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DOI: 10.1039/C4AN01616E
Spectral characterization of postage stamps printing inks by means of Raman spectroscopy Eleonora Imperio[b],Gabriele Giancane* [c] and Ludovico Valli [a]. [a]
Department of Biological and Environmental Science and Technologies, Università del Salento, Via
Monteroni, 73100 Lecce (Italy) [b]
Department of Innovation Engineering, Università del Salento, Via Monteroni, 73100 Lecce (Italy)
[c]
Department of Cultural Heritage, Università del Salento, Via D. Birago, 73100 Lecce (Italy) E-mail:
[email protected]
Table of Contents The instrumental apparatus used, by which it has been achieved a completely non-destructive analysis by the ATR-FTIR spectrometer and the filtered laser beam, focalized by the Raman microscope objective lens.
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ABSTRACT: A time travel through 150 years of Italian postage stamp issues has allowed defining the evolution with time of the recurring pigments in stamp designs by means of Raman and FTIR spectroscopy. Numerous exemplars have been analyzed, covering the whole production of stamps for both Italian Kingdom and Republic. Raman and FTIR spectra showed the changeover from Prussian blue to copper phthalocyanine inks in 1958. Also the entire succession for red inks was recognized to develop from the original cinnabar to red ochre and minium, and finally to red azo pigments. The changes in the orange printing ink proceeded on a similar path; the first orange Italian exemplar was printed employing a mixture of chrome orange and red ochre. In 1929 this combination was replaced by azo pigments. Green stamps belonging to the first issues instead entailed the choice of blue and orange inks, namely chrome orange and Prussian blue; later on an ink composed mainly of phthalocyanine was employed as the green dye. The merger of data coming from Raman microscopy and FTIR-ATR spectroscopy, both non-destructive techniques, has allowed the characterization of stamp designs and potentially provides direct and fast evidence for the recognition of forged exemplars. 1.
INTRODUCTION
The history of materials employed in the manufacture of stamps is still lacking information since a minor part of the world production has been investigated up to now. The knowledge of the employed substances is not based on analytical confirmations. Moreover the identification of pigments in the stamps is fundamental with the aim of clarifying particular doubts about conservation or counterfeiting. The aim of this paper is to identify the pigments used in the production of the Italian stamps throughout their overall history. Up to our knowledge, such a complete and thorough examination, covering more than 150 years, has not been reported yet. The characterization of constituents involved in the preparation of this complex class of composite materials - i.e. the stamps – is the starting point for the realization of a database on the substances employed in stamp production and the creation of a “time-line” for the rapid and unambiguous identification of counterfeit samples. Hence it is essential to use rigorous techniques of analysis that exploit every method to the uttermost. This comprehension is not immediate as an aftereffect of the great intricacy of the investigated matrices, the materials aging and the probable blending with incidental substances. Moreover this extractionless and non-invasive approach is notably suited for samples onto paper, parchment or raw-materials when a very thin layer of pigmentation has been immobilized onto the substrate.1,2,3 So Raman spectroscopy has been profitably employed to investigate natural pigments and many of the XIX and XX century synthetic colorants.4,5 Finally, in the last years some spectral databases of organic pigments have been realized, even though they need to be remarkably completed and up-dated.6, 7 Our whole research consists of two stages; in fact, in a previous communication the same complete collection of Italian stamps was characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) mapping8. Then in the second step, which is the objective of the present research, Raman
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spectroscopy has been employed in order to investigate the printing inks used in the overall production of stamps in the Italian Kingdom and Republic.
2.
RESULTS AND DISCUSSION
The approach employed during the whole research is the following: whenever any change in the ink composition was observed, the analyses kept on backward in time to identify the first exemplar exhibiting the same variation. With this method, it has been possible to identify the exact year of introduction of the innovation. 3.1 Blue pigment The FT-IR spectrum of the blue sample with a denomination of 20c, part of the first Italian issue in 1862, displays an intense peak at 2083 cm-1 (Figure 1, A). This signal is attributed to the characteristic stretching vibration of the triple bond in cyanide group (C≡N), belonging to the ferric ferrocyanide compound (Fe4{Fe(CN)6}3), commonly known as Prussian blue.9 Raman analysis confirms the presence of such a pigment, as visible from the spectrum in figure 1 B, in which two signals appear, at 2160 cm-1 and 2098 cm-1. They are both contribution of the same functional group, C≡N10 as it can be rationalized inspecting Figure 1 A: the two spectra exhibit the absorptions in the same wavenumber range, confirming that this vibration is active in both Raman and IR spectroscopies. Also another Raman study on early postage stamps of Mauritius11 has identified Prussian blue as the pigment used in the case of the two pence blue exemplar issued in 1847.
Figure 1. (A) FTIR-ATR and (B) Raman spectra of the blue stamp with denomination of 20 cent of the first issue of the Kingdom of Italy in 1862. In the inset, a microphotography of the examined blue exemplar. Getting on with years, Prussian blue was ascertained until 1957. Starting from 1958 blue printing ink was made with a copper phthalocyanine (CuPc). The Raman spectrum of the blue pigment of the specimen on the Universal Expo in
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Brussels of 1958 with a denomination of 60 Liras (Figure 2, A) exhibits a peculiar intense band at 1524 cm-1, corresponding to the deformation and breathing vibrations of the macrocycle.12 Also the band at 1451 cm-1 involves a deformation of the isoindole ring system and the strong signals at 684 and 749 cm-1 correspond to motions in which the coordinated nitrogen and the bridging nitrogens breathe symmetrically.13 The peak at 1338 cm-1 can be ascribed to aromatic ring vibration14 and the weak band at 236 cm-1 is due to the coordination with copper;6 these bands are very sensitive to the size of the central metal ion and are considered as ‘markers’ of crystal structure modifications.15 The group of copper phthalocyanines belongs to commercial blue pigments and they have been widely employed as dyes for many applications16
17
, because of their properties of excellent good solvent fastness, good dispersibility and
transparency. They are rather inexpensive pigments,6 this is the reason why this type of pigment was chosen and it is still used in blue printing ink on today postage stamps, as proved by the spectrum of the stamp issued in 2008 and reported in figure 2 B.
