In order to monitor the kinetics of the degradation, the papyri were analysed after 2, 3, and 6 weeks exposure to three types of aging:
• Heat and humidity according to the IPSO 5630/3: 80°C, 65% RH.
• Light irradiation: 1500-W xenon lamp at 25°C, 50% RH.
• Pollution in an atmosphere containing 20 ppm NO2 and 10 ppm SO2 at 30°C, 50% RH.
For describing the degradation of the aged papyri we measured the mechanical strength (zero span tensile strength), acidification (pH), optical constant (whiteness) and oxidation (copper number).
The results of pH measuring are presented in Table 3, those of whiteness and oxidation in Fig. 8. The following general conclusions can be drawn:
• Tensile strength: Because of the heterogeneity of the fibres in papyrus, the results obtained from the zero-span measurements are too dispersed to allow a reliable interpretation.
Fig. 8: Variations in the degree of whiteness and copper number of the modern white and brown papyrus.
• pH: After 6 weeks, pollution causes a significant acidification of the white papyrus, and a pronounced drop in pH for the brown papyrus. We noted that brown papyrus was also acidified after accelerated ageing with light (Table 3).
• Whiteness: Only the white papyrus yellowed under the action of heat. This appeared in the first week and then stabilized. The behaviour of the papyri upon exposure to light was very different. After 1 week's accelerated ageing, the white papyrus had yellowed, but then, with ongoing exposure to light, became progressively whiter. The brown papyrus rapidly became lighter from the beginning. (Fig. 8).
• Oxidation: The main observation was that the copper number of the non-aged brown papyrus was three times higher than that of the non-aged white papyrus. This indicated that the beating of the pith had led to a significant amount of oxidation in the material. Following accelerated ageing due to light, both papyri suffered significant oxidation. The oxidation of the brown papyrus, after 6 weeks exposure to pollution and to light, reached values too high to be measured (Fig. 8).
These results show that, during accelerated ageing, the white papyrus reacts much better than the brown papyrus. This can be explained by the high degree of oxidation of the brown papyrus, due to its method of manufacture.
The acidification caused by six weeks exposure to pollution appears to have little effect on the cellulose in the white papyrus, but has more effect on the brown. Light causes negligible degradation of both types.
This study has shed new light of the technique of manufacture of papyrus as a writing material. We can draw several conclusions.
• No glue was added for the adherence of the strips.
• The sheets made with the lower part of the plant are more translucent and stick together better than sheets made from the upper part of the plant.
• The sheets dried after beating are very brown and contain highly oxidized cellulose.
The analysis of the mineral and organic elements in the ancient and modem papyri provided an opportunity to test a number of different analytical methods. Considering the valuable nature of the documents concerned, we have obviously favoured non-destructive methods, or those which required very small samples. Some proved to be more appropriate for our studies, while others remain very promising.
We have, thanks to the PIXE method, analysed the mineral content of the papyri. The identification of the chemical content in the pollution laver of each papyri reveals the influence of the cleaning process. However, the analytical results are difficult to interpret because the mineral content of the papyrus itself can interfere and also the elementary analysis gives little information on the chemical compounds present in the pollution layer.
Regarding the organic compounds, we used thermogravimetric analysis to measure the respective concentrations of cellulose and lignin in some modern and ancient papyri. The results brought us to form the hypothesis that there is a correlation between the cellulose-lignin ratio and the condition of papyrus.
Our experiments will be continued and be extended, not only in order to confirm this hypothesis, but also in order to find a better understanding of the history of papyrus conservation, alterations and any treatment that such an object may need at present. We hope that this technique may also prove to be useful in dating and locating the origin of papyri.
This work has also brought a result for practical conservation. The behaviour of modern papyri during accelerated ageing has demonstrated the disastrous consequences of exposure to light. This very important point should be taken into account for exhibiting papyrus collections.
We would like to express our thanks to Prof. Blanchard, who kindly provided the ancient papyri. We thank M.C. de Bignicourt, who has done the analysis regarding starch, Prof. Roland, Michel Abadie and Monique Mousiniak, who helped with them and Prof. Vairon for his help in the interpretation of the thermograms. We also thank the team at the AGLAE accelerator of the C2RMF for their help during the PIXE analyses.