4.1 C and NC collections
The C and NC collections, stored in different environmental conditions, were considered separately. The books were divided into two pH categories: acidic (pH ≤ 6) and non-acidic (pH > 6). pH was chosen as a key factor because of its crucial role in paper degradation [59].
All books of the C collection were found to be made of rag paper with a measured pH value higher than 6. On the contrary, 65% of the books of the NC collection were estimated to be acidic. Table 1 reports the number of the acidic and non-acidic analysed books of the NC collection, and the proportional number of books represented by the samples in each stratum, calculated from the number of books gathered from the digital and paper catalogues [52–54].
Table 1. Acidic (pH≤6) and non-acidic (pH>6) books measured (sample size), percentages, and corresponding total number of books of the NC collection in each stratum of age.
NC Stratum
|
NC collection
|
Acidic
|
Non-acidic
|
Sample size
|
% of books
|
No of books
|
Sample size
|
% of books
|
No of books
|
1501-1550
|
1
|
8
|
103
|
11
|
92
|
1132
|
1551-1600
|
2
|
17
|
499
|
10
|
83
|
2497
|
1601-1650
|
6
|
50
|
640
|
6
|
50
|
640
|
1651-1700
|
3
|
25
|
293
|
9
|
75
|
878
|
1701-1750
|
2
|
17
|
431
|
10
|
83
|
2155
|
1751-1800
|
1
|
8
|
314
|
11
|
92
|
3453
|
1801-1850
|
2
|
17
|
706
|
10
|
83
|
3529
|
1851-1900
|
11
|
61
|
5787
|
7
|
39
|
3683
|
1901-1950
|
48
|
96
|
26430
|
2
|
4
|
1101
|
total
|
76
|
65
|
35203
|
76
|
35
|
19068
|
Table 1 shows that low pH values were usually observed in paper produced between 1850 and 1950, mainly due to the introduction of acidic sizing, as discussed in Part I [44]. The estimated proportion of acidic papers (65%) of the NC collection of the Classense Library is comparable, considering 10% of margin of error, with that of a typical Western library or archival collection, where the proportion of acidic papers is around 70–85% [60].
4.2 Isochrones
Isochrone curves (i.e., the points defined by pairs of T and RH values for which the expected lifetime is equal) were elaborated using the average pH and DP values for each group of acidic and non-acidic books, as reported in Table 2. As mentioned above, no acidic paper (pH<6) was measured in the C collection. However, in order to evaluate the degrading effect of iron gall ink on paper [61], given that acidity, in combination with high RH levels, is one of the main contributors to iron gall ink degradation [62], the estimated proportion of books with iron gall ink (72%, as determined by visual assessment [44]) of the C collection was considered, as a first approximation, as low quality acidic paper having pH 5 and DP 600 [24]. In order to be able to work with actual values, an NIR method would be needed for iron gall ink, and this currently does not exist in the frame of the SurveNIR instrument.
Table 2. Average pH and DP0 values for the groups of acidic and non-acidic books for the C and NC collection.
|
C collection
|
NC collection
|
Non-acidic
|
Acidica)
|
Non-acidic
|
Acidic
|
Average pH
|
6.8
|
5
|
6.4
|
5.4
|
Average DP0
|
1550
|
600
|
1640
|
1350
|
Proportion
|
28%
|
72%
|
35%
|
65%
|
a) books with iron gall ink considered as acidic books
|
In Figure 1, two sets of isochrones are calculated for the books of the C collection: pH 6.8, DP0 of 1550 for non-acidic books, and pH 5, DP0 of 600 for books with iron gall ink.
Figure 2 shows the isochrone plots for the non-acidic (pH 6.4, DP0 1640) and acidic (pH 5.4, DP0 1350) books of the NC collection. It is worth recalling that for the latter group, 40 books out of 76 are made of groundwood paper, for which DP values were not measured.
As expected, the reddish-orange areas corresponding to a lifetime of 50-200 years are notably reduced with increasing pH. The sets of isochrones of Figures 1 and 2 should be interpreted considering the thermo-hygrometric conditions of the storage environments to predict the lifetime in their actual context or in possibly different preventive conservation scenarios (e.g., lowering T and/or RH). It is evident that the books without iron gall ink of the C collection are predicted to survive the typical 500-year long-term planning horizon in their current storage environment (20 °C and 60% RH). In contrast, the books with iron gall ink and the acidic books of the NC collection are not predicted to survive the 500-year horizon even at the controlled conditions of the Caveau, which would lead to the DP threshold value within about 100 and 350 years, respectively. For the books with iron gall ink, substantial cooling (T < 10 °C at 60% RH) would be necessary to achieve the 500-year horizon. Dehumidification would achieve this only RH < 20% at 20 °C, which is already outside the safe RH limits for historic paper [20]. Alternatively, a possible remedial conservation scenario could involve deacidification or treatments with antioxidants for iron gall ink, and deacidification for acidic paper. The chelating agent phytate has been found [63] to stabilise inks with a substantial amount of transition metals, although antioxidants such as tetrabutylammonium bromide were found to have a more pronounced stabilisation effect, also on paper without inks [63], without affecting its brightness [64]. A combined deacidification and reduction treatment using borane tert-butylamine complex as the reducing agent was found to be effective and compatible with 19th-century paper [65,66]. Numerous options exist for conservation interventions, although none seem to be as available on a large-scale as deacidification.
