Non Destructive Detection of Iron Gall Inks by Means of Multispectral Imaging

Non Destructive Detection of Iron Gall Inks by Means of Multispectral Imaging

Part 1: Development of the Detection System by JOHN HAVERMANS, HADEEL ABDUL AZIZ & HANS SCHOLTEN

introduction

It is obvious that the final effect of iron gall inks on paper is destruction of the paper, everyone can see the holes, chemically burned into the support. However, identifying the ink before that destruction has taken place is somewhat more difficult and requires analytical tools.

One of the tools is the application of UV-light. At a certain stage of the degradation process, the area around the ink, and even the areas on the previous and the following pages in direct contact with it, will show a fluorescence. This effect has been shown, for example, by Neevel1. However, it is known that UV light may promote the formation of radicals, and knowing that the degradation process caused by iron gall inks is partly based on the formation of radicals, detection using UV-light is somewhat doubtful2.

Besides the fluorescence effect, visible effects can be described. Reissland3 showed the visible progress of paper ageing due to the presence of iron gall inks. Unfortunately there are quite a lot of ageing stages caused by different brown inks which the eye cannot distinguish.

Infrared spectroscopy is able to show, additionally to its degradation stage, the presence of the organic tannin and gum compounds in a paper.4. However, this method will not always prove the presence of a metal tannin ink. Moreover, it is quite sophisticated.

This is also true for the application of Proton Induced X-ray Emission (PIXE). Vodopivec and Budnar applied this method for the detection of metal ions5. They proved that PIXE could be applied as a non-destructive method for the analysis of iron gall inks.

More simple and therefore, more easily applied methods were presented by Jane Colbourne6. Infrared Reflectography (IR) can provide valuable information on naturally aged inks, especially if they have faded. False Colour Infrared Pho-

tography (FCIR) uses a normal photographic technique but removes, for example the blue light, which results in a change in colour especially of iron gall ink. It was shown that inks such as iron gall, sepia, ivory black and bistre could be identified. The technique, however, requires some special photographic equipment.

A piece of equipment that produces digital images and has been applied in the study of paintings and miniatures is the MUSIS-2007 (Multispectral Imaging System)7. It works in the area between ultraviolet and infrared (320-1550 run) and can be used in seven separate bands. It also includes a powerful software tool for imaging and the analysis of, for example, paintings and drawings.

With respect to the demand for non-destructive analysis and identification methods of metal tannin inks, multispectral imaging may offer a new tool. We, therefore used this method in our research programme relating to the preventive conservation of objects affected by iron gall ink corrosion. We used an ARTIST digital imaging camera; the arrangement is given in Fig. 1. Details of the equipment are given in the annexe.

materials

Metal tannin inks were made in the laboratory. The basic components of the test inks were Iron (II) Sulphate (0.15 mol/1), Copper(II)Sulphate (47.46 g/1), tannin (0.027 mol/1) and gum arabic (31.4 g/1). Ink lines were drawn onto two types of sample paper and submitted to accelerated ageing of 90°C and 50% RH for 8 and 16 days. Additionally, spots of each ink (the iron gall and the iron/copper ink) and each individual component were put on the sample papers using a pipette.

Two types of cellulose substrates were used, a paper made from bleached softwood pulp and one made from cotton linters pulp. They can be understood as being pure paper substrates and have been proved to behave so in previous research.

results and discussion

We first observed the metal gall ink lines, together with reference lines consisting of carbon made by a laser printer. After tuning the filters on the MUSIS apparatus, the metal gall ink became red while the carbon lines remained black. This phenomenon was observed with both test papers. Examining the samples with the naked eye, no clear distinction could be made between the carbon and the metal tannin ink lines.

Fig. 1: The system set-up for analysing the differences between metal-tannin inks.

Looking at the separate compounds, it was found that these compounds, too, caused some discoloration of the paper. In the FCIR region, however, there was a distinct difference from that caused by the metal tannin inks. Gum arabic resulted in some slight reddish-brown discoloration; tannin became slightly yellow (nearly invisible), copper sulphate blue and the iron sulphate yellow.

After accelerated ageing the reported effect in the FCIR region was intensified. It may be possible to determine the stage of ageing in this way, but more research is needed in order to establish scientifically sound results.

conclusion

We conclude that using FCIR non-destructive identification of metal tannin inks is possible, for both iron and copper based inks. The system using an ARTIST digital imaging camera is accurate and easy to use and may be seen as being an advisable tool for the identification of metal tannin inks in conservation workshops. There is even a good correlation between the density of the colour produced by the imaging process and the age of the ink, which means that FCIR

multispectral imaging can also provide some information on the age of an ink and paper. Further research will establish a well-defined relation between the density of colour seen on an FCIR image and the age of an original object.

