The molecules of color in Portuguese postage stamps (1857–1909)

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.

HPLC

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 [16]. 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 (λ_exc=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 [17].

Quantum yields

The fluorescence quantum yields (Φ_f) 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).

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.

Fig. 2:

Absorption spectra comparing the (a) 13_1857-62_Rep1, (b) 66_1884_Rep2 and (c) 160_1909_Rep2 postage stamps-paper: back of the stamp; dye: colored area (shown in the image by an ellipse) – with a standard, (a) carminic acid and (b)/(c) Eosin y–paper: filter paper painted with the standard; solid: standard in the powder form.

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.

Fig. 3:

HPLC chromatograms and absorption spectra of the (a) 13_1857-62_Rep2, (b) 66_1884_Rep2 postage stamp extracts and the standard, (a) carminic acid and (b) Eosin Y.

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 λ_exc=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.

Fig. 4:

Emission spectra comparing the (a) 141_1899_Rep3, (b) 160_1909_Rep3 postage stamps with the standard, Eosin Y, in DMSO.

Fig. 5:

Fluorescence decays of (a) Eosin Y in MeOH:H₂O, (b) 66_1884_Re2 postage stamp extract in MeOH:H₂O, (c) Eosin Y in DMSO and (d) 141_1899_Rep3 postage stamp extract in DMSO.

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 (C_i) 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 Φ_f 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.