Historic Stamps: Cracking down on counterfeits with Raman

Figure 1. Historical Envelope with three unknown ink areas

Historical stamps are cultural heritage objects that provide an invaluable amount of historical information.
Holding a high monetary value as a collector’s item there is an influx of counterfeit inks to fraud unsuspecting buyers in the pursuit of historical stamps. Fraudulent stamps must be identified and removed from the market; with the use of portable Raman fraudulent stamps can be identified.

A historical envelope from 1885 with three unknown colorants was analyzed with a Portable Raman System (Fig. 1). The i-Raman EX is a portable Raman device with a 1064 nm laser that is used because it minimizes the fluorescence of the ink that is overwhelming with a 785nm laser. The ability to lower the laser power down to 1% of the maximum intensity prevents sample burning and the Raman video microscope system analyzes the smallest of details. These features make the i-Raman EX a useful and nondestructive tool for cultural heritage analysis.

Figure 2. (a) Spectrum collected from area 1 consistent with carbon black (background corrected)

Figure 2. (b) Spectrum collected from area 2 consistent with vermilion and chrome yellow

The i-Raman EX with a Raman video microscope system identified distinguishing peaks for the three unknown historical ink samples. Figure 2a shows the Raman spectrum acquired from area 1. The broad peaks at ~1300 cm-1 and 1590 cm-1 are consistent for the D and G bands respectively of carbon black ink. Spectra acquired from area 3 are also consistent with carbon black ink. Figure 2b shows the spectrum acquired from the ink of area 2. The sharp peaks at 252 cm-1 and 344 cm-1 are consistent with the Raman signal of vermilion, a bright red pigment made of mineral containing mercury sulfide. The weak, broad peak at ~838 cm-1 is consistent with chrome yellow, a yellow pigment made of lead (II) chromate. The presence of chrome yellow with the red vermilion may account for the slightly more orange hue of the stamp than would be expected from only vermilion.

The i-Raman EX system is an effective portable Raman device for cultural heritage objects with numerous features for sample integrity and lowering fluorescence.

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