Ink corrosion - Chemistry
Prof. Dr. Gerhard Banik (1998)
A major issue is often overlooked in light of the drastic statistics on deterioration of library archival holdings: important historical objects in libraries and archives, i.e. documents, illuminated manuscripts, colored prints and maps, all suffer very serious damage as a result of the destructive effects of iron gall inks and are therefore acutally endangered.
The reasons for this destruction are not fully understood yet, although numerous theories have been proposed to explain the degradation of papyrus, paper and parchment objects. The desire to conserve these items necessitates the development of effective treatment methods, which can only be possible after determination of the chemical causes for deterioration. Thus, given the historical and material value of these threatened objects, every effort of scientific research to combat further progress of decay is justified. Recent scientific research undertaken at the Netherlands Institute for Cultural Heritage can be regarded as a decisive step forward for a deeper scientific insight in the decomposition mechanisms and created the basis for the development of a new approach for a suitable conservation intervention.
The origin of the destructive mechanism of inks is a result of a complex overlapping of different processes. Of importance in this context are the natural ageing of paper, the composition of the inks and their ability to create chemical reactions with the carrier or the medium. The reactions between the inks and the carrier materials are strongly influenced by environmental and storage conditions, especially temperature and humidity and - as in the case of paper as a support medium for drawings or writing, by sizing and inorganic fillers.
In principle deterioration of paper by iron gall inks is largely a result of the action of the inks consisting of iron of diverse other transition metal ions, e.g. such as copper or zinc. The damage to the support material goes through various stages. Firstly fluorescence in the immediate vicinity of the ink writing under UV-light is noticeable, which is followed by a brown discoloration of the support in this area. This especially occurs in case of broad pen or brush strokes. The brown discoloration spreads through the support and very often an offsetting to neighbouring pages is observable. Finally, the degradation of paper is so severe, that whole areas, especially along the written or drawn lines, fall apart and the information is lost. In both degradation processes, the relatively flexible support - paper and parchment - becomes brittle and friable as a result of ageing complicated by the influence of apparently destructive inks. The support material suffers an extensive decline in its natural properties which finally makes its further use as an information medium impossible.
As early as about 100 years ago the head of the Vatican Library F. Ehrle, worried about the problem of ink damage and warned of the impending destruction of numerous documents and manuscripts. The conference he called in to St. Gall in I898 was the starting point for the systematic and scientific research for an explanation of the causes of this degenerative process. Since then the scientific literature names the following reasons for ink degradation of paper:
the high acidity of some inks which contributes to the hydrolytic splitting of the cellulose;
the efficacy of soluble iron compounds as catalysts for the oxidative decomposition of cellulose.
Among the numerous publications which appeared until the 1970s and dealt with the chemical causes of ink degradation, the leading articles are the work of Haerting, Herzberg, and Brannahl & Gramse. Haerting investigated the interesting fact that the damage caused by ink occurred not only in objects with a high mineral-acid content i.e. sulphuric acid, but also in those which were totally acid-free. His research led to the conclusion that only inks containing iron(II) salts can cause ink degradation damage. The other components of the ink - even the sulphuric acid present, according to his research, do not cause noticeable damage to the support medium. Herzberg also explained the appearance of ink deterioration with the presence of iron(II) compounds, but limits Haerting's results in describing all iron gall inks as detrimental and considered the acidity of an ink as most important for the damage. Brannahl & Gramse were able to prove that in many iron gall inks excess FeSO4, (iron(II) sulphate) is present and the permanence of such 'unbalanced inks' is questionable.
Investigations which deal mainly with the additional influence of biological infestation on ink deterioration, lead to similar results. Kowalik in particular published a great deal on microbiological factors as causes for ink digestion. During growth, fungi can generate organic acids like citric acid, enzymes such as cellulose which are both paper splitting catalysts, or oxidizing agents, such as hydrogen peroxide. However further experiments undertaken could demonstrate, that fungi growth from iron gall ink is rather poor; and cellulose and organic acids have to be taken in account as biological destructive agents to a lesser extent. By far the strongest effect on deterioration has proved to be the radical mechanism catalyzed by iron(II) compounds, even under neutral conditions. The effect of iron(II) compounds was proved to be much stronger than the destructive action of acids.
