People have been concerned about the premature degradation of stored books and the poor quality of the paper for a long time (Murray 1824; Johnson 1891; MacAlister 1898).  Gradually it became known that acidity is a key factor in shortening the useful life of books (Kohler and Hall 1925; Barrow 1965, 1974; Moll 1965; Koura and Krause 1978; Sclawy and Williams 1981).   Barrow compared books manufactured during the years between 1500 and 1950 and observed a disturbing decline in folding endurance, especially during the period of manufacture between about 1670 and 1900.  During the same period, the typical pH values of manufactured books became lower than 6 and continued to decrease to a range between 4.5 and about 5.  Wilson and Parks (1980) reported related findings.  A correlation between pH and the rate of strength loss was found for both accelerated and natural aging tests of the same books.  Katuscak et al. (2010, 2012), who studied more recent specimens, observed a coincidence of low pH and low folding endurance of printing papers produced during the decades between 1900 and 1990.  By contrast, some books from previous centuries have been found to remain in relatively good condition, and such findings were generally correlated to higher pH values (Stephens et al. 2008a).  Especially low values of pH happened to be measured for samples manufactured in the 1960s and 1970s (Katuscak et al. 2012).  But the situation was very different for the samples that had been manufactured in the 1980s and 1990s, for which the average pH of the paper was near neutral and the number of double-folds before breakage was dramatically higher.  Not only were those relatively recent samples newer, but they also were not degrading as fast.  What happened, in order to bring about that change, was almost certainly the emergence of alkaline papermaking practices, which in just a few years has come to be dominant for the production of printing papers (Hubbe 2005).

In addition to acidity present in paper at the time of its manufacture, acids are formed continually both in acidic and alkaline cellulose material due to hydrolysis or oxidation reactions, which will be described later.  Also, some acidity accumulates through absorption of pollutants from the air, particularly in urban areas (Smith 1987).  However, the detailed effects are different, depending on whether the paper’s microstructure is acidic or alkaline:  The acids arising in the alkaline part of the microstructure, which contains alkaline reserve, presumably can be continually neutralized.  The degradation in the alkaline paper continues very slowly, and most probably the effects of the likely predominant peeling reaction can be neglected (Green et al. 1977; Ahn et al. 2012c, 2013; Testova et al. 2014). Peeling reactions cause only splitting off of a single monomer unit per reaction, thus decreasing the degree of polymerization (DP) by 1, for example  from DP = 1000 to DP = 999 per 1 split, or to the DP = 998 per 2 splits, etc. Such a process would result in a negligible difference of the DP, longevity, brittleness, folding endurance, usability, and other mechanical and chemical quantities of paper or books.  Another alkaline degradation mechanism, the beta-elimination reaction, could be detected in the deacidified book papers (Ahn et al. 2012c, 2013).  However, it had little influence on the molar mass of cellulose in the paper compared to the beneficial effects obtained by deacidification treatment.

In any acidic paper, even if it contains alkaline particles on its surface or within some of the larger pore spaces, there may be an interior acidic region (i.e. an acid core) or acid remaining in microstructures of the paper after nonuniform or incomplete deacidification (Buchanan et al. 1994; Katuscak et al. 2012; Hubbe 2015). Within such zones the pH would tend to be further decreased by continually arising acids.  Acid hydrolysis can cause rapid random cellulose degradation, in which a handful of breaks in a chain can reduce the cellulose DP in paper from an initial high value to the range 300 to 700, at which point the paper is subject to crumbling (Zou et al. 1996a; Smith 2004).  Zou et al. (1996a) observed a dramatic drop in tensile strength and folding endurance after the DP of cellulose chains within paper fell below about 700. Likewise, Smith (2004) suggested that the range of 400 to 500 is a critical range of DP, beyond which paper falls apart when folded.  According to Jeong et al. (2014) the critical value for DP was found to be about 1000 in the case of Hanji papers.  Thus, the level of DP needed to maintain the strength of paper may depend on the type of paper.  Zervos and Moropoulou (2005) found a high correlation between the DP of cotton cellulose and strength properties of paper made from the fibers.  When considering such data, however, it should be kept in mind that the correlation between cellulose DP and paper strength can be different for different test procedures.  Such results must be considered cautiously, since paper’s strength also can be highly affected by localized damage that within the fibers during pulping operations (Gurnagul et al. 1992).