The nature of the pigments or dyes used to dye the first Portuguese postage stamps has remained unknown until now. In this work, a study has been made of the inks used for red, rose, purple and orange colors in a selected number of Portuguese postage stamps from the period 1857 to 1909. This is based on analysis involving a variety of techniques (X-ray fluorescence, UV-Vis spectroscopy, HPLC-MS/HPLC-DAD, steady and time resolved fluorescence). It was found that the inks included, among others, the inorganic pigments cinnabar (HgS), lead oxide (Pb3O4) and chromate (PbCrO4), lead sulphide (PbS), and the organic compounds carminic acid and Eosin Y. The study demonstrated a non-destructive analysis method for identification of two molecules of color involving the UV-Vis (for carminic acid and Eosin Y) and fluorescence spectra, together with quantum yields and lifetimes (for Eosin Y).
Postage stamps can be considered both as a form of art and of history. There are numerous examples where this connection can be found. The history of chemistry behind a postage stamp is well illustrated in many textbooks and articles , , . From a previous analysis that we have carried out on some postage stamps of the United Kingdom we know that carminic acid, cochineal (for reds) and the iconic mauveine (for lilacs/purple) were used in them in the period from 1847 to 1901 . Indeed, we previously showed that the iconic mauveine dye was found in UK Victorian 6d (penny) postage stamps from 1867–1880 . In the present work we will focus on the dyes or pigments (the molecules of color) used to color some of the first Portuguese postage stamps.
The history of the first postage stamp dates back to the XIXth century when Sir Rowland Hill introduced the first Postal Stamp, the “penny black” on 8th of May 1840 , . Before that, the exchange of correspondence was carried out by letters containing two marks, a nominal stamp (for the mailing sender) and a fixed stamp (for the rate to be paid by recipient according to the weight and distance travelled).
One landmark previous to Sir Rowland’s creation was the postage-paid system used in 1653, Paris, by Jean Jacques Renouard de Villayer, a French employee of Louis XIV, to whom the king gave permission to carry and deliver letters. The postage was represented by a “postage paid ticket” and it consisted of a postage stamp, which was not glued to the letter. However, Villayer’s organization lasted only a short time, and by 1662 it no longer existed . In 1663, in London, a similar system, created by William Deckwra, was attempted . The letters delivered contained no covering; the letter was a sheet closed in itself. The first envelopes only appeared at the end of the XVIIIth century (in 1784), issued by the Post of the Austrian Empires. However, they were not in use for very long, and it was only later (in 1818) that the envelopes, commonly known as “cavallini” or “cavallotti” (designation due to the design of a horse in the front) came to be used, and continued until the end of 1837 , .
In parallel, in China, in 1823, letters (with postmarks but without postage stamps) circulated temporarily, enveloped with the following inscriptions: for internal use – “for 3 sapecas, this letter can circulate in all China’s provinces and only be stopped at the borders from the Ocean” and, for external use, “for 10 sapecas this letter can cross all seas and big mountains” .
The analysis of postage stamps, in particular of the pioneer samples (1847–1900) has been the subject of a few studies, aiming particularly at the detection of counterfeit or forged stamps , , . The relevance of these studies lay in the fact that some of these specimens can sell for values of hundreds of thousands, or even millions of dollars, making the forgery of these very profitable.
A philatelic specialist may carry out the analysis (namely its origin and authenticity) of a postage stamp by eye; however, a more scientific analysis must determine its contents (dyes, pigments, mordents) and the type of paper used.
Only a limited number of studies have been reported regarding the analysis of postage stamps. The most relevant include that of the iconic mauveine dye in UK 6d postage stamp, in use from 1867 to 1880 , the rare Hawaiian Missionary stamps which were analysed by Raman spectroscopy and compared with forgeries and modern reproductions , the 1847 1d orange–red Mauritian stamp , the 1847 2d deep blue unused stamp , the one-penny postage stamps between 1841 and 1880 , the first Chilean postage stamps , some Spanish stamps [Spanish 15 cents stamp from the reign of King Alfonso XIII (1889–1901)] , some Brazilian ones (printed between 1850 and 1922)  and recent work on Japanese pioneer samples .
Usually these studies involve non-invasive techniques, which include Raman and IR spectroscopies, together with X-ray fluorescence (XRF); although not completely unambiguous, these allow substantial conclusions to be made, particularly on the presence of inorganic pigments. In the case of the UK postage stamps, HPLC-DAD-MS analysis of the extracted mixture of dyes was performed, and led to more clear conclusions on the presence of Mauveine and carminic acid (Cochineal) dyes .
