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Raman analysis of iron gall inks on parchment
Vibrational Spectroscopy 41 (2006) 170–175 www.elsevier.com/locate/vibspec
Raman analysis of iron gall inks on parchment Alana S. Lee a,b,*, Peter J. Mahon c, Dudley C. Creagh a a
Cultural Heritage Research Centre, University of Canberra, Canberra, ACT 2601, Australia b National Archives of Australia, Mitchell, ACT 2911, Australia c Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia Received 4 November 2005; received in revised form 29 November 2005; accepted 30 November 2005 Available online 26 January 2006
Abstract For centuries, iron gall inks were the most commonly used black inks in the Western world. Many documents, manuscripts and artworks are now suffering varying degrees of degradation due to the corrosive nature and colour instability of the ink. Raman microspectroscopy has been used to analyze historic iron gall inks in situ on parchments, iron gall inks prepared following traditional recipes, separate ink components and various combinations of components. FT-Raman spectroscopy was also applied to the latter samples. This research investigates the optimization of the technique and some problems encountered in the collection of Raman spectra for iron gall inks. Preliminary results from sample analysis are presented. # 2005 Elsevier B.V. All rights reserved. Keywords: Raman microspectroscopy; Iron gall ink; Parchment; Ink analysis
1. Introduction Iron gall inks were used extensively from the late Middle Ages until the early part of last century. Libraries, archives and cultural institutions around the world now hold collections of historic iron gall ink documents, manuscripts and artworks on both paper and parchment supports. These range from everyday letters and indenture documents to medieval illuminated manuscripts, the musical compositions of J.S. Bach and sketches by Rembrandt and Van Gogh. Iron gall inks on parchment supports were used almost exclusively for Federation documents (c1900) of great importance to Australia’s cultural heritage held in the collection of the National Archives of Australia. Many iron gall inks have a corrosive nature and a tendency to undergo colour change from black to brown, often fading quite significantly. Numerous documents, manuscripts and artworks now stand in danger of severe deterioration, while others are in excellent condition. Certain combinations of environmental storage conditions, composition of the ink and/or support [1,2] have and will in many cases result in partial or total loss of the
* Corresponding author. Tel.: +61 2 6201 2121; fax: +61 2 6201 5419. E-mail address: [email protected] (A.S. Lee). 0924-2031/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2005.11.006
paper or parchment due to the ink effectively ‘eating’ its way through the support. The complexity of studying iron gall ink chemistry lies in the numerous recipes and ingredient sources used, resulting in differing compositions and component proportions [3,4]. The ink is essentially made by the combination of iron(II) sulfate with tannins (tannic or gallic acids either from plant gall extract or in later years as pure compounds) in various proportions. These ingredients were combined in water with the tannins breaking down to gallic or di-gallic acid to give rise to the ink complex. Gum arabic was commonly added to give the mixture fluid writing properties and keep the ink particles in suspension. Historic recipes record various preparative methods, including boiling and/or fermenting the mixture [4]. Initially, on combination of the iron(II) ions with the organic tannin component, an iron(II) complex is formed. This complex is water soluble and gives a pale grey purple colour to the ink [5–7]. Once written on the support and exposed to oxygen in the air, this iron(II) complex oxidizes so that the resulting compound responsible for the black ink is believed to be 1:1 iron(III)–pyrogallin [5] or 1:1 iron(III)–gallic acid [6] complexes. These iron(III) complexes are black and insoluble in water. Other compounds and dyes were sometimes added to alter tone, provide immediate visibility and adjust other physical properties of the ink [4,8].
