniques
Both methods used in this study gave the same information and were equally useful for all samples
analysed. Poor quality spectra are usually obtained when non-degraded samples of paper are
measured with the ATR microscope. Usually, several attempts have to be made, because the crystal
often touches a rougher region containing fibres and empty areas. As degraded ink samples have a
larger and more homogeneous surface, the ATR spectrum is normally easier to obtain.
It is true to say that for both methods, whether removing particles for transmission or pressing the
ATR crystal, the samples of ink on parchment usually give better spectra than those of ink on paper.
Fig. 1 shows two spectra of the same sample obtained using ATR and transmission. They show that
the ink composition is the same, not only on the surface but also in the region in contact with the
support.
Characteristic bands of standards
• Calcium oxalate hydrate (CaC2O4.H2O): Calcium oxalate shows very intensive bands at 1628,
1326 and 780 cm-1. All these bands are sharp.
• Iron(II) oxalate didydrate (FeC2O4.2H2O): This compound shows bands at 1623, 1358, 1316 and
814 cm-1.
• Iron(III) potassium oxalate trihydrate (FeK3C6O12.3H2O): This spectrum shows more
absorptions at 1676, with shoulders at 1711 and 1647 cm-1. Other bands are at 1393 and 1273, with
a shoulder at 1256 cm -1. Finally, there are two more bands at 895 and 804 cm "'. As cellulose is
absorbed at 895 cm -1, this is a good absorption only in parchment samples or samples where strong
bands of cellulose cannot be detected.
• Iron (II) sulfate (FeSO4): The most important bands lie at 1110 with a shoulder at 1150 cm -1 and
a very small and sharp characteristic band at 989 cm "', which establishes its difference from other
sulfates.
• Calcium sulfate (CaSO4): The most important band is situated at 1150, with shoulders at 1115
and 1095 cm-1. The small and sharp band is at 1008 cm"1.
• Calcium carbonate (CaCO3): The most important absorptions of calcium carbonate are at 1418
(strong and broad) and at 875 cm ' (weak but very sharp).
Other spectra of standards were also taken into account, such as potassium oxalate, iron sodium
oxalate and magnesium oxalate, which contain bands not observed in the samples measured.
Characteristic bands of samples
• Ml (pH 5.8, black and no corrosion): This sample showed some bands corresponding to iron
potassium oxalate, but it was difficult to identify the oxalate bands, as the spectrum of cellulose was
very intense. SEM showed medium absorptions of K, Fe and Ca.
• M3 (pH 6.5, light and no corrosion). The spectrum showed clear bands of calcium oxalate.
However, calcium carbonate was also detected (1418 and 875 cm"1), which was consistent with this
being a parchment support. SEM showed a little peak of K, and a strong presence of Ca and Fe.
• M4 (pH 2.5, black and heavy corrosion): This spectrum showed hardly any cellulose bands: the
sample was so degraded that cellulose fibres were not hit during the analysis. Iron potassium
oxalate bands were detected clearly.
• M5 (pH 6.6, light and no corrosion): The spectrum just showed the bands of calcium oxalate.
Absorption of cellulose was very strong. SEM showed a strong absorption of Ca and Fe.
• M6 (pH 6.4 dark and no corrosion): In this case, the bands observed corroborate iron oxalate
bands. Absorption of cellulose bands was very poor, which allowed a clear spectrum.
• M7 (pH 6.7, light and no corrosion): Calcium oxalate bands were detected even though cellulose
strongly absorbed. SEM showed a strong absorption of Ca and weak bands of K and Fe.
• M8 (pH 5.8, black and no corrosion): This parchment showed a very neat spectrum. Iron(II)
sulphate was detected perfectly, as well as iron potassium oxalate.
• M9 (pH 3.2, black and significant corrosion): This was one of the best spectra matching the
bands of iron potassium oxalate. SEM showed large absorptions of K and Fe.
• MIO (pH 4.8, black and slight corrosion): This sample showed some iron potassium oxalate
bands, but it was difficult to identify the oxalate bands as the cellulose spectrum was very intense.
SEM showed large absorptions of K and Fe, and some of Ca.
• Mil (pH 6.5, light and no corrosion): Calcium oxalate bands were detected even though cellulose
absorbed strongly. SEM showed a strong absorption of Ca and weak bands of K and Fe.
• M12 (pH 5.9, black and no corrosion): A large absorption of the calcium oxalate bands was seen.
Moreover, a small signal of iron potassium oxalate was also detected.
• M13 (pH 5.7, black and slight corrosion). Although cellulose absorption was very strong, some
signal in the range of iron potassium oxalate was observed.
• M75 (pH 4.3, black and corrosion): Bands matched those assigned to iron potassium oxalate.
SEM showed large absorptions of K and Fe.
Table 2: The samples ordered according to their pH.
VHC = very heavy corrosion, HC = heavy corrosion, C = corrosion, SC = slight corrosion, NC = no
corrosion
• M90 (pH 6.2, black and no corrosion): This spectrum was not very clear, but seemed to show
bands of calcium oxalate, iron potassium oxalate and calcium sulfate. SEM showed medium
absorptions of K and Fe, and large absorptions of Ca.
• M98 (pH 4.7, black and corrosion): The spectrum showed mostly cellulose, but some bands
could be assigned to iron potassium oxalate, iron oxalate and calcium oxalate.
