Historical Stamp Inks Identification with i-Raman EX

Significance of the Topic

Stamps are cultural artifacts rich in historical and monetary value. The rise of counterfeit inks threatens the integrity of philatelic collections and heritage preservation. Rapid, non-destructive techniques for identifying ink composition are essential to authenticate items, prevent fraud and support conservation efforts.

Objectives and Study Overview

This study demonstrates the use of a portable 1064 nm Raman spectrometer, the i-Raman EX®, to characterize colorants in a historical 1885 envelope. Three distinct ink areas were analyzed to differentiate black and red pigments and detect any mixtures, aiming to verify authenticity and gather compositional data without damaging the artifact.

Methodology and Instrumentation

The analysis employed an i-Raman EX coupled with a BAC151 Raman video microscope. Key parameters included:

• Laser wavelength: 1064 nm to minimize fluorescence interference.
• Objective: 50×, working distance 3.68 mm, spot size 42 μm.
• Laser power: 3–5 % of maximum (≈9.9–16.5 mW) to avoid sample damage.
• Integration times: 30–60 s with corresponding dark spectrum subtraction.
• Software: BWSpec® for data acquisition; KnowItAll® library for spectral identification.

Samples were placed on the microscope stage and focused individually for spectrum collection. Each measurement was background-corrected before library matching.

Results and Discussion

The investigation of three ink regions yielded the following findings:

• Area 1 and Area 3: Broad Raman bands near 1300 cm⁻¹ (D band) and 1590 cm⁻¹ (G band), characteristic of carbon black ink.
• Area 2: Sharp peaks at 252 cm⁻¹ and 344 cm⁻¹ matching vermilion (mercury sulfide). A weaker band at ~838 cm⁻¹ indicated the presence of chrome yellow (lead(II) chromate), explaining the slightly orange hue.

Spectral matches were quantified by Hit Quality Index (HQI) using KnowItAll®:

• Area 1: Carbon black, HQI 93.14.
• Area 2: Vermilion red, HQI 97.05; mixture analysis confirmed chrome yellow.
• Area 3: Carbon black, HQI 98.29.

These results confirm reliable pigment discrimination and identification in situ.

Practical Benefits and Applications

The i-Raman EX system offers:

• Non-destructive, in-field analysis ideal for cultural heritage and conservation.
• Suppression of fluorescence from aged organic media via 1064 nm excitation.
• Low-power operation to protect sensitive substrates.
• High-resolution Raman video microscopy for targeted spot analysis.

This approach supports authentication of historical stamps, counterfeit detection, and documentation of pigment usage.

Future Trends and Opportunities

Advancements may include:

• Integration of hyperspectral imaging for broader area mapping.
• Enhanced spectral libraries with regional and temporal pigment variations.
• Machine learning algorithms for automated pigment classification.
• Multimodal coupling with techniques such as XRF or LIBS for complementary elemental analysis.

Such developments will further streamline heritage analysis and expand applications across cultural institutions.

Conclusion

The portable i-Raman EX system effectively distinguishes historical stamp inks while preserving sample integrity. Its low-fluorescence excitation, adjustable power settings and video microscope capabilities make it a powerful tool for heritage authentication and research.