The aging behavior of ancient papyri is not entirely understood, although such understanding is crucial for tailored conservation concepts to preserve these precious historical documents for the millennia to come. In a study on accelerated aging, the effect of light on papyrus sheets was studied, and the consequences were monitored by a combination of non-invasive and invasive techniques. Papyrus sheets were aged for two months at 90 °C and 50% relative humidity, with and without the influence of light. Changes at the optical, mechanical, and molecular levels were monitored using CIELAB color measurements, tensile tests, Fourier transform-infrared spectroscopy (FTIR), and size exclusion chromatography—multi angle light scattering (SEC-MALS). Light exposure has severe consequences for the optical and mechanical properties of papyri, reflected by a strong decrease in cellulose chain length and tensile strength. SEC-MALS proved to be the best method to assess the conservation status of papyri due to its clear results and low sample demand. This publication seeks to highlight the effects of light-induced degradation of papyri and to facilitate an understanding of the underlying aging mechanisms, to support in future conservation measures. Graphical Abstract

Papyri belong to the oldest writing grounds in history. Their conservation is of the highest importance in preserving our cultural heritage, which is best achieved based on an extensive knowledge of the materials’ constituents to choose a tailored conservation approach. Thermogravimetric Analysis (TGA) has been widely employed to quantify cellulose and lignin in papyrus sheets, yielding reported lignin contents of 25% to 40%. In this work, the TGA method conventionally used for papyrus samples was repeated and compared to other lignin determination approaches (Klason-lignin and acetyl bromide-soluble lignin). TGA can lead to a large overestimation of the lignin content of commercial papyrus sheets (~27%) compared to the other methods (~5%). A similar overestimation of the lignin content was found for the pith and rind of the native papyrus plant. We concluded that the TGA method should, therefore, not be used for lignin quantification.