Raman Spectroscopy in Archaeological Studies

Significance of the Topic

The study of archaeological artefacts using noninvasive analytical methods is crucial for preserving cultural heritage. Raman spectroscopy offers a powerful means to identify pigments and mineral compositions directly on site. This knowledge assists in dating objects, confirming authenticity, and guiding conservation efforts without removing or altering precious samples.

Objectives and Study Overview

The primary goal of the presented work was to demonstrate the applicability of portable Raman instruments in archaeological contexts. Two case studies are highlighted: prehistoric rock art in Iberian open-air shelters and polychrome plaster decorations in the Alhambra’s Hall of Kings. Each example illustrates challenges of in situ measurement and the benefits of modern portable Raman systems.

Methodology and Instrumentation

Measurements were carried out with a 785 nm portable Raman spectrometer equipped with a cooled CCD detector and fiber optic sampling probe. Laser power settings as low as 3 mW allowed analysis of light-sensitive and dark pigments. Integration times up to 30 minutes enabled detection of weak signals.

• i-Raman Plus 785H portable spectrometer with spectral range from 65 cm-1 to 2800 cm-1
• Fiber optic probe for flexible access to complex surfaces
• Video microscope sampling head with coaxial illumination for precise targeting
• Manual tripod with fine XYZ positioning to stabilize the probe in hard-to-reach areas

Main Results and Discussion

In the rock art study, spectra acquired in daylight revealed peaks of haematite pigments alongside gypsum and whewellite crust deposits. A custom foam cap minimized ambient light interference. In the Alhambra plasterworks, Raman mapping of vault decorations identified gypsum substrates and a variety of pigments such as lapis lazuli, cinnabar, and minium. The presence of lazurite peak at 548 cm-1 distinguished natural and synthetic blue pigments and allowed inference of geographic origin. Degradation markers like calomel in degraded cinnabar and tin oxide deposits in darkened gilded areas were also detected, informing on materials aging and past restoration practices.

Benefits and Practical Applications

Portable Raman spectroscopy provides significant advantages for archaeological research and conservation:

• Noninvasive in situ analysis maintains artefact integrity
• Minimal sample preparation accelerates field investigations
• Fiber optic and tripod-mounted probes allow access to varied geometries
• Adjustable laser power addresses pigment sensitivity and fluorescence
• High spectral resolution yields detailed molecular fingerprinting

Future Trends and Potential Applications

Advances in portable Raman technology are expected to further enhance archaeological studies. Trends include integration with multivariate data analysis software for automated pigment classification, miniaturization of probes for microscopic mapping, and coupling with other spectroscopic modalities such as LIBS or infrared imaging. These developments will expand the scope of in situ diagnostics and support multidisciplinary conservation strategies.

Conclusion

Portable Raman spectroscopy emerges as a vital tool in archaeological science, enabling comprehensive, noninvasive characterization of pigments and substrates at heritage sites. Its flexibility, sensitivity, and minimal sample disturbance make it ideal for on-site investigations. Insights into material composition and degradation processes empower more informed preservation and restoration efforts, safeguarding cultural assets for future generations.

Reference

1 Spectroscopic characterisation of crusts interstratified with prehistoric paintings preserved in open air rock art shelters A Hernanz et al J Raman Spectrosc 2014 45(11) 1236-1243 doi 10.1002 jrs 4535
2 In situ noninvasive Raman microspectroscopic investigation of polychrome plasterworks in the Alhambra A Dominguez-Vidal M J de la Torre-Lopez R Rubio-Domene M J Ayora-Canada Analyst 2012 137(24) 5763-5769 doi 10.1039 c2an36027f
3 Gold in the Alhambra study of materials technologies and decay processes on decorative gilded plasterwork M J de la Torre-Lopez et al J Raman Spectrosc 2014 45(11) 1052-1058 doi 10.1002 jrs 4454