Figure 2. (A) Raman spectrum of the stamp with denomination of 60L on the Universal Expo in Brussels of 1958; (B) Raman spectrum of the polychrome stamp with denomination of 0,60 € issued in 2008. The enclosed image is a microphotography of the second specimen. 3.2 Red pigment
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The study on red printing inks has permitted the identification of the first red pigment chosen for the red Italian postage stamp in 1862. The Raman spectrum of the red stamp with denomination of 40c shows evidence for mercury sulfide pigment (HgS), known as Vermillion (otherwise Cinnabar), with bands at 350 and 260 cm-1, due to the trigonal mercury sulfide structure18. In Figure 3 A, its spectrum has been reported in comparison with the one obtained from a reference sample of Cinnabar powder. For the first issue of the Kingdom of Italy the same typology of stamps of the Kingdom of Sardinia was employed.
19
This is further substantiated by the spectrum of the red exemplar issued in 1851 with
denomination of 40c, containing the effigy of Vittorio Emanuele II: the same Cinnabar bands have been detected. Cinnabar has been utilized through the ages as a coloring material for ancient manuscripts. Also the application of this pigment on stamps was not rare; its employment was similarly discovered in red Britannia-type Mauritian exemplars during the same period of time (between 1858 and 1862). 11
Figure 3. (A) Raman spectra of red stamp (denomination of 40 cents) of the first issue of the Kingdom of Italy in 1862 (black) and of a reference sample of Cinnabar powder (red). In the inset, a microphotography of the same sample. (B) Raman spectrum of the red stamp of the Kingdom of Sardinia (denomination of 40 cents) issued in 1851. Carrying out the analyses, it was ascertained that on the following year Cinnabar was replaced by a red pigment composed of hematite (Fe2O3). The red stamp issued in 1863 (denomination of 2 cents) has displayed Raman signals
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in the region between 600 and 200 cm-1 (Figure 4, A). The bands at 615, 413 and 295 cm-1 are related to Fe-O symmetric bending and the signals at 248 and 229 cm-1 are assigned to Fe-O symmetric stretching.
20
The ratio
between the two principal bands at 295 and 229 cm-1 pinpoints the synthetic nature of this pigment, commonly known as Mars Red, a synthetic red iron oxide pigment produced in laboratory by the 18th century. The ratio between the two principal bands at 295 and 229 cm-1 points out the synthetic nature of this pigment, commonly known as Mars Red, a synthetic red iron oxide pigment produced in laboratory by the 18th century. A comparison between the Raman spectra reported in the literature3,21 of a natural Red Ochre and an artificial Mars Red pigment shows the variation of signal ratio between the peaks at 225 cm-1 and 290 cm-1. These kind of pigments showed all the properties, such as durability and permanence, of their natural counterparts. These kind of pigments showed all the properties, such as durability and permanence, of their natural counterparts. Cinnabar was an expensive pigment and its substitution with hematite could be probably attributed to an economic motivation.
22
Red ochre has been found in other red exemplars until 1913. Starting from that year different red ink
spectra have been recorded. The most intense bands in the Raman spectrum of the red stamp issued in 1913 (denomination of 2c superimposed onto 10c) are situated at 551, 470 ca, 393 and 316 cm-1 (Figure 4, B). The peak at 551 cm-1 is due to the PbO stretching vibrational mode and the other signals can be assigned to lead tetroxide (Pb3O4),23 identified as the pigment Minium, known since antiquity and employed in stamps printing inks of other countries.11
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Figure 4. (A) Raman spectrum of the specimen issued in 1863 with denomination of 2 cents; (B) Raman spectrum of the red exemplar issued in 1913 (denomination of 2 cents). A key change in the manufacture of red inks involves exemplars issued since 1923. Analyses on the painted area of the red postage stamp with denomination of 50c have evidenced Raman signals belonging to the azo pigments (Figure 5 A), the oldest and largest group of organic synthetic pigments. Following a protocol provided by Vandenabeele et al.,14 it has been possible to recognize the subclass of the azo pigment. It belongs to the naphthol AS pigments with fundamental vibration of azo pigments, the trans —N=N— group stretch at 1428 and 1398 cm-1; symmetrical bend is located at 1170 cm-1 and symmetrical stretching is at 1111 cm-1; a strong peak at 1592 cm-1 assigned to the benzene quadrant stretch and a band at 730 cm-1 assigned to the naphthalene mode are apparent.6 The strong peak at 1323 cm1
is related to the presence of an aromatic nitro group. 14 The 1926 red exemplar Raman spectrum (Figure 5, B) exhibits
different signals compared with the previous stamp of 1923, nonetheless its principal signals are distinctive of an azo pigment, too. So a band occurring at about 1400 cm-1 assigned to the N=N stretch and an aromatic stretch at around 1600 cm-1 can be observed.14 In this spectrum the two signals at 1616 and 1602 cm-1 are attributable to benzene stretch and the peak at 1449 cm-1 to the azo group stretch. Also the peak at 1346 cm-1 can be assigned to aromatic ring vibration.
14
Additionally, aromatic azo compounds in the trans form manifest a band at 1485 cm-1.24 The presence of
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the intense band at 723 cm-1, due to the existence of the naphthalene group and the signal at 605 cm-1, due to the aromatic ring deformation, allow associating the vibrational pattern to a specific azo compound class, the azo pigment lakes (β–naphthol type). Comparing this spectrum with the database supplied by Vandenabeele et al.
14
, a close
resemblance has been observed to PR 49:1 azo pigment. The first azo dyes were synthesized in 1861 and the first
-1
Figure 5. (A) 1500-450 cm region of the Raman spectrum of the red stamp with denomination of 50 cents issued in 1923; (B) Raman spectrum of the red exemplar issued in 1926 with denomination of 60c + 30c.
In another postage stamp of the same period, the red sample with denomination of 20c issued in 1936 (Figure 6 A), the Raman spectrum has led to the identification of an azo pigment belonging to the class of the naphthol AS pigments, likewise the case of the 1923 50c red stamp. For this sample the signals were pinpointed at 1607 and 1553 cm-1, attributed to the N=N group and naphthalene structure, respectively. The band at 727 cm-1 also matches a naphthalene mode. The peak at 1258 cm-1 is related to the amide III band, and can be considered a discriminant signal, because only the naphthol AS pigments have amide functionality in their structure.
