Chemistry of 19th-Century Swiss Postage Stamps

Abstract

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.

Graphical abstract

This is a visual representation of the abstract.

Keywords

Swiss postage stamps Raman spectroscopy attenuated total reflection Fourier transform infrared ATR FT-IR high-performance liquid chromatography coupled with diode array detection HPLC-DAD scanning electron microscopy with energy dispersive X-ray spectroscopy SEM-EDX organic red dyes

Introduction

Postage stamps, although often not larger than a few square centimeters, are precious cultural artifacts. Beyond their original function as a proof of payment, stamps reflect the technological, artistic, and political contexts of their time. The mid-19th century saw the start of a revolutionary practice in the postal sector with the introduction of the first postage stamp. The Penny Black, which featured Queen Victoria’s portrait, was the first official postage stamp issued in the United Kingdom in May 1840.¹ Three years later, Switzerland became the second country to adopt adhesive postage stamps. Following the Swiss Federal Constitution of 1848, the management of postal services, which had previously been handled by some of the largest cantons (Zurich, Geneva, Basel), was transferred to the Federal Government.^2,3 At the time, postal rates in Switzerland were based on classified distances, measured in “Wegstunden” (way hours) and called Rayon (postal zones).⁴ These first federal stamps, depicting the national coat of arms and printed in black and red on white wove paper with backgrounds of various colors (Figure 1, first row), were produced using hand-drawn lithographic stones by Carl Durheim in Bern.⁵ In 1854, the first mass-produced federal postage stamps depicting a sitting Helvetia (symbolic mother of Switzerland), symbolizing the new constitutional state, were introduced. They were manufactured using a letterpress printing process combined with uncolored embossed printing of the stamp’s design and text. The poor print quality often resulted in an unrecognizable laurel wreath on Helvetia’s head, making it look more like a tousled hairstyle, which earned these stamps the nickname “Strubel”, taken from a very popular German children’s book.⁴ This design was deemed unartistic, later versions portrayed Helvetia in a sitting or standing profile (Figure 1). The first stamps were imperforated until July 1863, when they were replaced by perforated versions (Figure 1, fourth row).

In Switzerland, as elsewhere, stamps have been produced using a range of materials and printing techniques that reflect technological advancements and changing aesthetic values. Although they have been extensively studied in the fields of philately and art history, little is known about their manufacture.^6–10 The lack of information is largely due to the secrecy maintained by printers to avoid counterfeiting and to the absence of historical documentation. Compared to other cultural heritage artifacts, postage stamps have not been the subject of equally in-depth scientific investigation. As a result, their material composition has received relatively little attention in the literature.

Figure 1.

A selection from the 77 different series of Swiss postage stamps studied in this work.

In recent years, studies have begun to address this gap employing techniques such as Raman spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) together with scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) to examine the chemical composition of stamps, establishing these methods as reliable analytical tools in philatelic material research.^11–14 A diverse set of approaches may be distinguished: some studies adopt a historical-geographic lens, as the 150 year study of Italian stamps by Imperio et al.¹² highlighting the limited pigment selection for ink printing in Italy between 1862 and 2011. Some chose a more targeted approach for their studies, focusing on a specific color shade.^13,15 A third angle explores forensic applications, in which pigments, fillers, and paper additives are scrutinized to detect counterfeits and authenticate rare specimens.^16–20 Previous research on postage stamps has largely relied on non-invasive techniques which, although they provide information on inorganic pigments, remain ambiguous for organic dyes. With the advent of the first synthetic dyes in the mid-19th century, the use of high performance liquid chromatography (HPLC) coupled with diode-array detection (DAD) or mass spectrometry (MS) proved particularly effective in identifying organic colorants in historical postage stamps. Analysis of Queen Victoria’s Lilac postage stamps revealed the presence of mauveine and carminic acid, while eosin Y and carminic acid were identified in a series of Portuguese stamps from 1850–1900, underscoring the value of chromatographic techniques for identifying organic red dyes.^15,21–23 Although these approaches differ in scope and methodology, they converge in their contribution to a deeper understanding of the material choices for stamp manufacturing tied to specific countries or time periods. Nonetheless, the number of such studies remains limited and systematic investigations are still rare. To date, only a small proportion of the worldwide stamp production has been examined, yet Switzerland not.

In this work, we present the first detailed non-invasive material analysis of 98 Swiss postage stamps manufactured between 1850 and 1908, using a combination of Raman spectroscopy, ATR FT-IR spectroscopy, and SEM-EDX. In specific cases, a destructive technique, HPLC-DAD, was used for the identification of organic dyes. This work not only benefits Swiss philately but also provides information on the availability and cost of pigments, as well as the wider economic and cultural contexts that led to these choices. The results obtained are put into perspective with those obtained on worldwide stamps, enriching our knowledge of stamp production in the 19th century.

Experimental

Materials and Methods

In this work, 98 Swiss postage stamps issued between 1850 and 1908 and representing 77 different series were analyzed. Of these, 74 monochrome stamps, covering the period from 1854 to 1908, which include imperforate Sitting Helvetia “Strubel” stamps, perforated Sitting and Standing Helvetia stamps, Cross and Numeral stamps, Due stamps, and finally the 25th anniversary of the Universal Postal Union (UPU) stamps. The remaining 24 polychrome stamps correspond to the first Swiss Federal “Rayon” issued between 1850 and 1854 (Figure 1). A note should be added: when certification of the stamp is available, its Zumstein number (popular Swiss catalog; hereinafter referred to as Zst), which identifies the specific series to which it belongs, is mentioned.

Raman Spectroscopy

Raman analyzes were carried out on 98 stamps using a LabRAM HR800 Raman spectrometer (Horiba JobinYvon, France) equipped with a 633 nm laser excitation source and a 100× objective lens, resulting in an estimated laser spot of approximately 0.9 µm in diameter. Each spectrum was collected with an acquisition time of 3 s and three accumulations with an estimated laser power of 1.04 mW, enabling samples to be measured with radiation damage reduced or controlled as much as possible.²⁴ Raman analyzes were also performed on seven stamps using a 532 nm laser excitation source and a 100× objective lens, resulting in an estimated laser spot of approximately 0.7 µm in diameter. Each spectrum was collected with an acquisition time of 3 s and three accumulations with an estimated laser power of 2.36 mW. Two to three point measurements were collected on each stamp. Data were processed using LabSpec 6 software and codes developed in the R statistical environment.

Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy

Attenuated total reflection Fourier transform infrared spectra were collected on 74 stamps using an ALPHA FT-IR spectrometer equipped with a 4 mm² Platinum-ATR (diamond crystal) sampling module (Bruker Optics, Germany). Each stamp was placed directly on the ATR window without further preparation, and optimum contact between the sample surface and the crystal was ensured by means of light pressure applied by a piston. The analysis was performed on both sides (front, i.e., colored and back) of the stamps, collecting two to three measurement points. For all spectra, 32 scans were acquired in the range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. Data were acquired using Opus 7.0 software without post-processing.

Scanning Electron Microscopy Energy Dispersive Spectroscopy (SEM-EDX)

Surface morphology and composition of 61 untreated stamps were studied using a Phenom ProX desktop SEM-EDX (Phenom-World, Thermo Fisher Scientific) with a CeB₆ electron source operating at 15 kV and under low vacuum (10 Pa). For electron imaging, a backscattered electron detector (BSD) was used. Data analysis was performed using the Phenom Pro Suite software.

High-Performance Liquid Chromatography-Diode Array Detector (HPLC-DAD)

Chemicals

High-performance liquid chromatography gradient grade acetonitrile and methanol from Fisher Scientific (Switzerland), 85% orthophosphoric acid from Supelco (MilliporeSigma, Merck KGaA, Germany), HCl 37% (Rotipuran, Carl Roth, Germany), and ultrapure water (Aqua Max Ultra 370 series water purification system, Young In Chromass, Korea) were used for all extractions and mobile-phase preparations. The reference material for carminic acid was obtained from Sigma Aldrich (USA), purpurin from Cayman Chemical Company (USA), alizarin from Acros Organics (Belgium) and eosin Y from Carl Roth (Germany).

Postage Stamp Extraction

Small fragments of the printed area (0.2–0.6 mg) were cut from 12 postage stamps and subjected to mild acid hydrolysis in 500 µL H₂O:MeOH:2% HCl (1:1:2, v/v/v) for 10 min at 100 °C to extract the organic colorant.²⁵ After quenching in an ice bath, the samples were evaporated to dryness under N₂ at 60 °C, reconstituted in 50 µL methanol, and filtered through a 0.22 µm PTFE syringe filter (BGB Analytik AG, Switzerland) prior to HPLC analysis. 10 µL of extract was then injected into the HPLC system.

Apparatus

Hydrolyzates from the postage stamps were analyzed on an Ultimate 3000 Dionex HPLC system equipped with a diode array detector (Thermo Fisher, Massachusetts, USA) and a Pinnacle DB C18 column (100×2.1 mm, 3 µm, Restek, USA). Gradient separation was performed using the following solvents: (i) Ultrapure water + 0.5% H₃PO₄ and (ii) acetonitrile at 0.35 mL/min starting at 10% B and increasing to 100% B over 8 min, hold for 1 min, down to 10% B in 1 min and re-equilibration for 5 min. Whenever possible, solutions of reference dyes were injected for the identification of unknown components. Liquid chromatography–mass spectrometry (LC-MS) analyses were carried out using a Waters UPLC ACQUITY H-Class quaternary system coupled to a SQD2 single quadrupole mass spectrometer (Waters Corporation, USA) under the same chromatographic conditions as those used for HPLC-DAD analysis, substituting phosphoric acid with 0.2% formic acid in the mobile phase.

Raman Imaging

Raman imaging was carried out on two stamps using a RAMANtouch (Bruker Optics, Germany) equipped with a 785 nm laser excitation source and a 20× objective lens. Spectra were collected with an acquisition time of 15 s and two accumulations and 1 s and one accumulation, respectively, with estimated laser powers of 16.6 mW and 13.5 mW.

Results and Discussion

The results are presented in two main sections: First the monochrome stamps are discussed separately according to their color, while polychrome stamps are addressed in a common subsection. The typical analytical workflow began with Raman spectroscopy, followed by ATR FT-IR and SEM-EDX as a complement. When the presence of organic dyes was suspected and with the consent of the stamp owner, a destructive HPLC analysis was performed. Detailed results are summarized in Table S2 (Supplemental Material). Raman spectroscopy offered higher sensitivity for detecting weak vibrational frequencies, as well as minimal interference from the paper substrate, with the added versatility of multiple excitation wavelengths to optimize pigment detection. However, there are inherent limitations due to fluorescence and small spot size (0.9 µm), raising questions about the representativeness of the analyzed area compared to the entire stamp. This issue was partially mitigated by collecting multiple spectra throughout the sample and by the complementary use of ATR FT-IR and SEM-EDX analyzes, which cover larger areas (4 mm²). SEM–EDX enabled elemental analyzes that supported pigment assignments and clarified compositional heterogeneity. However, these techniques also have specific limitations. Due to overlapping absorption bands, identifying mixtures can be challenging using FT-IR. SEM-EDX analysis requires that the sample fits into the sample holder (2.4 cm × 2.4 cm), which imposes restrictions on the size of objects that can be analyzed. In addition, a balance must be struck between time-consuming mapping and less representative spot measurements. HPLC-DAD analysis, although destructive, remains indispensable for the unambiguous identification of organic dyes. These constraints have been taken into account in the following discussion.

Paper Substrate

The ATR FT-IR analysis of the paper substrate of stamps reveals typical spectral signatures at 1162, 1104, 1053, and 1028 cm⁻¹ related to cellulosic fibers, which can be attributed to the stretching of C–O groups in glucose chains (e.g., Figure 2a). They all also show a peak at 1004 cm⁻¹ typical of C–O stretching, which could be attributed to starch chains rather than cellulose.¹¹ Peaks in the regions 3400–3200 cm⁻¹ and 3000–2800 cm⁻¹ correspond, respectively, to the stretching vibrations νOH and νC–H of the glucose chains in the cellulose fiber. Although this region is mainly dominated by the cellulose contribution, more defined peaks are observed at 2916 and 2856 cm⁻¹ on the front (colored) side of the stamps compared to the back side. Both bands are characteristic of C–H vibrations in aliphatic compounds, here attributed to the binder medium used in the ink formulation (Figure 2). A majority of spectra show primary and secondary amide bands at 1647 and 1543 cm⁻¹, characteristic of protein glue. ATR FT-IR analysis identified a peak at 3688 cm⁻¹ in 64 stamps, attributed to Si–OH stretching, suggesting the presence of kaolinite (Al₂Si₂O₅(OH)₄), used as a filler in paper manufacturing. Elemental analysis revealed the co-location of aluminum and silicon in 30 specimens, consistent with ATR FT-IR observations. As such, kaolinite was identified as a recurring component of the substrate, although it was not detected in all stamps. In addition, SEM-EDX analyses revealed the dilute but ubiquitous presence of calcium in unprinted areas. Together with the characteristic carbonate bands at 874 and 712 cm⁻¹ observed in the IR spectra of 15 stamps, this indicates that calcium is mainly present as calcite (CaCO₃). Although both kaolinite and calcite were identified, their role should be interpreted with caution, as these minerals may originate from the paper substrate, ink formulation, or both. Similarly, sulfate bands (660 and 606 cm⁻¹) were observed in several stamps, which could indicate the presence of gypsum (CaSO₄·2H₂O).

