Chapter 17 A study of ancient manuscripts exposed to iron-gall ink corrosion

Chapter 17

A study of ancient manuscripts exposed to iron-gall ink corrosion Ewa Bulska and Barbara Wagner

17.1

INTRODUCTION

In recent years growing attention has been focused on the use of various instrumental analytical methods to analyse works of art and support their conservation [1-8]. Historical artefacts realized on paper such as drawings, fine prints, watercolours, documents and manuscripts are more susceptible to destructive processes than the other works of art. The main factors determining the extent of damage can be divided into internal and external factors [9]. Internal factors are connected with paper composition and thickness as well as the presence of sizing and fillers; external factors are mostly connected with storage conditions and the use of the objects [9,10]. For drawings and manuscripts the composition of the ink should also be taken into account as one of the most important internal factors. It was found that documents written with iron-gall ink can be endangered by several destructive processes. These are collectively designated as ink corrosion [10-14]. Direct observation of various ancient manuscripts in all cases showed brown discoloration of the paper around the ink line, a phenomenon connected with its progressive deterioration. According to the most pessimistic scenario, scores of unique, written documents of artistic and cultural value are exposed to a degree of degradation that can lead to total destruction. Among the most spectacular examples, one can list famous works of Bach [15], Rembrandt [16], Guercino [17] or Gallileo [18]. Many other drawings and documents, however, also need immediate protection. This problem is very serious and concerns collections all around the world. Although ink corrosion phenomena have been thoroughly investigated, a truly successful conservation treatment for effectively halting or slowing down the degradation process has not yet been found [9,19,23-25]. Comprehensive Analytical Chemistry XLI1 Janssens and Van Grieken (Eds.) © 2004 Elsevier B.V. All rights reserved

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Ewa Bulska and Barbara Wagner Bringing into practice any new conservation procedure should be preceded by preliminary investigations devoted not only to the properties of the originals, but also to a detailed study of the phenomena that give rise to the damage. Examination of works of art requires a special strategy. In principle, when dealing with such an object one has to respect the physical integrity of the item investigated. Because objects of art are unique, the applied analytical methods should be non-destructive or if that is not possible, then micro-destructive [1-3]. Therefore, the decision concerning the chemical or physical examination of works of art should be done individually and with full awareness of its influence on the artefact. The principal rule says that valuable objects can only be investigated when the analysis does not result in any visible damage. Usually this completely eliminates sampling or limits it to very small amounts [26]. Among methods most often used for such purposes are spectroscopic methods, which are particularly useful in solid-state research [27]. They can be used either for structural investigation or for the determination of the elemental and molecular composition of the object. Iron-gall inks used in ancient manuscripts were basically produced by mixing iron salts (often containing traces of copper) with a gallotannin aqueous solution. Many historical formulae can be found in the literature with emphasis on the multiplicity of substrates included [28-31]. No exact formula for preparing those inks existed, and before the proper proportions were found, they had been made by arbitrarily mixing all reagents together. However, three components were always present: extract of gallnuts, an iron salt and Arabic gum [32-35]. According to the literature ink corrosion can be attributed to two main reasons: either the presence of free sulphuric acid, which can cause acid hydrolysis of cellulose or an excess of iron ions, which may catalyse the oxidative degradation of cellulose [9,10,36,37]. The aim of our work was to develop a procedure using a multi-technique approach for the physico-chemical examination of ancient manuscripts endangered by iron-gall ink corrosion. 17.1.1

Iron-gall ink

According to the literature [31], iron-gall inks were developed from carbon inks, which were already well known. Carbon-based inks were made by the suspension of burned organic materials added in the form of soot into an aqueous solution of gum. Although these inks were known to offer an intense black colour, they were also relatively easy to erase. They were sensitive to moisture and could easily be destroyed by contact with water. This was a 756

A study of ancient manuscripts exposed to iron-gall ink corrosion major drawback for important documents that needed to be stored for a long time. As the ability of tannins to form a black colour with iron ions was already known in antiquity, initially they may have been added to carbon ink to increase the permanence of the colour. Gradually black iron-tannin solutions replaced carbon ink. James [31] described the provenance of the names used for different kinds of ink. While the Latin name atramentumreferred to carbon black, the Greek name encaustun was used to describe metallo-gallic inks. The English word ink is etymologically derived from encoustum. However, in Western Europe confusion between the terms has been observed, and since the Renaissance the term incaustum has been used to describe any black ink. General acceptance of the use of iron-gall inks was connected with the conviction of its users concerning the stability of the colour of the ink after being deposited on a paper support. Unfortunately in some cases these inks negatively influence the stability of the paper by changing its colour and increasing its brittleness. For many years the reason for these phenomena was not clear, and much effort was exerted towards understanding the processes leading to the corrosion of the paper support. It was known that iron-gall inks was produced by mixing an aqueous solution of iron(II) sulphate (vitriol) and extracts from gallnuts (a swelling on plants caused by an insect's laid eggs) containing gallotannins [22,32]. The reaction between vitriol and tannins can be described by the following scheme [33]: FeSO 4 + H 2Tan vitriol

tannin

'

FeTan black coloured compound

+

H 2 SO4

sulphuric acid

As a by-product of the reaction leading to the creation of the colourful FeTan complex, sulphuric acid is formed. The presence of H 2 SO 4 in ink marks led to the formulation of a first explanation of ink corrosion, based on the acidic hydrolysis of the paper's cellulose, causing the latter scission of the polymeric strands. According to this hypothesis, the very first conservation treatments relied on the deacidification of the affected objects [25,36]. This, however, was not sufficient to diminish the corrosion processes; in many documents it was observed that even after the deacidification the paper became brown and brittle as a result of ink corrosion. The reactions leading to the formation of coloured compounds of ink were unknown until the first investigations described by Wunderlich [13,28]. He assumed that a complex of iron(III) with gallic acid was responsible for the colour. He concluded that the actual ink was formed by oxidation of the iron(II) 757

Ewa Bulska and Barbara Wagner to iron(III) in air, followed by the formation of a complex with gallic acid. The reversible transfer of an electron from gallic acid to the bound iron(III) ion upon absorption of light thus caused the black colour of the complex. Some authors [32,38] recall investigations done by Krekel [39], who showed that the gallic acid could lose carbon dioxide during the complex formation and change to pyrogallol, which then formed a 1:1 complex with iron(III). There is common agreement that the lack of a detailed formula for preparing iron-gall inks resulted in non-stoichiometric ratios between the substrate tannin and iron sulphate [13,28,32,38]. According to the literature [13,14,19,20], most historical recipes contained an excess of iron sulphate compared with gallotannins and gallic acid. So after many years or even centuries the ink may still contain substantial amounts of iron not bound in the FeTan form. 17.1.2

