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Fourier transform infrared spectroscopy applied to ink characterization of one-penny postage stamps printed 1841–1880
Analytica Chimica Acta 555 (2006) 161–166
Fourier transform infrared spectroscopy applied to ink characterization of one-penny postage stamps printed 1841–1880 N´uria Ferrer ∗ , Anna Vila Serveis Cientificot`ecnics, University of Barcelona, Lluis Sole Sabaris, 1, 08028 Barcelona, Spain Received 5 May 2005; received in revised form 25 August 2005; accepted 25 August 2005 Available online 10 October 2005
Abstract Fourier transform infrared spectroscopy (FTIR) was applied to the characterization of red inks used to print one-penny stamps in Britain during the period 1841–1880. Micro- and macro-accessories were compared in both transmission and reflection mode. The best spectra were obtained using a microscope with a diamond cell coupled to an infrared spectrometer. In this case, the extraction of a single paper fibre was enough to obtain a good spectrum. Calcium carbonate, calcium sulfate, lead chromate, cyano compounds, cellulose, and oil were identified. When bigger surfaces need to be analyzed, diffuse reflection or variable specular reflectance accessories can be used. Nevertheless, only cyano compounds and calcium carbonate can be characterized. Results are in agreement with those obtained using scanning electron microscopy. The chronological order of the 71 stamps studied showed that the chemical composition of the red ink changed during the period studied. © 2005 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; Characterization of inks; Composition of printing inks in stamps; “One-penny” stamps (GB)
1. Introduction Application of atomic and molecular spectroscopy analysis to solve problems related to cultural heritage has increased dramatically in the last few years. One reason is that restorers, artists and other people working in the field of the fine arts have become aware of and proficient in techniques of instrumental analysis. In addition, the demand for these kinds of analysis has increasingly involved spectroscopists and scientists in general in the study and development of new methodologies applied to samples from museums, archives, private collections, and other institutions. Because artworks are, in many cases, unique and valuable, stringent restrictions are required before removing even small amounts of material. Therefore, specific methodologies must be applied in order to minimize the amount of sample to be used and, consequently, to avoid any damage to the artwork. Other possibilities involve methods that do not damage the sample at all.
∗
Corresponding author. Tel.: +34 93 4021346. E-mail address: [email protected] (N. Ferrer).
0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.08.080
Not all methodologies are equally suitable for all samples, and comparative studies are required to identify the most appropriate in each case. In our laboratory, different samples, taken for the purpose of examining problems of conservation, restoration or fraud, have been analyzed in the last decade: inks and papers on manuscripts, pigments and dies on paintings, inks on engravings and plates, ceramics, textiles, and also polymers used by restorers. Analysis of inks on papers has the handicap of the strong absorption of the matrix. A spectrum of cellulose shows very intensive bands in the fingerprint region; therefore, it is necessary to try to remove particles where fibres are not present. Depending on the manual or mechanical processes used for printing, the mixture of ink and paper can change, and handling can also be more difficult. One of the most recent requests we received from a private collector of stamps was the analysis of ink composition in red “one-penny” stamps printed in Britain during the period 1841–1880. Stamps were printed intaglio from steel plates [1]. Depending on environmental conditions, stamps printed before 1857 showed a blueing effect on the white paper (Fig. 1). The official formula of the red ink was a mixture of rose pink,
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acterization of inks used for “one-penny” stamps printed during the 19th century. For this purpose a total of 71 stamps was analyzed. 2. Experimental 2.1. Apparatus
Fig. 1. One-penny stamp.
potassium prussiate (potassium hexacyanoferrate), potassium carbonate, cochineal, and oil. The blueing effect, which was assumed to be produced by potassium prussiate, was undesirable. In some cases the effect appeared many years after the printing. It seems that the cause of the blueing of the paper, which was unintended, was a result of the introduction of yellow prussiate of potash (potassium hexacyanoferrate) into the inks. The purpose of using this chemical was to make it more difficult for people to erase the cancellation of the stamp. The stamp printing ink had to be fugitive, so that fraudulent attempts to remove the postal cancellation would also damage the appearance of the stamp itself. Compounds based on prussiate of potash are totally destroyed by alkaline solutions which are frequently used as abstergents in picture-cleaning [2]. It is thought that at some time during 1857 a change in the formula of red ink stopped this blueing effect. The interest for collectors of red one-penny stamps was the value of the stamps printed both before and after the date when the ink composition changed. Infrared spectroscopy has been successfully applied to the analysis of inks [3–6]. Different methodologies and accessories can be used depending on the amount of sample, destruction, and the information required. Because of the value of stamps, nondestructive or quasi non-destructive sampling should be applied. Three different accessories were used: specular reflection, diffuse reflection and microscope. The microscope can be used either in transmission mode with a diamond cell or with an attenuated total reflectance (ATR) objective. This study has the purpose of using different types of sample preparation and infrared spectroscopy techniques for the char-