14
The other bands at 1488, 1452 and 1344
cm-1 are ascribed to the same vibrational modes as the azo pigment lakes specimen illustrated above. Forward in
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group of azo pigments became popular at the beginning of the 20th century. 7
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years, all the spectra collected from red ink of postage stamps exhibit signals attributed sometimes to AS pigments and sometimes to azo pigment lakes, proving the contemporaneous employment of both the azo pigment classes, still attested nowadays. As a further confirmation of our inference, in figures 6B and 6C the spectra of a stamp issued in 1970 with denomination of 90L and of another one issued in 1982 with denomination of 900L are illustrated and are ascribable to naphtol AS pigments. On the other hand, the spectrum in Figure 6D, concerning a postage stamp issued in 2010 with denomination of 0.60 € is imputable to azo pigment lake. The structures of the azo pigment lakes and Naphthol AS pigments are available in Scheme 1, Supporting Information.
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Figure 6. Raman spectra of (A) the red stamp with denomination of 20cent issued in 1936; (B) the stamp circulated in 1970 (denomination of 90L); (C) the sample distributed in 1982 with the value of 900L; and (D) the specimen emitted in 2010 with denomination of 0.60 €. The inset in figure 6D contains the relative microphotography.
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Scheme 1. (A) Structure of the azo pigment lakes and (B) Naphthol AS pigments.
3.3 Orange pigment The first orange Italian stamp was produced in 1863 with the value of 10c and portrays King Vittorio Emanuele II. The FT-IR spectrum of this sample (Figure 7 A) displays a broad band centered on a narrow peak at 849 cm-1 related to the stretching of the CrO42- ion in lead chromate (PbCrO4).24 Consequently it has been possible to recognize the pigment Chrome orange (PbCrO4 • PbO) as the one responsible of the orange color, as it can be seen in Figure 7A which displays the FT-IR spectrum of the stamp and the one collected from the reference pigment Chrome orange. Performing the Raman analysis, the occurrence of the lead chromate was confirmed by the weak peak at 842 cm-1 produced by the symmetric stretching mode of the chromate anion; 227 cm-1 are distinctive of the hematite pigment, Mars Red.
20
25
in addition, the signals at 616, 500, 296, 247 and
Thus, it was possible to understand how orange color
was obtained. Besides Chrome orange, it was added the hematite pigment to the printing ink, the same pigment used to attain the red ink in those years. This information is even clearer by inspection of the stamp by optical microscopy; the picture clearly evidences the presence of orange and red particles. The same printing mixture was employed during the following years, as apparent in the spectrum of the orange stamp issued in 1877 with denomination of 20c, in which the chromate signal is more discernible.
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Figure 7. (A) FTIR-ATR investigations of the orange stamp of the Reign of Italy issued in 1863 with denomination of 10 cents; (B) Raman spectrum of the same exemplar; and (C) Raman spectrum of the stamp distributed in 1877 with the value of 20 cents. In the insets, microphotographies of the corresponding orange exemplars. In 1929 a remarkable variation in the orange printing ink was ascertained. An example is provided by the orange stamp with denomination of 1.75L (Figure 8, A) which displays the distinctive signals of the azo pigments at 1616, 1423 and 725 cm-1, ascribed to the aromatic ring quadrant stretching, to the N=N stretch and to the naphthalene mode, respectively.14 Comparing the spectrum with the ones reported in a database,14 it was found that the orange ink was
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composed of the azo pigment lake red 49:1, exactly the same pigment employed in the manufacturing of the red exemplar emitted in 1925 with denomination of 60c + 30c. Therefore, as observed for the red inks and in the same historical period, also for the orange production an organic synthetic pigment was preferred. Moreover, a red tint was used in order to obtain an orange color, just as it was made earlier when the chrome orange was chosen in mixture with the iron red pigment. Orange inks were produced with azo pigments during successive decades as confirmed by the Raman spectra of several postage stamps. The Raman spectrum of the orange stamp produced in 1962 with value of 30L (reported in Figure 8, B) shows peaks typical of the naphtol AS pigments at 1610, 1445, 1341 and 838 cm-1.14
Figure 8. Raman spectra of: (A) the orange stamp issued in 1929 with denomination of 1.75 L; (B) the sample supplied in 1962 with the value of 30L. Still nowadays the same kind of inks is commonly used for both red and orange color inks. The fastness and the durability of these pigments are greater than the common natural pigments employed before. Indeed, the resistance to light, heat, solvents and other external agents is improved, making the printing ink on the stamps a more resisting coating. 26
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3.4 Green pigment
Examining green printing inks on the stamps of the first issues, signals from the Prussian blue and the Chrome orange pigments were found in the FT-IR spectra. As it can be seen in the spectrum of the green stamp circulating from 1895 with denomination of 45c (Figure 9 A), the typical peak at 2093 cm-1 is due to the C≡N group10 and the band at around 851 cm-1 can be attributed to the lead chromate anion CrO42-.
24
This information prompted us to suppose that, in order
to produce a green ink, it has been used a mixture of the blue and orange pigments. Raman analyses on coeval green stamps have allowed achieving an insight on the printing ink mixture by means of the 100x microscope. It has been possible to localize small particles, blue and orange colored. Pointing the laser on each particle, the Raman spectra have manifested the presence of Prussian blue10 and Chrome orange25 (Figure 9, B and C), confirming the previous
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hypothesis.
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Figure 9. (A) FTIR-ATR spectrum of the green stamp of the Kingdom of Italy issued in 1895 with denomination of 45 cents. Raman spectra of the same exemplar registered pointing the laser beam on the blue (B) and on the orange (C) particles; in the (B) and (C) sections the signals of Prussian blue and Chrome orange pigments are apparent, respectively. Going forward with time, it was found that the composition of the green ink changed in 1958. The Raman spectrum indicated the presence of copper phthalocyanine (CuPc).6 It is exactly the same kind of synthetic pigment employed for
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the blue printing inks on postage stamps, used starting from that year. The spectra in Figure 10 show the 15L green stamp and the blue areas of the 25L polychromatic stamp, both issued in 1958. The principal peaks at 1530 cm-1, ascribed to the N=N stretch, and at 753 and 686 cm-1, assigned to the breathing vibration of the macrocycle structure of the phthalocyanine, match.6 Copper phthalocyanines are the principal blue and green organic pigments used in printing inks as well as in the coloration of plastic and artists' paints.27,28 The same kind of pigments is in use still today in modern postage stamps for both the blue and the green inks, as it is clearly noticeable in Figure 10 B, where blue and green inks from modern stamps have been compared.
