Poster: Raman spectroscopy as a tool for analysing carbon inks from ancient palm-leaf manuscripts

This poster was presented at : International Conference on the Application of Raman Spectroscopy in Art and Archaeology (RAA) 2023

Abstract

The Palm-leaf Manuscript Profiling Initiative (PLMPI) project at the Centre for the Study of Manuscript Cultures (CSMC) aims at carrying out an innovative and comprehensive study of palm-leaf manuscripts by employing various non-destructive and non-invasive analytic methods that have not yet been fully or at all tested on such artifacts. The study of inks is of paramount importance to investigative matters of provenance, which remains one of the main research topics for the history of the book in South Asia. In Europe since the Middle Ages, the most common inks are iron gall inks, which are produced by the reaction between iron sulphate from natural vitriol and gall nuts extract, mixed with a binder. Such inks have been extensively studied in the last twenty years, using the non-destructive and non-invasive method developed by Hahn et al. This method involves micro X-ray fluorescence (XRF) analysis to measure the relative weight contributions of metallic sulphates characteristic of vitriol sources [1]. However, in the case of South Indian manuscripts on palm leaf, carbon inks are used, composed of carbonaceous material (soot or charcoal) mixed with water and a binder.

Since the detection of light element-containing compounds, for instance H- and C-bearing, is not viable by XRF, alternative non-invasive, non-destructive and preparation-free methods, such as Raman spectroscopy that can identify hydrous and carbonaceous species, are gaining popularity among the scientific community. Moreover, Raman spectroscopy can identify both crystalline and amorphous phases and has the great ability, compared to other non-destructive analytical methods to distinguish different phases in a polyphasic material. Meanwhile, Raman spectroscopy enables the determination of the exact elemental distribution within each structure type, as vibrational phonon modes are sensitive to both structure and chemistry. As a result, the obtained information can be further matched with other codicological features (e.g. types of palm leaves, hand style of specific scribes, available paleogeographical information, etc.) and thereby promote provenance studies.

So far carbon-bearing materials, particularly graphitic materials and carbonaceous nanomaterials have been thoroughly studied with Raman spectroscopy [2-3]. Therefore, it naturally appeared as a good candidate for the investigation of carbon inks. In this work, we compare the spectra obtained on mock-up samples of ink, made after antique recipes using different sources of carbon [4]. We also analyse inks from several palm-leaf manuscripts from Southern India. The investigation was conducted with two bench-top instruments (Renishaw in-via with a 532 nm laser and Horiba Jobin Yvon T64000 triple-monochromator system with a 514.5 nm green line) to check the reproducibility of the method and identify suitable conditions for distinguishing the different sources of inks depending on the spectrometer. All the spectra were fitted with pseudo-Voigt peak shape functions, to define the Raman spectral features (peak position; ω, full widths at half maximum; FWHMs, and integrated intensities) of the observed D (diamond-like) and G (graphite-like) peaks. These spectral parameters were then used: (a) to differentiate between the mock samples depending on their choice of precursors, (b) for the discrimination of various C-based inks, and (c) for the determination of the particle size of soot/carbonaceous material used for the production of C-based inks. The results clearly indicate that it is possible to distinguish between charcoal and soot-based carbon inks (Fig. 1). Additionally, it was possible to analyse the black inks present in palm leaf manuscripts under similar experimental conditions, and the results of this investigation indicate the use of soot-based carbon ink in palm leaf manuscripts with a graphite cluster diameter of 8-12 nm.