Has your ancient stamp been “regummed”? A FT-NIR study

Introduction
Since the beginning of postage stamp production, counterfeiters started their activities principally oriented to defraud not only the postal service, but also stamp collectors, by altering genuine stamps in some way to make them more desirable. These activities include the application of new gum on the back side of non-cancelled stamps, whose condition greatly influences the stamp value. If a stamp is identified as “regummed” by a philatelic expert, it is downgraded and considered as a stamp without gum, considerably decreasing its value1. Because these counterfeits are not always easily detected by philatelic expert visual inspection, a reliable scientific detection method is desirable.It is well documented that the three main glues used in Italian stamps during the period investigated were animal glue (before 1901), Arabic gum (1901-1973), and polyvinyl acetate glue (PVAc) (after 1974)2. It has also been reported that many counterfeits have been performed in recent years by applications of modern synthetic glues, such as PVAc, on ancient stamps2,3.
The aim of this work is to study and to develop, for the first time, a rapid and non-destructive methodology able to study the stamp gum composition and to unmask regummed stamps by using FT-NIR spectroscopy.
Experimental
NIR measurements were performed with a FT near-infrared spectrometer (BuchiNIRFlex N-500) in the 10 000 - 4000 cm−1 range at 4 cm−1 resolution.
A total number of 113 non-cancelled Italian stamps, covering the period 1861-2001 were analysed.
Moreover, eight ancient Italian stamps (period 1862-1932) were purposely regummed with a PVAc thin layer, to simulate counterfeit and used as test samples.
The samples were analysed in the reflectance mode and a total of 512 scans were averaged for every spectrum. Three replicate spectra were acquired on three different back side points for each stamp analysed, and the average spectrum was submitted to principal component analysis (PCA).

Results and discussion
The results obtained indicate that NIR spectroscopy, coupled with exploratory pattern recognition methods, is able to detect the compositional chemical differences between the different gum applied onto the stamp back side.
PCA results also show that the method proposed is able to detect ancient stamps artificially regummed with PVAc glue. The results are critically compared with those obtained by FT-Raman spectroscopy on the same sample set.

References
1 Bolaffi, Il catalogo dei francobolli 2014, Giulio Bolaffi Editore, Torino
2 E. Imperio, G. Giancane and L. Valli, Anal. Chem., 2013, 85, 7085–7093
3 Antonio Bandini Buti, Manuale di filatelia, Ugo Mursia Editore, Milano, 1973 terza edizione