Spectroscopic Investigations for the Dating of Paper from the Nineteenth Century.

The knowledge of the detailed material composition of paper can help art historians, archivist, librarians, paper historians, and conservators to determine the possible age of a document or a work of art. The dating of paper by the identification of specific paper components is especially applicable to paper from the 19th century. In this era, many changes in the paper production technology occurred and new raw materials were introduced in relatively short time intervals. ATR-IR and Raman spectroscopic measurements were used to analyze the chemical composition and structure of papers from the 19th century. The infrared spectra showed the general material composition of the papers. More specific information on the different paper components and their distribution in the paper were obtained by Raman microscopic measurements. The resulting Raman images visualize the detailed chemical structure of the papers including all components such as paper fibers, filler pigments, sizing agents, and color pigments. Special emphasis was made on the spectroscopic identification of different paper fiber types, like straw, esparto, and sulphite and sulfate chemical wood pulp, which were introduced to paper production during the second half of the 19th century. It could be shown that the specific spectral differences in the Raman spectra of the different paper fiber types are related to different amounts of hemicellulose xylan compounds on or in the cellulose fibers.

New method for the absolute dating of paper by radiocarbon measurements.

The absolute dating of documents is still one of the most important challenges for forensic document examiners (FDE). The potential difference between the date on a questioned document and the actual year of production of the used paper can be only 1 to 5 years. Until now, there was no analytical method with this accuracy available. This study demonstrates a method for an absolute dating of paper by using the 14 C bomb peak and dating the starch in the paper. Accelerator mass spectroscopy (AMS) radiocarbon measurements were performed on starch extracts and cellulose fibers from 50 paper samples with known production year from 1950 to 2018. For most of the paper samples, the measured 14 C concentration values in the starch extracts were highly correlated with the data of the 14 C bomb peak calibration curve. The differences between the calibrated ages and the actual harvest years of the starch-containing plants were only up to 3 years. The 14 C concentration in the paper fibers showed a lower but significant correlation with the production year of the papers and can be used to support the dating results of the starch extracts. In order to secure the accuracy of the dating, aside from the radiocarbon measurements of the extracted starch other analytical methods or data are used, like a detailed chemical analysis of the paper composition including fiber composition and the spectroscopic analysis of the purity of the starch extracts. In practice, only starch extracts without contaminations are used for the paper dating.

Analysis of starch distribution in the paper cross-section by Raman microscopy.

A new Raman microscopy approach was developed to analyze the starch distribution of paper cross-sections in a faster and more specific way than is possible with the currently used iodine-staining method. Raman images were recorded and analyzed from cross-sections of cellulose hand sheets surface-sized with 1% or 2% starch solutions and with different film thicknesses. In addition, Raman imaging analysis of the starch distribution was performed on two industrial papers, an abrasive base paper and a surface-sized recycling paper. The visualization and the quantitative analysis of the starch distribution were performed by using the intensity changes of the Raman starch band at 855 cm(-1) and by principal component analysis. Distribution curves were calculated from the intensity data and compared for the samples with different starch concentrations and with results obtained from iodine-stained cross-sections of the same samples. The results of this study demonstrate the great potential and the new possibilities of Raman microscopy for studying the z-distribution of chemical components and additives in paper.