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If the 1:1 ratio of iron(II) sulfate to gallic acid is not achieved by stoichiometric quantities of ingredients used (likely in the case of natural products such as oak galls), excess of an ingredient will occur. The survey of a number of historic ink recipes has found that an excess of iron(II) sulfate was common [9]. It is also known that copper is present as a contaminant in many historic inks [10]. Based on the extensive survey of historic ink recipes, additions of 1 and 10% (w/w) contamination of copper(II) sulfate in iron(II) sulfate have been included in two devised historically representative model iron gall inks [4]. Recent and current research being conducted on the mechanisms of iron gall ink corrosion on paper has shown that the degradation of the cellulose support is caused by two possible mechanisms: metal-catalyzed oxidation by the excess of uncomplexed iron(II) and any contaminant copper(II) ions and acid-catalyzed hydrolysis due to the high acidity of the ink mainly due to the sulfuric acid produced on formation of the complex [8,9]. Brown degradation products of the ink itself are believed to contain a complex mixture of phenols, such as purpurogallin or ellagic acid and various oxides of iron [5]. Identification of an ink on an object as iron gall is important from an historical and curatorial perspective, as well as for conservation treatment and preservation purposes. From a conservation science perspective, identification of an ink as iron gall, and the nature of the iron gall ink composition, is useful in surveying historic collections and documenting the correlation of the occurrence or type of iron gall ink corrosion with respect to certain ink compositions. Possible analysis of degradation products or surface accretions present in the ink also assists in understanding the mechanism of degradation. Iron gall inks range from brown to black tones. Despite typical characteristics of their appearance being different from those of other black and brown inks (i.e. sepia, bistre and carbon) enabling identification in ideal examples, simple visual identification is not always straightforward. Non-destructive analysis for identification of iron gall inks includes infrared reflectography and false colour infrared photography [11], multispectral imaging [12] and microchemical spot testing using bathophenanthroline Fe(II) indicator strips [13]. To further examine the composition of the ink, instrumental analysis applied to iron gall inks include both destructive and non-destructive elemental techniques, used to detect types and quantities of elements in the ink. These include proton-induced X-ray emission spectroscopy [10,14–16], scanning electron microscopy with energy dispersive X-ray microanalysis [17– 21], X-ray fluorescence spectroscopy [22–24], electron probe microanalysis [20], inductively coupled plasma mass spectroscopy [25,26] and atomic absorption spectroscopy [25]. In situ analysis of the oxidation state of the iron in inks has been determined by Mossbauer spectroscopy [27,28] and micro-Xray absorption near edge structure [22,29]. To complement elemental techniques, analysis of molecular components of iron gall inks has been performed by gas chromatography coupled with mass spectrometry [21,30] and Fourier transform infrared spectroscopy [17,31–33]. Despite its advantages, a technique absent from the list of those used regularly in the analysis of iron gall ink is Raman
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microspectroscopy. Using excitation sources of 514 and 633 nm, Raman microspectroscopy has been applied successfully to the in situ analysis of pigments and some coloured inks contained in illuminated manuscripts [34–42] and is now recognized as one of the best ways to identify pigments nondestructively. It seems timely therefore, to investigate the possibility of applying Raman analysis to iron gall inks, which are likely candidates for writing or drawing inks used in historic illuminated manuscripts, documents and artworks. Groups using 514 and 633 nm laser lines have previously regarded analysis of iron gall ink by Raman microspectroscopy as difficult. Only recently, the possibility of analysis of iron gall inks with near-infrared 782 nm excitation Raman microspectroscopy has been demonstrated, initially by removing traces of the inks from medieval manuscripts using dry cotton swabs [43] and then in situ [44]. The ability to use Raman microspectroscopy as an in situ technique has a huge advantage in the analysis of iron gall inks as it is non-invasive in terms of the integrity of these historically important objects. It has the additional benefit of being a nondestructive molecular analysis technique, with high spatial selectivity and sensitivity. 2. Experimental 2.1. Instrumentation Samples were analyzed at the University of Canberra by Raman microspectroscopy using a Renishaw 2000 system with a Peltier cooled CCD detector, and coupled to an Olympus BH2 confocal microscope. The excitation sources used with this instrument were 633 nm (HeNe) and 782 nm (NIR diode) laser lines. Some analyses were also carried out with a 514 nm (argon) laser using a Renishaw 2000 system and 1064 nm (Nd-YAG) laser excitation using a Bruker IFS FT-Raman spectrometer, both at the University of Sydney. Laser power at the sample was up to 2 mW for the newer inks and complexes formed but was reduced to around 1 mW, by neutral density filters on the Renishaw 2000 system, for analysis of the historic samples. The Raman signal was collected with a spectral resolution of 1 cm 1 over the range 100–2000 cm 1 (or 4000 cm 1) using 20 or 50 microscope objectives. 2.2. Iron gall ink samples 2.2.1. Ink complexes and components Approximate 1:1 iron(III) complexes were prepared from the combination of iron(II) sulfate with gallic acid, tannic acid or crushed oak galls in deionised water: (i) 1.22 g gallic acid (3,4,5-trihydroxybenzoic acid; Sigma) was dissolved in 150 mL heated deionised water. 2.00 g iron(II) sulfate (FeSO47H2O; Merck) was dissolved in a small amount of deionised water. The two solutions were combined and the reaction mixture left to oxidize with
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occasional stirring for a week. The mixture was then filtered and the collected black solid washed repeatedly with deionised water. It was initially dried under vacuum and then air dried. (ii) 1.22 g tannic acid (Sigma) was boiled in 150 mL deionised water and combined with 2.00 g iron(II) sulfate dissolved in deionised water. A black solid was collected following the procedure in (i). (iii) 3.20 g of crushed oak galls (Kremer Pigmente) were boiled in 150 mL deionised water and combined when cooled with 2.80 g iron(II) sulfate dissolved in deionised water. Once again, a black solid was obtained following the procedure in (i). The complexes and individual components were analyzed using 514, 633 and 782 nm laser excitation Raman microspectroscopy. FT-Raman using a 1064 nm source was also performed. 2.2.2. Prepared inks To test whether any differences in Raman spectra would be observed for different inks, ink solutions prepared in the laboratory 2 years ago using four different historic recipes that had been written on to new parchments, were analyzed by Raman microspectroscopy with excitation sources of 633 and 782 nm. As a further test, three simple inks were prepared to illustrate the effect of different proportions of the two major ink components with ratios of 1:3, 1:1 and 3:1 iron(II):gallic acid. Two percent (w/v) gum arabic (BDH) was added to each of these inks. The resulting inks were thoroughly mixed and then placed in sealed containers. The inks were applied to new parchment and allowed to dry.
