QUALITY OF THE RETURNED TREATED AND CONSERVED MATERIALSCONSERVED MATERIALS

Restoration workshop H performed conservation work on both irradiated and non-irradiated samples, while restoration workshop S performed conservation work only on the irradiated samples. The non-irradiated samples were only dry cleaned. The irradiated samples were surface cleaned. Both types of materials were flattened. Figure 12.2 shows some of the materials after all treatments.

Initially, all samples had a dry and brittle feeling; however, the non-irradiated samples took on a wetter feeling after two months of storage in the dark and at room conditions similar to those in which the irradiated samples were stored.

12.5. SurveNIR ANALYSIS — PART 1, MATERIAL ASSESSMENT

Before the SurveNIR analysis was performed, the following criteria were established for evaluating the treated materials: acidity (pH), risk of yellowing and risk of brittleness. Weight factors were given to the near infrared (NIR) correlated values for pH, tensile strength, tear after folding strength and the presence of lignin. As one of the three samples indeed contained lignin, degree of polymerization was not included in this evaluation stage. Maximum and minimum values were assigned to calculate the need for conservation.

Based on the results presented in Table 12.1, it is obvious that for all papers, severe degradation was observed as a result of the serious mould infection.

For both the softwood and cotton-containing paper, the overall qualification was ‘poor’, while for the groundwood-containing paper it was ‘critical’.

This qualification is in agreement with the recommendation of the restoration workshops to have the papers strengthened after the irradiation treatment.

After the disinfection by irradiation, using 8 ± 2 kGy, no serious changes were found in the evaluation by SurveNIR. These observations suggested that the paper degradation caused by up to 10 kGy of gamma radiation is minor compared to the degradation caused by mould.

Looking at the SurveNIR evaluation of the acidity of the groundwood paper after irradiation and flattening, it is observed that the acidity is lower after all treatments than before them. This might be due to the small amount of pH neutral water used for flattening the curled paper leaves.

As SurveNIR is based on a large database of original correlated characteristics, the SurveNIR software is able to estimate the characteristics of the paper, for example, the degree of polymerization, or the tensile strength index. The degree of polymerization of both cellulose types is presented in Figs 12.3 and 12.4. The values presented in these figures represent an average of 30 measurements; nevertheless, the estimated error in the presented mean value is about 10% due to non-homogeneity of the samples. It is demonstrated that mould degrades cotton cellulose paper less than softwood cellulose, probably owing to the higher degree of crystallinity of the cotton cellulose, while softwood cellulose contains more amorphous fractions that can easily be hydrolysed by factors such as mould, enzymes and acids. After irradiation with a maximum FIG. 12.2. Left: a sheet of mould infected groundwood paper after dry cleaning. Right: a sheet of mould infected groundwood paper after disinfection with 8 kGy of gamma radiation and removal of the mould dust.

TABLE 12.1. RESULTS OF THE SurveNIR ASSESSMENT Code Restoration workshop

Material typeTreatmentSurveNIR assessment pH Yellowing riskBrittleness risk P1NoneSoftwood celluloseNo mould, referenceFairFairFair P1 MHSoftwood celluloseMould, dry cleaned, flattenedPoorFairPoor P1 M GHSoftwood celluloseMould, irradiated, flattenedPoorFairPoor SSoftwood celluloseMould, irradiated, flattenedPoorFairPoor P2NoneCotton celluloseNo mould, referenceFairGoodFair P2 MHCotton celluloseMould, dry cleaned, flattenedPoorGoodFair P2 M GHCotton celluloseMould, irradiated, flattenedPoorGoodFair SCotton celluloseMould, irradiated, flattenedFairGoodFair P3NoneGroundwoodNo mould, referencePoorPoorCritical P3 MHGroundwoodMould, dry cleaned, flattenedCriticalCriticalCritical P3 M GHGroundwoodMould, irradiated, flattenedCriticalCriticalCritical SGroundwoodMould, irradiated, flattenedPoorCriticalCritical Note: ‘Good’ indicates that the material is in good condition and does not need any conservation action; ‘fair’ indicates that conservation action is recommended but not necessary; ‘poor’ indicates that there is a need for conservation; and ‘critical’ indicates that conservation action is necessary to avoid total loss of the material.

FIG. 12.3. The degree of polymerization of the softwood cellulose paper before mould and irradiation (P1), after infection by mould and dry cleaning (P1/M), and after infection by mould and irradiation using maximum 10 kGy and subsequent surface cleaning and flattening (P1/M GH and P1/M GS).

FIG. 12.4. The degree of polymerization of the cotton cellulose paper before mould and irradiation (P2), after infection by mould and dry cleaning (P2/M), and after infection by mould and irradiation using maximum 10 kGy and subsequent surface cleaning and flattening

degree of polymerization for either the cotton or softwood cellulose-containing papers.

12.6. SurveNIR ANALYSIS — PART 2, CHEMOMETRIC APPROACH

Another approach to measuring the effects of irradiation on paper is the application of principal component analysis (PCA). PCA is a statistical multivariate analysis to evaluate and describe a large amount of data using a small number of relevant variables. This application was first established by the European research project on the effect of air pollutants on the ageing of paper (STEP CT 90-0100) [12.6]. Experimental data can often be arranged as a table, a data matrix with p variables measured on n objects. In this case the objects are the paper samples and the variables are the measurements. PCA can be used to obtain an overview of the data structure in the data matrix. PCA reduces the dimensionality of data matrices that contain intercorrelated variables. A plot based on the two most significant components can be found in Fig. 12.5. The measurements are presented in a 2-D component weight plot. The further a method is away from zero on the axis, the better the method can be applied for determining the ageing effect. The two most significant components (component 1 and 2) are calculated based on a co-variance matrix using a statistical software program. In this analysis, the calculation of the component weights was done by means of a software program. The parameters for ageing used in the EU project

FIG. 12.5. PCA of the NIR spectra of the original samples, the mould infected samples (M) and the samples that were surface cleaned and flattened after irradiation with up to 10 kGy of gamma radiation (MG).

were comparable to those used by the SurveNIR project, and the PCA can be established based on these data, although the SurveNIR project did not generate a large quantity of data. The original NIR spectra of the samples were used for the evaluation. The results of the PCA evaluation are shown in Fig. 12.5.

Figure 12.5 shows not only components 1 and 2 for each sample, but also the direction of ageing (red arrow), which has been put into these graphs starting from the original least aged sample (e.g. paper P1, the softwood cellulose-containing paper). For the three samples it is clear that the arrow points from 1 to MG to M, and that the main deterioration of the materials is caused by mould (M) as this point lies farthest from the original point. This method of evaluation demonstrates again that irradiation using 8 ± 2 kGy of gamma radiation (which therefore has a maximum dose of 10 kGy) causes less degradation than mould.

12.7. CONCLUSIONS

The research described in this chapter demonstrated that the SurveNIR assessment method can be applied even to samples that contain mould.

Nevertheless, care should be taken in choosing the location for sampling the NIR spectrum, as papers with mould are extremely inhomogeneous. The sampling locations used for this research appeared visually identical.

Based on the SurveNIR assessment method, it can be concluded that mould caused the highest degree of degradation of the paper materials and that there was no significant difference between mould infected samples irradiated with 8 ± 2 kGy of gamma radiation followed by manual flattening and mould infected samples that were dry cleaned only.