A METHOD FOR THE AQUEOUS DEACIDIFICATION OF OXIDIZED PAPER
JOHN BOGAARD, HANNAH R. MORRIS, & PAUL M. WHITMORE
Treatment of acidic, oxidized paper can pose a difficult problem for conservators. The oxidation of cellulose has been found to increase the rate of hydrolytic degradation upon thermal aging (Whitmore and Bogaard 1995), magnifying the need for deacidifica-tion. But immersion of oxidized papers in alkaline solutions may cause some breakage of the cellulose chains near the oxidation sites in conditions as mild as pH 9 (Luetzow and Theander 1974). Treatment with dilute alkaline solutions may reduce this risk; however, photo-oxidized sheets have been found to be harder to stabilize than unoxidized ones (Bogaard and Whitmore 2001). It can be difficult to gauge the extent to which a paper has been oxidized unless such factors as the composition of the pulp furnish and the history of light exposure and bleaching are well known. Lacking this information, it is difficult to assess the risks of damage from alkaline treatments.
Chemical reduction of oxidized paper with sodium borohydride can stabilize the sheet (Tang 1986), as well as make it suitable for alkaline deacidi-fication. However, solutions of sodium borohydride can be highly alkaline, and some degradation of the paper may occur during treatment until the reduction is complete. The borohydride molecule also decomposes upon contact with an acidic paper and liberates hydrogen gas, sometimes with a very vigorous bubbling action that can damage friable ink or media, and this effect slows the reduction process (Varshney and Luner 1961). Neutralization of the paper prior to reduction is recommended (Burgess et al. 1989).
So the conundrum a paper conservator faces when treating acidic, oxidized objects is that the best treatments for imparting long-term stability may themselves cause some immediate degradation, the extent of which may be difficult to gauge ahead of time. A method of neutralization that reduces these risks is desirable.
In the study described here, a technique to address this problem was examined that utilizes the affinity of the cellulose polymer for behaving as an ion-exchange resin and attempts to deacidify paper by treatment with a concentrated neutral salt solution rather than an alkaline solution. When a paper sheet is immersed in an aqueous salt solution, ion exchange occurs as the acidic hydrogen ions in the cellulosic fibers exchange with the cations of the salt solution until equilibrium is reached. The rate of this process is controlled by diffusion into and out of the fibers, a process that is driven by the difference in ion concentrations between the fiber and the solution. When the concentration gradient is small, such as with deionized water, weakly alkaline solutions, or dilute salt solutions, the displacement of acids from the fibers proceeds very slowly. By using a stronger solution to increase the concentration gradient, the process can be accelerated (Helfferich 1962). This is the principle behind the modified pH test for paper suggested by Scallan (1990), in which the sample is soaked in a 0.1 molar solution of sodium chloride, rather than deionized water, to reach equilibrium faster and obtain a more accurate pH reading.
The equilibrium attained by soaking an acidic paper in a concentrated neutral salt solution may not remove all the acids from the paper fibers; alkalinity may be needed to neutralize the solution and drive the ion exchange to completion. Treatment with concentrated neutral salt solutions may work best as a first step in a sequence of treatments.
This study explored the possible use of concentrated neutral salt solutions in deacidification treatments for acidic, oxidized papers and assessed the benefits and risks of this technique. A variety of salt solutions were evaluated for their effectiveness at removing acids from photo-oxidized paper sheets. Calcium salts were focused on, due to the strong affinity of calcium ions for cellulose and their binding power to carboxylic acid groups on the cellulose chain (Ohlsson and Rydin 1975). Calcium chloride was found to effectively neutralize oxidized paper and was studied in greater detail as a possible first step in a conservation treatment. Since calcium ions carry a double positive charge, they are likely to bear another anion, generally from the original salt, when bound to a carboxylate, so an alkaline rinse was used to exchange the residual chloride ion with an hydroxyl ion as well as to complete the deacidification and leave an alkaline reserve. Comparisons were made between treatments with calcium chloride solutions alone, calcium chloride treatment followed by an alkaline rinse, and an alkaline rinse alone to determine the most effective procedure for slowing paper degradation. Concentrated calcium chloride solutions were also evaluated for use as a neutralization step prior to chemical reduction with sodium borohydride. The effects of these treatments were assessed by measuring chemical changes in treated papers that were subjected to accelerated thermal aging.