acknowledgements

Dennis Schipper of Art Innovation is gratefully acknowledged for his kind hospitality and for providing workspace for the analyses. The work on iron gall ink corrosion was made possible due to the founders of the research: Museum Boy-mans van Beuningen, Shell Nederland,. Weiss Enet b.v., Art Innovation, the Dutch State Archives and the Dutch Ministry of Economic Affairs.

annexe: the MUSIS equipment

The camera was equipped with an Electrophysics 25 mm, C-mount lens (flange distances 17,524 mm and flange to object-distance 274 mm). The light sources were two 30-Watt Halogen lamps with a distance of 750 mm, fitted 530 mm above the paper samples on an angle of 30 degrees to the normal (see Fig. 1). The system uses different filters for taking the digital images. Those applied on the ARTIST were from CVI Laser Corporation and Auburn8. The infrared (IR2) filter is a LongWavePass filter LPF-1000-1.00, cut on wavelength 1000 nm. The red filter is a CG-BG-39-1.00 with a Red Additive dichroic filter CP-AR-585. The green filter is a CG-BG-39-1.00 with a GreenRed Additive dichroic filter CP-AG-540. Images were stored on a normal computer system equipped with ARTIST9 and PAINT SHOP PRO 7 software10. For observing the probes first the red (585 - 675 nm) and green (505-575 nm) images were taken using a diaphragm of 2.8.

Afterwards the infrared image (1000-1150 nm) was taken using a diaphragm of 1.4. As reference printed carbon-lines from a laser printer were used. Subsequently the separate images were added using PAINT SHOP PRO 7. The results per sample were compared to each other and to the reference material.

summaries

Non Destrutive Detection of Iron Gatt Inks by Means ofMultispectral Imaging

False Colour Infrared Photography (FCIR) is demonstrated to be an appropriate method for analys-ing iron gallotannate ink or its components (iron- or copper sulphate, tannin, gum arabic). Its ad-vantage over other non-destructive optical methods (UV-light, Infrared spectroscopy, Proton Induced X-ray Emission) is that its apparatus is much simpler.

Detection non destructive d'encres galliques ä l'aide de l':analyse spectrale multiple

On a pu demontier que la photographie infrarouge de fausse couleur (FCIR) est une methode adaptee pour analyser l'encre gallique et ses composants (sulfate de fer ou de cuivre, tanin, gomme arabique). Son avantage par rapport aux autres methodes optiques non destructrives (rayons ultra violets, spectroscopie infrarouge, emission de rayons X induite par des protons) est qu'elle necessite un equipement beaucoup plus simple.

Nicht-materialzerstörender Nachweis von Eisengallustinte mit Hilfe Mehrfach-Spektralanalyse

Es wird gezeigt, daß Falschfarben-Infratrotphotographie (FCIR) eine geeignete Methode zum Nachweis von Eisengallustinte bzw. seiner Komponenten (Eisen- bzw. Kupfersulfat, Tannin, Gummi Arabicum) ist. Gegenüber den anderen nichtmaterialzerstörenden optischen Methoden (UV-Fluorenzenz, Infrarotspektro- und -reflektographie, protoninduzierte Röntgenstrahlemission) hat sie den Vorteil des wesentlich einfacheren apparativen Aufwands.

references

1. Neevel, J.G., SchadetijkeEffecten van IJzfr-Gallusinkten op Papier (Inktvraat). 1996, Centraal Laboratorium voor Onderzoek van Voorwerpen van Kunst en Wetenschap: Amsterdam.

2. Daniels, V. The chemistry of iron gall ink The Iron Gall Ink Meeting 2000 (I.G.I.M.). New-castle upon Tyne: University of Northumbria.

3. Reissland, B. Visibleprogress ofpaper degradation caused by iron gall inks. I.G.I.M. (cf. ref. 2).

4. Sistach, M.C., & N. Ferrer. Iron gall ink corrosion in manuscripts. I.G.I.M. (cf. ref. 2).

5. Vodopivec, J., & M. Budnar. Analyses of iron gall inks by PIXE. I.G.I.M. (cf. ref. 2).

6. Colborne, J. A survey of methods used in the technical examination and analysis of brown inks. I.G.I.M. (cf. ref. 2).

7. Teule, J.M. The use of the MuSIS 2007 for the analyses of easel paintings and miniatures. Art'99. 6™ International Conference on Non-Destructive Testing and Microanalvses for the Diagnostics and Conservation of the Cultural Environmental Heritage, Rome 1999.

8.

9. Art-Innovation: ARTIST Imaging Software. Hengelo 2001.

10. Jasc_Software Inc.: Paint Shop Pro 2000.

JohnHavermans & HadeelAbdulAzizTNOBuildings and Constructions Department of Healthy Buildings and Systems POBOX 49 2600 AA Delft The Netherlands

Hans Scholten Art Innovation

Westermaatsweg 11 7556 BW Hengelo The Netherlands