by Yun Liu
The impact of environmental fluctuations on paper degradation has always been of great interest to the conservation community. It is not only because paper is one of the heritage materials that are known as being sensitive to environmental conditions, but also because it is extremely challenging to keep microclimate constant, even under the protection of storage boxes and display cases [1, 2]. But how does paper respond to environmental fluctuations? How much should we worry about environmental fluctuations for book and paper collections?
A thorough review on the effects of temperature and relative humidity (RH) fluctuations regarding both mechanical strength and discolouration of paper is given by Menart et al. . Generally, from the viewpoint of chemical degradation, two types of empirical observations have been reported. Faster degradations were observed under cycling conditions than at steady median ones [4-7] or even those at the upper limits of the cycles . There has not been a generally accepted model explaining the observed behaviours. Some researchers argued that fluctuations in temperature and RH would initiate extra degradation mechanisms, such as development of local stresses, increase in accessibility to chemical reactants, and formation of cellulose crystallinity [5-8]. However, these behaviours could also be explained by the common thermodynamic theories where time spent as worse conditions has a greater impact of the state of paper than the time spent at better conditions , or by the hysteresis effect of moisture desorption which potentially shortens the time spent at better conditions [9, 10].
The most recent research demonstrated that moderate fluctuations in temperature (within 10 °C) and RH (within 20%) would not have significant accelerating effects on degradation of paper defined by degree of polymerisation (DP) . The results seem to be of more practical significance since the investigated conditions are more representative of the environmental conditions in real situations. However, it might be worth noting that the experiments were conducted at 90 °C. Although this is a widely used condition for accelerated degradation experiments, the fast rate of degradation promoted by such a high temperature could potentially mask some possible differences in behaviour induced by RH variations.
Another issue with chemical degradation is that induced by corrosive inks. Research has reported that fluctuating RH has the potential of increasing the migration of iron gall inks, which not only decreases the readability and contrast of the document but also accelerates the chemical degradation of paper . In addition, fluctuating RH could cause inorganic materials in iron gall ink to crystalize on the surface of ink during its natural ageing, which may cause recognizable shifts in the appearance of ageing iron gall ink work as well as mechanical damage [12, 13].
From the viewpoint of physical degradation, RH fluctuations seem to have a much greater effect than temperature fluctuations . Accelerated deformation induced by changing moisture content, defined as mechano-sorptive creep in literature, has been found in multiple cultural materials including paper. A comprehensive review of studies on mechano-sorptive creep of paper is provided by Alfthan . This indicates that RH fluctuations could potentially lead to higher rate of degradation caused by the strain resulted from accelerated creep. I have not found any studies on the acceptable RH range for the allowable strain range of paper, however, studies on other types of cultural materials, such as wood, ivory, adhesives, paints, etc., have suggested that although different materials show different responses to RH changes, moderate RH fluctuations, i.e. within about 30%, have not been found to be detrimental since the dimensional changes are within elastic ranges in most cases [16, 17]. This suggests in a real collection environment, where RH usually ranges within 20%, RH fluctuations may not initiate irreversible structural change in paper.
In addition, considering the probability of book and paper collections to experience fluctuating conditions ranging to extremes is rare, moderate fluctuations are usually well buffered by storage enclosures as well as stacked or packed materials , and the equilibration time of paper is relatively long [19, 20], it is unlikely to see large and abrupt change in moisture content of paper in real situations. Therefore, fluctuations may not be the most important thing to worry about.
As a summary, based on what I have read, fluctuations in temperature and RH could potentially contribute to accelerated degradations physically and chemically but the mechanisms have not been confirmed and the effects have not been found to be profound within a small range. Therefore, it is fair to say that in real environments for book and paper collections, where condition changes are moderate, fluctuations in temperature and RH generally do not matter. But it might be worth noting that paper degradation is a complex process and an evaluation of the synergistic effect of both chemical and physical degradations at room temperature may be necessary before making conclusive statements.
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