Micro-analysis was performed on a Bomem MB120 infrared spectrometer with a coupled Spectra Tech IR Plan Microscope. The spectrometer has a KBr beamsplitter and a Glowbar source. The microscope has an MCT (mercury cadmium telluride) detector refrigerated with liquid nitrogen and an ATR (attenuated total reflectance) objective of ZnSe. Infrared spectra were measured at a resolution of 4 cm−1 in transmission mode and 8 cm−1 in ATR mode. The range measured was between 4000 and 720 cm−1 . The spectral data were processed with the GRAMS/32 program. Macro-analysis was performed on a Bomem DA3 infrared spectrometer. The spectrometer has a KBr beamsplitter, a Glowbar source and an MCT(nr) detector refrigerated with liquid nitrogen. The accessories used were a Variable Angle Specular Reflectance Accessory, which was manufactured in our laboratory, [7] and a diffuse reflection accessory (DRIFT). The instrument works under vacuum. The range measured was between 4000 and 600 cm−1 with a resolution of 4 cm−1 . The spectral data were processed with the GRAMS/32 program. Scanning electron microscopy was performed on a Cambridge Instruments Stereoscan 360 microscope provided with microanalysis equipment Inca Energy 200. Spectra were recorded in the following conditions: 20 kV, 1 nA and a distance of 20–35 mm from sample to detector. Areas analyzed were between 20 and 50 m2 and, in some cases, punctual spots were measured. 2.2. Samples Stamps analyzed were dated between 1855 and 1868. Some of the stamps (14) were stuck down on envelopes, while the others (57) were not. Stamps show different tones of reds and some of them are cancelled and have black postmarks. The blueing effect can be seen in about 45% of the postmarks. 2.3. Sample handling for infrared measurements Diamond cell is one of the most frequently used methods in analysis of valuable samples. It is normally used when samples are very small but can be separated from the matrix. The particle or fibre, which can be as small as 5 m, is removed from the matrix with the help of tungsten needles. It is then placed in a diamond window and put under pressure with the help of another diamond window. The pressure of the cell allows the sample to spread over the diamond, increasing its surface area while decreasing its thickness. In this way, the infrared radiation is able to go through the sample and, therefore, its infrared spectrum can be obtained by transmission.
N. Ferrer, A. Vila / Analytica Chimica Acta 555 (2006) 161–166
Attenuated total reflectance is applied to samples where the composition of the surface needs to be measured. This is especially useful when the sample is either too thick or cannot be destroyed, separated or manipulated. It is applied to soft samples, which can achieve good contact with the crystal of the attenuated total reflectance objective. The only consideration to be borne in mind is the ability to obtain good optical contact between the surface of the sample and the ZnSe crystal. The specular or diffuse reflection accessories can be used if the sample is big enough and thus there is no need to focalize small areas. These accessories give us information on a large area of the surface (about 2 mm). The configuration of the Variable Angle Specular Reflectance accessory, allows the analysis of big samples. Stamps, even if they are stuck down, are placed on a black platform which contains a hole. The sample is placed faced-down on the platform and the light comes from a mirror under the sample. The configuration of the diffuse reflection accessory has some limitations when the samples are too big. Non-adhered stamps are placed in the sample hole, and the light comes from the mirror, which is positioned above the sample. The size of the sample, 2.5 cm by 2 cm, is the maximum size in order to avoid a lack of energy. 2.4. Sample handling for scanning electron microscopy The particle or fibre is removed from the matrix with the help of tungsten needles. It is then placed on a carbon adhesive over an aluminium stub. Samples are covered with carbon to make them conductive. 3. Results and discussion Four different methods of infrared measurements were applied: microscope with diamond cell, microscope with ATR objective, macro-sample compartment with specular reflectance accessory and macro-sample compartment with diffuse reflectance accessory. Triplicates of each sample measured with each methodology yielded a good reproducibility.
163
Fig. 2. Spectrum of a particle from a stamp containing cellulose.
Other representative bands that can be observed in nearly all the stamps are those corresponding to calcium carbonate. Bands at 1420, 875, 1795 and 2515 cm−1 can be clearly seen in many spectra. The strong bands at 1135, 1114 and 3406 were attributed to calcium sulphate dihydrate. Other bands at 1621 and 1690 are also due to this molecular absorption. Bands at 2920 and 2851 cm−1 in many spectra allow us to identify oil content in ink composition. They belong to the CH2 vibration on linear aliphatic chains. Fig. 3 shows the spectrum of a particle from a stamp containing absorptions corresponding to calcium carbonate, calcium sulphate and oil. Three bands at around 2080, 2060 and 2040 cm−1 can be seen in some spectra. Intensity ratio of these bands can be different. All the spectra that showed these bands correspond to the stamps with the blueing effect. Normally the 2060 cm−1 band is the most intensive. Due to the fact that this is a relatively neat range of the spectra and just a few vibrational modes can be seen, they have been assigned to the triple bond CN of cyano compound. The suspicion of potassium hexacyanoferrate (called yellow potash prussiate) in the ink composition was another point to be considered for this assignation. Ferric ferrocyanide (Fe4 {Fe(CN)6 }3 ) or Prussian blue was one of the suspected molecules present in the stamps and the one respon-
3.1. Microscope with diamond cell Infrared spectra obtained with the microscope and the diamond cell is normally good and show important information about the composition of a stamp. In fact, this was found to be the best way to obtain spectral information on the ink composition. Depending on the stamp, different absorptions can be detected in the spectra. During sample handling, it is possible to distinguish among those which have a significant layer of ink over the paper, and samples where paper fibres are on the surface. Some of the spectra show a very strong absorption of cellulose (1158, 1111, 1061, 1036 and 3346 cm−1 (Fig. 2)). In these cases, it proved difficult to remove ink without taking away some fibres.