Figure 2.

ATRFT-IR spectra of the front (colored lines) and back (black lines) sides of representative Swiss postage stamps: (a) The green 5C Cross and Numeral, (b) the orange 20C Strubel, (c) the yellow 10Rp Rayon II, and (d) the gray 2Rp Strubel.

Blue Monochrome Stamps

The study of four imperforate 10Rp Strubel issued in 1854 with Raman spectroscopy shows two stretching modes that can be assigned to the ν(CN) band, A_1g (2155 cm⁻¹), and E_g (2093 cm⁻¹), the symmetric and asymmetric vibrational modes, respectively. These peaks along with additional peaks at 526 and 281 cm⁻¹ are distinctive of Prussian blue ([Fe₄[Fe(CN)₆]₃· xH₂O]). These results were confirmed by the presence of a peak at 2087 cm⁻¹ in ATR FT-IR spectra, which can be attributed to the C≡N stretching in the cyanide group characteristic of this pigment. The four 10Rp Sitting Helvetia stamps from the later perforated edition (1862–1863), as well as the imperforated 2C Sitting Helvetia stamp (1861–1862), all show the use of Prussian blue as a pigment.

Raman spectra acquired on four perforated 30C Sitting Helvetia issued in 1867–1881 show characteristic stretching modes at 583 and 546 cm⁻¹, symmetric and asymmetric, respectively, which can be attribute to S₃⁻. These two peaks and the strong peaks at 1639, 1087, and 805 cm⁻¹, are characteristic of ultramarine (Na₇Al₆Si₆O₂₄S₃). An additional band at 390 cm⁻¹ is observed due to the presence of the disulfur ion S₂⁻. ATR FT-IR spectra show a more pronounced peak at 985 cm⁻¹ that can be attributed to the asymmetric stretching of Si–O–Si. However, this region is also governed by major vibrational modes attributable to the cellulose substrate. As such, FT-IR spectra alone do not identify with certainty of ultramarine, but they do rule out the use of Prussian blue, as no peaks related to the ν(CN) band could be detected.

The 12C Cross and Numeral stamp was used from its introduction in 1882 until 1924, with four separate issues: Zst 56 (1882), Zst 62A (1882), Zst 62B (1894), and Zst 84 (1906). All issues used the same printing, making it difficult to formally identify them. Two different chemical compositions were identified on the 13 stamps analyzed: three specimen labeled as Zst 56, Zst 62A, and Zst 62 (exact series unknown) showed the sole presence of ultramarine by Raman spectroscopy. These three stamps were used on 23.10.1883, 19.10.1887, and 25.12.1887, as indicated by their respective cancellation marks. Furthermore, SEM-EDX measurements revealed large amounts of Zn and Pb, probably in the form of zinc white and lead white pigments for color lightening (Figure S2e, Supplemental Material). For ultramarine, while Al and Si were easily detected, the main Na and S lines visible using a 15 keV electron energy (Na K_α: 1.040 keV, S K_α: 2.307 keV) overlap with Zn (Zn L_α: 1.011 keV) and Pb (Pb M_α: 2.345 keV) lines, making their identification more difficult. Interestingly, all 10 other stamps analyzed, in use between 1890 and 1906 (either Zst 62A or B), feature both ultramarine and Prussian blue (figure S2f and S3d, Supplemental Material). Furthermore, elemental analysis revealed no trace of Pb and Zn, which were replaced by Fe for Prussian blue, as well as Ba and S. The latter points to the use of baryte in blue ink, an extender commonly added to Prussian blue.²⁶ Although cancellation dates are not ideal chronological markers due to potential storage delays, the data nonetheless reveal a clear fracture in ink formulation that occurred in the late 1880s, several years before the first issue of the Zst 62B series in 1894, which only affected the control watermarks applied to the back of the paper. The partial replacement of ultramarine with Prussian blue may have been motivated by fluctuations in pigment prices, color adjustments, or ink workability considerations. The combined use of the two pigments may have resulted in a deeper and richer blue, as Prussian blue alone yields a rather dark hue. The addition of baryte probably helped to dilute the intensity of the pigment in the absence of identifiable white pigments, at a lower cost. This hypothesis could be further explored by examining the composition of Zst 84 certified specimens, which were not part of this research.

A similar change in pigment composition was observed in the 50C Helvetia 70D series manufactured from 1895 onwards (Prussian blue only) compared to the 70A series produced from 1882 (Prussian blue, ultramarine). Lead white was confirmed by FT-IR in both stamps due to the presence of peaks at 835 and 677 cm⁻¹. Analysis of three due stamps (1878–1882) also showed the joint use of Prussian blue and ultramarine.

The analysis of a perforated 25C Standing Helvetia stamp issued between 1899 and 1903, likewise highlighted the combined use of ultramarine and Prussian blue. In contrast, the editions produced from 1899 onwards (73D, 73E, 93A) showed the sole use of Prussian blue, which was also observed in the 25C UPU stamp (1900).