Iron-gall ink corrosion

Library collections all over the world are affected by iron-gall ink corrosion to a different extent. However, in all cases, visual observations prove that the first signs of ink corrosion are connected with the formation of brown edges at the inks' regions and the appearance of a brown colour at the verso side of the page [10,16,17,31]. Afterwards the brittleness of the paper increases, leading to the complete degradation of the paper support at the end of the process. Systematic research into ink corrosion began after a conference on this subject in St Gallen (Switzerland) in 1898. The presence of transition metals began to be taken into account when scientists discovered that the acid hydrolysis of cellulose could not be the only reason for the degradation of the paper. Transition metal ions were known to catalyse the oxidative degradation of cellulose and other organic substances present in the paper support. Interest was focused on the presence of non-bound iron ions that could accelerate oxidative scission of cellulose chains. The deterioration of the paper leading to extreme weakness of the material was explained by two processes: acid-catalysed hydrolysis of cellulose and metal-catalysed oxidation of cellulose. Both can either occur simultaneously or independently of each other [10,40]. After discovering the role of iron in ink corrosion processes, it became clear that an effective conservation treatment should include not only the deacidification step, but also the deactivation of the nonbound iron ions. Three different possibilities for achieving this were proposed by Neevel and Reissland [17,20]. Iron ions could either be (i) removed from the paper or (ii) bound into very stable complexes by specific chelating agents, which would be able to block the catalytic activity of iron ions.

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A study of ancient manuscripts exposed to iron-gall ink corrosion Another possible treatment was (iii) the application of antioxidants, such as lignin, that reacts faster with radicals than cellulose. Reissland [9] nicely demonstrated the importance of paper composition, which can influence the rate of corrosion processes to a great extent. The penetration of deposited ink is related to the amount of "sizing" present in the paper. Paper is sized (i.e., treated with appropriate Al-containing chemicals) to reduce the lateral spread of ink by diffusions during and after writing. It was observed that text written on thick and sized paper exhibited local corrosion effect while for text written on unsized paper the corrosion was spread around, because of the deeper ink penetration into the structure of the support. Neevel [19] described the oxidation of cellulose that takes place by contact with oxygen from the air. The reaction is catalysed by iron(II) ions, which convert oxygen into more reactive radicals. The iron(II) ions originate from iron(II) sulphate, which was always used as a basic component of iron-gall ink. When ink contained an excess of iron not bound in the coloured complexes with tannins, iron(II) ions can partially oxidize to insoluble hydrated iron(III) oxide (rust); in this form the iron is catalytically inactive. Paper and iron-gall ink, however, contain many substances that can reduce the iron(III) ions back to the iron(II) form. According to Neevel [19] iron(II) ions can accelerate oxidative degradation of cellulose by participating in two processes: 1. The formation of organic radicals followed by their oxidation: Fe 2 +

0+ 2

-

Fe 3+ +' O-0 CELL' + 02

Fe3 + +' 0-O - + CELLH i

, Fe 2+ + HOO' + CELL'

CELLOO'

CELLOO' + CELLH

, CELLOOH + CELL'

2. Formation of hydroxyl radicals from hydrogen peroxide according to the Fenton reaction: Fe 2+ + HOO' + H -1-' Fe2 + + H 2 0 2

, Fe 3+ + H 2 0 2

, Fe 3 + + HO' + OH-(Fenton reaction)

Iron(II) ions catalyse the oxidative degradation of cellulose by the formation of hydroxyl radicals (HO') from hydrogen peroxide (H 2 0 2 ). Hydrogen peroxide is formed during the reduction of molecular oxygen by 759

Ewa Bulska and Barbara Wagner iron(II) ions. Hydroxyl radicals are very reactive and they easily abstract hydrogen from cellulose, leading to the formation of organic radicals, which then react in a chain reaction with oxygen and the next cellulose molecule. Chain scission occurs when cellulose hydroperoxide reacts with iron(II) ions in a Fenton-like fashion. Fenton [41,42] described the reaction of tartaric acid and hydrogen peroxide in which coloured products are formed in the presence of minute amounts of a ferrous salt. Nowadays, the Fenton reaction is referred to as a process of the generation of the very reactive hydroxyl radicals from hydrogen peroxide catalysed by Fe(II) [43]. The sequence of reactions showing this catalytic activity is referred to as the Haber-Weiss reaction. The catalytic function of Fe(II) also implies that Fe(III) ions are involved. In general, only trace amounts of iron, or other transition metals such as copper, are needed for the hydroxyl radical formation [44]. The hydroxyl radicals are known to be highly reactive and can accelerate the oxidation of most organic compounds. In many publications concerning the catalytic oxidation of cellulose, authors have described the model of cellulose oxidation based on the Fenton reaction [19-22]. Although the radical mechanism of the Fenton reaction has been a dominant theory in the last 50 years, there is still some controversy over the nature of the intermediates [45]. Paper support and ink contain a variety of ingredients that could also participate in the reactions, and according to recent observation the mechanism of the Fenton reaction is not clear. Recent results [46] showed that the principal pathway of the Fenton reaction does not involve 'OH radicals, but that another strongly oxidizing species is formed instead. The mechanism of the reaction proposed by Kremer [46] is based on the formation of the ferryl ion FeO 2+ formation, which is the key intermediate involved both in the oxidation of Fe 2+ and in the evolution of 02. The formation of a mixed valence complex [FeOFe] 5 + formed from Fe3 + and FeO2 + was experimentally proven. It is clear that full understanding of the chemistry of reactions leading to ink corrosion requires further investigation. 17.1.3

Investigated artefacts

The aim of this chapter is to describe a multi-technique approach for the examination of ancient manuscripts endangered by iron-gall ink corrosion. Documentation and understanding of the ongoing corrosion processes, not to mention the selection of the most appropriate conservation method, can be benefits from such an investigation. Our studies were stimulated by a 760

A study of ancient manuscripts exposed to iron-gall ink corrosion co-operation with the National Library of Poland (Warsaw, Poland), where a number of manuscripts are endangered by the corrosion process [47-49]. This work was centred nearly exclusively on a manuscript dating back to the beginning of the 16th century, entitled Meditationes, passionis Domini nostri Iesu Christi (BN BOZ. 1113, National Library of Poland, Warsaw). The catalogue description of the manuscript is relatively short. Originating from the library of the Bernardine monastery in Bydgoszcz (Poland), it became part of the collection of the Zamoyski family in the 19th century. After the Second World War, the National Library of Poland took the document into its collection. According to the hypothesis of art historians, the manuscript could be one of many copies of Meditationes among many others that were written in the monastery. Some parts of the text are written in Latin while others are written in Polish. The manuscript is composed of 630 pages. The size of the contemporary leather binding is about 21.5 x 16 cm. Two of the inside pages of the manuscript are presented in Fig. 17.1. In this book most of the pages are subject to iron-gall ink corrosion, though different parts of the manuscript have been corroded to various

Fig. 17.1. Meditationes passionis Domini Nostri Iesu Christi (reproduced with kind permission of National Library of Poland). (For a colored version of this figure, see Plate 17.L.)