Fig. 3. Spectrum of a particle from a stamp containing calcium carbonate, calcium sulphate and oil.
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Fig. 4. Spectra of particles from stamps showing absorptions in the CN region.
sible for the blueing effect. Nevertheless, Prussian blue has a single strong absorption at 2080 cm−1 , and this does not explain the other two bands appearing in our spectra. Tetrapotassium hexacyanoferrate(II) trihydrate (K4 {Fe(CN)6 }·3H2 O) also has some absorptions [8] in the range between 2080 and 2040 cm−1 , showing a maximum at 2040 cm−1 . According to some authors [9], CN absorptions can change depending on the kind of cyanotype, but they always appear around 2075 cm−1 . Fig. 4 shows some spectra of particles from stamps which show absorptions in the 2200–2000 cm−1 region. Different relative intensities can be seen at 2080, 2060 and 2040 cm−1 . Some of the samples showed a weak absorption at 848 cm−1 with a shoulder at 834 cm−1 . These bands are very close to the 875 cm−1 carbonate band. The shape of these bands is very characteristic and they have been assigned to lead chromate, used as a red pigment in ink formulations. Lead chromates show a large variety of shades, from a pale yellow to a deep orange-scarlet, depending on the variations in the manufacture. They work well when mixed with oils and are used in cheap pigments. They turn darker and can react with other pigments [10]. Scanning electron microscopy corroborated these results. In fact, all the samples which show the chromate absorption also show lead and chromium under scanning electron microscopy. Looking through the microscope while preparing the sample for the best 100 m spot to analyze, some strong red particles could be seen. The intensity of lead chromate peaks had a good correlation with the visual perception of the red particles. Fig. 5 shows a spectrum of a particle from a stamp containing calcium carbonate and lead chromate bands. The last compound identified was silicon oxide. Due to the fact that only two samples showed this absorption, this could be explained as an artefact coming from environmental pollution.
Fig. 5. Spectrum of a particle from a stamp containing calcium carbonate and lead chromate.
pressure to the sample. Some stamps show a poor amount of ink over the surface and therefore it is not possible to see more than cellulose bands. On the other hand, the advantage of this technique is that any part of the stamp can be measured without removing any particle. Fig. 6 compares the spectra of the same sample using the ATR objective and the diamond cell. Although the same information can be obtained from both spectra, the intensity of CH2 and CN bands is much weaker using the reflection mode. Carbonate bands have a good signal, but they show the first derivative shape. 3.3. Specular reflectance accessory With this accessory it is possible to use the whole sample. The stage is a metallic support of approximately 6 cm by 3 cm. The spot of light can change from 1 to 5 mm diameter by means of a black mask. The angle of incidence can also change from 20◦ to 60◦ . The best signal was obtained using an angle of 25◦ . The sample is placed in such a way that the light enters it from below.
3.2. Microscope with ATR objective In order to obtain a good infrared spectrum using the ATR objective, a very good contact between crystal and sample should be achieved. Sometimes it is necessary to try to press different parts of the stamp before getting a good signal. Because of the fragility of the ZnSe crystal, care should be taken when applying
Fig. 6. Comparison of spectra of the same sample obtained with ATR objective and diamond cell.
N. Ferrer, A. Vila / Analytica Chimica Acta 555 (2006) 161–166
165
3.6. Statistical data treatment
Fig. 7. Specular reflectance spectra for samples: with CN (1) and without CN (2).
The spectra do not give so much information compared to those obtained with the microscope. Even so, clear absorptions of CN and calcium carbonate can be seen. Fig. 7 shows the spectra of samples with and without CN. All of them show the absorptions of calcium carbonate at 1795 and 2515 cm−1 . 3.4. Diffuse reflection accessory A special support for the sample was manufactured in order to hold the whole stamp. In this case the light enters from the mirror from above the sample. Spectra are similar to those obtained with the specular reflectance accessory. Only absorptions of CN and calcium carbonate are clear. Fig. 8 shows the spectra of samples with and without CN. All of them show the absorptions of calcium carbonate at 1795 and 2515 cm−1 . 3.5. Scanning electron microscopy Elements found using this technique were: calcium, iron, potassium, sulphur, aluminium, lead, mercury and chromium.
