Effect of Some Deacidification Agents on Copper-Catalyzed Degradation of Paper
Introduction
Paper conservators are all too familiar with artifacts in which iron gall ink or copper in verdigris pigment has caused severe deterioration of the paper base. But even trace concentrations of these metals, which are generally present in paper, contribute significantly to its aging by catalyzing oxidation reactions.1,2,3 Copper and iron contaminants can also cause foxing in paper.4,5
Williams and his co-workers observed that paper samples containing copper acetate, which degraded rapidly when aged at 90°C and 50°C relative humidity (RH), were stabilized appreciably after deacidification treatment with magnesium bicarbonate.3 However, deacidification by a Barrow two-step treatment with calcium hydroxide and calcium bicarbonate solutions did not stabilize against the degradative effect of copper acetate to a comparable extent. It was logical at that time to draw a parallel between the observed stabilizing influence of magnesium bicarbonate and the deactivation of iron species by "complexation" with magnesium carbonate in alkaline pulping processes.6,7,8,9 In these systems, too, calcium carbonate had been reported to be ineffective in preventing transition-metal catalyzed oxidative degradation of cellulose under similar process conditions.9
In separate work, Williams and Merrill showed that magnesium acetate does not inhibit copper-catalyzed oxidation of cellulose.10 This observation suggests that the formation of a complex between magnesium and copper species may not be the root cause of the observed inhibition of copper-catalyzed oxidative degradation of cellulose after a magnesium bicarbonate treatment. Nevertheless, deacidification treatments do appear to influence oxidative degradation of paper. Arney, Jacobs, and Newman have reported that deacidification of paper retards not only oxygen-independent degradation (acid hydrolysis) but also oxygen-dependent degradation (oxidation).11 Margaret Hey has shown that manuscripts with iron gall inks, which are prone to oxidative degradation, can be stabilized by a deacidification treatment.12
In earlier works, the authors studied the effect of neutralization of acidic species in paper on the catalytic activity of copper and iron, taking care not to introduce any metal ions which could be inferred to complex with the transition metal catalysts.13 To this end, soluble sodium bicarbonate was employed to neutralize acidic species, and any unreacted bicarbonate and reacted sodium salts were washed away. It was observed that paper containing copper or iron species aged more slowly after the neutralization treatment. In the same work, it was also shown that chelation of the catalytic transition metal species provided an uncertain solution. While iron acetylacetonate did not catalyze the degradation of paper at all, copper acetylacetonate was a more active catalyst than the copper species adsorbed and/or exchanged from a copper salt solution.
The most recent phase of this continuing work compared the effect of different aqueous deacidification agents on the aging of copper-doped paper. The deacidification treatments selected for this purpose were magnesium bicarbonate. Barrow's two-step treatment with calcium hydroxide followed by calcium bicarbonate, and zinc bicarbonate. As far as is known, this work represents the first attempt at using a zinc bicarbonate solution for deacidification of paper.
Besides comparing the effectiveness of different deacidification agents, the work has revealed the reason for the effectiveness of magnesium bicarbonate and the relative ineffectiveness of the Barrow two-step treatment in inhibiting the catalytic effect of copper on the oxidative degradation of paper.
Table of Contents - Introduction - Experimental Description - Testing and Analysis - Results and Discussion - References - Supporting Documents