Fig. 1. 782 nm Raman spectra of iron gall ink ingredients iron(II) sulfate and gallic acid, and the resulting complex formed. Spectra are vertically offset for clarity.
and 255 cm 1. No additional peaks were present beyond the range shown when the scan was extended to 4000 cm 1. The major Raman peaks observed for the iron gall ink complexes (i–iii) formed using the tannin ingredients, can be seen to be generally common to all three, shown in Fig. 2, with slight variation observed in peak shape and position. Although these analyses were performed in triplicate and have good reproducibility, analysis of further complexes following different preparative methods are needed to remark on ‘real’ or indicative correlations between the spectra and the composition or preparation of the inks. It was observed (when including prepared inks in the following section) that the
2.2.3. Historic samples The naturally aged samples used for this analysis consisted of a number of 19th and early 20th century common English indenture documents written in iron gall inks on parchment. These documents were available for destructive testing and 3 cm2 pieces of the parchment with iron gall ink writing were cut from the single sheets or booklets. Ink areas near the edge of the documents were also placed under the microscope and analyzed directly on the document. 3. Results and discussion 3.1. Ink complexes and components Fig. 1 shows the 782 nm Raman spectrum of the complex formed in (i) along with the Raman spectra of the original iron(II) sulfate and gallic acid components. Spectra were obtained for the complex with all four laser lines, the best being 633 and 782 nm, using a 20 objective, 2 mW laser power and five accumulations with 120 s integration. The spectra for the ink complex show Raman bands at 1575, 1470 (strong), 1425, 1315 (strong), 1230, 1095 (weak), 960, 815, 710 (weak), 620–500 (broad due to multiple peaks), 400
Fig. 2. 782 nm Raman spectra of complexes formed with iron(II) sulfate and (a) tannic acid, (b) crushed oak galls and (c) gallic acid. Spectra are vertically offset for clarity.
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3.3. Historic samples
Fig. 3. 782 nm Raman spectra of four inks prepared by different historic recipes. Spectra are vertically offset for clarity.
complexes formed with tannic acid and crushed galls gave a higher fluorescence baseline than those prepared with gallic acid alone. This is probably due to the inclusion of more complex organic compounds in the tannins. Complexes and inks formed using combinations of the above ingredients along with pyrogallol, iron(III) sulfate, copper(II) sulfate contamination and various added compounds and dyes will also be analyzed to compare different compositions representative of historic recipes. 3.2. Prepared inks Fig. 3 shows the 782 nm Raman spectra (20 objective, 1 mW at sample, five accumulations with 120 s integration) of the four inks that had been prepared following historic recipes and written on to new parchment 2 years prior to this analysis. Again, subtle but fairly reproducible variations on the general iron gall ink spectrum have been noticed. Collection of further spectra of known recipes should enable the determination of whether these are ‘real’ differences. In addition to reproducibility of runs for the same ink, observation of reproducibility between different inks prepared in the same way is essential. However it should be considered that spectral changes with ageing and degradation of the ink might limit the potential for this as a diagnostic tool for ink composition. The inks in Fig. 3 all contain gum arabic, and their spectra do not look markedly different from those of the complexes without gum arabic shown in Fig. 2. Given that the spectrum of a dried solution of gum arabic has no peaks above a fluorescent baseline observed with 782 nm excitation, it is likely that this compound does not contribute significantly to the spectra of freshly applied iron gall ink when used in quantities typical of many inks. Whether gum arabic contributes to the increased fluorescence present with most iron gall inks on ageing or degradation has not yet been investigated. Initial analysis of the three inks prepared with iron(II) sulfate and gallic acid in different proportions, analyzed as dried ink lines applied to parchment, did not show any significant difference between the spectra. However, gallic acid crystals were identified as formed on the surface of the 1:3 iron:gallic acid ink. An excess of iron(II) is better observed by elemental techniques.