A complete matrix of 38 stamps was prepared including results of molecular and atomic analyses. Molecular data columns include calcium carbonate, calcium sulphate, CH2 , CN, and lead chromate. Atomic data columns include calcium, iron, potassium, sulphur, aluminium, lead, mercury and chromium, which were the main elements detected using this technique. Samples ordered chronologically show that CN compounds were used at the beginning of the studied period until April 1857 (samples 1–32). In March 1857 (sample 25), lead chromate and mercury appeared. Lead chromate disappears in September 1857 (sample 42) but mercury remains until the last studied stamp. Mercury sulphide was commonly used as a red pigment at the time, but it cannot be measured by mid-infrared spectroscopy. Other compounds such as calcium carbonate appear in nearly all of the samples, while calcium sulphate and oil are distributed randomly in the chronological list. Using the SPSS statistical program, some good correlations could be obtained. A correlation matrix is obtained from the data matrix (38 files corresponding to the number of stamps and 13 columns corresponding to molecules or atoms). Using the factor analysis subroutine of the SPSS package, it is possible to obtain the eigenvalues and their contribution to the variance. After performing the Varimax rotation, the rotated matrix components show five factors. Each factor would represent a substance present in ink. Table 1 shows the matrix of rotated components. It can be seen that CN, K and Fe have a very good correlation which is in accordance with the characterization of potassium hexacyanoferrate. Lead chromate and chromium also show a very good correlation. Nevertheless lead and lead chromate are not correlated at all. This could be explained by assuming that all chromium comes from lead chromate (Chrome Red), but not all lead comes from this molecule. In fact lead oxide, which is not detected by infrared microscopy due to the low energy absorption of this molecule, is normally present in printing inks as Red Lead. Calcium seems to come basically from calcium carbonate. Although calcium is also present in calcium sulphate, Table 1 Values of the five factors obtained after the Varimax rotationa 1 CN K Fe Hg PbCrO4 Cr CaCO3 Ca CH2 S CaSO4 Pb Al
Fig. 8. Diffuse reflectance spectra for samples: with CN (1 and 2) and without CN (3).
a
0.907 0.893 0.848 −0.749 −0.218 −0.161 0.405 0.171 −0.247 0.198
2
3
−0.183 0.476 0.936 0.902 0.174 −0.101
−0.139 0.120
4
5
0.219
0.118
0.907 0.773 0.581 −0.230 0.122 −0.271
0.250 0.234
−0.310 0.786 0.741 0.325 −0.159
Each factor would represent a substance present in ink.
−0.179 0.291
0.769 0.761
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this molecule is correlated with sulphur. The amount of calcium sulphate in all the spectra is much less than calcium carbonate. Lead and aluminium have also a good correlation. 4. Conclusions Different compounds of ink used to print red one-penny stamps were characterized: calcium carbonate, calcium sulphate, oil, potassium hexacyanoferrate and lead chromate. The blueing effect was correlated with the presence of CN groups in the infrared spectrum. In order to discriminate the stamps according to the presence or absence of CN groups, macro-sample accessories such as diffuse reflection or specular reflection can be used. These techniques are not in any way destructive or harmful to the stamps. The measure of other absorption bands requires the use of a microscope. The optimal methodology, which gives better spectra, is to remove a particle and use the diamond cell in transmission mode. Good correlations were observed comparing infrared spectroscopy with scanning electron microscopy results. Potassium, iron, and CN are very well correlated. Chromium and lead chromate also show good correlation. Sulphur with calcium sulphate and calcium with calcium carbonate are also strongly correlated. Ordering the samples chronologically, it is possible to follow the changes that occurred in the ink composition.
Acknowledgments The authors wish to express their gratitude to Don Madden (MRCVS, FRPSL), who was the initiator of the investigation and the supplier of the stamps and who read early versions of the manuscript in addition to kindly providing funding. Thanks are also due to thank Ramon Fontarnau of the Scientific Technical Services for results obtained in the use of scanning electron microscopy. References [1] Bamber Gascoigne, How to Identify Prints, Thames and Hudson, 1998, p. 77. [2] Elisabeth West Fitzhugh (Ed.), Artists’ pigments, in: A Handbook of their History and Characteristics, vol. 3, 1997, p. 204. [3] M.T. Romero, N. Ferrer, Mikrochim. Acta 131 (1999) 237–245. [4] M. Carme Sistach, N. Ferrer, M.T. Romero, Restaurator (1998) 173–186. [5] N. Ferrer, Forensic science applications of infrared spectroscopy. Vibrational, rotational and Raman spectroscopies, in: John C. Lindon, George E. Tranter, John L. Holmes (Eds.), Encyclopedia of Spectroscopy and Spectrometry, Academic Press Ltd., 2000, pp. 603–615. [6] N. Ferrer, Mikrochim. Acta 14 (1997) 329–332. [7] M.C. Polo, N. Ferrer, M. Romero, J. P´erez, M. Quevedo, F. Vilardell, J. Vac. Sci. Technol. A 17 (1999) 1. [8] V. Caglioti, G. Sartori, M. Scrocco, J. Inorg. Nucl. Chem. 8 (1958) 87–92. [9] M. Ware, Cyanotype. The history, science and art of photographic printing in Prussian Blue Science Museum and National Museum of Photography, Film and Television (1999). [10] R. Mayer, The Artist’s Handbook of Materials and Techniques, Faber and Faber, London, 1991, p. 42.