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Ewa Bulska and Barbara Wagner degrees. It should be noted that some pages have been nearly totally ruined. From the pages that no longer could be restored, conservators decided to take a limited number of small samples for investigative purposes. The fragments of paper from selected pages were so brittle that any manipulation led to further fracture. Hence, this situation was considered a unique opportunity to undertake an investigation of the samples from an original manuscript already significantly advanced in the destructive corrosion process. Since it was anticipated that the corrosive action was caused by iron-gall ink, samples with written letters or fractions of such letters were investigated. Our preference was to employ micro-analytical methods since only minute pieces of material are required to perform these types of analyses. As an example, several sample pieces, with their dimensions, are shown in Fig. 17.2. Although it was possible to undertake a number of preliminary physicochemical investigations by using the samples originating from the manuscript, it was not possible to employ a large number of these samples. The systematic evaluation of a new conservation procedure requires many reproducible samples to allow for the execution of more detailed investigations. Therefore, the analysis results obtained from the ancient manuscript samples were used to reproduce the ink's elemental composition so that model samples could be prepared for use in further investigations. Those samples were also analysed by means of the analytical methods described below. Investigations concerning the Fe(II)/Fe(III) ratio by Mossbauer spectroscopy and pi-XANES (X-ray absorption near edge spectroscopy) were done not only for samples taken from Meditationes, passionisDomini nostri Iesu Christi (manuscript Ml), but also for single micro-samples from two other 16th century manuscripts (labelled manuscripts M2 and M3). The composition of paper differed visibly for the samples investigated, and only manuscript M1 was written on a poorly sized paper support. Surface sizing of the paper used for the two other manuscripts limited the ink penetration into

Fig. 17.2. Selected pieces of micro-samples from manuscripts M1, M2 and M3.

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A study of ancient manuscripts exposed to iron-gall ink corrosion the structure of the paper support, and in consequence influenced the corrosion effect. In the case of M1, the corrosion spread over the entire written area of the page whereas in the other two manuscripts corrosion is limited to the ink lines. 17.2

ANALYTICAL METHODS

Works of art have been subjected to various investigations concerning conservation problems and many methods have been used to study the phenomena occurring in the objects examined [50-54]. Examples of the use of micro-analytical techniques suitable for characterizing ancient and/or artistic objects were recently described by Adams et al. [3]. Ancient manuscripts endangered by ink corrosion have been examined with the use of numerous analytical methods depending on the information needed. Analytical methods used to diagnose and investigate phenomena occurring in ancient books or manuscripts, as well as aspects of conservation, are described in many publications [18,36,38,53,55]. Hey described the use of scanning electron microscopy (SEM) and its first application for documentation purposes in the field of paper conservation [4]. The difference between the model samples and samples taken from 16th century Venetian books damaged in the Arno flood (Florence, 1966) were documented on micrographs. SEM was also used by Schreiner and Grasserbauer for the morphological study of damaged fibres in paper destroyed by copper pigments [2]. The structure of the paper fibres in ancient manuscripts was also the subject of interest of Sistach, who examined manuscripts written with iron-gall ink [36]. The author assumed that the corrosion was caused by sulphuric acid present within the inked area of the investigated manuscript. Therefore, samples were treated with two different deacidification procedures. In order to compare the effectiveness of each method, manuscripts were analysed by SEM and X-ray micro-analysis. Another method used in this field was electron spin resonance (ESR). Attanasio et al. [50] studied the role of the paramagnetic impurities in the degradation of paper. The authors found that even trace amounts of copper are very destructive, whereas iron catalysed the degradation processes effectively only when found in a specific rhombic symmetry. Choisy et al. [5] studied ancient papers by using non-invasive FTIR and fluorescence techniques. They found an excess of iron and copper in the paper areas that differed from their surrounding in terms of colour and mechanical properties. Such phenomena are known as "foxing stains" and were also the 763

Ewa Bulska and Barbara Wagner subject of investigations performed by Tang using atomic absorption spectrometry [51]. Espadaler et al. [56] used SEM combined with energy-dispersive X-ray micro-analysis (SEM-EDX) to analyse manuscripts written with iron-gall ink. They described the distribution of iron, calcium and sulphur and demonstrated the relationship between acidity and the presence of iron compounds with the corrosion effect. Heller et al. [57] used X-ray fluorescence (XRF) to examine the linear distribution of iron and calcium across the area covered by iron-gall ink and found that the concentration of iron is greater on the edges of lines than in the middle part of them. The same effect was described by Vodopivec and Budnar, who used proton-induced X-ray emission (PIXE) [55]. They, however, were able to recognize different elemental patterns depending on the sampling area. In our investigation SEM was used for studying the topomorphology of the corroded and non-corroded area of the object [49]. The same samples were studied simultaneously by means of XRF for the major elemental composition, electron probe micro-analysis (EPMA) and laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) [47] to obtain their elemental distribution patterns. The samples were then analysed for trace element content by means of inductively coupled plasma mass spectrometry (ICP-MS) or graphite furnace atomic absorption spectrometry (GF-AAS) [48]. The applied sequence of the use of various analytical methods is very important (Fig. 17.3). In this case the first three methods (SEM, XRF and EPMA) are non-destructive; thus they could be applied to investigate exactly the same area of the object. The fourth method (LA-ICP-MS) is microdestructive. The fully destructive ICP-MS or GF-AAS methods were used last since the decomposition of the entire sample was required prior to the measurement. XRF was used for the elemental analysis of written and unwritten areas of the manuscript. Based on those results, several elements (Fe, Cu, Hg, Pb, Zn, Ca, S) were chosen to be investigated using EPMA. With the use of that method, elemental maps within micro-areas located across the border of a character were studied. Details of the local concentration of several elements and the correlation with the ink mark visible on the paper surface were examined in order to reveal the composition of the ink used in the investigated manuscript. Next, LA-ICP-MS was used to determine the distribution patterns of iron and copper along the ink line and its surroundings. The investigations done by EPMA and LA-ICP-MS were, nonetheless, only qualitative, and the content of specific essential elements had to be 764

A study of ancient manuscripts exposed to iron-gall ink corrosion SEM Observation of cellulose morphology NON-DESTRUCTIVE METHOD

g

ED XRF Elemental analysis NON-DESTRUCTIVE METHOD

EPMA Mapping of chosen elements on the samples surface NON-DESTRUCTIVE METHOD

M6ssbauer Spectroscopy Analysis of Fe(ll)/Fe(lll) on the samples surface NON-DESTRUCTIVE METHOD

XANES Analysis of Fe(ll)/Fe(lll) NON-DESTRUCTIVE METHOD

LA ICP-MS Distribution of chosen elements on the surface MICRO-DISTRUCTIVE METHOD

XGF AAS Quantitative analysis

NON-DESTRUCTIVE METHOD

ICPICP-MS MS Quantitative multielemental analysis DISTRUCTIVE METHOD

Fig. 17.3. Sequence of use of different analytical methods for the investigation of micro-samples from Meditationes passionis D.N.I.C. determined quantitatively. The minute samples available were sufficient for analysis by means of ICP-MS or GF-AAS. In the literature, the role of Fe(II) in the deterioration of paper by catalysing the redox depolymerization of cellulose was emphasized [9,21,32].