Our study shows that among the 38 monochrome blue stamps examined, Prussian blue is present in 17 of them (∼50%), ultramarine in nine, and both pigments in 12 (∼25%). Switzerland appears to use ultramarine much more than its European neighbors and the combined use of both blue pigments on a single stamp in such proportions also appears to be a Swiss specialty. Our results align with previous studies showing the predominance of Prussian blue in contemporary European stamps, confirming its status as the most prevalent blue pigment at that time.^{12,14,20,27–30} Discovered in 1704, Prussian blue rapidly spread to become the most widely used blue pigment in artworks in the 19th century.²⁶ Although exemplified by the study of Egyptian and Chinese stamps,^31,32 the identification of ultramarine in our stamps may seem surprising at first, given that it was long considered more valuable than gold, due to the high cost of importation of the natural mineral lapis lazuli from Afghanistan and the subsequent grinding process to reduce it to powder.³³ At the time, Prussian blue represented a considerably cheaper alternative to ultramarine, costing approximately ten-fold less.³⁴ The situation only changed after 1826 when Guimet developed a workable industrial process for manufacturing synthetic ultramarine at about one-tenth of the cost of the natural mineral, making its use more economically feasible.³³ The subsequent reintroduction of ultramarine in stamp production in Switzerland in the following years likely reflects the emergence and adoption of synthetic ultramarine, which became economically competitive with the natural mineral. Based on all available data, a provisional chronological trend emerges: Ultramarine was initially used alone (until ca. 1867), then mixed with Prussian blue until the 1900s, when it seems to have disappeared completely. Some discrepancies in this timeline are likely due to uncertainties or misattributions in the issue dates of specific specimens. A possible theory is that the rise of synthetic ultramarine production may have promoted its use in Swiss issues coinciding with the return of their printing to Bern. It is also possible that ultramarine was simply chosen to obtain specific shades or because of its coloring power or reactivity.

Recent studies employing time-of-flight secondary ion mass spectrometry (TOF-SIMS) and fiber optic reflection spectroscopy (FORS) have shown promising results in distinguishing between natural and synthetic ultramarine pigments.^35,36 Future work could build on these advances to explore whether the increased prevalence of ultramarine observed in later stamp issues might be associated with the introduction and progressive adoption of synthetic ultramarine.

Red Monochrome Stamps

Visual inspection of the 16 stamps under ultraviolet irradiation using a Reskolux UV LED set (Deffner and Johann, Germany) with a wavelength centered at 365 nm suggested the presence of organic red pigments in several of them, as evidenced by their characteristic orange to pink fluorescence emission. Raman spectroscopy further confirmed this observation, revealing that the monochrome red stamps fall into two categories based on their Raman spectrum. Some exhibit peaks at 343 and 254 cm⁻¹, characteristic of the trigonal mercury(II) sulfide structure (HgS), commonly known as vermilion,^37,38 while others show no discernible Raman bands but strong fluorescence background indicative of organic lake pigments. Differences in the visual appearance under optical microscopy also support this distinction, with organic reds appearing more vivid and forming large flat areas of color, in contrast to the granular appearance of vermilion (Figure S1, Supplemental Material). Raman analyzes using a 532 nm laser were also performed on seven stamps exhibiting strong fluorescence and no traces of vermilion were detected. In five of them, additional Raman bands could be identified, but they were not specific enough for reliable dye identification, while the other two showed no signal. Nine stamps suspected of containing organic dyes were further analyzed by HPLC (Figure 3; Table S1, Supplemental Material). Based on the respective retention time and absorbance spectrum, the main colorant could be identified in eight stamps while components in the other specimen could not be formally determined.

Figure 3.

Chromatograms collected at 480 nm of extracts from red monochrome stamps and their respective UV-Vis spectrum given as a function of relative absorbance and wavelength (λ) in insets. (a) The 15Rp Strubel in which carminic acid (CA) was identified, (b) the 10C Sitting Helvetia in which CA and eosin Y (EY) were detected, (c) the 10C Cross and Numeral Zst 61A pink hue in which EY was found, (d) the 15C Cross and Numeral Zst 64B in which alizarin (Ali) was evidenced, and (e) the 10C UPU in which both EY and an unknown component (Unk) were determined.

The predominance of vermilion as the main red pigment in the early Swiss issues corresponds to the findings of other studies conducted worldwide.^12,15,17,23 No iron- or lead-based reds, such as hematite, minium, or litharge, were found in the monochrome issues analyzed here, in contrast to an observation made in the context of Italian stamps, where vermilion was seen to be replaced by red iron oxides for economic considerations.¹² Instead, Switzerland appears to have switched directly from vermilion to synthetic organic reds from the 1880s onwards, reflecting technological and economic changes in pigment production.

An interesting example is the 15Rp Strubel stamp, issued between 1854 and 1862. Vermilion was identified on only one of the six stamps, while the other five specimens exhibited strong fluorescence indicative of an organic pigment. Peaks were observed at 1631, 1552, 1475, 1307, 1078, and 668 cm⁻¹, but none could be confidently attributed to a specific pigment, nor did the FT-IR spectra exhibit any particular features. SEM-EDX analysis confirms the exclusive presence of Hg and S in the vermilion containing stamp, whereas the five “organic stamps” showed Sn, Ca, Pb and Al. These elements are commonly associated with a cationic support for organic lake pigments.³⁹ Ca and Pb may alternatively originate from fillers such as calcium carbonate and lead white, with aluminum serving as a support for the lake. Raman spectroscopy did not identify the use of litharge or red lead, indicating that lead white was the most likely source of lead. Unfortunately, no IR band related to the latter was detected that could have supported this assertion with certainty. The additional identification of phosphorus hints at the use of a phosphate salt during pigment production. Analysis of one of these organic specimen by HPLC identified cochineal, a generic term for a crimson-dyeing scale insects dye, based on the detection of carminic acid (Figure 3a). Although the exact issue dates for the six stamps remain unknown, the wide variability in their composition probably reflects an experimental phase during which several ink formulations were tested during the production of the same issue.

The second case of change from inorganic to organic pigment between two editions is illustrated by the 10C Sitting Helvetia stamp. Three specimens denominated Zst 38 dating from 1867 display characteristic vermilion peaks (343, 254 cm⁻¹), while a single Zst 46 (1881) exhibits peaks at 1594 and 1496 cm⁻¹ with strong fluorescence, consistent with an organic pigment. HPLC analysis confirmed the presence of the latter on both series (Figure 3b). The DAD spectra at 4.08 min match carminic acid, while the double peak between 7 and 8 min, originates from the coexistence of eosin Y (EY) and its monodebrominated side product (dbEY). This attribution was supported by LC-MS data (m/z 649 [EY+H]⁺; m/z 569 [dbEY+H]⁺).⁴⁰ SEM-EDX analysis of the three Zst 38 stamps revealed a fairly constant composition, with Hg, S and Pb as the major components, and Al and Ca being less abundant. With the IR band at 3691 cm⁻¹ assigned to the stretching vibration of hydroxyl groups in the kaolinite structure, these last two elements can be linked to fillers, i.e., kaolinite or calcium carbonate, despite the absence of an additional spectroscopic feature that clearly identifies the latter. Alternatively, Ca, Al and Pb could be related to the use of mordant dyes. Interestingly, the Zst 46 stamp had an identical elemental composition, albeit with a lower HgS concentration, which could explain why it was not detected behind the strong fluorescence background in Raman spectroscopy. In the absence of a signal attributable to red lead in the Raman spectra, the presence of Pb could be related to lead white used as a filler or in the preparation of organic pigments, also known as mordant dyes. Historical sources indicate that a light-sensitive substitute for vermilion can be obtained by precipitating an eosin solution with “sugar of lead”, i.e., lead(II) acetate, highlighting the potential dual role of Pb as both a pigment component and a substrate in organic red lakes.³⁴ A total of three red pigments were thus identified in the Zst 46 series, as well as in one specimen of the Zst 38 series. The two remaining Zst 38 stamps showed no fluorescence under UV light irradiation, ruling out the presence of eosin Y. They were also not analyzed by HPLC, so the presence of carminic acid cannot be formally excluded. The reason why only one Zst 38 specimen contains organic pigments remains unclear and could reflect, for instance, an incorrect attribution of issue. Interestingly, Ferreira et al.¹⁵ also report vermilion and carminic acid, but no eosin Y in their study of an 1867 Swiss Sitting Helvetia stamp. This raises the question of whether their sample corresponded to a Zst 38 issue similar to ours, or whether the absence of eosin Y reflects differences in extraction procedures (oxalic acid versus HCl) prior to HPLC analysis (Figure S10, Supplemental Material).