Figure 10. (A) Raman spectra corresponding to the green stamp with denomination of 15 L and to the blue areas of the polychromatic stamp with value of 25 L, both distributed in 1958. (B) In blue the Raman spectrum of the stamp emitted in 1978 (value of 170 L) registered on the blue particles and in green the Raman spectrum of the stamp produced in 1980 (denomination of 80 L) registered on the green particles. The analyses testified evidence that this technique is valuable, fast and non-destructive in postage stamps inks evaluation. All these results have been briefed into a time line which reports the changes observed in stamp printing inks since 1862 until nowadays (Scheme 2). The aim of the suggested timeline is to easily identify the type of pigments
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employed in the stamps printing inks. It will allow dating postage stamps and determining the originality or the forgery of a given exemplar.
Scheme 2. Time line representation of principal technical innovations in the manufacturing of stamps printing inks, relatively to (a) blue pigments, (b) red pigments, (c) orange pigments and (d) green pigments used in the course of time since the unification of Italy. 1.
EXPERIMENTAL
Samples were analyzed by a Horiba XploRA micro-Raman spectrometer coupled to an Olympus BX-41 confocal microscope. The excitation source was a 785 nm (90-100 mW) laser. Its power at the sample surface was reduced to a value between 0.25 and 0.001 mW, by 25% and up to 0,1% maximum neutral density filters, depending on the type of samples. The estimated laser spot size was 1,064 µm. To prevent any alteration of the samples, the measurements were performed with a laser power density lower than 150 W/cm2. The Raman signal was collected over the range 200–2000 cm-1 using a 100x microscope objective. Some postage stamps were subjected to FT-IR analyses using a Varian 600-IR series instrument in ATR mode (the internal reflection element is composed of zinc selenide) for all the FT-IR measurements. The entire different types of stamp printing inks were analyzed. Four principal colors were chosen whose applications and evolution were followed through time, basing on their recurrence: blue, red, orange and green. Postage stamps were placed under the Raman microscope, no sample preparation was necessary. FT-IR
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analyses were performed placing the exemplars directly onto the ATR element. An optimal contact between the surface of the sample and the ATR crystal was ensured by means of a piston.8 The number of all the examined exemplars, grouped in different years, has been reported in the table below. This wide period of time covered a lot of post issues, thus samples were selected in order to inspect a specific typology of printing ink, relative to a particular year of production. In some cases indeed, a minor number of samples were inspected because of the repetitiveness of the same components observed in today’s exemplars. A full version of the table with the number of stamps examined from each year listed is contained in the supporting information. Number of analyzed Issue Course
specimens
1862 – 1889
13
1891 – 1922
11
1923– 1929
11
1929 – 1942
17
1945 – 1960
14
1962 – 1970
20
1973 – 1980
13
1981 – 1992
16
2008 – 2011
8
CONCLUSIONS Blue-, red-, orange- and green-colored Italian stamps have been investigated proceeding chronologically throughout the Italian history. All the collected spectra allow knowing the exact sequence of ink choices. Likewise the paper component and glues studied in our previous work 8, it has been achieved a time mapping of the printing inks employed during 150 years of history. Every stamp was characterized by means of Raman spectroscopy and no alteration to the specimens was caused. Different kinds of pigments and dyes have been clearly identified. This work allows a deep knowledge about any change made in the ink mixture. For example, the analyses showed that the first Italian blue stamps in 1862 were printed using Prussian blue. In its place phthalocyanine ink was introduced in 1958. Also green exemplars exhibit Prussian blue, in early productions and phthalocyanine inks in modern stamps, together with orange pigments, such as chrome orange. This last ink was employed to obtain orange color and has been substituted by azo pigments in contemporary times. This research also could be useful in the detection of forgery exemplars. The
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advantage of our study consists in the comprehension that a really fast and non-destructive methods of investigation allows the realization of a spectral library of the inks employed in the stamp manufacture.
AUTHOR INFORMATION Corresponding Author *Gabriele Giancane, Department of Cultural Heritage, University of Salento, Via D. Birago, 73100, Lecce, Italy E-mail: [email protected] Tel.: +39 0832 29 7372 - 7246
ACKNOWLEDGMENT This research was supported by the Projects PON 254/Ric. Potenziamento del “CENTRO RICERCHE PER LA SALUTE DELL'UOMO DELL'AMBIENTE” Cod. PONa3_00334, PRIN 2012 Nanostrutture gerarchiche fotosintetiche per la produzione di
energia
and
the
Project
PON
S.I.Mi.S.A.
Cod.
PON02_00186_3417512.,
Project
PON
Pro.Ali.Fun
Cod.
PON02_00186_2937475, by Regione Puglia, Costituzione di Reti di Laboratori Pubblici di Ricerca, Progetto Esecutivo 09, WAFITECH.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
R. J. Rice and R. L. McCreery, Analytical Chemistry, 1989, 61, 1637-1641. C. Fotakis, D. Anglos, V. Zafiropulos, S. Georgiou and V. Tornari, Lasers in the preservation of cultural heritage: Principles and applications, CRC Press, 2012. I. M. Bell, R. J. Clark and P. J. Gibbs, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 1997, 53, 2159-2179. M. Capasso, V. Filieri, G. Giancane, N. Pellè and L. Valli, Papyrologica Lupiensia, 2009, 18, 37-54. M. Capasso, icon, 2005, 39, 256011. M. d. R. Marcelino and V. S. Muralha, Journal of Raman Spectroscopy, 2012, 43, 1281-1292. H. Zollinger, Color chemistry: syntheses, properties, and applications of organic dyes and pigments, Wiley. com, 2003. E. Imperio, G. Giancane and L. Valli, Analytical chemistry, 2013, 85, 7085-7093. N. Ferrer and A. Vila, Analytica chimica acta, 2006, 555, 161-166. P. Larkin, Infrared and Raman spectroscopy; principles and spectral interpretation, Elsevier, 2011. T. D. Chaplin, A. Jurado‐López, R. J. Clark and D. R. Beech, Journal of Raman Spectroscopy, 2004, 35, 600-604. C. Jennings, R. Aroca, A. M. Hor and R. O. Loutfy, Analytical Chemistry, 1984, 56, 2033-2035. A. Bovill, A. McConnell, J. Nimmo and W. Smith, The Journal of Physical Chemistry, 1986, 90, 569-575. P. Vandenabeele, L. Moens, H. G. Edwards and R. Dams, Journal of Raman Spectroscopy, 2000, 31, 509-517. T. V. Basova, V. G. Kiselev, B. E. Schuster, H. Peisert and T. Chassé, Journal of Raman Spectroscopy, 2009, 40, 2080-2087. L. Giotta, G. Giancane, D. Mastrogiacomo, T. Basova, P. Metrangolo and L. Valli, Physical Chemistry Chemical Physics, 2009, 11, 2161-2165. W. Herbst and K. Hunger, Industrial organic pigments, Wiley. com, 2006. H. Riccius and K. Siemsen, The Journal of Chemical Physics, 1970, 52, 4090.