Excitation with the 782 nm laser line has proven to be the most successful for analysis of the aged inks. Both 514 and 633 nm source analyses were limited by fluorescence of the ink itself which obscured any Raman signals in most samples. It is believed that no interference from the parchment support is present when analyzing inks of typical layer thickness. Future work will assess the further suitability of 1064 nm FT-Raman to historic samples as a decrease in fluorescence with higher wavelength excitation is anticipated. Analysis of historic iron gall inks on parchment was also found to vary somewhat with certain instrumental parameters and sample target area. Generally, when using the 782 nm laser, the recommended parameters to achieve optimum spectra were found to be the following. A 50 microscope objective gave improved Raman signal, due to increased collection angle and increased spatial selectivity over a 20 objective. This allowed avoidance of cracks in the ink and crystal inclusions or surface accretions that could contribute to fluorescence. Fortuitously, this spatial selectivity will also allow selective analysis of non-fluorescent inclusions and accretions. Approximately 1 mW laser power at the sample. It was found that both lower and higher powers yielded poorer spectra and 2 mW in some cases caused micro-cracking and ‘burning’ of the target. Flat, homogeneous surface target area, again to increase signal collection and avoid interference from fluorescent inclusions or accretions. It is also best to choose thicker over thin layers of ink to increase the Raman signal due to the ink and avoid influence from the support. This will be of greater concern to inks on paper supports due to the increased absorption of the ink into the paper matrix. Preventing movement with light weights around the sample target area is necessary especially at high magnification since parchment and paper are responsive to the heat generated by the laser or white light of the microscope. This movement causes the target area to shift out of focus and leads to poorly resolved spectra. Consideration of these conditions with 10 accumulations of 40 s integration, enabled spectra with good reproducibility to be obtained in situ at different locations on many of the historic ink samples. Fig. 4 shows duplicate spectra collected for inks written on two 19th century parchment documents. In general, the four largest bands were observed and used as positive identification of an iron gall ink in the historic samples. These occured around 1475, between 1310 and 1350 (variable), 490–640 (combined broad band) and 400 cm 1. The less intense bands were present to a lesser and more varied extent. Results also indicate that natural ageing or degradation causes an increase in fluorescence of the ink as compared to that of freshly prepared inks, resulting in poorer resolution and broader, weaker bands above a higher fluorescence baseline. In the case of larger objects such as manuscripts, books, or large drawings, re-direction of the beam to a support holding the object vertically is possible to facilitate in situ analysis.
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Carter and Koman Tam (University of Sydney) for assistance and input during the use of the Raman instrumentation at the Vibrational Spectroscopy Facility. This research is being carried out under an Australian Research Council Linkage Project Grant, in which the University of Canberra, Australian National University, National Archives of Australia, National Museum of Australia and National Film and Sound Archive are participants. References
Fig. 4. 782 nm Raman spectra obtained from inks on two historic documents (a) 1877 indenture document at two locations and (b) 1859 indenture document at two locations. Spectra are vertically offset for clarity.
Remote Raman spectroscopy using fibre optic instrumentation may also be possible. 4. Conclusions This preliminary study has demonstrated the successful collection of Raman spectra for iron gall inks in situ on parchment supports with 782 nm laser excitation. Recommended conditions suggested by preliminary analyses include the use of a 50 microscope objective and laser power of around 1 mW at the sample. Analysis with sources of 514 and 633 nm laser excitation was successful on freshly applied inks and the prepared ink complexes, but was limited in the analysis of historic samples due to increased fluorescence of the ink. In future work on historic samples, the use of 1064 nm FTRaman will be considered, as will the application of this technique to more ink samples covering a wider time span. Further analysis of prepared inks of known composition is required to see if spectral differences are diagnostic or relevant. Monitoring changes in iron gall ink chemistry during corrosion and colour change degradation processes will also be considered. It is hoped that Raman analysis will be able to assist in the identification of iron gall inks on historic objects and to complement other techniques used to study ink degradation and other areas of investigation into the degradation of the support. An understanding of the degradation mechanisms of iron gall ink corrosion is essential for development of successful conservation treatments to preserve the extensive range of documents, manuscripts and artworks affected by this process. Acknowledgements We would like to thank Caroline Whitley and Ian Batterham (National Archives of Australia), Dr. Peter Vandenabeele (Ghent University), Maria Kubik (Australian National University), Robin Tait (National Museum of Australia), Dr. Vincent Otieno-Alego (Australian Federal Police) for their contribution to and enthusiasm for the project; Dr. Elizabeth
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[38] P. Vandenabeele, B. Wehling, L. Moens, B. Dekeyzer, B. Cardon, A. Von Bohlen, R. Klockenkamper, Analyst 124 (1999) 169. [39] G.D. Smith, R.J.H. Clark, Rev. Conserv. 2 (2001) 92. [40] M. Clarke, Rev. Conserv. 2 (2001) 3. [41] K.L. Brown, R.J.H. Clark, J. Raman Spectrosc. 35 (2004) 181. [42] S. Pages-Camagna, A. Duval, H. Guicharnaud, J. Raman Spectrosc. 35 (2004) 628. [43] P. Vandenabeele, L. Moens, in: K. Janssens, R. Van Grieken (Eds.), NonDestructive Microanalysis of Cultural Heritage Materials (Comprehensive Analytical Chemistry XLII), Elsevier, Amsterdam, 2004, p. 635. [44] A.S. Lee, P. Vandenabeele, P.J. Mahon, D.C. Creagh, Raman analysis of iron gall ink, in: Third International Conference on the Application of Raman Spectroscopy in Art and Archaeology, Paris, 2005, p. 89 (poster presentation).