The Cross and Numeral stamps issued from 1882 onwards in various denominations ranging from 5C to 15C and in different shades of red are all characterized by the use of an organic pigment. The Raman spectrum of the 5C Zst 60A stamp exhibits peaks at 1548, 1497, 1424, 1378, 1185, 721, and 520 cm⁻¹. These characteristics and the absence of vibrational modes associated with the amide function and SO₃⁻groups suggested the possibility of a naphthol based pigment (Figure S4b, Supplemental Material). However, on this basis, no formal identification of the pigment could be made. HPLC-DAD analysis did not result in any successful identification either. Despite the detection of four chromatographic peaks in the 5C Zst 60A stamp, their retention times and spectra did not match any known reference (Table S1, Supplemental Material). Although the precise identity of the red pigment remains unknown, a blue component present in the mixture was identified as Prussian blue, based on its characteristic FT-IR band (2090 cm⁻¹).

The Raman spectrum of the 10C Zst 61A series with a red hue has peaks at 1617, 1501, 1276, 1177, 840, 716, 640, 478, and 345 cm⁻¹, while the same series with a pink hue shows only fluorescence background (Figure S5b, Supplemental Material). For the series issued from 1894, an additional peak at 1659 cm⁻¹ is visible. Prussian blue was identified in both issues due to its IR band at 2089 cm⁻¹. In the case of the pink shaded 10C Cross and Numeral Zst 61A, the corresponding chromatogram matches the profile of eosin Y (Figure 3c). The two other stamps in the red-shaded series show a similar chromatographic profile, confirming the presence of eosin Y. Minor signals, eluting earlier, were also observed, but their identities remain uncertain and could correspond to degradation or other debrominated products of the eosin Y dye.^41,42

Finally, the burgundy red 15C Cross and Numeral Zst 64B Raman spectrum shows peaks at 1668, 1566, 1479, 1326, 1291, 1161, 934, and 656 cm⁻¹ (Figure S4d, Supplemental Material). While some peaks could indicate the presence of aromatic nitro group, no conclusive identification could be made. The chromatogram exhibits a single peak at 6.743 min with maximum absorbance at 248, 278 and 428 nm, consistent with the presence of alizarin (Figure 3d). The color shift to a deeper purple-burgundy hue results from the admixture of Prussian blue (FT-IR, 2091 cm⁻¹; Raman, 2152 cm⁻¹) with the red dye. In both the 10C Zst 61A and 15C Zst 64B Cross and Numeral stamps, the presence of Ba and S detected by SEM–EDX is most likely associated with barium sulfate. While in the blue 12C Cross and Numeral stamps, BaSO₄ was probably used mainly as a filler or extender, here its presence could also reflect its utilization as a substrate in the preparation of organic lake pigments. Consequently, its role cannot be unequivocally assigned to a single function within the stamp manufacturing process.

The 10C UPU stamp created for the Universal Postal Union’s jubilee in 1900 also contains an organic pigment with Raman bands at 1623, 1500, 1339, 1283, 1179, 714, 642, 480, 296 and 212 cm⁻¹ (Figure S4c, Supplemental Material). In addition, FT-IR analysis identified the presence of Prussian blue (2091 cm⁻¹) and calcium carbonate (CaCO₃; 875 cm⁻¹). Eosin Y was identified in its composition together with an unknown red dye (Figure 3e; λ_max = 229, 327, and 483 nm).

It appears that 1882 marked the beginning of a transition in the use of red pigments for Swiss stamps, from predominantly inorganic to organic reds. Many stamps produced from that date onwards already exhibit the use of organic colorants, a trend that became more pronounced with the 1894 and 1900 issues. Of the 16 red stamps analyzed, 13 were found to contain organic dyes, where seven were identified by HPLC. These results are consistent with the work of Ferreira et al.¹⁵ on red stamps, who reported that carminic acid, alizarin (synthesized in 1868), and eosin Y (commercialized in the 1880s) were the predominant red organic dyes in postage stamps from the same historical period in different countries. Although economic considerations may have contributed to the progressive replacement of vermilion, cost alone does not fully explain the adoption of organic red pigments. Early synthetic dyes such as eosin Y were initially expensive, around 800 German Marks per kilogram in the 1880s and still pricier than other synthetic organics like alizarin.³⁴ As production volumes of the xanthene dye increased, multiplying fifty-fold in 15 years, prices fell significantly, making these organic reds increasingly attractive for printing applications. Aesthetic considerations may also have played a role. Eosin Y, for instance, can be precipitated with aluminum, lead, or potassium salts to form geranium lakes with brilliant red hues, from pink to bright crimson tones that could not be achieved with inorganic pigments.

