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Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF
Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF M. Hložek a, T. Trojek b,n a b
Methodical Centre of Conservation – Technical Museum in Brno, Purkyňova 105, 612 00 Brno, Czech Republic Czech Technical University in Prague, Department of Dosimetry and Application of Ionizing Radiation, Břehová 7, 115 19 Prague, Czech Republic
H I G H L I G H T S
Tiny remains of plating on the surface of coins were recognized with micro-XRF. More techniques of counterfeiting of coins were revealed. The determined low fluorescence ratio Kα/Kβ of copper was an evidence of plating.
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2015 Received in revised form 12 August 2016 Accepted 13 August 2016
Archaeological surveys and metal detector prospecting yield a great amount of coins from the medieval period. Naturally, some of these are counterfeit which an experienced numismatist can determine without using chemical methods. The production of counterfeit coins in the middle ages took place in castles, caves or other remote areas where waste from this activity can still be found today – copper sheets, technical ceramics and counterfeit coins. Until recently, it has been assumed that medieval counterfeit coins are made by silver-plating copper blanks using an amalgam. However, the performed analyses reveal that there are many more techniques of counterfeiting of coins. Other techniques were based on e.g. tin amalgam plating of the blanks or alloying so-called white metal with silver-like appearance from which the coins were minted. Current chemical analyses indicate that the coins were often tinned by hot dipping with no amalgamation. Micro-X-ray fluorescence analysis has been chosen as a suitable non-destructive method to identify present chemical elements in investigated artifacts and to quantify their concentrations. In addition, a quick technique telltale the plating was applied. This technique utilizes the detected fluorescence ratio Kα/Kβ of copper, which is the main ingredient of a lot of historical metallic materials. & 2016 Elsevier Ltd. All rights reserved.
Keywords: X-ray fluorescence Microanalysis Counterfeit coin Plating
1. Introduction Amalgam gold-plated metal artefacts are often objects of study in the field of cultural heritage. These artefacts are mostly found during archaeological digs or they occur as medieval or modern pieces of art. It is less known that amalgam silver-plating and tinplating was used as well (Vlachou et al., 2002). Amalgam silverand tin-plating can mostly be found in forgeries of medieval coins. In gold-plating (silver-, tin-plating) in fire, an amalgam of gold (silver, tin), i.e. metal dissolved in mercury, was used. Mercury evaporates by the heat and a thicker layer of gold, silver or tin forms on the surface of the metal-plated object. At present, this technique is not used due to toxicity of mercury. n
Corresponding author. E-mail address: tomas.trojek@fjfi.cvut.cz (T. Trojek).
Money counterfeiting workshops can mainly be found in castles, caves (the Koněprusy Caves), and possibly in other remote places in the Czech Republic. The workshop from about 1440 in the Skály Castle from where a large number of blanks and forgeries come from was the most preserved. In the second half of the 15th century, there was a workshop in the Křídlo Castle in eastern Moravia. Unique discoveries of waste from the equipment of a counterfeiting workshop in the Žampach Castle in eastern Bohemia were made also. A fake coin can be recognized by means of a combination of elemental and microstructural analyses (Bartoli et al., 2011). X-ray fluorescence (XRF) analysis is one of the most frequently used analytical methods in investigating old metallic artefacts (Ferretti, 2014). Besides its non-destructivity, the XRF measurements and data analyses are usually quick and easy to perform (Milazzo and Cicardi, 1997). Till today, it has been successfully applied for characterization of mint and issue period of silver Roman coins
http://dx.doi.org/10.1016/j.radphyschem.2016.08.013 0969-806X/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i
M. Hložek, T. Trojek / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
(Gorghinian et al., 2013) or ancient and modern copper based fakes were identified (Giumlia-Mair and Lucchini, 2005). It seems that the counterfeit and original coins differ in concentrations of major and minor elements or the counterfeit coins were manufactured by means of coating of a metaling substrate. In this paper, we are dealing with the recognition of the plating of copper or copper-based alloys using XRF analysis. There are several XRF methods/modalities available for coating identification. These include confocal XRF analysis, a method based on the internal X-ray ratios of present elements, and a depth-profiling technique based on at least two measurements performed under different experimental conditions. The depthprofiling with confocal XRF requires a special instrumentation including a micro-focus X-ray tube with focusing X-ray optics and collimating optics on the detector side of the confocal XRF setup. It is useful especially for analysis of paintings consisting of layers with different pigments (Kanngießer et al., 2004). An approximate depth distribution of an element can be also determined with its internal X-ray ratios (Kα/Kβ or Lα/Lβ) acquired with an X-ray detector (Cesareo et al., 2009). Thanks to different absorption of particular X-ray lines in a sample matrix, the deeper the element is located under the surface the more these ratios are affected. Last but not least, when two XRF measurements are performed under different conditions analysing different surface thicknesses, the coating can be identified also (Trojek et al., 2007) or (Trojek and Hložek, 2012).