As a result, it was of interest to investigate whether both Fe(II) and Fe(III) ions are present in ancient manuscripts. Mssbauer spectroscopy is a suitable method for the determination of the Fe(II) to Fe(III) ratio in a bulk sample, especially in paramagnetic materials. This technique was already used for the estimation of the Fe(II) content in the 15th and 18th century manuscripts [19]. In our work, beside Mossbauer spectroscopy [58], microXANES was used to the investigate the ratio and distribution of both Fe(II) and Fe(III) on the surface of the manuscript with high lateral resolution [59]. The results from these investigations proved that Fe(II) is present at the level of a few to tens of percent of the total iron content.

765

Ewa Bulska and Barbara Wagner 17.2.1

Inspection by scanning electron microscopy

Surface characterization of the morphology of the cellulose fibres within corroded and non-corroded parts of the manuscript was performed by SEM. A Camebax SX 50 electron microprobe (Cameca, France) was used for SEM and spatially resolved analysis. The investigated samples were mounted onto a small metal stub by means of double-sided adhesive tape. This was found to hold the paper sufficiently firmly in the proper position for inspection. The samples were examined using a 10 kV electron beam. These conditions were chosen intentionally in order to obtain a high lateral resolution of the top fibre surface, although the depth resolution was sacrificed. Among the collected samples (Fig. 17.2), it was possible to find some areas partially covered by iron-gall ink that had greatly suffered from the corrosion process. The pictures presented in Fig. 17.4a show scanning electron micrographs from a surface covered by iron-gall ink, while those in Fig. 17.4b show equivalent micrographs from a noncovered surface. Since both investigated samples were taken from the same manuscript, they exhibit the same history of influencing external factors. The difference of the fibres' conditions is clearly visible in Fig. 17.4. Broken fibres are seen together with ink particles in Fig. 17.4a while in Fig. 17.4b the fibres are not broken. For the non-corroded sample the fibre density appears to be less than in the corroded area, where a compression of paper fibres seems to have taken place by the action of writing. It is clear that the corrosive action of the ink induced the fibre breakage, increasing the brittleness of the paper.

17.2.2 Compositional analysis by X-ray fluorescence spectrometry The elemental composition of selected parts of the manuscript was determined by energy dispersive X-ray fluorescence analysis (EDX), which was chosen as the instrumental analytical technique for the elemental analysis of solids with minimal sample treatment. An energy dispersive X-ray fluorescence spectrometer X-Lab 2000 (Spectro Analytical Instruments, Germany) was used for XRF measurements. In order to overcome the Pb-M, and S-K, overlap, a spectral deconvolution program was used. Small samples of approximately 1 cm 2 were carefully placed into the sample holder of the X-ray spectrometer, directly onto a 7 m 766

A study of ancient manuscripts exposed to iron-gall ink corrosion (a)

(b)

Fig. 17.4. Scanning electron micrographs of manuscript paper showing its morphological structure: (a) an area covered with ink; (b) an unwritten area (reprinted from Ref. [491 with kind permission of Springer). Mylar foil while an aluminium cylinder was placed on top to keep the samples in position when a vacuum was applied to the sample chamber. Two types of samples were investigated. The first originated from the written area of manuscript M1 and was subject to corrosion. The second sample originated from the non-written area of the page and represents the non-corroded paper. The differences in elemental composition between a nonwritten and written area of the paper (subject to corrosion) are summarized in Table 17.1. The most pronounced differences were found for Fe, Cu, Hg, Zn, S and Pb; other elements are present at the same level in both samples. The higher amount of Hg, Zn and Pb detected on the written part of the page can be explained by the presence of impurities in the ink. The presence of Zn was also described by Vodopivec and Budnar [55], although they attributed it to the composition of the paper rather than to that of the ink.

767

Ewa Bulska and Barbara Wagner TABLE 17.1 Semi-quantitative XRF results on element concentrations in written and unwritten areas Investigated Written area Unwritten area Investigated Written area Unwritten (Ig/g) area element (pug/g) (,glg) element (tLg/g) Fe Cu Pb Hg Zn S Ca Mg Na As K

7540 6320 3890 3340 3290 1460 840 380 <300 290 280

630 37 27 7 48 220 570 330 490 11 <100

Si P Al Cl Mn Sb Ti V Cr Co Ni

240 140 <100 <50 42 26 <20 <15 <15 6 <5

350 150 160 130 <10 <2 <20 <15 <15 <3 9

The results obtained from XRF measurements were useful for selecting the elements chosen for further inspection by EPMA (see section 17.2.3). It must be pointed out, however, that the results obtained with the use of XRF provide information about a relatively large sample area (2 cm 2 ). This is not only because of the limited resolution of this method, but also because of its limited sensitivity in absolute sense. When only minute amounts of samples were available and elemental composition of areas covered and non-covered by iron-gall ink were of interest, another method had to be found. For this purpose EPMIA and LA-ICP-MS were employed. 17.2.3

Electron probe micro-analysis

EPMA was used for a more detailed study of written areas of the paper surface. This technique allows for a morphological characterization of the surface while elemental distribution maps can also be obtained. The investigations were performed with the use of a Camebax 50 electron microscope (Cameca, France) equipped with three vertically mounted wavelength-dispersive spectrometers and one horizontally mounted WDX system, all incorporating gas flow proportional counters. It also comprised an energy-dispersive X-ray spectrometer from Princeton Gamma Tech. (Princeton, USA), with an Si(Li)-detector with 150 eV resolution at Mn-K, 768

A study of ancient manuscripts exposed to iron-gall ink corrosion and additionally a secondary electron (SE) and backscattered electron (BSE) detector for morphological studies. Before the EPMA inspection the nonconductive paper samples were coated with a 10 nm gold film; these films did not disturb the EPMA measurements of the light elements present above the 0.5% m/m concentration level. The first observations of the inserted micro-sample were performed in BSE mode and a transition area as shown in Fig.17.5 was selected. This is part of a letter "b." On the right side the ink layer can clearly be seen because of the considerable difference in density. In the part without ink, the structure of the cellulose is similar to the one shown in Fig. 17.4, which was obtained by a scanning electron microscope. X-ray spectra were registered at selected points in both areas (Fig. 17.6). It is clear that the signal intensity for Fe, Cu, Hg, Pb and Zn differs. Therefore, for those elements as well as for S and Ca, elemental maps over the chosen surface were collected by moving the electron beam across the sample in steps of 10 /im, covering an area of 500 ,tmx 500 ,um in total. At each beam position, EDX spectra was collected and data for Fe, Cu, Pb, Zn, S, Hg and Ca were acquired (see Fig. 17.5). In the inspected areas of the ancient manuscript under study, the ink layer is characterized by a high amount of Fe, Cu, Hg, Pb and Zn. Conversely, both S and Ca are nearly uniformly distributed over the investigated area. This effect has already been described by many authors [9,23,27,38] and can

Fig. 17.5. Backscattered electron (BSE) micrograph and element distributions obtained from a micro-sample taken from the manuscript M1 (reprinted from Ref. [49] with kind permission of Springer).

