How to stabilise the ink?

In order to develop a much needed stabilisation treatment for iron gall ink corroded paper, the main reactions leading to degradation need to be identified.

It is by now firmly established, that iron gall ink corrosion is caused by the combined degradative activity of sulphuric acid and iron ions on the support material. In the case of paper, the acids lead to hydrolysis of the cellulose, whereas the iron ions catalyse the oxidation. Both processes lead to gradual depolymerisation of cellulose, resulting in embrittlement of paper. A complete treatment thus has to remove or neutralise the acid, as well as minimise the oxidative degradation promoted by the transition metal ions.

The problem of acid hydrolysis has been identified in the previous century and the first patent on the use of deacidification solutions to stabilise the acidic paper dates from 1936. Till now, a wide variety of aqueous as well as non-aqueous deacidification solutions have been developed.

On the other hand, stabilisation of oxidative degradation catalysed by iron ions have only recently been attempted.

In order to prevent oxidative decay of substances, antioxidants are usually employed. According to their mode of action, they may be classified as preventive or chain breaking (also called primary antioxidants, or radical scavengers). Primary function of the former ones is prevention of the radical formation, either by deactivation of corrosive metal ions (e.g. phytate) or by heterolytic decomposition of peroxides (e.g. KI), while chain breaking antioxidants react with radicals forming more stable products, thus interfering with the chain propagation step. Lignin belongs into this group of antioxidants.

The first antioxidant to be proven to have a marked stabilisation effect (theoretically extending paper lifetime by the factor of 2) on paper containing iron gall ink was a preventive antioxidant, iron chelating agent phytate. Due to its strong stabilising activity, the method has been quickly adopted and developed to a stage, where it can be considered safe to be used by conservators. Unfortunately, phytate is only able to decrease the catalytic properties of iron ions, while some other transition metal ions (e.g. copper), which are also included in some historical ink recipes remain largely unaffected. In the same chemical environment (e.g. pH 8), Cu(I) produces ~105x more hydroxyl radicals than iron, so that even if it is present in minute proportions, its effect cannot be overlooked.

While important steps were made towards stabilisation of ink-corroded paper through the aqueous treatments, the much needed non-aqueous method is still confided to deacidification only. In addition, all of the existing non-aqueous treatments involve magnesium-based alkali, which due to its high pH results in strong lightening of the iron gall ink, rendering the existing non-aqueous methods unsuitable for stabilisation of ink corrosion. A non-aqueous approach is important, due to some disadvantages of the aqueous treatments: possible migration of ink compounds, changes of ink colour and the overall tonal balance of the object, change of dimensions and surface characteristics of paper, necessary dismounting of bound volumes, long drying times. A non-aqueous treatment for ink corrosion, would evade these disadvantages, while at the same time provide the means to treat large quantities of endangered archival material.

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