Fig. 1. Photograph of coin #1 – forgery of the coin of Ferdinand I of Habsburg.
2. Experimental setup and data analysis The coins were analyzed with a micro-XRF system because some of them were damaged and only tiny remains of their plating were found. The micro-XRF device consists of a molybdenum anode X-ray tube (XOS, Power Flux PF) with polycapillary focusing optics (minimal FWHM 15 mm for Mo-Kα line) and an SDD detector (Amptek, 25 mm2 0.5 mm). The X-ray tube was operated at a maximum voltage of 50 kV and a maximum current of 1 mA. The detector was located approximately 10 mm from a sample. The geometry arrangement was 90°/45°. The acquisition time was 30 s for individual analyses and only 2 s per spot in the case of XRF scanning. The goal of the XRF analysis was to identify present elements and to identify the coating. The approximate depth distribution of an element can be determined with its internal X-ray ratios acquired with an X-ray detector, e.g. Kα/Kβ. In this particular case, the Kα/Kβ ratio of copper, the major element in all specimens, was evaluated. This ratio is about 6–7 for most thick homogeneous copper alloys. Its value depends on attenuation of these copper K-lines in a matrix of a certain alloy. If such alloy or pure copper is plated, the ratio is reduced in most of cases. The X-ray fluorescence spectra were evaluated with the code WinAXIL that made it possible for us to determine the net peak areas of both Kα and Kβ lines independently of each other. The code MCNPX version 2.4.0 (MCNPX™ 2.4.0 user's manual, 2002) was used for calculating the Cu Kα/Kβ ratio for the given experimental conditions and the specimen composition.
Fig. 2. XRF spectrum of coin #1; analyzed area marked in Fig. 1.
Fig. 3. Photograph of coin #2 – forgery of the pfennig of Ottokar IV of Styria.
3. Results Several dozen old coins suspected to be counterfeits were analyzed with the micro-XRF system at the Czech Technical University in Prague but only three of them are discussed in detail in this article. These three coin specimens and their XRF spectra are shown in Figs. 1–6. These coins are supposed to be forgeries of the following coins:
Fig. 4. XRF spectrum of coin #2; analyzed area marked in Fig. 3.
Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i
M. Hložek, T. Trojek / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 5. Photograph of coin #3 – forgery of the coin of Franz Joseph I of Habsburg.
Fig. 6. XRF spectrum of coin #3; analyzed area marked in Fig. 5.