Fourier transform infrared spectroscopy applied to ink characterization of one-penny postage stamps printed 1841–1880 N´uria Ferrer ∗ , Anna Vila Serveis Cientificot`ecnics, University of Barcelona, Lluis Sole Sabaris, 1, 08028 Barcelona, Spain Received 5 May 2005; received in revised form 25 August 2005; accepted 25 August 2005 Available online 10 October 2005
Abstract Fourier transform infrared spectroscopy (FTIR) was applied to the characterization of red inks used to print one-penny stamps in Britain during the period 1841–1880. Micro- and macro-accessories were compared in both transmission and reflection mode. The best spectra were obtained using a microscope with a diamond cell coupled to an infrared spectrometer. In this case, the extraction of a single paper fibre was enough to obtain a good spectrum. Calcium carbonate, calcium sulfate, lead chromate, cyano compounds, cellulose, and oil were identified. When bigger surfaces need to be analyzed, diffuse reflection or variable specular reflectance accessories can be used. Nevertheless, only cyano compounds and calcium carbonate can be characterized. Results are in agreement with those obtained using scanning electron microscopy. The chronological order of the 71 stamps studied showed that the chemical composition of the red ink changed during the period studied. © 2005 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; Characterization of inks; Composition of printing inks in stamps; “One-penny” stamps (GB)
1. Introduction Application of atomic and molecular spectroscopy analysis to solve problems related to cultural heritage has increased dramatically in the last few years. One reason is that restorers, artists and other people working in the field of the fine arts have become aware of and proficient in techniques of instrumental analysis. In addition, the demand for these kinds of analysis has increasingly involved spectroscopists and scientists in general in the study and development of new methodologies applied to samples from museums, archives, private collections, and other institutions. Because artworks are, in many cases, unique and valuable, stringent restrictions are required before removing even small amounts of material. Therefore, specific methodologies must be applied in order to minimize the amount of sample to be used and, consequently, to avoid any damage to the artwork. Other possibilities involve methods that do not damage the sample at all.
∗
Corresponding author. Tel.: +34 93 4021346. E-mail address: [email protected] (N. Ferrer).
0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.08.080
Not all methodologies are equally suitable for all samples, and comparative studies are required to identify the most appropriate in each case. In our laboratory, different samples, taken for the purpose of examining problems of conservation, restoration or fraud, have been analyzed in the last decade: inks and papers on manuscripts, pigments and dies on paintings, inks on engravings and plates, ceramics, textiles, and also polymers used by restorers. Analysis of inks on papers has the handicap of the strong absorption of the matrix. A spectrum of cellulose shows very intensive bands in the fingerprint region; therefore, it is necessary to try to remove particles where fibres are not present. Depending on the manual or mechanical processes used for printing, the mixture of ink and paper can change, and handling can also be more difficult. One of the most recent requests we received from a private collector of stamps was the analysis of ink composition in red “one-penny” stamps printed in Britain during the period 1841–1880. Stamps were printed intaglio from steel plates [1]. Depending on environmental conditions, stamps printed before 1857 showed a blueing effect on the white paper (Fig. 1). The official formula of the red ink was a mixture of rose pink,
162
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acterization of inks used for “one-penny” stamps printed during the 19th century. For this purpose a total of 71 stamps was analyzed. 2. Experimental 2.1. Apparatus
Fig. 1. One-penny stamp.
potassium prussiate (potassium hexacyanoferrate), potassium carbonate, cochineal, and oil. The blueing effect, which was assumed to be produced by potassium prussiate, was undesirable. In some cases the effect appeared many years after the printing. It seems that the cause of the blueing of the paper, which was unintended, was a result of the introduction of yellow prussiate of potash (potassium hexacyanoferrate) into the inks. The purpose of using this chemical was to make it more difficult for people to erase the cancellation of the stamp. The stamp printing ink had to be fugitive, so that fraudulent attempts to remove the postal cancellation would also damage the appearance of the stamp itself. Compounds based on prussiate of potash are totally destroyed by alkaline solutions which are frequently used as abstergents in picture-cleaning [2]. It is thought that at some time during 1857 a change in the formula of red ink stopped this blueing effect. The interest for collectors of red one-penny stamps was the value of the stamps printed both before and after the date when the ink composition changed. Infrared spectroscopy has been successfully applied to the analysis of inks [3–6]. Different methodologies and accessories can be used depending on the amount of sample, destruction, and the information required. Because of the value of stamps, nondestructive or quasi non-destructive sampling should be applied. Three different accessories were used: specular reflection, diffuse reflection and microscope. The microscope can be used either in transmission mode with a diamond cell or with an attenuated total reflectance (ATR) objective. This study has the purpose of using different types of sample preparation and infrared spectroscopy techniques for the char-