769

Ewa Bulska and Barbara Wagner

e

Zn

OF

C

U~~~~~~~~~~~~~~~~~~~~~

gLA_

0 0

5, 0

10.0

15.0

keV

t~~i CA'

W's

0. 0

5.0

10.0

15S0

key

Fig. 17.6. XRF spectra taken from paper samples: (A) Non-corroded area. (B) Corroded area of manuscript (reprinted from Ref. [49] with kind permission of Springer). be explained by the ability of sulphur to migrate in the presence of moisture. The uniform distribution of Ca is connected with the presence of fillers in the paper's structure. 17.2.4 Laser ablation inductively coupled plasma mass spectrometry Heller et al. [57] found that the concentration of iron across a letter written with iron-gall ink may be irregular and was higher on the edge of the line than in the central part of it. A similar observation was made by Vodopivec and Budnar [55], who explained it by the occurrence of a diffusion process at

770

A study of ancient manuscripts exposed to iron-gall ink corrosion the wet/dry interface. Consequently, it was of interest to investigate whether such a phenomenon also occurs in the case of Meditationes,passionisDomini nostri Iesu Christi with special attention given to iron and copper. For this purpose we used LA-ICP-MS, which is considered a micro-destructive method with very good detection limit for elemental analysis. The combination of laser ablation sampling of minute amounts of material from the surface of the sample followed by ICP-MS measurements has been successfully applied for the determination of trace elements in different types of solid materials [60,61]. However, to the best of our knowledge, none of these publications have been dedicated to the analysis of paper samples. In this work, LA-ICP-MS was used for the observation of the distribution patterns of both metals across a line of ink in the ancient manuscript [47]. This method allowed to register the changes of the metal content within the analysed area. In general, dimensions of analysed samples are limited by the size of the analytical cell (9 cm 2 ), which was not, however, a limiting factor in the case of the investigated micro-samples. Microscopic inspection of the sample, done after measurement, showed that the destructive effect caused by the ablation appeared as micro-holes along a laser path. Although invisible to the naked eye, LA-ICP-MS needs to be thought of as a micro-destructive method. A PlasmaQuad 3 ICP-MS (VG Elemental, UK) in combination with a UV MicroProbe laser ablation system was used to perform element profiles across the character and its surroundings. The laser energy is provided by a horizontally mounted Nd/YAG laser, operated in the UV region at 266 nm. The laser beam is folded through 90 ° onto the sample surface via an Olympus microscope to enable optimal viewing quality. The ablation target can be viewed through a colour CCD camera for precise location of the site of analysis. The measurements were performed simultaneously for 57 Fe and 6 5Cu isotopes. Although there is a lack of standard samples for paper analysis, experimental conditions such as plasma gas flows, torch position and voltage of the lenses were optimized with respect to the SIN ratio of the 59 Co isotope by using a NIST SRM 612 glass standard having a Co concentration of approximately 35 ppm. A low frequency laser beam (1 Hz) with limited power (2 mJ) was used in order to reduce penetration into the sample. A sample of manuscript M1 was mounted on a cell on a motorized highprecision XYZ stage. The laser was firing while the sample was moved in the range of pre-defined co-ordinates. Prior to the first laser shot, a background signal was measured for 15 s. An optical micrograph of the laser path on the surface of the paper sample is shown in Fig. 17.7. The ablation was

771

Ewa Bulska and Barbara Wagner

3x10 6

.-

2X106 xl

6

6 1I x10 xlO

500xlO

0 0,5

110

1,5

2,0

2,5

Distance, mm. Fig. 17.7. Distribution pattern of Fe and Cu across the written area. Above: optical micrograph of the character across which the distribution pattern was measured.

performed over a distance of 2.5 mm along a line perpendicular to the inked area, beginning from the outside of the line, passing through the area covered by ink and finishing at the opposite side out of the inspected line. Although only qualitative, from our results it could be concluded that there is no evidence of higher concentration of both investigated elements at the edges of the written characters in the M1 sample. In the samples analysed, the distribution of iron and copper across the line was found to be fairly uniform. In our opinion the observed scattering of the results

772

A study of ancient manuscripts exposed to iron-gall ink corrosion can be explained by the non-uniform thickness of the ink layer along the ablated line.

17.2.5 Elemental analysis by inductively coupled plasma mass spectrometry The results obtained by the above-described methods were either structural or qualitative. Only XRF measurements offered semi-quantitative information, but that was limited to major elements and bulk analysis. For the determination of trace elements a more sensitive method was required. For this purpose, ICP-MS was chosen because it offers excellent detection limits (at the ng/l level) and, additionally, allows for a multi-elemental determination while only requiring minute samples taken from the artefact. Precise micro-sampling by means of a titanium micro-blade allowed to obtain results with an appropriate spatial resolution. Micro-samples having masses between 0.725 mg and 0.800 mg were removed from manuscript samples collected by conservators and donated for study. These micro-samples were distinguished according to the visible ink marks on the surface. A Maxidigest MX350 (Prolabo, France) microwave system was used for digestion (in 1:1 HNO 3 :water) of the paper samples prior to the ICP-MS measurements. Samples and blank solutions were digested with the same microwave program. A sector field ICP-MS (with high-resolution capabilities, Finigan MAT GmbH, Germany) equipped with a 200 ,l micro-concentric nebulizer was used. ICP-MS in low or medium resolution mode was used, depending on the spectroscopic interferences that could be expected for the element being determined. The low resolution mode (m/Am = 300) was used for 2 3 Na, 107Ag, 109 Ag, 2 06Pb, 2 07pb, 2 08 pb and 2 09 Bi since no detectable interferences occur at those masses. The medium resolution mode (m/Am = 3000) was required for 24 Mg, 27A1, 55Mn, 56 Fe, 57 Fe, 6 3 Cu, 65 Cu, 6 6 Zn and 68Zn. Only those elements in which the concentration was above the detection limit were taken into account. The results listed in Table 17.2 show the difference between both groups of samples with respect to the content of selected elements. In particular, the concentration of Fe, Cu, and Pb varies between both investigated parts of the manuscript, and depends on the presence of ink on the paper surface. The samples covered by ink are characterized by a higher concentration of all these elements when compared with the non-covered areas. The concentrations of Na, Ag, Cd, and Al were found to be independent of the presence of ink.