#1: coin of Ferdinand I of Habsburg (1526–1564) #2: pfennig of Ottokar IV of Styria (1164–1192) #3: coin of Franz Joseph I of Habsburg (1869–1916) For each coin, at least 2 areas with a diameter of approximately 100–200 mm were analyzed. That should reduce the effect of sample heterogeneity. The plating identification based on the copper Kα/Kβ ratio is demonstrated in Table 1 which summarizes the XRF analyses of these counterfeit coins and three standard reference materials (SRM). The brass SRM was used as a comparative specimen of known composition for quantitative analysis and Monte Carlo simulations. The applied Monte Carlo method requires at least one SRM with the same element(s) which would be determined in analyzed samples (Trojek and Čechák, 2007). Two other SRM were measured and simulated to validate the Monte Carlo calculation method. The acquired XRF spectra were evaluated with the code WinAXIL and the Cu Kα/Kβ ratio was determined (third column in Table 1). The uncertainty of this ratio is derived from the fitting of
an individual spectrum. The Cu Kα/Kβ ratio is quite low for coins #1 and #2 and thus it indicates plating of copper-based alloys. On the other hand, the Cu Kα/Kβ ratio of the third coin is much higher than for the brass SRM. The following procedure was performed to prove the hypothesis about the plating of the two coins and to explain the high value of the Cu Kα/Kβ ratio for the last one. The elemental concentrations (second column in Table 1) were calculated with the code WinAxil using fundamental parameter method with normalization of concentrations. The specimens were considered to be thick and homogeneous, i.e. without plating. For the composition determined in this manner the corresponding Cu Kα/ Kβ ratio was calculated with the MCNPX code (forth column in Table 1). The uncertainty of this ratio is derived from the uncertainties of the Monte Carlo simulation and the measurement of the calibration brass SRM. The measured Cu Kα/Kβ ratio is considerably lower than the calculated ratio for the homogeneous material for those two coins. Thus, the results prove the plating. The last coin #3 has a bright hue but noble metals are absent. Since the calculated and measured ratios are almost identical, it indicates homogeneous distribution of copper, the coin is not plated, and the element concentrations in the second column are correct. This is an evidence of a forgery made of Cu-Zn-Ni alloy. Such high value of Cu Kα/Kβ ratio (in comparison with brass SRM) is caused by the high concentration of nickel which has the X-ray absorption edge just between the energies of Kα and the Kβ lines of copper, i.e. the Kβ line of copper is much more absorbed than the line Kα in a matrix with nickel. If a copper-zinc alloy is plated with nickel, the Cu Kα/Kβ ratio would be even higher. Since the measured and calculated Cu Kα/Kβ ratios are almost identical, the coin #3 has obviously homogeneous composition and the determined elemental concentrations imply that it is an example of modern forgery. The last coin #4 described in this article is a forgery from the counterfeiting workshop in the Starý Světlov Castle (15th century). Its whole surface was scanned with our micro-XRF system with a step of 200 mm. It is made of copper substrate and traces of silver and mercury were found in 2 areas on its surface. The copper Kα/ Kβ ratio decreases in those places where the remains of the coating have been preserved, see Fig. 7. Its value shows also small statistical fluctuations outside these areas because of short acquisition time (2 s per spot). The results demonstrate that medieval silver coins were counterfeited by means of several techniques. The most widespread technique of counterfeiting workshops was metal-plating of copper blanks using an amalgam of silver and tin. Archaeological finds of production waste are the main signs of coin counterfeiting in castles. These are first of all remains of copper sheets, which blanks and blanks with traces of metal-plating were made from. Analyses of coins, where surface traces of silver and mercury were identified by XRF spectrometer, could be the evidence of silver-plating using an amalgam. Other techniques used the principle of so called white metal preparation (Richtera et al., 2014); see the examples of modern-times coins. Chemical analyses show that fire metal-plating (tin-plating) without the use of an
Table 1 Measured and calculated ratios of copper Kα/Kβ lines for analyzed specimens and SRM; their difference indicates plating. Specimen
SRM SRM SRM Coin Coin Coin
– Brass – Pure copper – Silver #1 #2 #3
Element concentrations [%] (assuming homogeneity)
3
Cu Kα/Kβ ratio measured Cu Kα/Kβ ratio calculated with MCNPX (assuming homogeneity)
Certified: Cu 78.8, Zn 14.5, Sn 1.9, Pb 1.9, Ni 1.1, Fe 0.5 6.83 70.03 Cu 100 6.777 0.03 Ag 92.5 Cu 7.5 6.02 70.04 Ag 61.6, Cu 8.9, Hg 29.5 5.40 70.06 Cu 24.6, Sn 51.9, Pb 23.5 4.85 70.07 Cu 66.7, Zn 22.8, Ni 10.5 8.32 70.08
– 6.727 0.04 5.90 70.04 6.157 0.04 5.96 70.04 8.50 70.04
Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i
M. Hložek, T. Trojek / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
forgeries. We were able to distinguish between authentic coins and forgeries very quickly and effectively. Based on the results of the analyses, in forgeries, we are able to state if the blank was metal-plated with an amalgam or using fire-technique. Using micro-XRF, it was further proved that it is possible to identify the metal-plating traces even if the plates are covered with a layer of green-coloured corrosion products. The evaluation of the copper Kα/Kβ ratio provided us with the approximate information on depth distribution of this element and thus, the plating of a copper-based material can be recognized.