In the case of Portuguese postage stamps, there is little literature on the dyes used. There is a single reference to a box with printing inks, originating from England, received on 23rd of April of 1853 by the Portuguese Coin House, without any reference to its content . However, this suggests that some similarities may exist between those and the UK postage stamps. To fill in this gap in our knowledge, we report a study of the inks used in some early Portuguese postage stamps.
X-ray fluorescence (XRF)
The analysis was performed with a Hitachi X-Ray Fluorescence model EA6000VX. For the elementary analysis the measurement conditions were 60 s, collimator with 1.2×1.2 mm, current at 1000 μA, and voltages of 15 kV (measurement without filter) and 50 kV (filter for Pb). For the mapping of some elements the conditions were 60 μm/pixel, 50.00 ms per pixel, collimator with 0.2×0.2 mm, current 1000 μA, for some elements we used a voltage of 15 kV, some of them with a filter for Cr, and voltage of 50 kV, some analysis with a filter for Pb.
UV-Vis absorption spectra (solution and solid state)
For the solid state UV-Vis absorption spectra of the postage stamps and standards (carminic acid, cinnabar, hematite, Eosin Y, fuchsine, lead chromate, lead oxide, lead sulphite) were analysed in two forms, solid (powder) and gouache (the standard powder was dissolved in water and potato starch and afterwards was painted onto filter paper). For these analyses, the absorption spectra were obtained by diffuse reflectance using a Cary 5000 UV-Vis-NIR spectrophotometer equipped with an integrating sphere. Before spectra of postage stamps and solid standards were recorded, a baseline was obtained with barium sulphate. For the gouache samples, the baseline was acquired with filter paper.
Postage stamp extraction
The procedure used in this work is similar to that used in Ref. , with some minor changes. A few pieces of postage stamp were extracted with a 200 μL solution of 0.2 M oxalic acid/methanol/acetone/water (1:3:3:4, v/v/v/v) in a 1.5 mL Eppendorf at 60°C in a water bath with constant agitation until the paper lost its color (approximately 30 min). After extraction, the samples were dried in a nitrogen line under heating with a dryer, the residues were reconstituted in 50 μL methanol, ultrasounds and 50 μL water before HPLC-MS and HPLC-DAD analysis.
The postage stamp extractions were analysed on a DioneX UltiMate 3000 Pump – Thermo Scientific apparatus using a TOF Kinetex C18 column (2.1 mm×150 mm, 1.7 μm, 100 A). A solvent gradient with acetonitrile (A) and acid water (formic acid, 0.1% v/v) (B), with a flow rate of 1.5 mL/min for the chromophore separation: 0min: 5% A/95% B, 1.5 min: 15% A/85% B, 8 min: 50% A/50% B, 10 min: 70% A/30% B, 18 min: 100% A, 28 min: 5% A/95% B, 30 min: 5% A/95% B, with a flow rate of 0.150 mL/min at 35°C, and the chromatographic spectra were obtained by LC-HRMS/ESI(–).
The extracted dyes from the postage stamps were analysed on an Elite Lachrom HPLC-DAD system with L-2455 Diode Array Detector, L-23000 Column Oven (RP-18 endcapped column), L-2130 Pump and a L-2200 Auto Sampler . A solvent gradient was performed with acetonitrile (A) and acid water (formic acid, 0.1% v/v) (B), with a flow rate of 1.5 mL/min for the chromophore separation: 0 min: 5% A/95% B, 1.5 min: 15% A/85% B, 8 min: 50% A/50% B, 10 min: 70% A/30% B, 18 min: 100% A, 28 min: 5% A/95% B, 30 min: 5% A/95% B, with a flow rate of 1.5 mL/min at 35°C. The HPLC-DAD chromatograms were acquired at 460 nm and 524 nm.
Time-resolved fluorescence measurements
Fluorescence decays were measured using a home-built picosecond time correlated single photon counting, TCSPC, apparatus (3 ps time resolution) described elsewhere . The fluorescence decays and the instrumental response function (IRF) were collected using a time scale of 1024 channels, until 5×103 counts at maximum were reached. The excitation source consisted on a PicoLED (λexc=451 nm) from PicoQuant. For the solution sample of Eosin Y emission was at 560 nm whereas in the solid postage stamps and paper dyed with Eosin Y the emission wavelength was 600 nm. Deconvolution of the fluorescence decay curves was performed using the modulating function method, as implemented by Striker in the SAND program, as previously reported in the literature .
