Non Invasive Techniyues for the Investigation of Foxing Stains on Graphic Art Materials
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0034-5806 |
| Online Access: | http://www.viks.sk/chk/res_3_97_131_152.doc |
| Abstract: | CONCLUSION. Fluorescence is a full non invasive characterisation of foxing stains. Maxima of excitation andemission of fluorescence show that the molecules responsible for fluorescence belong to one family and the fine differences may be explained by the different substitutions of a same chemical skeleton. The surface fluorescence analysis is a semi-quantitative measurement which provide information on the evolution (area and intensity) of fluorescent foxing stains. If the fluorescent stains were precursors of the brown stains, surface fluorescence would be the best method to improve preventive conservation in graphic art conservation. FTIR as a routine analysis is more difficult to set up. The presence of water tightly bond to cellulose absorb in a large band which overlap other fundamental information (1650 cm-1). The most secure way to get rid of this band is to allow a full deuterium exchange. Then the difference between the spectrum of a stained area and a non stained area is normalised (stained area spectrum - (factor x non stained area spectrum)) in the way to neutralise the peak of 2500 cm-1 (D2O). The same procedure can be followed without deuterium exchange but, in this case, the normalisation factor has to be automatically determined by the software (GRAMS 386) which uses an algorithm called "dewiggle". Nonetheless it seems that this is the only non invasive method which can provide useful chemical information on foxing stains. We have set up the back-bone of a taxonomy based on FTIR spectra, made of three main categories: ˙ Non conjugated ketones which may come from the degradation of cellulose ˙ Unsaturated compounds (=C, C=N, C=O) ˙ Conjugated systems of C=C and C=O or C=N linked to sugars. The question arises whether foxing stains studied here in a limited set of paper samples belong to one superfamily or may be divided into categories. If foxing stains would belong to the same superfamily, the different spectra would only be relevant of different states of the same process of staining. During the first steps, fluorogenic compounds appear, then chromogenic compounds appear in conjugated chromophores, and eventually, full colour is reached with free ketones. The latter remains a hypothesis. Only the extraction, the purification and the analysis of the chemicals found in foxing stain area would provide the answer. Another approach would be to use the same analytic procedures applied to in vitro models of foxing. The two complementary research tasks are in progress. SUMMARIES. The yellow/brown stains of circular or irregular shape known as foxing spots have been fully described in conservation literature but still, this phenomenon do not find any scientific agreement since many hypotheses have been raised concerning their origin but we deplore a lack of chemical and experimental evidences. In this work we have used two non invasive techniques for the chemical identification of foxing stains in order to define objective keys for a taxonomy of these stains. Fluorescence gives poor chemical information but if fluorogeniccompounds would be the precursors of the brown end products, the quantitative measurement of fluorescence would be of major interest for preventive conservation. FTIR spectroscopy provides more chemical information and as such, FTIR is a very relevant tool for a taxonomy of foxing stains. Since many chemical bonds may overlap in the same region, the interpretation of FTIR spectra are discussed. |
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| ISSN: | ISSN 0034-5806 |