Micro-analysis was performed on a Bomem MB120 infrared spectrometer with a coupled Spectra Tech IR Plan Microscope. The spectrometer has a KBr beamsplitter and a Glowbar source. The microscope has an MCT (mercury cadmium telluride) detector refrigerated with liquid nitrogen and an ATR (attenuated total reflectance) objective of ZnSe. Infrared spectra were measured at a resolution of 4 cm−1 in transmission mode and 8 cm−1 in ATR mode. The range measured was between 4000 and 720 cm−1 . The spectral data were processed with the GRAMS/32 program. Macro-analysis was performed on a Bomem DA3 infrared spectrometer. The spectrometer has a KBr beamsplitter, a Glowbar source and an MCT(nr) detector refrigerated with liquid nitrogen. The accessories used were a Variable Angle Specular Reflectance Accessory, which was manufactured in our laboratory, [7] and a diffuse reflection accessory (DRIFT). The instrument works under vacuum. The range measured was between 4000 and 600 cm−1 with a resolution of 4 cm−1 . The spectral data were processed with the GRAMS/32 program. Scanning electron microscopy was performed on a Cambridge Instruments Stereoscan 360 microscope provided with microanalysis equipment Inca Energy 200. Spectra were recorded in the following conditions: 20 kV, 1 nA and a distance of 20–35 mm from sample to detector. Areas analyzed were between 20 and 50 m2 and, in some cases, punctual spots were measured. 2.2. Samples Stamps analyzed were dated between 1855 and 1868. Some of the stamps (14) were stuck down on envelopes, while the others (57) were not. Stamps show different tones of reds and some of them are cancelled and have black postmarks. The blueing effect can be seen in about 45% of the postmarks. 2.3. Sample handling for infrared measurements Diamond cell is one of the most frequently used methods in analysis of valuable samples. It is normally used when samples are very small but can be separated from the matrix. The particle or fibre, which can be as small as 5 m, is removed from the matrix with the help of tungsten needles. It is then placed in a diamond window and put under pressure with the help of another diamond window. The pressure of the cell allows the sample to spread over the diamond, increasing its surface area while decreasing its thickness. In this way, the infrared radiation is able to go through the sample and, therefore, its infrared spectrum can be obtained by transmission.
N. Ferrer, A. Vila / Analytica Chimica Acta 555 (2006) 161–166
Attenuated total reflectance is applied to samples where the composition of the surface needs to be measured. This is especially useful when the sample is either too thick or cannot be destroyed, separated or manipulated. It is applied to soft samples, which can achieve good contact with the crystal of the attenuated total reflectance objective. The only consideration to be borne in mind is the ability to obtain good optical contact between the surface of the sample and the ZnSe crystal. The specular or diffuse reflection accessories can be used if the sample is big enough and thus there is no need to focalize small areas. These accessories give us information on a large area of the surface (about 2 mm). The configuration of the Variable Angle Specular Reflectance accessory, allows the analysis of big samples. Stamps, even if they are stuck down, are placed on a black platform which contains a hole. The sample is placed faced-down on the platform and the light comes from a mirror under the sample. The configuration of the diffuse reflection accessory has some limitations when the samples are too big. Non-adhered stamps are placed in the sample hole, and the light comes from the mirror, which is positioned above the sample. The size of the sample, 2.5 cm by 2 cm, is the maximum size in order to avoid a lack of energy. 2.4. Sample handling for scanning electron microscopy The particle or fibre is removed from the matrix with the help of tungsten needles. It is then placed on a carbon adhesive over an aluminium stub. Samples are covered with carbon to make them conductive. 3. Results and discussion Four different methods of infrared measurements were applied: microscope with diamond cell, microscope with ATR objective, macro-sample compartment with specular reflectance accessory and macro-sample compartment with diffuse reflectance accessory. Triplicates of each sample measured with each methodology yielded a good reproducibility.
163
Fig. 2. Spectrum of a particle from a stamp containing cellulose.
Other representative bands that can be observed in nearly all the stamps are those corresponding to calcium carbonate. Bands at 1420, 875, 1795 and 2515 cm−1 can be clearly seen in many spectra. The strong bands at 1135, 1114 and 3406 were attributed to calcium sulphate dihydrate. Other bands at 1621 and 1690 are also due to this molecular absorption. Bands at 2920 and 2851 cm−1 in many spectra allow us to identify oil content in ink composition. They belong to the CH2 vibration on linear aliphatic chains. Fig. 3 shows the spectrum of a particle from a stamp containing absorptions corresponding to calcium carbonate, calcium sulphate and oil. Three bands at around 2080, 2060 and 2040 cm−1 can be seen in some spectra. Intensity ratio of these bands can be different. All the spectra that showed these bands correspond to the stamps with the blueing effect. Normally the 2060 cm−1 band is the most intensive. Due to the fact that this is a relatively neat range of the spectra and just a few vibrational modes can be seen, they have been assigned to the triple bond CN of cyano compound. The suspicion of potassium hexacyanoferrate (called yellow potash prussiate) in the ink composition was another point to be considered for this assignation. Ferric ferrocyanide (Fe4 {Fe(CN)6 }3 ) or Prussian blue was one of the suspected molecules present in the stamps and the one respon-
3.1. Microscope with diamond cell Infrared spectra obtained with the microscope and the diamond cell is normally good and show important information about the composition of a stamp. In fact, this was found to be the best way to obtain spectral information on the ink composition. Depending on the stamp, different absorptions can be detected in the spectra. During sample handling, it is possible to distinguish among those which have a significant layer of ink over the paper, and samples where paper fibres are on the surface. Some of the spectra show a very strong absorption of cellulose (1158, 1111, 1061, 1036 and 3346 cm−1 (Fig. 2)). In these cases, it proved difficult to remove ink without taking away some fibres.