4.3 Demographic curves
The effects of different storage conditions and deacidification on the time required for the collections to become unfit for manual use were modelled using the demographic curves, which show the lifetime profiles of collections stored at certain values of T and RH. In the numerous rooms of the Library, where no air-conditioning system is in operation, summer and winter monitoring campaigns were carried out [45,46]. Pronounced seasonal trends of T and RH were observed, although the building showed high daily thermal inertia. Moderate fluctuations of T and RH (ΔT=10 °C and ΔRH=20%) do not contribute significantly to the degradation processes of paper [67], while large fluctuations potentially damage various parts of books to different extents, beeswax seals, glues, wooden covers and iron gall inks being among the materials most sensitive to RH fluctuations [29]. However, as recently reported, although it is preferable to maintain stable conditions, gradual changes in T and RH from 13 °C and 35% to 23 °C and 60% may be acceptable as a result of seasonal cycling, if they occur over a month or more, or if items are packaged [29].
Winter (12 °C and 65% RH) and summer (27 °C and 56% RH) environmental scenarios were considered for the NC collection in order to compare how books behave in the two conditions, but also to explore if the winter environmental scenario could represent a possible conservation strategy if applied throughout the year. An alternative environmental scenario includes significant dehumidification of the atmosphere (20 °C and 35% RH). For the NC collection, two additional scenarios based on the Caveau conditions (20 °C and 60% RH), and an intervention strategy (mass deacidification) were considered in order to assess the possible conservation alternatives for more acidic books in less favourable environments.
For the C collection, two scenarios based on its current storage environment were explored, one considering the degrading impact of iron gall ink. Since the SurveNIR system [49,50] used to measure pH and DP of paper was calibrated and validated to measure properties of paper without ink, it was necessary to estimate the current DP of paper along the ink lines (DPink), which is expected to be much lower than that outside the ink. The initial DP of each book at the time of production, calculated from the current DP of paper (i.e., measured avoiding ink) on the basis of the rate of rag paper degradation (4.2·10-7 year-1) as estimated in Part I [44], was calculated and used to estimate DPink using the rate multiplication factor of 1.59 as suggested in the literature [61].
Figure 3 shows the lifetime profiles of the C and NC collections in the scenarios mentioned above.
It can be noted that the profiles of the two predictions for the C collection (left) are significantly different. Neglecting the acceleration factor for paper impregnated with ink, nearly all books of the C collection are predicted to remain in a fit-for-use state for 500 years. In contrast, the adverse effect of iron gall ink is potentially a significant concern as more than a half of the C collection is predicted to remain unfit in 500 years. Using the rate acceleration factor, the minimum calculated lifetime is ~180 years (see Fig. 3), whereas it is ~100 years if we assume that the rate of iron gall ink degradation can be approximated as acidic paper (pH 5, DP 600, Fig. 1). Such an assumption can thus lead to an overestimation of the degrading effect of such inks.
As deduced from the isochrones (Fig. 2), the Caveau conditions (20 °C and 60% RH) will not ensure fitness for use throughout the long term-planning horizon for most (about 65%) of the NC collection. As expected, the worst prediction for the NC collection is for the summer scenario, in which no book is predicted to be safely handleable in the long term. The winter and dry scenarios, which would necessitate significant cooling and drying of the storage environment throughout the year, respectively, would lead to preservation of ~90% of the NC collection for 500 years (see Fig. 3). Moreover, it is to be noted that other book components, such as beeswax seals, can be adversely affected by temperatures below 12 °C, which can cause formation of surface crystals [29]. While RH values higher than 65% can result in mould development, a dry environment (RH < 30%) can preserve archive materials but can also make folded parchment and paper documents, acid-decayed papers, iron gall ink papers, acid-decayed leather (red rot) and adhesive book structures less flexible [29]. Additionally, T and RH fluctuations between the reading and storage environments should be designed to avoid condensation phenomena [29].
In terms of proportion of the collection that remains fit-for-purpose, the most favourable scenario is provided by mass deacidification, more than 95% of the books of the NC collection being predicted to survive 500 years even in summer conditions (Fig. 3).
As far as interventive and preventive conservation are concerned, the above results present an argument supporting the use of deacidification or continuous cooling/drying, or their combination, as a one-off or a long-term investment, respectively. However, combinations of different measures (e.g., deacidification and cooling) or their staggered introduction could be defined as the most feasible strategy [68], once the environmental and economic sustainability implications are evaluated.
The lifetimes predicted in the stable environment of the Caveau as a function of publication date with different age intervals are explored in the box plots in Figure 4, with the data for the C collection including the effects of iron gall ink. The lower points in Figure 4 (left) are associated with the books with iron gall ink, clearly demonstrating its adverse effect.
The results indicate that the median lifetimes of the books of the C collection increase with increasing publication date, while a sharp decreasing trend is evident for the books of the NC collection dated after the second-half of the 19th century. The predicted median lifetime of the books of the NC collection dated between 1801-1850 is ~700 years, while that of the most recent books is less than a half (~300 years).