Yellow Monochrome Stamps

The single yellow monochrome 15C Cross and Numeral stamp, first issued in 1882, displays characteristic stretching modes at 842 and 360 cm⁻¹ corresponding to the lead chromate anion ( ${CrO}_{4}^{2 -}$ ; ν1) and the symmetric bending (ν4), respectively, both attributable to chrome yellow, a lead(II) chromate pigment (PbCrO₄).^38,43 It has already been identified in contemporary stamps from other countries, such as Japanese (1873–1875)²⁸ and Ottoman stamps (1880–1890).¹⁴

Green Monochrome Stamps

Green hues in artworks and printed materials can be achieved either through the use of a single green pigment or by mixing various blue and yellow pigments in different proportions, resulting in a wide green spectrum. Raman analyzes of the selected stamps point to the latter approach, as distinct signals attributable to both blue and yellow pigments were identified. Specifically, the 40Rp Strubel, the 5C Cross and Numeral (1882 and 1899) and the 25C Sitting and Standing Helvetia display the characteristic Raman bands corresponding to Prussian blue (2153, 2091, 530 and 280 cm⁻¹) and chrome yellow (840, 360 cm⁻¹), which is confirmed by the IR bands at 2085 and 596 cm⁻¹ (Figure 2a), with no evidence of a green pigment. In some cases, an additional IR band characteristic of calcium carbonate (874 cm⁻¹) was observed. The 25C Sitting Helvetia Zst 40 (1867–1881) and the Standing Helvetia Zst 67D (1894) series show a similar ink formulation despite being produced 18 years apart (Figure S6b–c, Supplemental Material). This pigment combination persisted for more than a decade, as evidenced in the 5C UPU stamps from 1900 analyzed by Raman spectroscopy and ATR FT-IR, which revealed the characteristic peaks of individual component. This observation aligns with previous studies on stamp ink formulations of the 19th century around the world, where green stamps seem to have been consistently produced with a mixture of Prussian blue and chrome yellow, also known as "chrome green".^{12,14,17,20,28,32}

Orange Monochrome Stamps

Of the three orange 20Rp Strubel analyzed, two exhibits Raman bands at 848, 826, 435, 382, and 343 cm⁻¹, associated with chrome orange, a basic lead(II) chromate (PbCrO₄·PbO), with the first strong vibrational mode at 826 cm⁻¹ corresponding to the symmetric stretching mode of the chromate anion. The third specimen, in contrast, displays peaks at 550, 353, and 152 cm⁻¹ ascribed to the stretching vibrational mode of lead tetra oxide (Pb₃O₄) and lead oxide (PbO), characteristic of red lead. However, spectra alone do not allow us to determine whether the pigment comes from the natural mineral, commonly known as minium, or its synthetic equivalent. These results were corroborated in the corresponding FT-IR spectra, distinct absorption bands at 844 and 831 cm⁻¹ were observed for the two stamps containing chrome orange, while these characteristics were absent in the red lead stamp (Figure 2b).

Complementary SEM-EDX analysis confirmed the presence of Pb and Cr in the two stamps bearing chrome orange, and only Pb in the red lead specimen, thus validating these identifications. Traces of cadmium were also identified in the former, hinting at the adjunction of small quantities of a Cd-based pigment, e.g., PY37, PO20, or PR108. The unknown issue date of these stamps prevents linking their compositions to specific series, but determining whether the red lead-based example predates or postdates the two chrome orange specimens could reveal ancient practices in pigment selection.

Brown Monochrome Stamps

The 5Rp Strubel, issued between 1854–1862, shows a mixture of hematite (Fe₂O₃; Raman bands at 403, 292, and 225 cm⁻¹) and carbon black (1592 and 1329 cm⁻¹), together with a small amount of kaolinite (Al₂Si₂O₅(OH)₄), supported by the elemental identification of Fe, Al, and Si.

In contrast, the 2C Cross and Numeral 59A is composed of a heterogeneous mixture that readily reveals the characteristic Raman bands of vermilion (344, 252 cm⁻¹), hematite (281, 220 cm⁻¹), and carbon black (1594, 1349 cm⁻¹). Interestingly, Ba, S, and Pb, but no Hg were observed using SEM-EDX. The two analyzed 2C Cross and Numeral stamps exhibit only the vibrational modes of carbon black (1589, 1398 cm⁻¹), a surprising result given their brown hue, which would normally require an additional red pigment. Under excitation with a 532 nm laser, weak signals consistent with an organic red dye were observed, with peaks at 1585, 1369, 1080, 969, 618, and 515 cm⁻¹ in the 58A series, while bands at 2425, 2050, 1568, 1354, 1072, 936, 667, and 525 cm⁻¹ were observed in the other stamp. Unfortunately, these features are not sufficiently distinctive to allow unambiguous identification of the pigment. SEM-EDX analysis identified the presence of Ba and S, consistent with the presence of baryte as filler or organic pigment support material, as also observed in the red 10C and 15C Cross and Numeral stamps. HPLC analysis attested the presence of alizarin in the case of 3C, while it was not possible to determine with certainty the nature of the pigment used in both 2C. The two series of 2C differ in their chromatographic profile, consistent with earlier Raman observations, indicating different ink formulations. In the case of the 58A series, an unidentified organic red pigment was used based on the absorbance peaks (λ_max: 450–480 nm), while a λ_max of 360–380 nm for the other series indicates the use of a yellow pigment instead.

Gray Monochrome Stamps

The single gray 2Rp Strubel shows characteristic broad Raman bands at 1590 and 1373 cm⁻¹ indicating the use of carbon-based black pigment, which could have been used in a dilute concentration or together with a white pigment to achieve a lighter hue. Elemental analysis reveals the presence of both Pb and Ca. The presence of lead points to the presence of lead white, further supported by the Raman band at 1048 cm⁻¹, together with the FT-IR bands ascribed to the symmetric stretching of the carbonate anion C–O at 1045 cm⁻¹ and in plane bending at 676 cm⁻¹ (Figure 2d). The additional peak at 837 cm⁻¹ indicates the presence of lead white, more specifically neutral lead carbonate PbCO₃. The absence of lead in paper background measurements suggests that it was used to lighten the hue of the black pigment. For the latter, bone black can be excluded due to the absence of phosphorus, leaving carbon black as the most likely candidate. However, the exact carbon source is impossible to identify further. The homogeneous signal obtained for calcium may therefore hint at the use of calcium carbonate (CaCO₃) or sulfate (CaSO₄) as a paper filler or color enhancer, but the absence of diagnostic bands prevents a clear attribution.

Polychrome Stamps

The study encompassed the typical red Swiss cross surmounted by a post horn in black on a cream (Local Post), dark blue (Rayon I; Zst 15), light blue (Rayon I; Zst 17), yellow (Rayon II), and red background (Rayon III).

