Convergence and Divergence of Some Methods Used to Assess the State of Preservation of Paper in Old Books

Abstract

The convergence or divergence of results for pH, yellowness, and mechanical strength were considered for papers from three old books. Good convergence was found for pH measurement results from the three books. The lack of convergence in some pH measurements between the surface method and the extraction–pH meter method may result from differences in pH of individual pages of old books, inherently lower pH values obtained using the surface methods, as well as from lower pH values measured in extracts of old papers prepared under hot conditions using a pH meter. Yellowness testing revealed overall good level of convergence between visual assessment and instrumental measurement (Elrepho 2000 apparatus). Strength tests indicated convergence between the results of the number of double folds (NDF) and tensile-after-six folds (TSF) tests, while divergence was observed when these methods were compared with the number of page turns (NPT) test. High NPT values may result from the papers of the examined books retaining yet relatively significant resistance to tensile and tearing stresses. Further research is needed to explain the relatively small damage to book pages in the NPT test in contrast to alarming results of the NDF and TSF tests.