773

Ewa Bulska and Barbara Wagner TABLE 17.2 Quantitative ICP-MS results on element concentrations in written and unwritten areas Investigated element

Written area (g/lg)

Unwritten area (g/g)

Fe Cu Pb Zn Na Mg Al Mn Ag

13,359 99 6970 + 31 7123 + 55 3452 + 125 3393 + 18 1979 + 30 1398 + 54 204 + 2 7±2

955 6 148 + 3 2787 + 33 292 + 14 3160 + 30 808 + 10 1962 + 81 73 + 1 4+1

From the results presented in Table 17.2, some elements (Fe, Cu, Pb, Zn, Mg, Ca, Mn) were chosen for data visualization in samples of manuscripts M1, M2 and M3. For this purpose the proportions between selected elements in the ink were recalculated. It is clear that in all investigated samples, iron is the major element while the concentration of copper differs between the samples. While in the M1 and M2 samples the concentration of Fe and Cu as well as the ratio between both elements are similar, the investigated M3 sample mainly contains iron (above 90%). According to Attanasio [50], even traces of copper could influence the degradation of the paper. The total concentration of other elements (Pb, Zn, Mg, Ca, Mn) is less than 10% and their proportions differ from one sample to another. Because the proportions between elements were quite different for each of the investigated manuscripts, only one was chosen as a prototype for model samples. In order to reproduce the composition of the ink used in manuscript Meditationes, passionis Domini Nostri Iesu Christi (M1), the content of Fe and Cu in model samples were controlled (see section 17.3.3).

17.2.6

Graphite furnace atomic absorption spectrometry

The direct analysis of solid samples is also possible with the use of GF-AAS. Although many publications describe the direct analysis of solid or slurry samples [62-65], only a few of them concern the analysis of paper [51]. It is obvious that methods requiring the preparation of a slurry need to be regarded as destructive. However, the very good detection limit of GF-AAS allows for determinations on the basis of minute samples and limits the 774

A study of ancient manuscripts exposed to iron-gall ink corrosion destructiveness of the method to the micro-scale. GF-AAS was used for the determination of Fe and Cu either in the slurry or in solution. In this work, GF-AAS was used mainly for investigations dealing with the preparation of the proper conservation method, and for this purpose was found to be even more convenient than ICP-MS. The results of this investigation are described in detail in section 17.3.3. An atomic absorption spectrometer model 4100 ZL equipped with a THGA graphite furnace (Perkin Elmer, Germany) with longitudinal Zeeman background correction was also used. Slurry samples were introduced into the atomizer with an AS-70 autosampler. Hollow cathode lamps for Cu (Beckman, UK) and Fe (Narva, Germany) were run at 3 and 11 mA, with recording of analytical lines at 324.8 and 248.3 nm using spectral bandwidths of 0.7 and 0.2 nm, respectively. An ultrasonic bath, model Cu-6 (Branson, USA), was used for agitation of the slurries. An optical microscope, model PME 3 (Olympus, Japan), was used to observe the size distribution of paper particles in the slurries. Calibration curves were constructed by adding a known amount of each element to the slurries prepared from chromatographic paper. Real samples were taken from the manuscript Meditationes, passionisDomini nostri Iesu Christi. As it was used for the methods described above, two different locations were analysed: samples which had almost no ink on their surface and samples with as much ink as was possible to find. The content of Fe and Cu was found to vary according to the area of the manuscript from which samples were taken and could be correlated with the amount of ink on the paper area. For samples taken from the unwritten parts of the manuscript the content of both metals was in the range of several mg/g (Fe: 3.3 ± 1.3 mg/g; Cu: 1.6 ± 0.8 mg/g). The mean content of metals for the micro-samples completely covered by ink was 32.8 ± 0.9 mg/g of Fe and 15.2 ± 0.5 mg/g of Cu, respectively. 17.2.7

Mssbauer spectrometry

Mossbauer spectroscopy (MS) was developed after the first observation of recoilless nuclear resonance absorption of gamma rays undertaken by Rudolf L. Mossbauer in 1958 [66]. This effect has been observed for over 40 elements, but not all are suitable for measurement. The chemistry of iron is by far the most extensively explored when compared with other Mossbaueractive elements. The Mossbauer effect of 57 Fe is relatively easy to observe. The spectra are well resolved and reflect important information about bonding and structural properties [67]. 775

Ewa Bulska and Barbara Wagner Mossbauer spectroscopy was found to be a powerful tool for the determination of the Fe(II)/Fe(III) ratio, especially in paramagnetic materials [68]. Additional information can be obtained on the local environment of the Fe atoms, which can be used as a fingerprint of the chemical phases that are present. Usually, in M6ssbauer spectroscopy the signal is measured as the resonant absorption of y radiation by the sample in transmission geometry. For this standard set-up, the mass density has to be in the order of 20 mg/cm 2 combined with a sample diameter of 5 mm. This was not sufficient, however, for the measurement of the Fe(II)/Fe(III) ratio in the samples taken from the ancient manuscripts where the ink forms a thin layer on the surface of the paper support. Another possibility is the detection of conversion and Auger electrons that result from the Mossbauer absorption process (also called the CEMS technique). Due to the limited travelling range ofthese electrons (ofthe order of 100 nm in pure Fe), the mass that is required for this technique is of the order of 80 Ag/cm 2 . Thus, the characterization of the iron-gall ink in valuable ancient manuscripts is feasible, although-as a thin layer-the ink constitutes only a small fraction of the analysed material (mainly paper). The prerequisite to get a meaningful CEMS signal is the abundance of Fe phases within a 100 nm thick surface layer. In the first step, three samples from manuscripts M1, M2 and M3 were analysed by means of the CEMS technique. The area covered by ink on each sample was about a few mm2 . A reference sample was also measured. From the preliminary results performed within a 28 h exposure time, it turned out that only in the case of the M2 and M3 samples, a fairly small signal could be detected. Measuring times in the order of several days with a 30 mCi 57 Co(Rh) source were required in order to obtain statistically meaningful signals. A high-sensitive parallel plate avalanche counter was used for this purpose. Even under these conditions no signal could be detected from the M1 sample. Figure 17.8 shows the M6ssbauer spectra derived from samples of M2 and M3 were appropriate; one can see that the sub-spectra of the different charge states are well separated. The fraction of Fe 2+ is equal to about 15% in both cases, with the statistical error at + 3%. In M6ssbauer spectroscopy only information from the Fe present at the surface is obtained. This explains why it was not possible to register the spectra for the M1 sample, which was taken from a document written on poorly sized paper. As described in section 17.1.3, the presence of sizing in the paper structure influences the penetration of the ink solution during the writing process and as a consequence influences the spreading of the corrosion effect. This has to be taken into account when investigations of model samples are performed. In this work, most of the model samples were

