Raman Spectroscopy in Archaeological Studies

Importance of the Topic

Raman spectroscopy provides a nondestructive approach for characterizing pigments, minerals and inorganic materials in archaeological artefacts. Its capability to probe low-frequency molecular vibrations enables polymorph differentiation and detailed identification of pigments such as haematite, lapis lazuli and cinnabar. The portability of modern Raman spectrometers empowers in situ analyses that preserve site integrity and guide conservation strategies.

Objectives and Study Overview

This work highlights applications of portable Raman spectroscopy in two archaeological contexts: prehistoric rock art in the Iberian Peninsula and polychrome plasterworks in the Alhambra Hall of Kings. Goals include on-site pigment identification, assessment of degradation processes and extraction of information on materials and techniques used in artefact creation.

Methodology and Instrumentation

Analyses were conducted with a fiber-optic-coupled portable instrument offering adjustable laser power, high spectral resolution and low-noise detection. Key instrumentation:

• i-Raman Plus 785H Portable Raman Spectrometer: TE-cooled CCD detector, spectral range 65–2800 cm⁻¹, integration times up to 30 minutes, laser power adjustable to 3 mW.
• Raman Video Micro-Sampling Head (785 nm): coaxial LED illumination, integrated video camera, compatible with standard microscope objectives.
• Manual Raman Video Sampling Tripod: stabilized mount with fine XYZ adjustments for hard-to-reach sampling, adapter for microscope head mounting.

Key Results and Discussion

In situ measurements of prehistoric finger-dot paintings revealed Raman peaks of haematite alongside crust components whewellite and gypsum despite environmental interferences such as sunlight and dust. Deployment of a foam rubber cap minimized stray light and improved signal quality.

At the Alhambra, noninvasive spectra of vault plaster decorations identified:

• Blue pigments: characteristic lazurite peak at 548 cm⁻¹, enabling differentiation of natural and synthetic lapis lazuli and inference of geographical origin.
• Red pigments: cinnabar and minium detected on gypsum substrates, with calomel degradation products appearing in altered samples.
• Gilded areas: tin oxide signals suggested historical use of tin foil in restoration phases, highlighting complex conservation history.

These findings demonstrate the method’s sensitivity to both original materials and subsequent chemical alterations.

Benefits and Practical Applications

Portable Raman spectroscopy offers:

• Minimal impact on heritage sites through noninvasive, in situ analysis.
• Flexibility to probe various sample geometries without preparation.
• Capability to monitor pigment degradation and inform conservation decisions.

Its rapid, contactless operation and adjustable laser power make it ideal for a broad range of archaeological substrates and environmental conditions.

Future Trends and Opportunities

Advances in detector technology and laser sources will enhance sensitivity for ultra-dark or highly fluorescent samples. Integration of multivariate data analysis and spectral libraries will streamline automated material identification. Combined spectroscopic imaging approaches promise spatially resolved compositional maps, further refining our understanding of artefact manufacturing techniques and deterioration pathways.

Conclusion

Fiber-optic-based portable Raman instruments have transformed archaeological science by enabling detailed, nondestructive compositional analyses directly on site. Their versatility in sampling and laser control supports robust pigment identification and monitoring of alteration processes, thereby aiding preservation and restoration of cultural heritage.

Reference

1. A. Hernanz et al., J. Raman Spectrosc., 2014, 45(11), 1236–1243.
2. A. Dominguez-Vidal et al., Analyst, 2012, 137(24), 5763–5769.
3. M.J. de la Torre-Lopez et al., J. Raman Spectrosc., 2014, 45(11), 1052–1058.