A METHOD FOR THE AQUEOUS DEACIDIFICATION OF OXIDIZED PAPER
JOHN BOGAARD, HANNAH R. MORRIS, & PAUL M. WHITMORE
A method for the aqueous deacidification of paper by immersion in a concentrated solution of a neutral salt has been examined. This technique increases the rate at which acidic ions diffuse out of the paper fibers in an ion-exchange process. Papers are quickly neutralized; however, no alkaline reserve is left by this treatment.
Photo-oxidized filter papers treated with this technique showed little or no chemical damage, and when subjected to thermal aging they were stabilized compared to untreated sheets. However, if the salt is not rinsed out of the sheet after treatment, there may be some yellowing. Following the concentrated salt solution with a dilute alkaline treatment to rinse out the salt and leave an alkaline reserve stabilized the sheets against thermal aging more than either treatment alone and mitigated the excess yellowing.
Treatment of acidic, oxidized paper with a concentrated neutral salt solution may also be appropriate as a first step before chemical reduction. Photo-oxidized paper was greatly stabilized to thermal-aging by a multistep treatment that involved neutralizing the paper with the concentrated salt solution, rinsing out the salt, then reducing the oxidation by treatment with sodium borohydride, and washing with an alkaline solution. By neutralizing the paper first, the treatment with sodium borohydride was made safer and more efficient by slowing its decomposition and subsequent release of hydrogen gas bubbles.
The salt examined most closely in this study was calcium chloride. This salt should be appropriate for most circumstances.1 However, it is not recommended for papers that contain a significant amount of silver or lead, due to the possibility of forming insoluble chloride salts. A survey of other salts showed that concentrated solutions of many neutral salts could also be effective.
This study examined the effects of concentrated salt treatments on cellulose alone and did not look at interactions with other paper components or with ink or media. More research needs to be done to assess the effects of these treatments on commercial paper as well as on printing and artistic materials. And, of course, for heavily sized or coated papers that do not wet easily, a completely different strategy to remove acidity and stabilize the sheet must be devised. Nonetheless, for papers that are sensitive to alkalinity and that can be subjected to aqueous treatments, this technique could be a safe, effective, and rapid method for removing acids from them and stabilizing them.