Spectral database for postage stamps by means of FT-IR spectroscopy.

A Fourier transform infrared (FT-IR) spectroscopy study on the entire Italian postage stamps production is presented in this work. Crossing 150 years of issues from the unification of Italy until today, a time line of the major components constituting the stamps has been defined, based on the wide spectral database built on the basis of the numerous analyzed exemplars. Even though it is easy to find reports about stamps' issues history, information arising from these investigations contributes to throw light upon the substances incorporated in the stamps, which could be described as hybrid or composite materials (a sort of undisclosed or hidden story). As a result of the whole spectra acquired in attenuated total reflectance (ATR) mode, changes in paper composition showed the transition from the protein sizing glue to starch sizing; also the employment of kaolin varied through time. First it was used as the extender in the pigment-medium mixture, and finally it constituted the coating on the stamp surface. Also the chemical composition of the adhesive gum on the rear side of stamps has been subjected to modifications, as well as the front side. The earliest back glue was a protein-based adhesive; then it was replaced by gum arabic first and by poly(vinyl acetate) (PVAC) later. FT-IR spectroscopy, supported by the detailed database developed, has been applied, for the first time, in the very useful detection of two counterfeit samples: a fake of the famous Gronchi Rosa, issued in 1961, and a regummed 2 cent red stamp, issued in 1865. The information held in the whole spectral data has been selected and employed in the principal component analysis (PCA) statistical analysis.