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Paper Yellowing
Yellowing of paper is a very important degradation process. For fundamental research on this subject, the National Archives store reference paper sheets under different conditions for several years, and measure periodically the progress of yellowing. The SEPIA quantitative hyperspectral imager was used to measure and compare the local variation of the yellowing of papers that had been stored for several years under different conditions. A comparison has been made between two sheets of acid mechanical pulp paper which have been stored in the same depot since 1997 at the National Archives: one sheet as part of a bound volume and the other sheet stored in a special cardboard box.
For each paper the spatial distribution of the yellowness index (as defined by the DIN 6167 norm) was determined (Fig. 1). In Fig. 2, the yellowing index along the cross section indicated in Fig. 1 is shown. The green and the red curves, corresponding to the loose and bound paper sheet, respectively, fluctuate around the same yellowness index of about 19 inside the paper sheets. For both papers, an approximately 20 mm wide border area can be identified, where the yellowness increases drastically. For the bound paper, however, this increase is much stronger than for the loose paper, reaching a value of about 38 at the sheet border, as compared to the value of about 30 for the loose paper.
Figure 1: False-colour images visualizing variation of the yellowness index over the paper sheet for A: loose and B: bound paper. Green colour corresponds to a low, red colour to a high yellowness index. The dashed lines indicate where cross sections were extracted.
Figure 2: Cross sections of the yellowness index from the centre towards
the border of the paper sheets. Red curve: yellowness index of bound
paper. Green Curve: yellowness index of loose paper.
A close inspection of the false-color images in figure 1 reveals that locally strong variations of the yellowness index occur also in the inside areas of both paper sheets, seen as small red spots on a green background. These variations of the measured yellowness are caused by the inhomogeneity of the mechanical pulp paper, which contains clearly visible bits of wood of up to several millimeters in size. In such cases of an inhomogeneous substrate, measurements of the yellowness or color with a conventional photospectrometer typically suffer from an unsatisfactory reproducibility of representative results. This is because with such instruments, measurements can be performed at only a few locations, and each measurement yields values averaged over an often not very well-defined small area. As opposed to the conventional single-point instruments, the hyperspectral imager measures with high resolution at millions of points simultaneously. This is a great advantage, because the local variations of the paper color can be accurately measured and taken into account during the data analysis, so that the measurements should become much more reproducible and comparable.