Climate-induced risk of paper hydrolysis in libraries and archives: the Time Weighted Expected Lifetime (TWEL)

The microclimate in libraries and archives can affect the conservation of paper collections. In times of climate change, a scenario of elevated air temperatures is expected with a likely consequence of reducing the paper durability, for the effect of accelerated rates of chemical deterioration (e.g., cellulose hydrolysis). This study aimed to explore the use of specific dose-response functions to outline the possible climate-induced risks of paper deterioration in libraries and archives. The rate of cellulose hydrolysis (at the conditions of dark storage) can be modelled as a function of both paper features (i.e., acidity (pH), and degree of polymerisation (DP)) and indoor air temperature (T) and relative humidity (RH) [1]. A synthetic index named as Time Weighted Expected Lifetime (TWEL) [2] can be effectively employed to account for an average risk of paper chemical deterioration on a seasonal and yearly basis. The TWEL allows to evaluate the durability of different paper collection types (based on their pH and DP), also considering the typical response time of paper to microclimate changes. In addition, since the TWEL by its definition is more influenced by adverse microclimate conditions, the cellulose hydrolysis risk for paper collections is investigated underlying a worst-case scenario. The storage facility of the Alessandrina library in the Sapienza University Campus in Rome was selected as the case study. The microclimate observations collected in Alessandrina over a year were used to characterise the microclimate behaviour and to understand the complex mutual interactions between microclimate and collections [3]. The TWEL index highlighted the impact of adverse conditions (e.g., T> 20°C reached naturally in summer and artificially in winter) on the preservation of severely deteriorated acidic collections (pH = 5.2, DP=600) typical of the XIX-century collection. Moreover, the TWEL was used to compare the expected lifetime of these collections at average T and RH conditions in the library to different scenarios where T, RH, or paper pH were changed to quantitatively explore the effect on the resulting cellulose hydrolysis risk of different mitigation measures (i.e., climate control and deacidification). A reduction in T (particularly if combined with a reduction in RH) resulted to be the most effective measure to enhance paper durability, followed by deacidification treatments that however involve high economic and time efforts. This study showed an outstanding example of the use of the TWEL index as a tool for diagnostics and prognostics to comparatively assess conservation strategies for paper collections.