• Word M (pH 6.2, dark and no corrosion): This had a very clear spectrum of iron(II) sulfate. In
addition, iron oxalate bands were detected as was a small signal of iron potassium oxalate.
Selected spectra are shown in Figs. 2-5.
We tried to establish some correlation between different oxalate salts and the support (paper),
acidity of ink, darkness and age.
Age and paper type did not seem to be related to oxalate salts.
Fig. 2: Spectrum of a light ink without corrosion on parchment (M3): bands of calcium oxalate and
calcium carbonate
Fig. 3: Spectrum of a black ink without corrosion on parchment (M8): bands of iron potassium
oxalate and iron sulfate.
Fig. 4: Spectrum of a dark ink without corrosion on paper (M6). Clear bands of iron oxalate.
Fig. 5: Spectrum of a black ink with corrosion on paper (M9): iron potassium oxalate bands.
Arranging the samples by pH gave some interesting results (Table 2). There seems to be a
correlation between pH and colour, changing from black, to dark to light as the pH increases.
Corrosion also changes from very heavy corrosion at very low pH to heavy corrosion, slight
corrosion and no corrosion as the pH increases.
Comparing the oxalate salts with these results, it was seen that iron potassium oxalate appeared at
the lowest pH values, from 2.5 to 6.2. Higher pH values clearly indicated calcium oxalate, even
though it coexisted with iron potassium oxalate and iron oxalate in some samples.
Calcium oxalate seemed to be related to samples with no corrosion at higher pH values.
In general, large peaks of K and Fe using SEM confirmed the presence of iron potassium oxalate.
Large peaks of Ca correlated to calcium oxalate.
All the samples analysed by GC-MS showed oxalic acid in their composition.
CONCLUSIONS
FTIR allows for the rapid analysis of ink samples on paper or parchment, using both transmission
and reflection microscopic techniques.
Different compounds were detected in an ink, depending on the state of a sample's pH, corrosion
and colour. Calcium oxalate, calcium carbonate, iron sulfate, iron oxalate and iron potassium
oxalate were analysed. These findings can provide a useful contribution to the better understanding
of the reaction mechanisms involved in the formation and degradation of iron-gall inks.
SUMMARIES
Characterisation by FTIR Spectroscopy of Ink Components in Ancient Manuscripts
Fourier transform infrared spectroscopy was applied to the characterization of ink components in
ancient manuscripts. A selection of samples were chosen according to their age, acidity and support
(paper or parchment). Due to the tiny amount of sample involved, two techniques in conjunction
with an infrared microscope, were used. Reflection techniques, such as attenuated total reflectance
(ATR) and transmission techniques, such as diamond cell, were compared.
A close correlation between the acidity of ink and oxalate salts with different cations was found.
Iron(II) sulfate was also detected in some samples. Comparison with other techniques, such as
scanning microscopy and gas chromatography combined with mass spectrometry, gave good results
and corroborated infrared spectra findings.
Caractérisation par spectroscopie FTIR des composants de l'encre de manuscrits anciens
La technique de transformation par infrarouge de Fourier a été utilisée afin de caractériser les
composants de l'encre dans des manuscrits anciens. Une sélection d'échantillons a été retenus selon
leur âge, leur degré d'acidité (pH) et leur support (papier ou parchemin). En raison du petit nombre
d'échantillons concernés deux genres différents de techniques ont été utilisés conjointement avec un
microscope infrarouge. Des techniques de réflexion telles que la réflexion totale atténuée (ATR) et
des techniques de transmission telles que la cellule de diamant ont été comparées.
Une étroite corrélation a été observée entre le degré d'acidité de l'encre et les sels oxalates avec
différents cations. Du sulfate de fer(II) a également été détecté dans certains échantillons. La
comparaison avec d'autres techniques d'analyse (microscopie par scanning et Chromatographie
gazeuse) combinées avec la spectrométrie de masse ont donné de bons résultats et corroborent ceux
obtenus par la spectroscopie infrarouge.
Charakterisierung der Komponenten von Tinte in alten Handschriften mit Hilfe von Fourier
Transform Infrarot-Spektroskopie
Fourier Transform Infrarot-Spektroskopie wurde zur Beschreibung der Bestandteile von Tinten in
alten Handschriften eingesetzt. Die Proben waren nach Alter, Säuregrad (pH) und Beschreibstoff
(Papier oder Pergament) ausgewählt worden. Da nur sehr kleine Mengen der jeweiligen Proben zur
Verfügung standen, wurden verschiedene Techniken, nämlich abgeschwächte Totalreflexion (ATR)
und Transmissionstechniken in Verbindung mit Infrarotmikroskopie eingesetzt. Es wurde eine enge
Korrelation zwischen Säuregrad der Tinte und verschiedenen Oxalaten gefunden. In einigen Proben
wurde auch Eisen(II)sulfat gefunden. Ein Vergleich mit anderen Analysetechniken (ScanningMikroskopie zusammen mit Gaschromatogaphie) erbrachte gute Ergebnisse und bestätigte die der
Spektroskopie.
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Núria Ferrer*
Serveis Cientificotècmcs. Universität de Barcelona
08028 Barcelona
Spain
Lluis Sole i Sabaris, 1
Tel: 34-93-4021346
Fax:34-93-4021398
E-mail: nferrer@sct.ub.es
Ma Carme Sistach
Arxiu de la Corona d'Aragô
Almogàvers, 77
08018 Barcelona (Spain)
* Author to whom correspondence should be addressed.