These results have been strongly supported by later research. Especially Neevel could evaluate from analytical investigations the frequent use of unbalanced iron gall inks on historical documents. Furthermore he could clearly prove a correlation between excess iron(II) sulphate in the inks and the occurrence of ink decay on paper documents. It is a well-known phenomenon that organic materials are oxidized by the transformation of Fe2+ to Fe3+. This kind of redox-reaction in an aqueous system or a moist atmosphere leads to the formation of unstable complexes of the metal ions with molecular oxygen which can in turn lead to the formation of free radicals in the presence of an organic substrate. In the equations below iron (Fe) stands for a metal ion which is involved in the mechanism in the oxidation states 2+ and 3+. R-H indicates an organic compound with at least one hydrogen atom e.g. cellulose. The organic radicals (R ·) and the perhydroxyl radicals (HOO ·) formed, start further radical chain reactions thus decomposing the organic substrate. Without going into details, it should be stated that research results suggest this type of mechanism for the decomposition of wood cellulose through iron(II) salts in a slightly acid environment. A similar reaction process can be proposed for the decomposition of paper by iron gall inks.
1. Formation of organic radicals
Fe2+ + O2 Fe3+ + O2·
Fe3+ + O2· + RH --> R· + HOO· + Fe2+
R· + O2 --> ROO·
ROO· + R'H --> RCOOH + R'·
2. Formation of hydrogenperoxide
Fe2+ +HOO· + H+ Fe3+ + H2O2
H2O2 decomposed by iron(II) into a hydroxyl radical and a hydroxide ion
Fe2+ + H2O2 --> Fe3+ + HO· + OH-
Although the knowledge about the chemical mechanisms of ink destruction is still incomplete some important conclusions can be drawn. The deterioration of paper is a result of the oxidizing action of soluble transition metal compounds - iron and in addition minor contaminants or trace elements, such as copper; zinc and others on the cellulose.
The presence of acidity is an important factor; as an acid environment accelerates the decomposition reactions. Nevertheless, deacidification alone will not stop the decay without removal of soluble transition metal compounds from the paper web or at least their conversion to chemically inert compounds.
This essay is part of a lecture presented at the Iron Gall Ink Corrosion Symposium in the Museum Boijmans Van Beuningen in Rotterdam, in April 1997. The complete text of this presentation can be found in the published symposium proceedings.
Neevel,J.G., Schadelijke Effecten van ljzer-Gallusinkten op Papier (lnktvraat), Report. Centraal Laboratorium voor Onderzoek van Voorwerpen van Kunst en Wetenschap. Amsterdam 1996
Sistach, M.C., "Scanning Electron Microscopy and Energy Dispersive X~Ray Microanalysis Applied to Metallogallic Inks," Preprints to the 9th Triennial Meeting or the ICOM Committee for Conservation, Vol. 2, Dresden (1990) 489 - 490
Banik C.. 'Phänomene und Ursachen von Farb- und Tintenfrass," Wiener Berichte uüber Naturwissenschaft in der Kunst, A. Vendl, B. Pichler eds., Vol. I ,Wien (1984) 188 - 213
Stachelberger, H., Banik, C., Schreiner, M., Mairinger, F., "Die Verteilung von Übergangsmetall-lonen über den Querschnitt Tintentrassbefallener Trägermaterialien von Schriften und graphischen Kunstwerken," 16. Kolloquium des Arbeitskreises für Elektronenmikroskopische Direktabbildung und Analyse von Oberflächen EDO; 11-16. Nov 1983, Antwerpen, in: Beitrag elektronenmikroskop. Direktabb. Oberfl. 16 (1983) 321 - 327
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Kowalik R., "Microbiodeteroration of Library Materials," Restaurator 4 (1980) 99 - 114 & 135-219
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Neevel, J. C., "Phytate: a Potential Conservation Agent for the Treatment of Ink Corrosion caused by Iron gall Inks," Restaurator 16 (1995) 153 - 160
Emery, J. A., Schröder, H. A., "Iron-Catalyzed Oxidation of Wood Carbohydrates," Wood Science and Technology 8 (1974) 123-137
Posse, 0., "Handschriften-Konsevirung. Nach den Verhandlungen der St. Gallener Internationalen Konfernenz zur Erhaltung und Ausbesserung von Handschriften von 1898 und der Dresdner Konferenz Deutscher Archivare von 1899," Restaurator, supplement I, Reprint (1969) 8
Neevel, J.C., "Phytate als chemische Inhibitoren von Tintenfrass auf Papier," Postprint to internationale Expertentagung Tintenfrasschaden und ihre Behandlung. Ludwigsburg. 14/15 April 1997, in press.
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