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19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
F. Zeri, Francobolli italiani, Skira, 2006. M. Legodi and D. De Waal, Dyes and Pigments, 2007, 74, 161-168. A. M. Jubb and H. C. Allen, ACS Applied Materials & Interfaces, 2010, 2, 2804-2812. H. Edwards, D. Farwell, E. Newton, F. Rull Perez and S. Jorge Villar, Journal of Raman Spectroscopy, 2000, 31, 407-413. K. Castro, M. Rodríguez‐Laso, L. Fernandez and J. Madariaga, Journal of Raman Spectroscopy, 2002, 33, 17-25. G. Socrates and G. Socrates, Infrared and Raman characteristic group frequencies: tables and charts, Wiley Chichester, 2001. R. L. Frost, Journal of Raman Spectroscopy, 2004, 35, 153-158. J. V. Koleske, Paint and coating testing manual, ASTM International, 1995. R. Marchessault, Pure and Applied Chemistry, 1962, 5, 107-130. I. Capron, P. Robert, P. Colonna, M. Brogly and V. Planchot, Carbohydrate Polymers, 2007, 68, 249-259.
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Spectral characterization of postage stamps printing inks by means of Raman spectroscopy Eleonora Imperio[b],Gabriele Giancane* [c] and Ludovico Valli [a]. [a]
Department of Biological and Environmental Science and Technologies, Università del Salento, Via
Monteroni, 73100 Lecce (Italy) [b]
Department of Innovation Engineering, Università del Salento, Via Monteroni, 73100 Lecce (Italy)
[c]
Department of Cultural Heritage, Università del Salento, Via D. Birago, 73100 Lecce (Italy) E-mail:
[email protected]
Table of Contents The instrumental apparatus used, by which it has been achieved a completely non-destructive analysis by the ATR-FTIR spectrometer and the filtered laser beam, focalized by the Raman microscope objective lens.
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ABSTRACT: A time travel through 150 years of Italian postage stamp issues has allowed defining the evolution with time of the recurring pigments in stamp designs by means of Raman and FTIR spectroscopy. Numerous exemplars have been analyzed, covering the whole production of stamps for both Italian Kingdom and Republic. Raman and FTIR spectra showed the changeover from Prussian blue to copper phthalocyanine inks in 1958. Also the entire succession for red inks was recognized to develop from the original cinnabar to red ochre and minium, and finally to red azo pigments. The changes in the orange printing ink proceeded on a similar path; the first orange Italian exemplar was printed employing a mixture of chrome orange and red ochre. In 1929 this combination was replaced by azo pigments. Green stamps belonging to the first issues instead entailed the choice of blue and orange inks, namely chrome orange and Prussian blue; later on an ink composed mainly of phthalocyanine was employed as the green dye. The merger of data coming from Raman microscopy and FTIR-ATR spectroscopy, both non-destructive techniques, has allowed the characterization of stamp designs and potentially provides direct and fast evidence for the recognition of forged exemplars. 1.
INTRODUCTION
The history of materials employed in the manufacture of stamps is still lacking information since a minor part of the world production has been investigated up to now. The knowledge of the employed substances is not based on analytical confirmations. Moreover the identification of pigments in the stamps is fundamental with the aim of clarifying particular doubts about conservation or counterfeiting. The aim of this paper is to identify the pigments used in the production of the Italian stamps throughout their overall history. Up to our knowledge, such a complete and thorough examination, covering more than 150 years, has not been reported yet. The characterization of constituents involved in the preparation of this complex class of composite materials - i.e. the stamps – is the starting point for the realization of a database on the substances employed in stamp production and the creation of a “time-line” for the rapid and unambiguous identification of counterfeit samples. Hence it is essential to use rigorous techniques of analysis that exploit every method to the uttermost. This comprehension is not immediate as an aftereffect of the great intricacy of the investigated matrices, the materials aging and the probable blending with incidental substances. Moreover this extractionless and non-invasive approach is notably suited for samples onto paper, parchment or raw-materials when a very thin layer of pigmentation has been immobilized onto the substrate.1,2,3 So Raman spectroscopy has been profitably employed to investigate natural pigments and many of the XIX and XX century synthetic colorants.4,5 Finally, in the last years some spectral databases of organic pigments have been realized, even though they need to be remarkably completed and up-dated.6, 7 Our whole research consists of two stages; in fact, in a previous communication the same complete collection of Italian stamps was characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) mapping8. Then in the second step, which is the objective of the present research, Raman
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spectroscopy has been employed in order to investigate the printing inks used in the overall production of stamps in the Italian Kingdom and Republic.
2.
RESULTS AND DISCUSSION
The approach employed during the whole research is the following: whenever any change in the ink composition was observed, the analyses kept on backward in time to identify the first exemplar exhibiting the same variation. With this method, it has been possible to identify the exact year of introduction of the innovation. 3.1 Blue pigment The FT-IR spectrum of the blue sample with a denomination of 20c, part of the first Italian issue in 1862, displays an intense peak at 2083 cm-1 (Figure 1, A). This signal is attributed to the characteristic stretching vibration of the triple bond in cyanide group (C≡N), belonging to the ferric ferrocyanide compound (Fe4{Fe(CN)6}3), commonly known as Prussian blue.9 Raman analysis confirms the presence of such a pigment, as visible from the spectrum in figure 1 B, in which two signals appear, at 2160 cm-1 and 2098 cm-1. They are both contribution of the same functional group, C≡N10 as it can be rationalized inspecting Figure 1 A: the two spectra exhibit the absorptions in the same wavenumber range, confirming that this vibration is active in both Raman and IR spectroscopies. Also another Raman study on early postage stamps of Mauritius11 has identified Prussian blue as the pigment used in the case of the two pence blue exemplar issued in 1847.
