Historical Stamp Inks Identification with i-Raman EX

Significance of the Topic

Historic postage stamps preserve valuable cultural and monetary heritage but are vulnerable to counterfeit inks. Non‐destructive, in situ analysis of ink composition is essential to authenticate documents, guide conservation strategies, and protect collectors and institutions from fraud.

Study Objectives and Overview

This case study aimed to identify unknown colorants on an 1885 envelope using portable Raman spectroscopy. Three discrete ink areas were analyzed to distinguish black, red, and composite pigments, ensuring minimal interference from fluorescence and avoiding damage to the historic artifact.

Analytical Methodology

• Sample Preparation and Focus

• Three regions on the envelope were targeted without sampling removal.
• A 50× microscope objective provided a 42 µm laser spot and 3.68 mm working distance.

• Spectral Acquisition

• Raman measurements with integration times of 30–60 s.
• Laser power set to 3–5 % of maximum (approx. 9.9–16.5 mW) to prevent thermal damage.
• Dark scans subtracted for background correction.

Used Instrumentation

• i-Raman EX® portable Raman spectrometer equipped with a 1064 nm laser to suppress fluorescence.
• BAC151 Raman video microscope module for precise sample imaging and alignment.
• BWSpec® software for spectral capture and KnowItAll® software for library matching and hit quality index (HQI) calculation.

Main Results and Discussion

• Area 1 and Area 3: Broad peaks at ~1300 cm⁻¹ and ~1590 cm⁻¹ correspond to the D and G bands of carbon black. HQI values: 93.14 (Area 1) and 98.29 (Area 3).
• Area 2: Sharp peaks at 252 cm⁻¹ and 344 cm⁻¹ matched vermilion (mercury sulfide), with a weaker band at ~838 cm⁻¹ indicating chrome yellow (lead chromate). HQI for vermilion: 97.05.
• The co-presence of vermilion and chrome yellow explains the orange‐tinted red hue observed.

Contributions and Practical Applications

• Demonstrated that 1064 nm Raman reduces fluorescence in aged ink systems, yielding clear fingerprint spectra.
• Established a rapid, non‐invasive protocol for on‐site authentication of philatelic and archival materials.
• Provided quantitative hit quality metrics to support confident pigment identification.

Future Trends and Opportunities

• Integration of machine learning for automated spectral classification and counterfeit detection.
• Expansion of portable Raman databases to include synthetic and modern ink formulations.
• Hybrid approaches combining Raman with other non‐destructive techniques (e.g., XRF, hyperspectral imaging) for multi‐modal authentication.

Conclusion

The i-Raman EX® portable Raman system effectively identified historical stamp inks without sample removal, distinguishing carbon black, vermilion, and chrome yellow pigments. This non‐destructive approach supports cultural heritage conservation and counterfeit prevention.