Raman Spectroscopy In Archaeological Studies.

Introduction

Archaeological research plays a vital role in reconstructing human history and understanding the development of past societies. Through the meticulous examination of artifacts and cultural remains, archaeologists strive to piece together narratives from fragments of the past. However, traditional analytical methods often present limitations. Destructive sampling techniques can damage irreplaceable artifacts, while others may struggle to definitively identify the materials used in their creation.

Fortunately, advancements in scientific instrumentation have introduced a powerful tool for archaeologists: Raman spectroscopy. This non-destructive technique offers a unique window into the molecular composition of artifacts. By analyzing the interaction of light with a material, Raman spectroscopy provides a detailed fingerprint, revealing the specific components present without causing any harm to the object itself.

What is Raman spectroscopy?

Figure 1: Principle of Raman spectroscopy.

Raman spectroscopy is a powerful technique used to analyze the molecular composition of a material. It works by focusing a monochromatic light source, usually a laser, onto the sample. When light interacts with the molecules in the sample, it can scatter in two ways:

1. Rayleigh Scattering: This is the predominant form of scattering, where the scattered light retains the same wavelength as the incident light.
2. Raman Scattering: In a small fraction of scattering events (1 in 1 million photons), the light interacts with the vibrations of the molecules within the sample. This interaction causes a slight shift in the wavelength of the scattered light. This shift, known as the Raman shift, is unique for each type of molecule and acts as a fingerprint of its vibrational energy levels.

By analyzing the Raman shift of the scattered light using a spectrometer, Raman spectroscopy provides detailed information about the molecular composition of the sample.

Application of Raman spectroscopy in archaeological

Raman spectroscopy has revolutionized the field of archaeology by providing a non-destructive means to analyze the composition of historical artifacts. Here's how archaeologists leverage Raman spectroscopy:

• Unveiling Artistic Secrets: Analyze pigments in ancient paintings and sculptures to understand artistic techniques and cultural exchange. Differentiate between natural and synthetic pigments and even identify forgeries.
• Deciphering Stone and Ceramics: Identify specific stone types in sculptures and architectural elements, revealing provenance and construction techniques. Analyze the composition of pottery and ceramics, unlocking information about manufacturing processes and trade routes.
• Examining Organic Materials: Identify organic materials present in artifacts, such as binding media in paintings, textiles, or even food residue, providing insights into past cultural practices and resource utilization.

By unveiling the material story within artifacts, Raman spectroscopy empowers archaeologists to gain deeper insights into past technologies, artistic practices, and the societies they represent.

Result

Figure 2: Mummified cats in the Egyptian exhibition in the British Museum.

In research about ancient resin from Egypt dynasty, there is an interesting discovery. An “resin” eye-bead was found in the eye socket of a mummified cat from the XVIIIth dynasty, dating back to around 1350 BC. The bead was analyzed using Raman spectroscopy to determine its composition. Initial speculation suggested that it could be glass or amber resin. The analysis revealed that the bead was not made of amber or glass but was like a cat’s claw, indicating it was a keratotic material. The bead underwent some thermal processing to shape it, suggesting a possible ancient technological application. The purpose of the bead, whether as a false eye during the cat’s life or a funerary practice, remains unclear.

Figure 3: Raman spectrum of cat mummy eye-bead (A) and amber resin (B).

Figure 3 displays the Raman spectra of the cat mummy eye-bead (A) and amber resin (B). In Figure 3A, the spectral signature corresponds to a keratotic specimen. Notably, characteristic spectral peaks appear at 1660 cm, 1450 cm, and 1250 cm. Additionally, a strong, sharp band at 1003 cm indicates the presence of phenylalanine. The Raman spectrum of the amber resin (B) led to the conclusion that the eye-bead is neither resin nor glass.

Figure 4: Raman spectrum of cat mummy eye-bead (A) and modern cat’claw (B).

Then the eye-bead was compared to a contemporary cat’s claw sample. Due to the presence of similar spectral peaks, it can be confirmed that the mummy cat’s eye beads originated from cat claws.

Measurement system

Ocean Insight products, particularly the QEPro spectrometer and Raman laser light sources, offer significant advantages for Archaeological Raman Spectroscopy. The QEPro spectrometer is a high-performance instrument that stands out for its exceptional sensitivity and dynamic range, making it particularly suitable for the precise needs of archaeological studies. Its signal-to-noise ratio (SNR) exceeds 1000:1, which allows for the detection of very low light levels and contributes to high-quality spectral data. The dynamic range, typically around 85,000:1, enables the spectrometer to capture a wide range of intensities, ensuring detailed analysis of varied samples. Additionally, the thermoelectric cooler (TEC) maintains the detector at a stable temperature, which is crucial for long-term measurements and prevents spectral distortion, enhancing the reliability of the data collected during delicate archaeological investigations.

Figure 5: QEPro spectrometer.

A Raman laser light source with a wavelength of 532nm is highly beneficial for archaeological studies due to its narrow spectral line, which ensures high precision in detecting molecular vibrations. This specific wavelength is optimal as it provides a strong signal with minimal fluorescence, allowing for clear identification of mineral and organic compounds within artifacts. The narrow spectral line width, often less than 0.2nm, is crucial for resolving fine spectral features, which can reveal detailed information about the composition and structure of archaeological samples.

Figure 6: Raman laser light source.

Conclusions

Raman spectroscopy has revolutionized archaeological research by providing a non-destructive method to analyze the molecular composition of artifacts. This technique allows archaeologists to uncover the secrets of ancient materials, from pigments in paintings to the composition of ceramics and organic residues. By preserving the integrity of precious artifacts, Raman spectroscopy not only enhances our understanding of past societies but also ensures that these cultural treasures remain intact for future generations. The discovery of the keratotic eye-bead in an Egyptian cat mummy exemplifies the profound insights this technology can offer, shedding light on ancient technologies and practices. As Raman spectroscopy continues to evolve, its applications in archaeology will undoubtedly expand, offering even deeper insights into our shared human history.