Local Post

The study of a single specimen of the 2½Rp Poste Locale using Raman spectroscopy shows characteristics peaks at 343 and 254 cm⁻¹ associated with vermilion for the red coat of arms, as well as those typical of carbon black (1585 and 1347 cm⁻¹) for the outlines (Figure 4a). Upon visual inspection, the paper background appeared slightly yellowish. However, no signal related to the use of a specific pigment was identified by Raman spectroscopy or FT-IR. Nevertheless, the IR band at 874 cm⁻¹ confirms the presence of calcium carbonate, which could have acted as a paper filler or whitening agent. At this point, the “cream” hue seems to be related to the choice of chiffon paper used to produce this stamp and not to a pigment, although additional data would be needed to confirm this hypothesis. The quality and thickness of the paper varied, as did the gum, which ranged from brownish (gum arabic and dextrin) for “OrtsPost” to light yellowish (gum arabic) for “Poste Locale”.⁴⁴ SEM-EDX analysis confirmed the sole presence of Hg and S in the red area.

Figure 4.

Raman spectra of iconic Rayon stamps showing the use of vermilion for (a) the 2½ Rp, ultramarine for the blue area, and the combined use of vermilion and chrome orange for the red area for the (b) 5Rp issued in 1850, (c) Prussian blue for the blue area and vermilion and chrome orange for the red area for the 5Rp issued in 1851, (d) chrome yellow for the yellow area and vermilion, red lead and chrome orange for the red area for the 10Rp, and (e) vermilion for the 15Rp. Experimental conditions: λ_ex= 633 nm, laser power: 1.04 mW, acquisition time 3 s, and three accumulations.

Blue Rayon I

A total of 11 5Rp Rayon I stamps were studied, five dark blue (1850) and six light blue (1851). The dark blue background of the 1850 issue is ambivalent. The five analyzed stamps, all show the characteristic Raman peaks associated with ultramarine (1639, 1095, 803, 548, and 256 cm⁻¹), while two also exhibit peaks related to Prussian blue. Elemental analysis confirmed that ultramarine was the only blue pigment used in three of the stamps, while FT-IR confirms the combined use of Prussian blue in the remaining two stamps. The additional presence of Pb was observed in all three specimen, while Ba and S was observed in only one. All five show the use of carbon black (1590 and 1368 cm⁻¹) for the outlines (Figure 4b). FT-IR analysis revealed characteristic IR bands at 836 and 674 cm⁻¹, confirming the presence of lead white in the three stamps containing only ultramarine. Four stamps also have a characteristic IR band at 875 cm⁻¹, linked to the presence of calcium carbonate. Regarding the red pigments used, while Raman spectroscopy showed the presence of vermilion in all five stamps, it revealed the presence of chrome orange in only one stamp (826 and 381 cm⁻¹). However, another stamp shows a characteristic IR band at 826 cm⁻¹ that can also be assigned to chrome orange. While Hg and S were successfully identified in the red area using SEM-EDX, no Cr could be detected in this specific specimen. One stamp is quite distinct from the others, as it shows the combined use of both Prussian blue and ultramarine for the blue background and two other stamps show the combined use of vermilion and chrome orange for the coat of arms. However, it should be noted that Raman imaging attested the co-localized use of the two blue pigments but showed no trace of chrome orange in the red areas (Figure S7, Supplemental Material). Overall, it seems that the composition of Zst 15 stamps underwent several major changes over the short period in which they were produced. These modifications probably reflect attempts to balance color intensity and cost, with varying degrees of success, which ultimately led to the creation of the light blue Rayon I Zst 17 stamp in 1851.

The light blue stamps issued in 1851 display the characteristic bands of Prussian blue (2154, 530 and 280 cm⁻¹), as well as broad bands (1590 and 1365 cm⁻¹) related to carbon black for the outlines (Figure 4c). These results were confirmed by ATR FT-IR in the peak visible at 2076 cm⁻¹. Only Hg, S (red area), and Fe (blue area) were detected by elemental analysis, thus excluding the presence of ultramarine in the lighter blue series, unlike in the first Rayon I series. The change in the background color of the Rayon I stamp was motivated by economic considerations, as the first three-color edition on a dark blue background proved very costly to print, prompting the printer to experiment with color changes while keeping the existing printing plates.⁵ Rayon I was therefore changed to light blue on white paper with a red shield. These economic considerations were reflected in a change in composition, i.e., the switch from ultramarine to Prussian blue, associated with a less intense blue color, thus requiring a lower concentration of pigments. In addition, the commonly used black and red cancellation marks were much better recognizable. However, greater diversity was found in red pigments. Although vermilion was identified in all six stamps, chrome orange (848 and 826 cm⁻¹) was also detected in two of them using Raman spectroscopy . This difference was first attributed to the use of a specific printing stone, C2. Indeed, all stamps printed with C1 stones contain only one pigment, vermilion, whereas stamps printed with C2 stones contain both. However, an exception to the rule was identified with the 5Rp Rayon I C2 RU Type 5 stamp, which only has vermilion and no trace of chrome orange (Figure S8, Supplemental Material). Therefore, with the exception of that specimen, all stamps printed with the C1 stone contain only vermilion, while all C2 stamps contain both vermilion and chrome orange. Both SEM-EDX and FT-IR analysis confirmed this observation, thus excluding the omission of chrome orange during Raman analysis (Figure 5). Another possibility could have been the erroneous attribution of a C2 stone instead of a C1 stone stamp, which was ruled out after a second philatelic evaluation of the specimen. At this stage, it is unclear whether there is an underlying reason for using one or two pigment, such as cost and availability or technical constraints related to the printing process. Imperio et al.¹² already stressed the replacement of cinnabar with red mars or hematite for economic reasons. Raman imaging was conducted to understand the combined use of the vermilion and chrome orange, in order to determine whether they have been used jointly on the entire stamp or only on a few areas. The two pigments were co-localized and distributed relatively homogeneously over the surface of the coat of arms (Figure 6). We can therefore hypothesize that they were mixed at the time of the ink production and then applied to the printing stone, rather than applied in successive layers. It therefore seems that the type of stone used for printing is not the only parameter influencing the composition. It should be noted that in this study, only Rayon I stamps produced with the most abundant C1 and C2 stones were analyzed.

Figure 5.

SEM-EDX analysis of the 5Rp Rayon I C2 RO Type 6 stamp. SEM images of (a) red and (b) blue areas of the stamp, and (c–d) their respective average elemental spectra. Scale bars: 200 µm.

Figure 6.

Raman imaging of an area (black rectangle) of a 5Rp Rayon I issued in 1851 showing the co-located use of vermilion and chrome orange red pigments (yellow rectangle), as evidenced by the respective Raman spectrum of each pigment. Experimental conditions: λ_ex= 785 nm, laser power: 13.5 mW, acquisition time 1 s, and 1 accumulation. Scale bars: 100 µm.