776

A study of ancient manuscripts exposed to iron-gall ink corrosion

C) 0

Velocity [mm/s]

== Z3 (I,

0

c)

Velocity [mm/s] Fig. 17.8. M6ssbauer spectra derived from samples of manuscripts M2 and M3.

intentionally prepared by means of Whatman paper (see section 17.3.1) in order to avoid the influence of modern sizing agents. Whatman paper also proved not to be a suitable substrate for M6ssbauer measurements. This is a good example of problems that can arise during investigations of unique items: (i) the specificity of the methods always need to be considered for the

777

Ewa Bulska and Barbara Wagner evaluation of the results; (ii) the interpretation of the results for real ancient objects requires detailed knowledge of its properties. 17.2.8 Investigation of Fe(II)/Fe(III) by X-ray absorption near edge spectroscopy The main drawback of classical wet-chemical techniques for obtaining valence state information on analytes and to some extent also of Mbssbauer spectroscopy, is their bulk analysis character, requiring a large quantity of homogeneous matter to be available for analysis [69]. XANES is a synchrotron-based technique for obtaining information about the crystal and electronic structure, oxidation states and composition in the near-surface region (see Chapter 4). Micro-XANES offers structural information with high lateral resolution, especially for amorphous materials [70-72]. The advantages of this method are mainly based on the fact that the energy position and shape, together with the pre-edge feature in XANES spectra, vary with oxidation state, geometry, spin state and neighbours of absorbing atoms [73]. The basic process of X-ray absorption is the excitation of electrons from a deep core level of a selected atom, by the absorption of photons. XANES spectroscopy incorporates the structure below as well as above the ionization potential. Spectral features below the ionization potential are attributed to transitions of the excited electron to non-occupied molecular orbitals. The region above the ionization potential is dominated by multiple scattering effects of the outgoing electron wave. This part of the spectrum contains information on the geometrical arrangement of atoms around the absorbing atom and the electronic structure of this atom [74]. The most important observation is that the energy of the absorption edge depends on the valence state of the investigated compound. Accordingly, for higher valence states, a shift towards higher energies of the edge is observed [75]. R-XANES measurements were executed at HASYLAB (Hamburg, Germany) at beamline L with a lateral resolution of 30-50 Am in order to determine the local Fe(II)/Fe(III) ratios in fragments of original manuscripts as well as in model samples. The specimens are viewed with a horizontally mounted microscope equipped with a CCD camera. Since the XANES region of the spectrum cannot be described analytically, the available information has to be extracted by comparing the spectra of reference compounds with the spectrum of the investigated compound. In order to be able to extract quantitative information on the Fe(II)/Fe(III) ratio from the Fe-K profiles, FeSO 4 and Fe 2(SO 4) 3 were used to record the reference profiles. For this purpose, finely ground sulphate powders were diluted to 1% with

778

A study of ancient manuscripts exposed to iron-gall ink corrosion

boron-nitride powder and pressed into ca. 0.5 mm thick pellets. XANES spectra were obtained by measuring the Fe-K, fluorescence intensity as a function of incident beam energy. The latter was scanned from 7060 eV (below the Fe-K absorption edge) to 7310 eV (above the edge). The reference profiles are shown in Fig. 17.9. No indication of self-absorption was found in the reference profiles or in the profiles obtained for the manuscript samples. In all investigated samples, the Fe-K XANES profiles from the ink could be expressed as a linear combination of the FeSO 4 and Fe 2 (SO4 )3 XANES profiles. In the examined documents from the 16th century a variety of Fe(II)/Fe(III) ratios could be observed. It should be pointed out that, in opposition to Mossbauer measurements, meaningful results were obtained for samples taken from the manuscripts M1, M2 and M3. This means that XANES measurement was not limited by the composition or nature of the paper support. An overview of the average content of Fe(II) and Fe(III) obtained in this manner is shown in Table 17.3. The Fe(II) content varies from a few percent (M3) up to about 50% in M1. In order to establish whether a relation exists between the distance of a particular position relative to the borders of a character and the local Fe oxidation state, Fe(II)/Fe(III) specific distributions were recorded from a sub-area of a single character (Fig. 17.10). The results taken from different points indicate that, depending on the exact location inside a character, the oxidation of iron progressed to a different extent. In this case, around the outside borders of the character, iron is predominantly present as Fe(III) while in the inner part of the character more Fe(II) is present. SCA

o6000 C

4000

o

2000.

O 0 ·

/)

.

FeS04

7060 7110 7160 7210 7260 7310

SCA

6000

Fe

2

(S

4) 3

LL 4000 2000. 0 7060 7110 7160 7210 7260 7310

Fig. 17.9. XANES spectra derived from reference compounds FeSO 4 and Fe(SO 4) 3. 779

Ewa Bulska and Barbara Wagner TABLE 17.3 Average Fe(II) and Fe(III) content for three 16th century manuscripts Investigated manuscript

Fe(II) (wt%)

Fe(III) (wt%)

M1 M2 M3

48 9 21+9 6 2

52 9 79 9 94 2

Results were calculated from data obtained for six different measuring points.

17.3 17.3.1

SEARCHING FOR THE CONSERVATION TREATMENT Reconstitution of manuscript by model samples

The sampling of historical artefacts poses a crucial problem in archaeometry. The ongoing discussion takes into account the unique character of the analysed artefacts and the limited amount of matter that can be taken for any investigation [2,3]. The removal of samples needed for certain analytical purposes is often unacceptable to art historians, conservators or curators of museum collections. Nevertheless, the systematic study and identification of specific transformation processes, or other destructive phenomena occurring in artefacts, often requires the availability of a much larger number of samples or amounts of sampled material than what is ethically acceptable. In such situations, it is appropriate to perform experiments on model samples that mimic the relevant properties of the original artefacts as closely as possible. The possibility of using model samples with deposited iron-gall ink solutions was evaluated since it is clearly not possible to exclusively employ samples of ancient manuscripts during the detailed investigations that are necessary for the development of conservation strategies. The support for the model samples was a Whatman chromatographic paper chosen according to Whitmore and Bogaard [73]. Two series of model samples were prepared. Series Awas used for rendering inked areas and for this purpose 5 ul of a solution of self-prepared iron-gall ink was deposited onto a paper disk of 6 mm diameter. Series B, intended for rendering free iron ions, was prepared by depositing a solution of FeC13 on similar paper disks. All paper samples were dried under an IR lamp for 15 min, and then stored separately in closed vessels. Iron-gall ink was prepared according to a recipe reflecting the historical formulae [19]. Ferrous sulphate (4.20 g), tannin (4.92 g) and gum Arabic (3.14 g) were mixed together and diluted to a final volume of 100 ml. The solution was filtered after 24 h and analysed

780

A study of ancient manuscripts exposed to iron-gall ink corrosion for Fe and Cu content. In order to reflect the composition of the ink used in manuscript M1 (see Fig. 17.11) the concentration of copper in the ink solution was increased. This was done by addition of blue vitriol (copper sulphate) to the ink. The final concentration of copper in ink solution was established to be around 50% of the concentration of iron (Fig. 17.10). The strategy of the preparation of the model samples is presented in Fig. 17.12.

re-

mage

Fig. 17.10. Fe(II) and Fe(III) distribution maps within the character area (data from i-XANES). (For a colored version of this figure, see Plate 17.11.)