Raman analysis of iron gall inks on parchment Alana S. Lee a,b,*, Peter J. Mahon c, Dudley C. Creagh a a
Cultural Heritage Research Centre, University of Canberra, Canberra, ACT 2601, Australia b National Archives of Australia, Mitchell, ACT 2911, Australia c Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia Received 4 November 2005; received in revised form 29 November 2005; accepted 30 November 2005 Available online 26 January 2006
Abstract For centuries, iron gall inks were the most commonly used black inks in the Western world. Many documents, manuscripts and artworks are now suffering varying degrees of degradation due to the corrosive nature and colour instability of the ink. Raman microspectroscopy has been used to analyze historic iron gall inks in situ on parchments, iron gall inks prepared following traditional recipes, separate ink components and various combinations of components. FT-Raman spectroscopy was also applied to the latter samples. This research investigates the optimization of the technique and some problems encountered in the collection of Raman spectra for iron gall inks. Preliminary results from sample analysis are presented. # 2005 Elsevier B.V. All rights reserved. Keywords: Raman microspectroscopy; Iron gall ink; Parchment; Ink analysis
1. Introduction Iron gall inks were used extensively from the late Middle Ages until the early part of last century. Libraries, archives and cultural institutions around the world now hold collections of historic iron gall ink documents, manuscripts and artworks on both paper and parchment supports. These range from everyday letters and indenture documents to medieval illuminated manuscripts, the musical compositions of J.S. Bach and sketches by Rembrandt and Van Gogh. Iron gall inks on parchment supports were used almost exclusively for Federation documents (c1900) of great importance to Australia’s cultural heritage held in the collection of the National Archives of Australia. Many iron gall inks have a corrosive nature and a tendency to undergo colour change from black to brown, often fading quite significantly. Numerous documents, manuscripts and artworks now stand in danger of severe deterioration, while others are in excellent condition. Certain combinations of environmental storage conditions, composition of the ink and/or support [1,2] have and will in many cases result in partial or total loss of the
* Corresponding author. Tel.: +61 2 6201 2121; fax: +61 2 6201 5419. E-mail address: [email protected] (A.S. Lee). 0924-2031/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2005.11.006
paper or parchment due to the ink effectively ‘eating’ its way through the support. The complexity of studying iron gall ink chemistry lies in the numerous recipes and ingredient sources used, resulting in differing compositions and component proportions [3,4]. The ink is essentially made by the combination of iron(II) sulfate with tannins (tannic or gallic acids either from plant gall extract or in later years as pure compounds) in various proportions. These ingredients were combined in water with the tannins breaking down to gallic or di-gallic acid to give rise to the ink complex. Gum arabic was commonly added to give the mixture fluid writing properties and keep the ink particles in suspension. Historic recipes record various preparative methods, including boiling and/or fermenting the mixture [4]. Initially, on combination of the iron(II) ions with the organic tannin component, an iron(II) complex is formed. This complex is water soluble and gives a pale grey purple colour to the ink [5–7]. Once written on the support and exposed to oxygen in the air, this iron(II) complex oxidizes so that the resulting compound responsible for the black ink is believed to be 1:1 iron(III)–pyrogallin [5] or 1:1 iron(III)–gallic acid [6] complexes. These iron(III) complexes are black and insoluble in water. Other compounds and dyes were sometimes added to alter tone, provide immediate visibility and adjust other physical properties of the ink [4,8].