Yellow Rayon II

Raman analyzes of the yellow and black areas of 11 10Rp Rayon II stamps issued in 1850 show the characteristic signatures of chrome yellow (840 and 360 cm⁻¹) and broad bands at 1589 and 1372 cm⁻¹ corresponding to carbon black (Figure 4d). Half of them exhibit a characteristic IR peak associated with the presence of calcium carbonate (874 cm⁻¹). SEM-EDX further identified Pb in the yellow area, regardless of printing stone type. This is consistent with the presence of chrome yellow in these areas. For the red coat of arms, all stamps exhibit the characteristic vibrational band of vermilion at 343 and 254 cm⁻¹ (Figure S9, Supplemental Material). Compared to stones A1 and B, specimens printed with a stone A2 have a more complex ink formulation. In fact, all A2 specimens showed ternary ink, with the presence of chrome orange (848, 826, 424, and 382 cm⁻¹) and red lead (550, 360, 159 and 122 cm⁻¹). The underlying cause of this variability remains unclear at this stage of the research. It could be linked to changes over time in ink formulations, to adjustments in composition motivated by economic reasons, as already been observed in the manufacture of Swiss monochrome stamps, or to technical problems specific to the A2 printing stone.

Red Rayon III

The study of the single specimen of the 15Rp Rayon III shows characteristics peaks at 344 and 253 cm⁻¹ associated with vermilion, as well as those typical of carbon black (1582 and 1354 cm⁻¹) for the outlines (Figure 4e). The composition of this stamp is consistent with Swiss red monochrome stamps. A weak IR peak at 592 cm⁻¹ may suggest the presence of red lead, but the spectra are insufficient for a confident identification. It should be noted, however, that even when present in small proportions, vermilion is readily detected and may mask the spectral features of other red pigments, including red lead, which could explain why it was not identified by Raman spectroscopy.

Conclusion

This work enabled the comprehensive characterization of 98 Swiss postage stamps dating from the 19th to early 20th century. Capitalizing on the differences in spatial resolution and sensitivity to cellulose substrate between Raman spectroscopy and ATR FT-IR, this combination identified most of the pigments present in these stamps. For the remaining ones, HPLC-DAD, although destructive, unambiguously identified the organic dyes. SEM-EDX elemental analyses confirmed these results while providing clues as to the composition of the mordants used in the manufacture of organic dyes. Blue stamps were predominantly colored with Prussian blue and ultramarine, either used alone or in combination, consistent with the formulations reported for other contemporary postage stamps worldwide. A correlation was established between production cost constraints and the modification of the pigment formulation used to manufacture the two different issues (1850 and 1851) of the 5Rp Rayon I stamps. Red shades were initially achieved using vermilion for monochrome stamps and adding chrome orange and lead red for polychrome stamps, then gradually replaced by synthetic organic dyes, including carminic acid, alizarin, and eosin Y from 1882 onwards. The change in red pigments was probably driven not only by economic factors, as in blue inks, but also by aesthetic considerations, as organic dyes enabled a much broader and brighter palette of reds to be achieved. Yellow pigments were based on chrome yellow, which was mixed with Prussian blue to produce different shades of green. Orange tones were mainly obtained using chrome orange and red lead, while brown shades combined carbon black with inorganic and organic red pigments. Gray tones were typically achieved through mixtures of carbon black together with either calcite or lead white as white pigment. The presence of white or colorless compounds such as kaolinite, calcite, lead white or barium sulfate was also recurrent, though their precise function as part of the paper substrate as fillers, or linked to the ink formulation as extenders or lake substrates, or both, could not always be unambiguously determined. BaSO₄ appears to have been employed across several color formulations of the Cross and Numeral stamps, i.e., 12C (blue), 10C, and 15C (red), and 3C and 2C (brown), albeit potentially serving different functions. In the blue issues, BaSO₄ was likely used primarily as a filler or extender, whereas in the red and brown stamps, where organic lakes are present, it may additionally have had a role as a substrate in the lake pigment preparation. Its occurrence therefore cannot be assigned unambiguously to a single role within the stamp materials. Further work should focus on verifying its occurrence in a broader range of color formulation, in order to assess whether BaSO₄ could serve as a meaningful compositional marker within Cross and Numeral series. The knowledge gained about the chemical composition of these stamps not only contributes to a better understanding of the technological evolution of print materials in Switzerland in the 19th and 20th centuries, but also highlights the similarities in pigment choices between European countries at that time. In addition, knowing the composition of stamps will help conservators-restorers to better define the optimal conditions for their storage and display in order to ensure their long-term preservation.

Supplemental Material

sj-pdf-1-asp-10.1177_00037028261436069 - Supplemental material for Chemistry of 19th-Century Swiss Postage Stamps

Supplemental material, sj-pdf-1-asp-10.1177_00037028261436069 for Chemistry of 19th-Century Swiss Postage Stamps by Clémence Iacconi, Lionel Rumpf, Peter Fankhauser, Cyril Portmann and Laura Hendriks in Applied Spectroscopy

Footnotes

Acknowledgments

The authors thank Yohan Challande and Adeline Page (HEIA-FR) for their valuable contributions to the acquisition of spectroscopic data during the end of their apprenticeship. We acknowledge preliminary investigations carried out by Diana Reguin and Raphaël Siegenthaler (HEIA-FR) as part of internal 3rd year Bachelor semester projects. Their dedicated work laid an important foundation for this study. We thank Samuel Roth (HEIA-FR) for his technical support and instrumental expertise throughout this project. We warmly thank Curdin Maissen and Fabian Knechtel (Bruker Optics) for the possibility of performing Raman imaging. We would like to thank Jennifer Poulin and Stephanie Barnes (Canadian Conservation Institute) who provided invaluable assistance with the art historical data, particularly regarding the sources and pricing of pigments. Their expertise greatly enriched the contextual depth of this study.

Declaration of Conflicting Interests

The authors declare that they have no conflict of interest.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by a Branco Weiss Fellowship — Society in Science administered by ETH Zurich, granted to L.H., as well as internal Ra&D HES-SO//FR—R funding. The authors gratefully acknowledge financial support from the Swiss National Science Foundation (SNSF) via an SNSF Ambizione fellowship granted to L.H. (Grant no. 216127).

Supplemental Material

All supplemental material mentioned in the text accompanies this paper online.

ORCID iDs

Clémence Iacconi

Lionel Rumpf

Peter Fankhauser

Cyril Portmann

Laura Hendriks

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