Traditionally, conservation researchers performing accelerated aging studies have allowed the paper pH to drop gradually as acidic species emerge within the paper. In practice, this means that researchers are conducting accelerated aging studies over a pH range. In the case of papers containing alkaline reserves, these alkaline species do not buffer the pH in a rigorous sense, but merely hold the pH at a specific point during aging according to their dissociation constants (Ka). Furthermore, this effect ceases when the alkaline species are exhausted—if they ever are—as reactants. Aging paper at a number of specific and constant pH levels, however, is important not just to study the well-known Brønsted-acid-catalyzed cellulose hydrolysis ( “acid hydrolysis”), but also to study the effective ranges of other catalysts for hydrolysis and oxidation, since these catalyses are pH dependent.
Similarly, although conservation researchers have performed oxygen-free accelerated aging to study the efficacy of anoxic storage and display environments, they have not used anoxic accelerated aging to disentangle the oxidative from the hydrolytic mechanisms of paper degradation. Measuring the rates of degradation mechanisms separately is important in determining the prevailing mode of degradation for a particular paper formulation. Furthermore, determining the rates of degradation mechanisms acting separately can elucidate how they “fit together” in, for example, the spiraling degradation of oxidation working in tandem with hydrolysis.
Heritage Science for Conservation, a part of the Department of Conservation and Preservation at The Sheridan Libraries, The Johns Hopkins University, has developed two new approaches to accelerated aging studies of paper that address the limitations noted above. First, we have employed common-ion-effect buffers, which hold the paper pH constant for the duration of the aging experiment. Second, we have employed oxygen-free, but not moisture-free, aging environments to control for oxidative mechanisms of degradation.
Known or suspected catalysts for the hydrolytic degradation of paper that we are studying include the aluminum(III), magnesium(II), and iron(II) cations, which may be present from gelatin or rosin-alum sizing, aqueous and non-aqueous deacidification treatments, and iron gall ink, respectively. Known or suspected catalysts for the oxidative degradation of paper are the aluminum(III), iron(II), and copper(II) cations—copper being present in some pigments and inks.
By studying primary mechanisms of paper degradation, we aim to provide new information that is useful for the following objectives: (1) to enable more fully informed choices of conservation materials and treatment techniques, (2) to assist in the development of specifications for producing permanent, durable papers, and (3) to assess the efficacy of accelerated aging to predict the permanence of paper.