Figure 1. (A) FTIR-ATR and (B) Raman spectra of the blue stamp with denomination of 20 cent of the first issue of the Kingdom of Italy in 1862. In the inset, a microphotography of the examined blue exemplar. Getting on with years, Prussian blue was ascertained until 1957. Starting from 1958 blue printing ink was made with a copper phthalocyanine (CuPc). The Raman spectrum of the blue pigment of the specimen on the Universal Expo in
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Brussels of 1958 with a denomination of 60 Liras (Figure 2, A) exhibits a peculiar intense band at 1524 cm-1, corresponding to the deformation and breathing vibrations of the macrocycle.12 Also the band at 1451 cm-1 involves a deformation of the isoindole ring system and the strong signals at 684 and 749 cm-1 correspond to motions in which the coordinated nitrogen and the bridging nitrogens breathe symmetrically.13 The peak at 1338 cm-1 can be ascribed to aromatic ring vibration14 and the weak band at 236 cm-1 is due to the coordination with copper;6 these bands are very sensitive to the size of the central metal ion and are considered as ‘markers’ of crystal structure modifications.15 The group of copper phthalocyanines belongs to commercial blue pigments and they have been widely employed as dyes for many applications16
17
, because of their properties of excellent good solvent fastness, good dispersibility and
transparency. They are rather inexpensive pigments,6 this is the reason why this type of pigment was chosen and it is still used in blue printing ink on today postage stamps, as proved by the spectrum of the stamp issued in 2008 and reported in figure 2 B.
Figure 2. (A) Raman spectrum of the stamp with denomination of 60L on the Universal Expo in Brussels of 1958; (B) Raman spectrum of the polychrome stamp with denomination of 0,60 € issued in 2008. The enclosed image is a microphotography of the second specimen. 3.2 Red pigment
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The study on red printing inks has permitted the identification of the first red pigment chosen for the red Italian postage stamp in 1862. The Raman spectrum of the red stamp with denomination of 40c shows evidence for mercury sulfide pigment (HgS), known as Vermillion (otherwise Cinnabar), with bands at 350 and 260 cm-1, due to the trigonal mercury sulfide structure18. In Figure 3 A, its spectrum has been reported in comparison with the one obtained from a reference sample of Cinnabar powder. For the first issue of the Kingdom of Italy the same typology of stamps of the Kingdom of Sardinia was employed.
19
This is further substantiated by the spectrum of the red exemplar issued in 1851 with
denomination of 40c, containing the effigy of Vittorio Emanuele II: the same Cinnabar bands have been detected. Cinnabar has been utilized through the ages as a coloring material for ancient manuscripts. Also the application of this pigment on stamps was not rare; its employment was similarly discovered in red Britannia-type Mauritian exemplars during the same period of time (between 1858 and 1862). 11
Figure 3. (A) Raman spectra of red stamp (denomination of 40 cents) of the first issue of the Kingdom of Italy in 1862 (black) and of a reference sample of Cinnabar powder (red). In the inset, a microphotography of the same sample. (B) Raman spectrum of the red stamp of the Kingdom of Sardinia (denomination of 40 cents) issued in 1851. Carrying out the analyses, it was ascertained that on the following year Cinnabar was replaced by a red pigment composed of hematite (Fe2O3). The red stamp issued in 1863 (denomination of 2 cents) has displayed Raman signals
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in the region between 600 and 200 cm-1 (Figure 4, A). The bands at 615, 413 and 295 cm-1 are related to Fe-O symmetric bending and the signals at 248 and 229 cm-1 are assigned to Fe-O symmetric stretching.
20
The ratio
between the two principal bands at 295 and 229 cm-1 pinpoints the synthetic nature of this pigment, commonly known as Mars Red, a synthetic red iron oxide pigment produced in laboratory by the 18th century. The ratio between the two principal bands at 295 and 229 cm-1 points out the synthetic nature of this pigment, commonly known as Mars Red, a synthetic red iron oxide pigment produced in laboratory by the 18th century. A comparison between the Raman spectra reported in the literature3,21 of a natural Red Ochre and an artificial Mars Red pigment shows the variation of signal ratio between the peaks at 225 cm-1 and 290 cm-1. These kind of pigments showed all the properties, such as durability and permanence, of their natural counterparts. These kind of pigments showed all the properties, such as durability and permanence, of their natural counterparts. Cinnabar was an expensive pigment and its substitution with hematite could be probably attributed to an economic motivation.
22
Red ochre has been found in other red exemplars until 1913. Starting from that year different red ink
spectra have been recorded. The most intense bands in the Raman spectrum of the red stamp issued in 1913 (denomination of 2c superimposed onto 10c) are situated at 551, 470 ca, 393 and 316 cm-1 (Figure 4, B). The peak at 551 cm-1 is due to the PbO stretching vibrational mode and the other signals can be assigned to lead tetroxide (Pb3O4),23 identified as the pigment Minium, known since antiquity and employed in stamps printing inks of other countries.11
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Figure 4. (A) Raman spectrum of the specimen issued in 1863 with denomination of 2 cents; (B) Raman spectrum of the red exemplar issued in 1913 (denomination of 2 cents). A key change in the manufacture of red inks involves exemplars issued since 1923. Analyses on the painted area of the red postage stamp with denomination of 50c have evidenced Raman signals belonging to the azo pigments (Figure 5 A), the oldest and largest group of organic synthetic pigments. Following a protocol provided by Vandenabeele et al.,14 it has been possible to recognize the subclass of the azo pigment. It belongs to the naphthol AS pigments with fundamental vibration of azo pigments, the trans —N=N— group stretch at 1428 and 1398 cm-1; symmetrical bend is located at 1170 cm-1 and symmetrical stretching is at 1111 cm-1; a strong peak at 1592 cm-1 assigned to the benzene quadrant stretch and a band at 730 cm-1 assigned to the naphthalene mode are apparent.6 The strong peak at 1323 cm1
is related to the presence of an aromatic nitro group. 14 The 1926 red exemplar Raman spectrum (Figure 5, B) exhibits
different signals compared with the previous stamp of 1923, nonetheless its principal signals are distinctive of an azo pigment, too. So a band occurring at about 1400 cm-1 assigned to the N=N stretch and an aromatic stretch at around 1600 cm-1 can be observed.14 In this spectrum the two signals at 1616 and 1602 cm-1 are attributable to benzene stretch and the peak at 1449 cm-1 to the azo group stretch. Also the peak at 1346 cm-1 can be assigned to aromatic ring vibration.