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If the 1:1 ratio of iron(II) sulfate to gallic acid is not achieved by stoichiometric quantities of ingredients used (likely in the case of natural products such as oak galls), excess of an ingredient will occur. The survey of a number of historic ink recipes has found that an excess of iron(II) sulfate was common [9]. It is also known that copper is present as a contaminant in many historic inks [10]. Based on the extensive survey of historic ink recipes, additions of 1 and 10% (w/w) contamination of copper(II) sulfate in iron(II) sulfate have been included in two devised historically representative model iron gall inks [4]. Recent and current research being conducted on the mechanisms of iron gall ink corrosion on paper has shown that the degradation of the cellulose support is caused by two possible mechanisms: metal-catalyzed oxidation by the excess of uncomplexed iron(II) and any contaminant copper(II) ions and acid-catalyzed hydrolysis due to the high acidity of the ink mainly due to the sulfuric acid produced on formation of the complex [8,9]. Brown degradation products of the ink itself are believed to contain a complex mixture of phenols, such as purpurogallin or ellagic acid and various oxides of iron [5]. Identification of an ink on an object as iron gall is important from an historical and curatorial perspective, as well as for conservation treatment and preservation purposes. From a conservation science perspective, identification of an ink as iron gall, and the nature of the iron gall ink composition, is useful in surveying historic collections and documenting the correlation of the occurrence or type of iron gall ink corrosion with respect to certain ink compositions. Possible analysis of degradation products or surface accretions present in the ink also assists in understanding the mechanism of degradation. Iron gall inks range from brown to black tones. Despite typical characteristics of their appearance being different from those of other black and brown inks (i.e. sepia, bistre and carbon) enabling identification in ideal examples, simple visual identification is not always straightforward. Non-destructive analysis for identification of iron gall inks includes infrared reflectography and false colour infrared photography [11], multispectral imaging [12] and microchemical spot testing using bathophenanthroline Fe(II) indicator strips [13]. To further examine the composition of the ink, instrumental analysis applied to iron gall inks include both destructive and non-destructive elemental techniques, used to detect types and quantities of elements in the ink. These include proton-induced X-ray emission spectroscopy [10,14–16], scanning electron microscopy with energy dispersive X-ray microanalysis [17– 21], X-ray fluorescence spectroscopy [22–24], electron probe microanalysis [20], inductively coupled plasma mass spectroscopy [25,26] and atomic absorption spectroscopy [25]. In situ analysis of the oxidation state of the iron in inks has been determined by Mossbauer spectroscopy [27,28] and micro-Xray absorption near edge structure [22,29]. To complement elemental techniques, analysis of molecular components of iron gall inks has been performed by gas chromatography coupled with mass spectrometry [21,30] and Fourier transform infrared spectroscopy [17,31–33]. Despite its advantages, a technique absent from the list of those used regularly in the analysis of iron gall ink is Raman
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microspectroscopy. Using excitation sources of 514 and 633 nm, Raman microspectroscopy has been applied successfully to the in situ analysis of pigments and some coloured inks contained in illuminated manuscripts [34–42] and is now recognized as one of the best ways to identify pigments nondestructively. It seems timely therefore, to investigate the possibility of applying Raman analysis to iron gall inks, which are likely candidates for writing or drawing inks used in historic illuminated manuscripts, documents and artworks. Groups using 514 and 633 nm laser lines have previously regarded analysis of iron gall ink by Raman microspectroscopy as difficult. Only recently, the possibility of analysis of iron gall inks with near-infrared 782 nm excitation Raman microspectroscopy has been demonstrated, initially by removing traces of the inks from medieval manuscripts using dry cotton swabs [43] and then in situ [44]. The ability to use Raman microspectroscopy as an in situ technique has a huge advantage in the analysis of iron gall inks as it is non-invasive in terms of the integrity of these historically important objects. It has the additional benefit of being a nondestructive molecular analysis technique, with high spatial selectivity and sensitivity. 2. Experimental 2.1. Instrumentation Samples were analyzed at the University of Canberra by Raman microspectroscopy using a Renishaw 2000 system with a Peltier cooled CCD detector, and coupled to an Olympus BH2 confocal microscope. The excitation sources used with this instrument were 633 nm (HeNe) and 782 nm (NIR diode) laser lines. Some analyses were also carried out with a 514 nm (argon) laser using a Renishaw 2000 system and 1064 nm (Nd-YAG) laser excitation using a Bruker IFS FT-Raman spectrometer, both at the University of Sydney. Laser power at the sample was up to 2 mW for the newer inks and complexes formed but was reduced to around 1 mW, by neutral density filters on the Renishaw 2000 system, for analysis of the historic samples. The Raman signal was collected with a spectral resolution of 1 cm 1 over the range 100–2000 cm 1 (or 4000 cm 1) using 20 or 50 microscope objectives. 2.2. Iron gall ink samples 2.2.1. Ink complexes and components Approximate 1:1 iron(III) complexes were prepared from the combination of iron(II) sulfate with gallic acid, tannic acid or crushed oak galls in deionised water: (i) 1.22 g gallic acid (3,4,5-trihydroxybenzoic acid; Sigma) was dissolved in 150 mL heated deionised water. 2.00 g iron(II) sulfate (FeSO47H2O; Merck) was dissolved in a small amount of deionised water. The two solutions were combined and the reaction mixture left to oxidize with
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occasional stirring for a week. The mixture was then filtered and the collected black solid washed repeatedly with deionised water. It was initially dried under vacuum and then air dried. (ii) 1.22 g tannic acid (Sigma) was boiled in 150 mL deionised water and combined with 2.00 g iron(II) sulfate dissolved in deionised water. A black solid was collected following the procedure in (i). (iii) 3.20 g of crushed oak galls (Kremer Pigmente) were boiled in 150 mL deionised water and combined when cooled with 2.80 g iron(II) sulfate dissolved in deionised water. Once again, a black solid was obtained following the procedure in (i). The complexes and individual components were analyzed using 514, 633 and 782 nm laser excitation Raman microspectroscopy. FT-Raman using a 1064 nm source was also performed. 2.2.2. Prepared inks To test whether any differences in Raman spectra would be observed for different inks, ink solutions prepared in the laboratory 2 years ago using four different historic recipes that had been written on to new parchments, were analyzed by Raman microspectroscopy with excitation sources of 633 and 782 nm. As a further test, three simple inks were prepared to illustrate the effect of different proportions of the two major ink components with ratios of 1:3, 1:1 and 3:1 iron(II):gallic acid. Two percent (w/v) gum arabic (BDH) was added to each of these inks. The resulting inks were thoroughly mixed and then placed in sealed containers. The inks were applied to new parchment and allowed to dry.
