These experiments were carried out using a TGA7 Perkin-Elmer thermobalance, which operates in the 50 to 600°C temperature range with a heating grate of 10°C per minute. The purge gas is a mixture of oxygen (20%) and nitrogen (80%) and is injected at a pressure of 1.35 bar26. The sample weighs about 1 mg.
We worked on samples of dated papyri (Sorb. 5, Sorb. 6, Sorb. 7 and Abousir Papyrus E 25416). Two modern papyri manufactured in Egypt were also analysed. One was very light; it was made in a press. The other, which was brown, was beaten before being dried.
In the thermograms presented below, we can see the rate of degradation of cellulose and lignin, which is characterized by the slope of the curves or the
Fig. 6: Thermograms for cellulose and lignin. The broken line is the "First Derivation" of the thermogravimetric curve.
height of the peaks in the derivative curve. We can also see the relative quantities of the various components, by calculating the area of each peak. In order to identify the precise nature of each peak, we did the same thermogravimetric experiment with pure cellulose and lignin (Fig. 6).
Because of the large amounts of mineral compounds detected on the ancient papyri, the relative concentrations of organic compounds was calculated following deduction of the percentages of ashes and water linked to each sample (Table 2). Fig. 7 gives the thermograms of modern white, modern brown, the Abousir Papyrus (Louvre E 25416) and the papyrus Sorbonne 3. The following can be observed. • The thermal degradation of papyrus is divided into three stages corresponding to the loss of the following products:
• between 100 and 250°C: bound water
• between 250 and 380°C: cellulose
• between 380 and 600°C: lignin
Table 2: Relative values (percent)of cellulose and lignin (after deduction of bound water and ash) in modem and ancient papyri.
• When the pure products are analysed decomposition occurs in a narrow temperature range and the peak in the derivative curve is very sharp. Cellulose provides a striking example of this (Fig. 6).
• The percentages of cellulose and lignin found in the modern papyri (Table 2) are in agreement with the results found by others25. The brown, highly oxidized papyrus has a different thermogram from the white papyrus. During the ageing of papyrus, lignin degrades first. The decomposition products of lignin become volatile in the same temperature range as cellulose (Fig 8: brown pap.). The peak due to cellulose is thus broader, and that due to lignin narrower.
• It would be premature to draw, on the base of the results found in this research, any conclusions for the ancient papyri. Nevertheless, we can hypothesize. The papyri E 25416, Sorb. 5, 6 and 7 have percentages of cellulose and lignin similar to the modem white papyrus and appear to have undergone little oxidation. However, Sorb. 3 (Fig. 7) appears to be more degraded and the very low concentration of lignin in Sorb. 4, in which the pollution layer seems to have been cleaned with acid (recognizable by the low concentration of ash), reflects a strong deterioration.
These preliminary results are very promising, but should be interpreted with caution, considering that the initial cellulose-lignin ratio can vary from one papyrus to another depending on a number of parameters such as the season of harvest, geographic origin, the technique of manufacture and the current state of the manuscript.
In order to provide information to help date ancient papyri or determine their place of origin, we are planning to study the relative concentrations of cellulose and lignin in fresh papyrus stems grown in different regions (France, Egypt, and Sicily) and harvested in either spring or autumn. We will also analyse different parts of the stem. It is also important to extend our collection of samples and study many more papyri of known date and location.
Fig. 7: Thermograms for the modern and antique papyri. The broken line is the "First Derivation" of the thermogravimetric curve.
Table 3: pH of the modern papyri after accelerated ageing by moist heat (80°C 65% RH), light (25°C 50% RH) and pollution (SO2 + NO2).
study of the degradation of papyrus
To define the best conservation conditions for ancient papyri, we decided to study the behaviour of papyrus in the presence of the principal agents causing physical and chemical degradation: heat, humidity, light, and pollution. We have thus analysed the behaviour of two modern papyri (white and brown) in the presence of these various aggressive agents.