14
Additionally, aromatic azo compounds in the trans form manifest a band at 1485 cm-1.24 The presence of
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the intense band at 723 cm-1, due to the existence of the naphthalene group and the signal at 605 cm-1, due to the aromatic ring deformation, allow associating the vibrational pattern to a specific azo compound class, the azo pigment lakes (β–naphthol type). Comparing this spectrum with the database supplied by Vandenabeele et al.
14
, a close
resemblance has been observed to PR 49:1 azo pigment. The first azo dyes were synthesized in 1861 and the first
-1
Figure 5. (A) 1500-450 cm region of the Raman spectrum of the red stamp with denomination of 50 cents issued in 1923; (B) Raman spectrum of the red exemplar issued in 1926 with denomination of 60c + 30c.
In another postage stamp of the same period, the red sample with denomination of 20c issued in 1936 (Figure 6 A), the Raman spectrum has led to the identification of an azo pigment belonging to the class of the naphthol AS pigments, likewise the case of the 1923 50c red stamp. For this sample the signals were pinpointed at 1607 and 1553 cm-1, attributed to the N=N group and naphthalene structure, respectively. The band at 727 cm-1 also matches a naphthalene mode. The peak at 1258 cm-1 is related to the amide III band, and can be considered a discriminant signal, because only the naphthol AS pigments have amide functionality in their structure.
14
The other bands at 1488, 1452 and 1344
cm-1 are ascribed to the same vibrational modes as the azo pigment lakes specimen illustrated above. Forward in
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group of azo pigments became popular at the beginning of the 20th century. 7
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years, all the spectra collected from red ink of postage stamps exhibit signals attributed sometimes to AS pigments and sometimes to azo pigment lakes, proving the contemporaneous employment of both the azo pigment classes, still attested nowadays. As a further confirmation of our inference, in figures 6B and 6C the spectra of a stamp issued in 1970 with denomination of 90L and of another one issued in 1982 with denomination of 900L are illustrated and are ascribable to naphtol AS pigments. On the other hand, the spectrum in Figure 6D, concerning a postage stamp issued in 2010 with denomination of 0.60 € is imputable to azo pigment lake. The structures of the azo pigment lakes and Naphthol AS pigments are available in Scheme 1, Supporting Information.
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Figure 6. Raman spectra of (A) the red stamp with denomination of 20cent issued in 1936; (B) the stamp circulated in 1970 (denomination of 90L); (C) the sample distributed in 1982 with the value of 900L; and (D) the specimen emitted in 2010 with denomination of 0.60 €. The inset in figure 6D contains the relative microphotography.
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Scheme 1. (A) Structure of the azo pigment lakes and (B) Naphthol AS pigments.
3.3 Orange pigment The first orange Italian stamp was produced in 1863 with the value of 10c and portrays King Vittorio Emanuele II. The FT-IR spectrum of this sample (Figure 7 A) displays a broad band centered on a narrow peak at 849 cm-1 related to the stretching of the CrO42- ion in lead chromate (PbCrO4).24 Consequently it has been possible to recognize the pigment Chrome orange (PbCrO4 • PbO) as the one responsible of the orange color, as it can be seen in Figure 7A which displays the FT-IR spectrum of the stamp and the one collected from the reference pigment Chrome orange. Performing the Raman analysis, the occurrence of the lead chromate was confirmed by the weak peak at 842 cm-1 produced by the symmetric stretching mode of the chromate anion; 227 cm-1 are distinctive of the hematite pigment, Mars Red.
20
25
in addition, the signals at 616, 500, 296, 247 and
Thus, it was possible to understand how orange color
was obtained. Besides Chrome orange, it was added the hematite pigment to the printing ink, the same pigment used to attain the red ink in those years. This information is even clearer by inspection of the stamp by optical microscopy; the picture clearly evidences the presence of orange and red particles. The same printing mixture was employed during the following years, as apparent in the spectrum of the orange stamp issued in 1877 with denomination of 20c, in which the chromate signal is more discernible.
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Figure 7. (A) FTIR-ATR investigations of the orange stamp of the Reign of Italy issued in 1863 with denomination of 10 cents; (B) Raman spectrum of the same exemplar; and (C) Raman spectrum of the stamp distributed in 1877 with the value of 20 cents. In the insets, microphotographies of the corresponding orange exemplars. In 1929 a remarkable variation in the orange printing ink was ascertained. An example is provided by the orange stamp with denomination of 1.75L (Figure 8, A) which displays the distinctive signals of the azo pigments at 1616, 1423 and 725 cm-1, ascribed to the aromatic ring quadrant stretching, to the N=N stretch and to the naphthalene mode, respectively.14 Comparing the spectrum with the ones reported in a database,14 it was found that the orange ink was
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composed of the azo pigment lake red 49:1, exactly the same pigment employed in the manufacturing of the red exemplar emitted in 1925 with denomination of 60c + 30c. Therefore, as observed for the red inks and in the same historical period, also for the orange production an organic synthetic pigment was preferred. Moreover, a red tint was used in order to obtain an orange color, just as it was made earlier when the chrome orange was chosen in mixture with the iron red pigment. Orange inks were produced with azo pigments during successive decades as confirmed by the Raman spectra of several postage stamps. The Raman spectrum of the orange stamp produced in 1962 with value of 30L (reported in Figure 8, B) shows peaks typical of the naphtol AS pigments at 1610, 1445, 1341 and 838 cm-1.14
Figure 8. Raman spectra of: (A) the orange stamp issued in 1929 with denomination of 1.75 L; (B) the sample supplied in 1962 with the value of 30L. Still nowadays the same kind of inks is commonly used for both red and orange color inks. The fastness and the durability of these pigments are greater than the common natural pigments employed before. Indeed, the resistance to light, heat, solvents and other external agents is improved, making the printing ink on the stamps a more resisting coating. 26
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3.4 Green pigment
Examining green printing inks on the stamps of the first issues, signals from the Prussian blue and the Chrome orange pigments were found in the FT-IR spectra. As it can be seen in the spectrum of the green stamp circulating from 1895 with denomination of 45c (Figure 9 A), the typical peak at 2093 cm-1 is due to the C≡N group10 and the band at around 851 cm-1 can be attributed to the lead chromate anion CrO42-.
