Browning of paper Fluorescense changes accompanying the discolouration of artificially aged paper
| Main Authors: | , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 1563-2628 |
| Online Access: | http://www.viks.sk/chk/pap_S_00_47_54.doc |
| Abstract: | SUMARRIES. The development of brown stains on the surface of paper is almost invariably an undesired alteration to paper objects of intrinsic value. Discolouration has explicit aesthetic implications to those objects, and it indicates that chemical changes have occurred. Examples of brown discolouration are the irregularly shaped stains generally referred to as "foxing", iron-gall ink corrosion brown discolouration developing around ink lines and migrating into the paper upon ageing, and the permanent brown lines (tidelines) formed at the interface between wet and dry areas due to the local action of water evaporating from paper. A common feature associated to brown stains, irrespective of their origin, is that discoloured areas frequently fluoresce when examined under ultraviolet radiation. Enhanced fluorescence has been observed in all the three examples mentioned before: in connection with foxing stains (Hey et al 1992: 244; Cain and Miller 1982: 54; Gallo and Pasquariello 1989: l39;Choisy et al 1997:131), around iron-gall ink writings (ReiRland 1997; Baker 1983: 159), and at the tideline in water Stains (Hutchins 1983: 57; Dupont 1994; Eusman 1995; Hofenk de Graaff 1994:21). Fluorescent areas are usually larger than the visibly stained areas, and in some cases fluorescence occurs without any visible discolouration. Fluorescence seems to appear before discolouration, to increase in intensity and change colour from white-blue to green-yellow as degradation proceeds. When discolouration has developed into dark brown stains, fluorescence can no longer be observed. The common pattern in these observations suggests a common underlying mechanism at some level for all these cases. Paper conservators often use UV-light as a diagnostic tool that is simple and generally available. The interpretation of visually observed fluorescent patterns seen under UV-illumi-nation, however, is not straightforward. Is it generally true that fluorescent areas always develop into brown discoloura-tions? Can further conclusions be drawn from observations of specific perceived fluorescence colours? There is still a lack of knowledge on the precise mechanisms causing the phenomena, both on the chemistry level and on the spectral level. This paper is the first in a series where the question of a common mechanism for fluorescence preceding browning will be investigated. In this study two types of additive-free papers were subjected to accelerated ageing conditions. The changes in fluorescence and the discolouration of the test papers were assessed visually and checked against their spectral behaviour. At this stage no attempt has yet been made to elucidate the chemical structures of possible fluorescent species involved. Instead, this study aims at describing the fluorescence and browning phenomena at a spectroscopic level. Consistency of spectroscopic data is checked against visual observations with colour calculations. CONCLUSION. Paper submitted to ageing conditions that promote hydrolytic and oxidative degradation of cellulose exhibits increased overall fluorescence, which shows a single emission maximum at 445-450 nm (excitation wavelength = 365 nm). Fluorescence increases rapidly in the beginning of the ageing treatment and then levels off or decreases slightly for longer ageing periods. The initial fluorescence observed for the unaged test samples is believed to result from the processing and natural ageing of cellulose. A discernible change in the colour of fluorescence of the chemical pulp paper examined under UV-illumination at 365 nm with protective glasses was observed, which developed from a low-intensity bluish colour into a more intense green and, later on. yellow colour. This behaviour has been shown to be consistent with the formation of new fluorescent species within the paper re-emitting in the green-yellow region of the visible spectrum. No significant colour change in the fluorescence of Whatman No. 1 filter paper could be shown from the calculation of the CIE chromaticity co-ordinates from the emission spectra. The results of fluorescence spectroscopy show that the reported observations by paper conservators concerning the increase in intensity and the colour change in fluorescence of paper affected by degrading factors such as iron-gall inks, wet-dry interfaces, and foxing agents can be reproduced (at least to some extent) when additive-free paper is submitted to ageing conditions that promote hydrolytic and oxidative degradation of cellulose. The local fluorescence observed in connection to those degrading factors on paper may therefore arise from a locally more extensive degradation of cellulose promoted by them. The hypothesis that fluorescence precedes brown discolouration in paper could not be verified under the experimental conditions used in the present study. The results showed that fluorescence and discolouration have developed simultaneously in the course of artificial ageing of the test papers, and that only minor changes in the spectral distribution of the fluorescence emission has occurred. The perceived colour of fluorescence under UV-illumina-tion was found to be affected by the presence of a visible component in the light generated by the lamp. It is believed that the spectral distribution of most ultra-violet lamps used will contain some blue light component. As a result, the colour impression of paper emitting fluorescent light under an ultraviolet lamp will depend on the specific illumination conditions. Perceived colour changes could be due to changes in the fluorescence emission spectrum but could also be due to changes in the reflectance of paper. |
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| ISSN: | ISSN 1563-2628 |
