Degradation of Cellulose at the Wet/Dry Inter face The Effect of Some Conservation Treatments on Brown Lines
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0034-5806 |
| Online Access: | http://www.viks.sk/chk/res_1_96_1_21.doc |
| Abstract: | Conclusions. The experiments reported were carried out in order to approximate the nature of the brown degradation products forming on cellulose at the wet/dry boundaries, and to approach the conditions in which a brown line is likely to form in usual paper conservation treatments. Our results are consistent with earlier works carried out on textile, indicating oxidation of the cellulose at the wet/dry interface.1'7'9"13 However, they are complemented with artificial ageing, sodium borohydride reduction testing, and usual conservation treatments. We focused as much as possible on working in the conditions more often encountered in wet treatments in paper (and textile) conservation. For instance, organic solvents were chosen because of their frequent use in local treatments for the removal of stains (adhesive tapes, grease marks, etc.). Water-stains are sources as well as consequences of damage to cellulose: browning not only is unsightly, but it also is an indication of oxidation to some extent. Both washing and bleaching with the reduction agent sodium borohydride were found effective to remove or attenuate the browning on brown lines freshly formed. However, it was shown that degradation did not only lead to cleaved compounds from the cellulose chain, but also to oxidized cellulose with new end-groups attached to the macromolecule. Washing and reducing with sodium borohydride could be complementary treatments of the brown lines. Upon ageing, the solubility of the brown degradation products is found to decrease, not to say brown lines become insoluble. Both washing and bleaching with sodium borohydride appear to be more or less ineffective on aged brown lines. It was also found that wet treatments may be potentially harmful to paper artifacts. Any process including local wetting or creation of uneven humidity content of the artifact represents a potential source of oxidation if wet/dry boundaries are allowed to form. From this point of view, the drying of cellulosic material should also be emphasized as a step to follow very closely during a conservation treatment. The conservator must be aware that use of the suction table requires a very accurate control of the humidity content of the artifact. The removal of adhesive tapes, grease stains and other dirt marks as local treatments is also pointed out as susceptible to promote oxidation. UV fluorescence appeared to be very sensitive to detect fluorescent oxidized areas which could further develop brown stains on ageing, and to prevent them by considering a washing or a reduction treatment when possible. Since it is a very handy method, we recommend to use it as much as possible in the paper and textile conservation field, for instance, for evaluation reports of the artifacts before treatment, as well as a tool to follow the course of a treatment. This work does not attempt to answer all the questions on the formation of brown lines. The chemical analysis of the degradation compounds formed at the wet/dry interface is explored elsewhere by the author8 and gives more information on the nature of the phenomenon of the brown line formation. Summaries. This article contains results from the study of the formation of brown lines on filter paper at the wet/dry interface with water, ethanol or acetone. The effect of ageing and the effect of the conservation treatments of washing and bleaching with the reduction agent sodium borohydride were investigated. Qualitative evaluation of the degradation of the paper at the location of the brown line is done in daylight, under UV at 366 nm and staining with methylene blue dye. The results showed that any wet/dry interface, even a brief exposure, may be a potential cause of degradation. Ageing resulted in a drastic darkening of the brown line and insolubility of the brown compounds. It was also confirmed that degradation led not only to distinct compounds cleaved from the cellulose chain, but also to oxidized cellulose with new end-groups attached to the macromolecule. The compounds formed at the wet/dry interface were identified as containing carbonyl and acid groups. Reduction with borohydride was found effective to decrease the browning of freshly formed brown lines exclusively. Washing was also found effective only on recent brown lines and had to be done in the liquid that originated the brown line (water or organic solvents). Results showed that conservation processes involving local or global wetting of paper documents are a potential source of browning. Practical recommendations are given for the conservator, such as the frequent observation of paper artifacts under UV wavelengths to detect where browning could appear on ageing. |
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| ISSN: | ISSN 0034-5806 |