The fluorescence quantum yields (Φf) for Eosin Y in the solid state and in the postage stamps was obtained by the absolute method using a Hamamatsu absolute PL quantum yield spectrometer Quantaurus C11347 (integrating sphere).
Results and discussion
Twelve groups (each with 2–3 replicas) of Portuguese postage stamps were analysed by various techniques, with the aim of identifying the molecule(s) of color present in each specimen. A total of 34 postage stamps (including replicas) were investigated. The identification (with acronym) of all samples is based on their catalogue number (13-160), the production year (1857–1909) and replica number (1-3); as an example, the acronym 13_1857-62_Rep1 describes the postage stamp with a catalogue number of 13, produced between the years of 1857 and 1862 and is the replica #1.
Initial analysis aimed to differentiate between the organic and inorganic origin of the molecules of color present .
For the inorganic molecules, XRF was selected as the technique of identification. For the inorganic ions (and bromine) found in major amount, mapping of the entire postage stamp was performed (see Fig. 1; Figs. SI1 and SI2); In particular in the case of Fig. SI2 an illustrative example of the mapping of Hg and Br is presented. Mapping shows that the Hg ion (sulfur is also present indicating that the pigment is HgS or Cinnabar) is present only in the red part of the postage stamp. In the case of the bromine mapping, this suggested an organic dye in which Br is present, which was found to be Eosin Y (see below). Figure 1 shows a more clear and illustrative example of the mapping for the lead ion, which was the only ion present in significant amount in this postage stamp, which lead to the conclusion that it must be associated to the dyeing with lead oxides. The presence of other inorganic ions (S, Fe, Cu, Cr, Zn, etc.) is summarized in Table SI1, in the Supplementary Information (SI).
For the postage stamps where no inorganic ion was detected, and, therefore, all the dyeing molecules of color were of organic origin, an analysis was performed consisting of the extraction of the dye from the postage stamp followed by a HPLC-MS/HPLC-DAD analysis. Carminic acid and Eosin Y were found to be the only organic dyes in the postage stamps investigated. In order to develop a non-destructive technique, we have additionally obtained the UV-Vis absorption spectra of the postage stamps and (in the case where the dyes were luminescent) the fluorescence spectra and lifetimes. Table 1 summarizes the organic/inorganic origin of the dyes/pigments found in the postage stamps studied.
UV-Vis absorption spectra of the postage stamps
The HPLC-MS/HPLC-DAD extract showed that carminic acid was present as the major coloring compound in the 13_1857-62, 16_1862-66, 30_1867_Rep2/Rep3 and 40_1870-79 postage stamps. For the postage stamp 66_1884_Rep2 the extract showed that Eosin Y was the major compound. The UV-Vis spectra of these postage stamps was obtained and further compared with a mixture of the dyes with potato starch and water, which was used and painted over an ashless filter paper aiming to mimic the same solid state found in the postage stamps.
With the postage stamps, the absorption spectra was obtained from the best colored area considered by visual observation, i.e. a highly colored area and, when possible, without the postmark label. The absorption spectra of carminic acid and Eosin Y were obtained in solid form (powder) and over an ashless filter paper. Illustrative examples of these are depicted in Fig. 2.
From the chromatograms in Fig. 3 it can be seen that the dye extracted from the postage stamp 13_1857-62_Rep2 and a solution of carminic acid in methanol display the same retention time (see Table SI2) and absorption spectra, confirming that this is the dye in the postage stamp. Additionally, the UV-Vis absorption spectra of the postage stamp and of the dyes embedded in a starch media painted on an ashless paper filter show a very good overlap in the visible region. The lack of overlap in the UV region is due to the cut-off of the paper, as can be seen in Fig. 2. An identical procedure and conclusion applies to the postage stamp 66_1884_Rep2, where Eosin Y was found to be the major dyeing compound. The presence of carminic acid and Eosin Y in the various postage stamps analysed is summarized in Table 1 and the spectral and photopysical data summarized in Tables 2 and 3.
