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 non-corrosive 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 (Scanning-Mikroskopie 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.