Academic journal article Heritage Science

Investigation of the Stabilization of Verdigris-Containing Rag Paper by Wet Chemical Treatments

Academic journal article Heritage Science

Investigation of the Stabilization of Verdigris-Containing Rag Paper by Wet Chemical Treatments

Article excerpt

Authors: Kyujin Ahn [1]; Andreas Hartl [2]; Christa Hofmann [2]; Ute Henniges [1]; Antje Potthast (corresponding author) [1]

Introduction

Paper objects with copper pigments are threatened since copper ions that migrate from the pigments can cause the accelerated degradation of cellulose and discoloration of the paper. In severe cases, the paper is so fragile that even handling of the object becomes critical. Major parameters for the severe degradation of paper in the presence of copper acetate pigment (verdigris) found in historic objects remain unclear due to a variety of parameters of the paper substrates as well as of verdigris bound in an organic medium and the usually unknown storage conditions. Several studies show that oxidation is a dominant degradation pathway of cellulose when copper(II) is present compared to iron(III), which mainly causes hydrolysis in the acidic conditions [1, 2]. The formation of reactive oxygen species catalyzed by the presence of copper ions, which may cause further radical chain reactions, was detected [3, 4]. The catalytic activity of copper in the generation of hydroxyl radicals is higher than that of any other transition metals at pH?7 [5]. Its activity is highly dependent on pH: when the pH is higher than pH?7.5, the rate of formation of hydroxyl radicals increases greatly in the presence of Cu(II) [5].

The extensive production of hydroxyl radicals due to the presence of copper ions under alkaline conditions may lead to a need for caution in deacidification treatments of copper pigment-containing papers. Thus, an additional treatment with an antioxidant that reacts as a radical scavenger or peroxide decomposer is often recommended when copper compounds are present [6, 7]. In particular, halide antioxidants were found to be effective in reducing the generation of hydroxyl radicals [8, 9]. For verdigris-containing paper, tetrabutylammonium bromide (TBAB) with or without calcium hydrogen carbonate was found to be positive as a result of visual assessment [10]. Ethyl methylimidazolium bromide with non-aqueous deacidification using a Bookkeeper? spray [11] and non-aqueous magnesium propylate mixed with alkyl p-hydroxybenzoate as antioxidant [12] also yielded retardation of cellulose degradation.

The use of a chelating agent as a remedial chemical for copper pigment-containing paper was also tested. A short-chain gelatin and the recombinant protein isolated by immobilized metal ion affinity chromatography (IMAG) showed inhibition of copper pigment corrosion, indicating potential for the formation of a stable complex with copper(II) ions [13]. Calcium phytate treatment in combination with calcium hydrogen carbonate, which has become a secure conservation treatment for iron-gall ink-containing paper, could also stabilize the paper even when it contains additional copper ions [6, 14], which raises speculation regarding the possible chelating of phytate with copper ions. The concept of complexing copper ions using 1H-benzotriazol (BTA) has also been proposed [15]. 1H-benzotriazol has been known as one of the best copper corrosion inhibitors for metallic objects. Its low vapor pressure at room temperature and solubility in water as well as in alcohol can widen its application for Cu-containing paper.

Concerning deacidification treatment, despite the high catalytic activity of Cu(II) under alkaline conditions, alkaline earth carbonates still bring about the stabilization of copper-catalyzed degradation of paper. In particular, magnesium hydrogen carbonate treatment consistently resulted in a good contribution to the stabilization of copper-containing paper [4, 16, 17]. Not only does neutralization take place but a beneficial effect of magnesium ions also seems to occur. Non-aqueous deacidification treatments, which can readily deposit more magnesium carbonate and a higher pH than aqueous deacidification treatments [18], also gave rise to a potential treatment for copper pigment-containing paper [12]. …

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