24
This information prompted us to suppose that, in order
to produce a green ink, it has been used a mixture of the blue and orange pigments. Raman analyses on coeval green stamps have allowed achieving an insight on the printing ink mixture by means of the 100x microscope. It has been possible to localize small particles, blue and orange colored. Pointing the laser on each particle, the Raman spectra have manifested the presence of Prussian blue10 and Chrome orange25 (Figure 9, B and C), confirming the previous
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hypothesis.
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Figure 9. (A) FTIR-ATR spectrum of the green stamp of the Kingdom of Italy issued in 1895 with denomination of 45 cents. Raman spectra of the same exemplar registered pointing the laser beam on the blue (B) and on the orange (C) particles; in the (B) and (C) sections the signals of Prussian blue and Chrome orange pigments are apparent, respectively. Going forward with time, it was found that the composition of the green ink changed in 1958. The Raman spectrum indicated the presence of copper phthalocyanine (CuPc).6 It is exactly the same kind of synthetic pigment employed for
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the blue printing inks on postage stamps, used starting from that year. The spectra in Figure 10 show the 15L green stamp and the blue areas of the 25L polychromatic stamp, both issued in 1958. The principal peaks at 1530 cm-1, ascribed to the N=N stretch, and at 753 and 686 cm-1, assigned to the breathing vibration of the macrocycle structure of the phthalocyanine, match.6 Copper phthalocyanines are the principal blue and green organic pigments used in printing inks as well as in the coloration of plastic and artists' paints.27,28 The same kind of pigments is in use still today in modern postage stamps for both the blue and the green inks, as it is clearly noticeable in Figure 10 B, where blue and green inks from modern stamps have been compared.
Figure 10. (A) Raman spectra corresponding to the green stamp with denomination of 15 L and to the blue areas of the polychromatic stamp with value of 25 L, both distributed in 1958. (B) In blue the Raman spectrum of the stamp emitted in 1978 (value of 170 L) registered on the blue particles and in green the Raman spectrum of the stamp produced in 1980 (denomination of 80 L) registered on the green particles. The analyses testified evidence that this technique is valuable, fast and non-destructive in postage stamps inks evaluation. All these results have been briefed into a time line which reports the changes observed in stamp printing inks since 1862 until nowadays (Scheme 2). The aim of the suggested timeline is to easily identify the type of pigments
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employed in the stamps printing inks. It will allow dating postage stamps and determining the originality or the forgery of a given exemplar.
Scheme 2. Time line representation of principal technical innovations in the manufacturing of stamps printing inks, relatively to (a) blue pigments, (b) red pigments, (c) orange pigments and (d) green pigments used in the course of time since the unification of Italy. 1.
EXPERIMENTAL
Samples were analyzed by a Horiba XploRA micro-Raman spectrometer coupled to an Olympus BX-41 confocal microscope. The excitation source was a 785 nm (90-100 mW) laser. Its power at the sample surface was reduced to a value between 0.25 and 0.001 mW, by 25% and up to 0,1% maximum neutral density filters, depending on the type of samples. The estimated laser spot size was 1,064 µm. To prevent any alteration of the samples, the measurements were performed with a laser power density lower than 150 W/cm2. The Raman signal was collected over the range 200–2000 cm-1 using a 100x microscope objective. Some postage stamps were subjected to FT-IR analyses using a Varian 600-IR series instrument in ATR mode (the internal reflection element is composed of zinc selenide) for all the FT-IR measurements. The entire different types of stamp printing inks were analyzed. Four principal colors were chosen whose applications and evolution were followed through time, basing on their recurrence: blue, red, orange and green. Postage stamps were placed under the Raman microscope, no sample preparation was necessary. FT-IR
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analyses were performed placing the exemplars directly onto the ATR element. An optimal contact between the surface of the sample and the ATR crystal was ensured by means of a piston.8 The number of all the examined exemplars, grouped in different years, has been reported in the table below. This wide period of time covered a lot of post issues, thus samples were selected in order to inspect a specific typology of printing ink, relative to a particular year of production. In some cases indeed, a minor number of samples were inspected because of the repetitiveness of the same components observed in today’s exemplars. A full version of the table with the number of stamps examined from each year listed is contained in the supporting information. Number of analyzed Issue Course
specimens
1862 – 1889
13
1891 – 1922
11
1923– 1929
11
1929 – 1942
17
1945 – 1960
14
1962 – 1970
20
1973 – 1980
13
1981 – 1992
16
2008 – 2011
8
CONCLUSIONS Blue-, red-, orange- and green-colored Italian stamps have been investigated proceeding chronologically throughout the Italian history. All the collected spectra allow knowing the exact sequence of ink choices. Likewise the paper component and glues studied in our previous work 8, it has been achieved a time mapping of the printing inks employed during 150 years of history. Every stamp was characterized by means of Raman spectroscopy and no alteration to the specimens was caused. Different kinds of pigments and dyes have been clearly identified. This work allows a deep knowledge about any change made in the ink mixture. For example, the analyses showed that the first Italian blue stamps in 1862 were printed using Prussian blue. In its place phthalocyanine ink was introduced in 1958. Also green exemplars exhibit Prussian blue, in early productions and phthalocyanine inks in modern stamps, together with orange pigments, such as chrome orange. This last ink was employed to obtain orange color and has been substituted by azo pigments in contemporary times. This research also could be useful in the detection of forgery exemplars. The
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advantage of our study consists in the comprehension that a really fast and non-destructive methods of investigation allows the realization of a spectral library of the inks employed in the stamp manufacture.
AUTHOR INFORMATION Corresponding Author *Gabriele Giancane, Department of Cultural Heritage, University of Salento, Via D. Birago, 73100, Lecce, Italy E-mail: [email protected] Tel.: +39 0832 29 7372 - 7246
ACKNOWLEDGMENT This research was supported by the Projects PON 254/Ric. Potenziamento del “CENTRO RICERCHE PER LA SALUTE DELL'UOMO DELL'AMBIENTE” Cod. PONa3_00334, PRIN 2012 Nanostrutture gerarchiche fotosintetiche per la produzione di
energia
and
the
Project
PON
S.I.Mi.S.A.
Cod.
PON02_00186_3417512.,
Project
PON
Pro.Ali.Fun
Cod.
PON02_00186_2937475, by Regione Puglia, Costituzione di Reti di Laboratori Pubblici di Ricerca, Progetto Esecutivo 09, WAFITECH.
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DOI: 10.1039/C4AN01616E
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