Fig. 1. 782 nm Raman spectra of iron gall ink ingredients iron(II) sulfate and gallic acid, and the resulting complex formed. Spectra are vertically offset for clarity.
and 255 cm 1. No additional peaks were present beyond the range shown when the scan was extended to 4000 cm 1. The major Raman peaks observed for the iron gall ink complexes (i–iii) formed using the tannin ingredients, can be seen to be generally common to all three, shown in Fig. 2, with slight variation observed in peak shape and position. Although these analyses were performed in triplicate and have good reproducibility, analysis of further complexes following different preparative methods are needed to remark on ‘real’ or indicative correlations between the spectra and the composition or preparation of the inks. It was observed (when including prepared inks in the following section) that the
2.2.3. Historic samples The naturally aged samples used for this analysis consisted of a number of 19th and early 20th century common English indenture documents written in iron gall inks on parchment. These documents were available for destructive testing and 3 cm2 pieces of the parchment with iron gall ink writing were cut from the single sheets or booklets. Ink areas near the edge of the documents were also placed under the microscope and analyzed directly on the document. 3. Results and discussion 3.1. Ink complexes and components Fig. 1 shows the 782 nm Raman spectrum of the complex formed in (i) along with the Raman spectra of the original iron(II) sulfate and gallic acid components. Spectra were obtained for the complex with all four laser lines, the best being 633 and 782 nm, using a 20 objective, 2 mW laser power and five accumulations with 120 s integration. The spectra for the ink complex show Raman bands at 1575, 1470 (strong), 1425, 1315 (strong), 1230, 1095 (weak), 960, 815, 710 (weak), 620–500 (broad due to multiple peaks), 400
Fig. 2. 782 nm Raman spectra of complexes formed with iron(II) sulfate and (a) tannic acid, (b) crushed oak galls and (c) gallic acid. Spectra are vertically offset for clarity.
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3.3. Historic samples
Fig. 3. 782 nm Raman spectra of four inks prepared by different historic recipes. Spectra are vertically offset for clarity.
complexes formed with tannic acid and crushed galls gave a higher fluorescence baseline than those prepared with gallic acid alone. This is probably due to the inclusion of more complex organic compounds in the tannins. Complexes and inks formed using combinations of the above ingredients along with pyrogallol, iron(III) sulfate, copper(II) sulfate contamination and various added compounds and dyes will also be analyzed to compare different compositions representative of historic recipes. 3.2. Prepared inks Fig. 3 shows the 782 nm Raman spectra (20 objective, 1 mW at sample, five accumulations with 120 s integration) of the four inks that had been prepared following historic recipes and written on to new parchment 2 years prior to this analysis. Again, subtle but fairly reproducible variations on the general iron gall ink spectrum have been noticed. Collection of further spectra of known recipes should enable the determination of whether these are ‘real’ differences. In addition to reproducibility of runs for the same ink, observation of reproducibility between different inks prepared in the same way is essential. However it should be considered that spectral changes with ageing and degradation of the ink might limit the potential for this as a diagnostic tool for ink composition. The inks in Fig. 3 all contain gum arabic, and their spectra do not look markedly different from those of the complexes without gum arabic shown in Fig. 2. Given that the spectrum of a dried solution of gum arabic has no peaks above a fluorescent baseline observed with 782 nm excitation, it is likely that this compound does not contribute significantly to the spectra of freshly applied iron gall ink when used in quantities typical of many inks. Whether gum arabic contributes to the increased fluorescence present with most iron gall inks on ageing or degradation has not yet been investigated. Initial analysis of the three inks prepared with iron(II) sulfate and gallic acid in different proportions, analyzed as dried ink lines applied to parchment, did not show any significant difference between the spectra. However, gallic acid crystals were identified as formed on the surface of the 1:3 iron:gallic acid ink. An excess of iron(II) is better observed by elemental techniques.