Fig. 3. Spectrum of a particle from a stamp containing calcium carbonate, calcium sulphate and oil.
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Fig. 4. Spectra of particles from stamps showing absorptions in the CN region.
sible for the blueing effect. Nevertheless, Prussian blue has a single strong absorption at 2080 cm−1 , and this does not explain the other two bands appearing in our spectra. Tetrapotassium hexacyanoferrate(II) trihydrate (K4 {Fe(CN)6 }·3H2 O) also has some absorptions [8] in the range between 2080 and 2040 cm−1 , showing a maximum at 2040 cm−1 . According to some authors [9], CN absorptions can change depending on the kind of cyanotype, but they always appear around 2075 cm−1 . Fig. 4 shows some spectra of particles from stamps which show absorptions in the 2200–2000 cm−1 region. Different relative intensities can be seen at 2080, 2060 and 2040 cm−1 . Some of the samples showed a weak absorption at 848 cm−1 with a shoulder at 834 cm−1 . These bands are very close to the 875 cm−1 carbonate band. The shape of these bands is very characteristic and they have been assigned to lead chromate, used as a red pigment in ink formulations. Lead chromates show a large variety of shades, from a pale yellow to a deep orange-scarlet, depending on the variations in the manufacture. They work well when mixed with oils and are used in cheap pigments. They turn darker and can react with other pigments [10]. Scanning electron microscopy corroborated these results. In fact, all the samples which show the chromate absorption also show lead and chromium under scanning electron microscopy. Looking through the microscope while preparing the sample for the best 100 m spot to analyze, some strong red particles could be seen. The intensity of lead chromate peaks had a good correlation with the visual perception of the red particles. Fig. 5 shows a spectrum of a particle from a stamp containing calcium carbonate and lead chromate bands. The last compound identified was silicon oxide. Due to the fact that only two samples showed this absorption, this could be explained as an artefact coming from environmental pollution.
Fig. 5. Spectrum of a particle from a stamp containing calcium carbonate and lead chromate.
pressure to the sample. Some stamps show a poor amount of ink over the surface and therefore it is not possible to see more than cellulose bands. On the other hand, the advantage of this technique is that any part of the stamp can be measured without removing any particle. Fig. 6 compares the spectra of the same sample using the ATR objective and the diamond cell. Although the same information can be obtained from both spectra, the intensity of CH2 and CN bands is much weaker using the reflection mode. Carbonate bands have a good signal, but they show the first derivative shape. 3.3. Specular reflectance accessory With this accessory it is possible to use the whole sample. The stage is a metallic support of approximately 6 cm by 3 cm. The spot of light can change from 1 to 5 mm diameter by means of a black mask. The angle of incidence can also change from 20◦ to 60◦ . The best signal was obtained using an angle of 25◦ . The sample is placed in such a way that the light enters it from below.
3.2. Microscope with ATR objective In order to obtain a good infrared spectrum using the ATR objective, a very good contact between crystal and sample should be achieved. Sometimes it is necessary to try to press different parts of the stamp before getting a good signal. Because of the fragility of the ZnSe crystal, care should be taken when applying
Fig. 6. Comparison of spectra of the same sample obtained with ATR objective and diamond cell.
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3.6. Statistical data treatment
Fig. 7. Specular reflectance spectra for samples: with CN (1) and without CN (2).
The spectra do not give so much information compared to those obtained with the microscope. Even so, clear absorptions of CN and calcium carbonate can be seen. Fig. 7 shows the spectra of samples with and without CN. All of them show the absorptions of calcium carbonate at 1795 and 2515 cm−1 . 3.4. Diffuse reflection accessory A special support for the sample was manufactured in order to hold the whole stamp. In this case the light enters from the mirror from above the sample. Spectra are similar to those obtained with the specular reflectance accessory. Only absorptions of CN and calcium carbonate are clear. Fig. 8 shows the spectra of samples with and without CN. All of them show the absorptions of calcium carbonate at 1795 and 2515 cm−1 . 3.5. Scanning electron microscopy Elements found using this technique were: calcium, iron, potassium, sulphur, aluminium, lead, mercury and chromium.
