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Written Artefact Profiling Guide
Written ArtefactProfiling Guide

Photo: CSMC

Written Artefact Profiling Guide

Crack-Enhanced Weathering in Engraved Marble

Stylianos Aspiotis

In the present study, the sensitivity Raman spectroscopy was explored for the determination of detectable material differences between inscribed and non-inscribed regions of marble-based written artefacts. These differences hold the potential to be utilised in visualising obscured or completely vanished inscriptions via suitable Raman mapping. Here, we analysed engraved marble segments dating back approximately 2000 years from Asia Minor (present-day western Turkey), as well as marble gravestones with inscribed text, which is 66 ± 14 years old from the Ohlsdorf cemetery in Hamburg, Germany. To thoroughly examine the effects of diverse environmental conditions, varying grain sizes, and the age of inscriptions on the nature and penetration depth of marble deterioration, Raman spectroscopy was employed alongside complementary analytical methods including X-ray diffraction, wavelength-dispersive electron probe microanalysis, and Fourier-transform infrared spectroscopy.

The findings of this study demonstrate that environmental conditions play a crucial role in the formation of the dominant types of weathering changes in engraved marble. In particular, marbles from Hamburg showed carotenoid molecular inclusions resulting from lichen activity, whereas those from Asia Minor exhibited deposits of amorphous carbon. Moreover, the detection of gypsum at greater depths beneath inscriptions compared to non-inscribed areas in the Hamburg samples suggests the role of an urban environment where SO2-rich gases react with calcite leading to the formation of this weathering-related product. The degree of alteration was observed to be more significant in medium- and coarse-grained marbles compared to fine-grained marbles. However, the presence of letter colouring helped mitigate these alterations. Where letter colouring was absent, weathering-related products were found to be more prevalent beneath the engraved areas in comparison to non-engraved regions for both ancient and modern inscribed marbles, with deeper penetration observed due to the enhancement of fissures and micro-cracks around the engraved areas (Fig. 1).

The results also show that the Raman intensity ratio (Fig. 1) between the strongest peak of the weathering-related product (ν(C=C) ~1520 cm-1 for carotenoids or the G peak ~1595 cm-1 for soot-like carbon) and the strongest peak of calcite CO₃ stretching roughly at 1087 cm-1), as the most abundant mineral in marble, can serve as a quantitative marker. This ratio can be used to indirectly map the lateral distribution of cracks induced during the inscribing process, thereby offering a method to potentially recover and trace lost text on deteriorated marble inscriptions. While this technique shows promising application for marble, additional research is needed to identify analogous weathering-related products in other rock types, which would enable the broader application of this method.

Figure 1: Raman intensity ratios of I1595/1086 (a) and I1520/1087 (b) plotted against the distance to the surface for inscribed and non-inscribed areas, measured along lines, as those indicated in the insert photos. The data points in (a) were fitted with a lognormal function, while in (b) with an exponential decay function. The figure is after Aspiotis et al. 2021.