Excitation with the 782 nm laser line has proven to be the most successful for analysis of the aged inks. Both 514 and 633 nm source analyses were limited by fluorescence of the ink itself which obscured any Raman signals in most samples. It is believed that no interference from the parchment support is present when analyzing inks of typical layer thickness. Future work will assess the further suitability of 1064 nm FT-Raman to historic samples as a decrease in fluorescence with higher wavelength excitation is anticipated. Analysis of historic iron gall inks on parchment was also found to vary somewhat with certain instrumental parameters and sample target area. Generally, when using the 782 nm laser, the recommended parameters to achieve optimum spectra were found to be the following. A 50 microscope objective gave improved Raman signal, due to increased collection angle and increased spatial selectivity over a 20 objective. This allowed avoidance of cracks in the ink and crystal inclusions or surface accretions that could contribute to fluorescence. Fortuitously, this spatial selectivity will also allow selective analysis of non-fluorescent inclusions and accretions. Approximately 1 mW laser power at the sample. It was found that both lower and higher powers yielded poorer spectra and 2 mW in some cases caused micro-cracking and ‘burning’ of the target. Flat, homogeneous surface target area, again to increase signal collection and avoid interference from fluorescent inclusions or accretions. It is also best to choose thicker over thin layers of ink to increase the Raman signal due to the ink and avoid influence from the support. This will be of greater concern to inks on paper supports due to the increased absorption of the ink into the paper matrix. Preventing movement with light weights around the sample target area is necessary especially at high magnification since parchment and paper are responsive to the heat generated by the laser or white light of the microscope. This movement causes the target area to shift out of focus and leads to poorly resolved spectra. Consideration of these conditions with 10 accumulations of 40 s integration, enabled spectra with good reproducibility to be obtained in situ at different locations on many of the historic ink samples. Fig. 4 shows duplicate spectra collected for inks written on two 19th century parchment documents. In general, the four largest bands were observed and used as positive identification of an iron gall ink in the historic samples. These occured around 1475, between 1310 and 1350 (variable), 490–640 (combined broad band) and 400 cm 1. The less intense bands were present to a lesser and more varied extent. Results also indicate that natural ageing or degradation causes an increase in fluorescence of the ink as compared to that of freshly prepared inks, resulting in poorer resolution and broader, weaker bands above a higher fluorescence baseline. In the case of larger objects such as manuscripts, books, or large drawings, re-direction of the beam to a support holding the object vertically is possible to facilitate in situ analysis.
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Carter and Koman Tam (University of Sydney) for assistance and input during the use of the Raman instrumentation at the Vibrational Spectroscopy Facility. This research is being carried out under an Australian Research Council Linkage Project Grant, in which the University of Canberra, Australian National University, National Archives of Australia, National Museum of Australia and National Film and Sound Archive are participants. References
Fig. 4. 782 nm Raman spectra obtained from inks on two historic documents (a) 1877 indenture document at two locations and (b) 1859 indenture document at two locations. Spectra are vertically offset for clarity.
Remote Raman spectroscopy using fibre optic instrumentation may also be possible. 4. Conclusions This preliminary study has demonstrated the successful collection of Raman spectra for iron gall inks in situ on parchment supports with 782 nm laser excitation. Recommended conditions suggested by preliminary analyses include the use of a 50 microscope objective and laser power of around 1 mW at the sample. Analysis with sources of 514 and 633 nm laser excitation was successful on freshly applied inks and the prepared ink complexes, but was limited in the analysis of historic samples due to increased fluorescence of the ink. In future work on historic samples, the use of 1064 nm FTRaman will be considered, as will the application of this technique to more ink samples covering a wider time span. Further analysis of prepared inks of known composition is required to see if spectral differences are diagnostic or relevant. Monitoring changes in iron gall ink chemistry during corrosion and colour change degradation processes will also be considered. It is hoped that Raman analysis will be able to assist in the identification of iron gall inks on historic objects and to complement other techniques used to study ink degradation and other areas of investigation into the degradation of the support. An understanding of the degradation mechanisms of iron gall ink corrosion is essential for development of successful conservation treatments to preserve the extensive range of documents, manuscripts and artworks affected by this process. Acknowledgements We would like to thank Caroline Whitley and Ian Batterham (National Archives of Australia), Dr. Peter Vandenabeele (Ghent University), Maria Kubik (Australian National University), Robin Tait (National Museum of Australia), Dr. Vincent Otieno-Alego (Australian Federal Police) for their contribution to and enthusiasm for the project; Dr. Elizabeth
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