Crack-enhanced Weathering in Engraved Marble

This study explores the sensitivity of Raman spectroscopy for the determination of detectable material differences between inscribed and non-inscribed regions of marble artefacts with potential applications in visualizing obscured or completely vanished inscriptions. Here, we analyzed engraved marble segments dating back approximately 2000 years from Asia Minor and inscribed marble gravestones (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.

Our findings demonstrate that environmental conditions influence the formation of the dominant weathering-related products in engraved marble. Particularly, marbles from Hamburg showed carotenoid molecular inclusions resulting from lichen activity, whereas those from Asia Minor exhibited deposits of amorphous carbon. Moreover, gypsum was detected at greater depths beneath inscriptions compared to non-inscribed areas in the Hamburg samples, suggesting SO₂-rich urban conditions contributed to marble degradation. Medium- and coarse-grained marbles were more altered than fine-grained ones, although letter coloring mitigated some changes. Where letter coloring was absent, weathering-related products were found to be more prevalent beneath engravings for both ancient and modern inscribed marbles, facilitated by enhanced fissures and micro-cracks (Fig. 1).

Additionally, 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), 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.