Stabilising heritage Iron gall ink corrosion on paper and the eternal search for an ideal treatment

Abstract

This study has been conducted to investigate the complexities of iron gall ink, its degradation mechanisms and possible stabilisation methods. Iron gall ink represents a writing medium that was commonly used from the Middle Ages to the 20th century and is frequently found in historical documents in archives. The condition of iron gall ink documents can vary greatly, from very good to a state of degradation where the corrosion rate causes substantial material and informational loss. Extensive research has been carried out the past three decades to nuance the understanding of the ink, its historical variations in composition and degradation patterns. A part of this study has been directed towards providing an overview of the experimental work conducted within the cross- disciplinary field of iron gall ink research, as well as identifying current practices of treating iron gall ink objects within conservation-restoration. To this end, a survey was created and internationally distributed to practising conservator-restorers. The results from the study indicate a gap in communication and knowledge transfer between the experimental context of conservation science and the heritage management of conservation-restoration practice. Guiding principles of the contemporary conservation profession could be defined as continuously evolving in accordance with the discursive flows of contemporary heritage theory. A significant part of the study was focused on evaluating a proposed stabilisation method, based on a modified adhesive of Klucel G dissolved in an ethanol dispersion of nano-calcium hydroxide with the addition of an antioxidant, tetrabutylammonium bromide (TBAB). This was applied to groups of mock-ups that were created using Whatman No.1 paper and hand-made rag paper in combination with two sorts of iron gall ink, one containing copper and one without. Pre-aged and not-pre-aged mock-ups were after treatment submitted to processes of accelerated aging for different durations. For comparison, additional groups of mock-ups were treated with the proposed method in its decomposed variations: Klucel G in ethanol, Klucel G in an ethanol-based dispersion of nano-calcium hydroxide, Klucel G in ethanol with TBAB but was also compared with an aqueous method based on gelatine, dissolved in water and diluted with ethanol, with an addition of calcium propionate. After aging, the effects of the treatments were analysed using tensile testing, colorimetry, pH measurements, SEM-EDS, μ-XRF, bathophenanthroline test strips and ocular examination. The results indicated positive effects from both the proposed method and the aqueous treatment with gelatine, but with some effects on visual appearance that would be important to consider when planning a conservation action.