Analysis of Postage Stamps by Proton-Induced X-Ray Emission Spectrometry

Subtle variations in ink color and paper of postage stamps can affect their value and desirability to collectors and investors. Protoninduced Xray emission spectrometry (PIXE) is a generally nondestructive forensic analysis tool that reveals the concentrations of chemical elements in a sample. PIXE has been used for more than 30 years, sometimes in conjunction with other analytical methods, to differentiate philatelic printing techniques; to quantify differences between different shades, varieties, and reprints of stamps; and to separate genuine stamps from forgeries. This paper reviews the history of philatelic paper and ink analysis by PIXE through a focus on case studies from the 1980s to the present. For example, PIXE has been used to show distinct chemical signatures between genuine stamps from China, Czechoslovakia, Eastern Silesia, South Africa, SouthWest Africa, and Taiwan and forgeries thereof. PIXE analysis showed how variations in ink chemistry control color shades of stamps from Mexico and reveal the origin (country of printing) of Iranian stamps, suggesting early (pretwentieth century) utilization of organic inks on Europeanprinted stamps from both nations. The technique was utilized to determine variations in paper types on stamps from China and Taiwan and how original People’s Republic of China stamps from the early 1950s can be separated from modern reprints. The paper concludes with a PIXE investigation of Mexico’s 1895–1898 “Mulitas” stamps. The analysis reveals that unlike previous Mexican issues, the Mulitas were printed with organicbased inks on largely clayfree papers. Differing amounts and/or formulations of metal (zinc, lead, and titanium) oxide whiteners apparently controlled the presentday color shade of the stamps, either directly or indirectly, with bluish inks having relatively more zinc and lead and greenish inks having relatively more titanium.