Development of dose-response functions for historic paper degradation using exposure to natural conditions and multivariate regression

Abstract

Many collections of documents, manuscripts, and works of art on paper are prone to degradation due to a complex interplay of extrinsic and intrinsic factors. The aim of this study was to examine the simultaneous effect of multiple degradation agents on selected non-model types of paper in natural environments, and the relative effect of environmental parameters (heat, humidity, light and pollution) compared to material parameters (pH, fibre composition and presence of additives). An exposure experiment was set up to investigate visual and chemical changes of 12 different types of paper in real time in different environmental conditions over a 1.5-year period at 11 sites across Europe and North Africa, sheltered from UV light and precipitation. Suitable environmental monitoring equipment, such as data loggers and gas samplers, and analytical methods to characterise sample degradation, specifically spectrocolorimetry and capillary viscometry, were used to estimate alterations in visual appearance and degree of cellulose polymerisation, which are the most important properties of paper in the heritage context. Multiple linear regression and principal component regression were used to interpret the large volume of data and calculate a set of dose-response functions. The results of this study not only suggested that most of the considered variables are of significance in relation to changes in colour and in average molecular weight, but also revealed a number of meaningful interactions between these variables. Based on the assessment of the relative contributions of environmental and material-related variables to the natural ageing processes of paper, the dose-response functions proposed in this study enable prioritisation of degradation factors in environmental management of paper-based collections and in historic paper degradation studies; however, further work is required to increase accuracy and understanding of the chemistry of degradation.