781

Ewa Bulska and Barbara Wagner -. Mg

Mg

N Ca

nCa

nM

.Mn

nFe

Fe

Ca

;I Cu

M

Mg -.C Mn e Fe Cu

.Zn

Zn

Zn

=Pb

Pb

, Pb

Fig. 17.11. Diagrams showing the relative distribution within major elements of three 16th century manuscripts (M1, M2 and M3). 17.3.2

Requirement for conservation treatment

According to Neevel and Reissland [20-22], iron bound to gallic acid does not participate in the degradation of paper whereas iron ions are likely to be responsible for the metal-catalysed oxidation of cellulose. Therefore, in order to diminish the unwanted destruction of cellulose chains, a conservation treatment of the paper should include not only a deacidification, but also a

O

,

I~

~--1 I~=

m

4

Fig. 17.12. The strategy for preparation of model samples series A and B.

782

A study of ancient manuscripts exposed to iron-gall ink corrosion deactivation of the non-bound iron ions that are present [17,20]. This can be achieved either by removal of the active ions from the paper or by binding them into very stable complexes. It has been shown, however, that only a few chelating agents are able to block the catalytic activity of iron ions [17,20]. An additional possibility is the application of antioxidants (such as lignin) that react with radicals faster than cellulose does [74]. In recent years, several successful deacidification methods have been proposed for the conservation of individual items and for mass conservation [12,25]. Yet, it appears that these procedures are not sufficient to ensure a total protection of manuscript papers written with iron-gall ink. Indeed, it was found that metal-catalysed oxidation can take place independently from acid hydrolysis and that therefore a special conservation strategy needs to be developed for iron-gall ink corrosion [19,21,74]. In this work, special attention was devoted to the extraction of iron ions from paper by means of aqueous solutions of complexing agents. The optimal procedure should be able to effectively extract the iron deposited in the form of FeC13 from model samples, while iron deposited in the form of ink should remain on the model samples without destroying or otherwise altering the ink's colour. The applied solution should influence the paper material neither by destroying the cellulose structure nor by colouring the paper. The complexing agents that could potentially be used for the conservation of manuscripts endangered by iron-gall ink corrosion should fulfil several requirements (see Table 17.4). The first criterion is that they should form stable complexes with iron, though these should be less stable than the iron-gallate complex, so as not to bleach away the colour of the original ink. TABLE 17.4 Requirements for complexing agents used for the extraction of iron from paper Should form stable complexes with iron ions Should not be destructive to cellulose Should be stable in neutral or slightly alkaline conditions Should preferably be colourless Should dissolve in water or water/alcohol Should be easy to prepare and apply

...that are less stable than ink compounds so as not to weaken the ink colour

... therefore easily washed-out from the paper

783

Ewa Bulska and Barbara Wagner These complexes, preferably colourless, should be easily washed-out from the paper, stable in a broad pH range while they should also be non-destructive to cellulose. The last but not least requirement concerns the manner of application, which should be as simple as possible for routine use. 17.3.3

Investigation of the model samples

Both sets of the model samples (series A and B) were used for the investigation of the behaviour of iron in the presence of various complexing agents when the sample was immersed in the washing solution. The total amount of iron extracted from the paper into the solution of complexing agents was measured by GF-AAS (as described in section 17.2.6). Simultaneously, the efficiency of iron complex formation in the washing solution was investigated by means of UV/VIS spectroscopy by using the wavelength of maximum absorbance for iron complexes with the complexing agent being investigated. This method was used for the determination of the fraction of the total iron content that was bound to the complexing agent. After extraction, ICP-MS was used to determine the residual amount of iron and other elements in the model samples. In all experiments, paper disks were immersed into a solution of a specific complexing agent. During the extraction a few micro-litres of the aliquots were sequentially pipetted and analysed with respect to the iron content by GF-AAS. After extraction, all paper dots were dried and subjected to microwave digestion (as described in section 17.2.5), and then an elemental analysis was performed using ICPMS. In most cases, the sum of the iron amounts determined by GF-AAS in the solution and by ICP-MS in the paper disks after extraction was close to 100% of the total amount of iron deposited on the model samples. The combined use of UV/IS, GF-AAS and ICP-MS allowed to monitor the distribution of iron between the model samples and the washing solutions for different chelating agents and as a function of pH and exposure time. Thus, this strategy could be used for the evaluation of the conservation treatments without the necessity of taking material from the artefacts. However, it should be pointed out that any new conservation treatment, before it is brought into practice, has to be checked for its usefulness by a special test of the samples exposed to accelerating ageing tests. 17.4

CONCLUDING COMMENTS

Analytical investigations have become an important part of the labour devoted to cultural heritage. They are useful for the purposes of diagnosis as 784

A study of ancient manuscripts exposed to iron-gall ink corrosion well as conservation. In this project various modern instrumental techniques (SEM, XRF, EPMA, LA-ICP-MS, ICP-MS, GF-AAS, Mbssbauer spectroscopy, p-XANES) were used to obtain structural and chemical information from paper samples obtained from historical manuscripts endangered by irongall ink corrosion [47-49,58,59]. On the basis of the results obtained, model samples were prepared. Those samples were then used for the evaluation of the proposed procedure for the extraction of non-bound iron ions from the inked part of the manuscript. It is believed that such an investigation can be useful for the elaboration of suitable conservation treatments for iron-gall ink corrosion. Acknowledgements The authors want to express their thanks to the Conservation Division of Manuscripts of the National Library of Poland for supporting them with original paper samples used for investigation. We wish to thank Prof. A. Hulanicki and Prof. H.M. Ortner for many valuable discussions and critical remarks during all stages of the project. We thank Prof. K. Janssens for his interest in this project and for jointly performing XANES investigations. We greatly acknowledge support from Prof. W. Wegscheider. We also thank T. Meisel, M. Heck and B. Stahl for their help with ICP-MS (T.M.), XRF (M.H.) and Mossbauer (B.S.) measurements. REFERENCES 1 2 3 4 5 6 7 8 9 10

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