Raman Spectroscopy in Archaeological Studies

Significance of the Topic

The study of archaeological artefacts demands precise chemical identification to reveal pigment composition, production techniques and cultural context without damaging valuable items. Portable Raman spectroscopy offers a nondestructive approach for in situ analysis, enabling researchers to determine mineral phases, polymorphs and organic compounds directly on site. By capturing low-frequency vibrational modes below 500 cm–1, this technique excels at characterizing inorganic pigments and assessing material authenticity and state of preservation.

Objectives and Study Overview

This work demonstrates the application of portable Raman instruments in two key archaeological investigations:

• Characterisation of prehistoric rock paintings at Abrigo de los Chaparros under open-air conditions, addressing challenges such as ambient light, wind and surface crusting.
• In situ analysis of polychrome plaster and decorative pigments in the Hall of the Kings at the Alhambra, identifying original materials and assessing their degradation over centuries.

Methodology and Instrumentation

Measurements were performed with the i-Raman Plus 785H portable spectrometer equipped with a fiber-optic probe and TE-cooled CCD detector. Key specifications and accessories included:

• Spectral range from 65 to 2800 cm–1 with high resolution and low noise even at long integration times.
• Adjustable laser power down to 3 mW in 1% increments, enabling safe analysis of heat-sensitive dark pigments.
• Optional video microscope sampling head with coaxial LED illumination and a manual tripod mount for precise positioning.
• Multivariate analysis using BWIQ software and database identification via BWID.

Main Results and Discussion

1. Abrigo de los Chaparros rock art

• Detection of haematite peaks confirming red pigment use.
• Identification of surface crusts composed of whewellite and gypsum, highlighting environmental alterations.

2. Alhambra Hall of the Kings plasterwork

• Gypsum-based stucco decorated with blue and red pigments.
• Lazurite peak at 548 cm–1 indicating lapis lazuli for blue tones and enabling provenance studies.
• Co-occurrence of cinnabar and minium for red hues, with calomel formation identified as a degradation product.
• Observation of tin oxide residues in gilded areas, suggesting historical use of tin foil in restoration.

Benefits and Practical Applications

Portable Raman spectroscopy delivers several advantages for heritage science:

• Noninvasive, in situ analysis preserves artefact integrity and site context.
• Minimal sample preparation and flexible probe access accommodate irregular surfaces.
• Broad spectral range and low laser-power operation support analysis of diverse pigments.
• Rapid identification aids conservation planning, provenance research and authenticity assessment.

Future Trends and Potential Applications

Emerging developments are set to enhance the field:

• Further miniaturisation and integration of handheld confocal Raman imaging systems.
• Combining Raman data with 3D scanning and GIS mapping for comprehensive site documentation.
• AI-driven spectral interpretation and automated pigment databases for real-time decision support.
• Multimodal in-field analysis coupling Raman with other spectroscopic and imaging techniques.

Conclusion

Portable Raman spectroscopy has proven to be a powerful, nondestructive tool for archaeological research, enabling detailed pigment identification and degradation assessment directly on site. Its flexibility, low-power operation and compatibility with advanced sampling probes support comprehensive studies of cultural heritage materials, ultimately informing more effective conservation and restoration strategies.

References

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