An additional way of identification and validation of the presence of Eosin Y involved the use of fluorescence spectroscopy. Since Eosin Y is known to be strongly fluorescent, the fluorescence spectra of the extract (in DMSO) of the postage stamps (141_1899_Rep3 and 160_1909_Rep3) were compared with the spectra of a solution of Eosin Y in DMSO (see Fig. 4). As can be seen, there is a good match between the two spectra confirming once more the presence of Eosin Y in the above-mentioned postage stamps. These results were complemented by the fluorescence decays of the two solutions, where a single exponential decay with a lifetime of 3.25 ns was obtained in both cases, as can be seen in Fig. 5. Eosin Y and the extract obtained from the postage stamps from DMSO and MeOH:H2O solutions were studied, the fluorescence spectra recorded with λexc=450 nm, see Fig. 4 (for the extract in DMSO), and the fluorescence decay times obtained, Fig. 5. The almost identical values obtained from Eosin Y solutions in two different solvents and the extract from the postage stamps in these same solvents further confirms the dye in these as being Eosin Y.
Very interesting is also to note in Table 1 the fact that the 22_1866 postage stamp, replica 1 (rep 1) was dyed with HgS whereas the two other replicas (rep 2 and rep 3) with carminic acid. A similar situation is found for the postage stamp 30_1867 in which the first two replicas, rep 1 and rep 2, were dyed with HgS and carminic acid respectively and Rep 3 with a mixture of the two molecules. The reason for this is not clear. It may result from a dyeing procedure where the dyer used the two sources to obtain the color or simply used what was available, as a source of red, for dyeing the postage stamps.
In addition, to validate the above analyses based on a non-destructive methodology, the fluorescence spectra and decays of the 160_1909_Rep3 postage stamp and of the mimetic paper painted with Eosin Y were obtained. The fluorescence spectra are presented in Fig. SI3 again showing a good overlap; the spectral and photophysical data are summarized in Tables 2 and 3.
The fluorescence decays of the analysed postage stamps are multiexponential, mirroring the different environments probed by Eosin Y in a solid support (see Table SI3 and Fig. SI4). Analysis of the decays shows that these are best-fitted with a tri-exponential decay law.
From Fig. 5 it can be clearly seen that the fluorescence lifetime obtained from the extract, from the postage stamps (in methanol:water and DMSO solutions), is identical to the values of Eosin Y in solution, thus once more confirming the nature of the color in these postage stamps.
As mentioned the fluorescence decays of the postage stamps (Table SI3) are fitted with tri-exponential decays mirroring the heterogeneity of the solid media. However, in the case of the 160_1909 postage stamps, the major components, as given by the fractional contribution (Ci) of each species, is associated to the two longer components (with more than 90% of the contribution); the longer components although displaying a value that is ca. one half of the solution lifetime can be associated to an excited species associated to the “isolated” Eosin Y. In the case of the postage stamps 62_1887 and 66_1884 it is the intermediate component that dominates the decay with more than 50% of contribution.
An additional remark regards the fluorescence quantum yields of the postage stamps in Table 2. For the three different replicas of the 62_1887 postage stamps, the Φf value ranges from 0.012 to 0.02, whereas for the two replicas of the 66_1884 postage stamps the values are 0.003 and 0.006 and finally for the three replicas of 160_1909 the values range from 0.005 to 0.012. For these postage stamps, with Eosin Y, the absorption and emission wavelength maxima are basically found at 529–532 nm and 565–572 nm respectively. The differences in the quantum yield values are likely to mirror a fade of Eosin Y (with consequent formation of less emissive photodegradation products). A more detailed study on the nature of these products is, however, outside the scope of this work.
A detailed analysis of the dyes or pigments (molecules of color) used in some of the first Portuguese postage stamps has been undertaken. Inorganic and organic materials have been identified as the coloring matter used in these postage stamps. The use of these molecules of color, in particular of the inorganics, contrasts with those used in the printing of United Kingdom postage stamps. A non-destructive methodology for the identification of the dyes present in the postage stamps, which includes the UV-Vis and fluorescence spectra (and fluorescence lifetimes), has been developed and further validated by comparison with the spectra (and decays).
A special issue containing invited papers on Chemistry and Cultural Heritage (M.J. Melo, A. Nevin and P. Baglioni,editors).
Acknowledgments are due to the Chemistry Centre of Coimbra, supported by the Foundation for Science and Technology (FCT) through Project PEst-OE/QUI/UI0313/2014, to the ERDF, to COMPETE and to the “Coimbra Trace Analysis and Imaging Laboratory”. We also thank the Eng. J. Miranda da Mota for providing us with of some of the postage stamps investigated.
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The online version of this article offers supplementary material (https://doi.org/10.1515/pac-2017-0701).
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