Mechanism of Acidification that Progresses in Library Collections of Books Made of Alkaline Paper

Acidification mechanisms of paper were examined, studying 120 pairs of identical copies of books published from 1971 to 2010 housed in two Japanese university libraries. The pH at the edges of the alkaline paper lowered from 7.5 to 6.4 after 5-10 years and to 5.8 after 10-15 years after publication. The pH bottomed at 5.4 at the edges and at 7.0 in the centre of the sheet. Acid paper showed higher pH at the edges than in the centre of the sheet. The edge pH of the alkaline paper of books stored on an above-ground floor was lower due to more exposure to incoming open air with pollutants compared to paper stored in the basement. Elemental analysis suggested nitrogen oxide deposition, but could not prove sulphur oxide deposition at the edges of the paper. The position-dependent acidification is presumed to relate to the deposition tendency of air pollutants contained in open air, and accordingly the edge pH dropped dramatically. In the 1980s, alkaline paper was increasingly used in the production of books, and it was assumed that no more book acidification was to occur; however, this study concludes that even in alkaline paper, acidification processes will take place over time.