Chemistry of 19th-Century Swiss Postage Stamps

Despite their small size, postage stamps are culturally significant artifacts that reflect the artistic, technological, and political contexts of their production. Although the material composition of stamps from many European countries, including Italy, Portugal, and the United Kingdom, has been studied, Swiss postage stamps remain largely unexplored. In this study, we present the first systematic material characterization of 98 Swiss stamps issued between 1850 and 1908 using a combination of non-invasive analytical techniques including Raman spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR) together with scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). In specific cases, high-performance liquid chromatography coupled with diode array detection (HPLC-DAD) was also applied. Our results highlight the predominant use of Prussian blue as a blue pigment, whereas ultramarine appears more frequently in early editions. There is greater diversity in red inks, which consist of both inorganic pigments such as vermilion, chrome orange, and red lead, and organic dyes such as eosin Y, carminic acid, and alizarin from 1882 onwards. Yellow and green inks reflect standard formulations, the latter resulting from the combination of Prussian blue and chrome yellow, broadly corresponding to those used in contemporary foreign issues. Brown and gray inks were obtained from a mixture of carbon black and a red or white pigment, respectively. This study sheds new light on the material history of Swiss postage stamps and highlights the value of combining spectroscopy, imaging, and chromatography techniques in philatelic research and, more broadly, in heritage science.