A complete matrix of 38 stamps was prepared including results of molecular and atomic analyses. Molecular data columns include calcium carbonate, calcium sulphate, CH2 , CN, and lead chromate. Atomic data columns include calcium, iron, potassium, sulphur, aluminium, lead, mercury and chromium, which were the main elements detected using this technique. Samples ordered chronologically show that CN compounds were used at the beginning of the studied period until April 1857 (samples 1–32). In March 1857 (sample 25), lead chromate and mercury appeared. Lead chromate disappears in September 1857 (sample 42) but mercury remains until the last studied stamp. Mercury sulphide was commonly used as a red pigment at the time, but it cannot be measured by mid-infrared spectroscopy. Other compounds such as calcium carbonate appear in nearly all of the samples, while calcium sulphate and oil are distributed randomly in the chronological list. Using the SPSS statistical program, some good correlations could be obtained. A correlation matrix is obtained from the data matrix (38 files corresponding to the number of stamps and 13 columns corresponding to molecules or atoms). Using the factor analysis subroutine of the SPSS package, it is possible to obtain the eigenvalues and their contribution to the variance. After performing the Varimax rotation, the rotated matrix components show five factors. Each factor would represent a substance present in ink. Table 1 shows the matrix of rotated components. It can be seen that CN, K and Fe have a very good correlation which is in accordance with the characterization of potassium hexacyanoferrate. Lead chromate and chromium also show a very good correlation. Nevertheless lead and lead chromate are not correlated at all. This could be explained by assuming that all chromium comes from lead chromate (Chrome Red), but not all lead comes from this molecule. In fact lead oxide, which is not detected by infrared microscopy due to the low energy absorption of this molecule, is normally present in printing inks as Red Lead. Calcium seems to come basically from calcium carbonate. Although calcium is also present in calcium sulphate, Table 1 Values of the five factors obtained after the Varimax rotationa 1 CN K Fe Hg PbCrO4 Cr CaCO3 Ca CH2 S CaSO4 Pb Al
Fig. 8. Diffuse reflectance spectra for samples: with CN (1 and 2) and without CN (3).
a
0.907 0.893 0.848 −0.749 −0.218 −0.161 0.405 0.171 −0.247 0.198
2
3
−0.183 0.476 0.936 0.902 0.174 −0.101
−0.139 0.120
4
5
0.219
0.118
0.907 0.773 0.581 −0.230 0.122 −0.271
0.250 0.234
−0.310 0.786 0.741 0.325 −0.159
Each factor would represent a substance present in ink.
−0.179 0.291
0.769 0.761
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this molecule is correlated with sulphur. The amount of calcium sulphate in all the spectra is much less than calcium carbonate. Lead and aluminium have also a good correlation. 4. Conclusions Different compounds of ink used to print red one-penny stamps were characterized: calcium carbonate, calcium sulphate, oil, potassium hexacyanoferrate and lead chromate. The blueing effect was correlated with the presence of CN groups in the infrared spectrum. In order to discriminate the stamps according to the presence or absence of CN groups, macro-sample accessories such as diffuse reflection or specular reflection can be used. These techniques are not in any way destructive or harmful to the stamps. The measure of other absorption bands requires the use of a microscope. The optimal methodology, which gives better spectra, is to remove a particle and use the diamond cell in transmission mode. Good correlations were observed comparing infrared spectroscopy with scanning electron microscopy results. Potassium, iron, and CN are very well correlated. Chromium and lead chromate also show good correlation. Sulphur with calcium sulphate and calcium with calcium carbonate are also strongly correlated. Ordering the samples chronologically, it is possible to follow the changes that occurred in the ink composition.
Acknowledgments The authors wish to express their gratitude to Don Madden (MRCVS, FRPSL), who was the initiator of the investigation and the supplier of the stamps and who read early versions of the manuscript in addition to kindly providing funding. Thanks are also due to thank Ramon Fontarnau of the Scientific Technical Services for results obtained in the use of scanning electron microscopy. References [1] Bamber Gascoigne, How to Identify Prints, Thames and Hudson, 1998, p. 77. [2] Elisabeth West Fitzhugh (Ed.), Artists’ pigments, in: A Handbook of their History and Characteristics, vol. 3, 1997, p. 204. [3] M.T. Romero, N. Ferrer, Mikrochim. Acta 131 (1999) 237–245. [4] M. Carme Sistach, N. Ferrer, M.T. Romero, Restaurator (1998) 173–186. [5] N. Ferrer, Forensic science applications of infrared spectroscopy. Vibrational, rotational and Raman spectroscopies, in: John C. Lindon, George E. Tranter, John L. Holmes (Eds.), Encyclopedia of Spectroscopy and Spectrometry, Academic Press Ltd., 2000, pp. 603–615. [6] N. Ferrer, Mikrochim. Acta 14 (1997) 329–332. [7] M.C. Polo, N. Ferrer, M. Romero, J. P´erez, M. Quevedo, F. Vilardell, J. Vac. Sci. Technol. A 17 (1999) 1. [8] V. Caglioti, G. Sartori, M. Scrocco, J. Inorg. Nucl. Chem. 8 (1958) 87–92. [9] M. Ware, Cyanotype. The history, science and art of photographic printing in Prussian Blue Science Museum and National Museum of Photography, Film and Television (1999). [10] R. Mayer, The Artist’s Handbook of Materials and Techniques, Faber and Faber, London, 1991, p. 42.