References
Fig. 7. Kα/Kβ ratio of copper obtained with XRF scanning of the coin #4. Two circles show the areas with remains of silver-plating. Scanned area: 15 10 mm2.
amalgam, when copper blanks were metal-plated in a copper kettle in molten tin, was very common.
4. Conclusions In XRF analyses of coin forgeries, there are certain complications regarding interpretations of the obtained analytical data. When using a commercial XRF spectrometer, which does not enable a selection of the microscopic spot to be analyzed, it is impossible to find out whether a blank is metal-plated. In this case, average information of copper and tin (possibly silver) contents in the examined area will be obtained. If all the areas representing the remains of metal-plating are sufficiently small and locally damaged, the results of the analyses of copper- and tin-plated blanks are the same. Application of micro-XRF approach, which enables selective spot analysis or elemental mapping of a larger area of a coin blank, is recommendable. At common silver coinages, there are also certain difficulties regarding misinterpretation due to the effect of technology or corrosion. Silver coin blanks were relatively often whitened with wine stone before minting. Whitening of coins increased the impression and acted as corrosion protection at the same time. The obtained results of all the above mentioned methods applied on well preserved coins are always overvalued. Usually, more than 90% of Ag is detected, which does not correspond to the real purity of the coins (Richtera and Zmrzlý, 2013). The performed analyses showed that the micro-XRF analysis can be successfully used for non-destructive examination of coin
Bartoli, L., et al., 2011. Combined elemental and microstructural analysis of genuine and fake copper-alloy coins. Quantum Electron. 41 (7), 663–668. Cesareo, R., Rizzutto, M.A., Brunetti, A., Rao, D.V., 2009. Metal location and thickness in a multilayered sheet by measuring Kα/Kβ, Lα/Lβ and Lα/Lγ X-ray ratios. Nucl. Instrum. Methods Phys. Res. B 267, 2890–2896. http://dx.doi.org/10.1016/ j.nimb.2009.06.119. Ferretti, M., 2014. The investigation of ancient metal artefacts by portable X-ray fluorescence devices. J. Anal. . Spectrom. 29, 1753–1766. http://dx.doi.org/ 10.1039/C4JA00107A. Gorghinian, A., et al., 2013. XRF analysis of Roman imperial coins. Nuclear Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At. 309, 268–271. http://dx.doi. org/10.1016/j.nimb.2013.02.022. Giumlia-Mair, A., Lucchini, E., 2005. Surface analyses on modern and ancient copper based fakes. Surf. Eng. 21 (5–6), 406–410. http://dx.doi.org/10.1179/ 174329305 64402. Kanngießer, B., et al., 2004. 3D Micro-XRF investigations of paint layers with tabletop set-up. Spectrochimica Acta Part B At. Spectrosc. 60 (1), 41–47. http://dx. doi.org/10.1016/j.sab.2004.10.012. MCNPX™ 2.4.0 user's manual, 2002. Monte Carlo N-Particle Transport Code System for Multiparticle and High Energy Applications. Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA. Milazzo, M., Cicardi, C., 1997. Simple methods for quantitative X-Ray fluorescence analysis of ancient metal objects of archaeological interest. X-Ray Spectrom. 26, 211–216, doi: 10.1002/(SICI)1097–4539(199707)26:4 o 211::AID-XRS214 43.0. CO;2-G. Richtera, L., et al., 2014. Falza rakousko-uherských zlatníků, Folia numismatica: supplementum Ad Acta Musei Moraviae/Brno. Morav. zemské Muz. 28 (2), 191–221. Richtera, L., Zmrzlý, M., 2013. Identifikace způsobů pokovení mincovních falz a možnost jejich citlivého ošetření, Ochrana mincí a medailí v muzejních sbírkách. MCK, Brno, Technické muzeum v Brně, 7-58. ISBN: 978-80-87896-00- 6. Trojek, T., Čechák, T., Musílek, L., 2007. Techniques for depth heterogeneity identification in X-ray fluorescence. Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At. 263 (1), 76–78. http://dx.doi.org/10.1016/j. nimb.2007.04.064. Trojek, T., Čechák, T., 2007. Use of MCNP code in energy dispersive X-ray fluorescence. Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At. 263 (1), 72–75. http://dx.doi.org/10.1016/j.nimb.2007.04.063. Trojek, T., Hložek, M., 2012. X-ray fluorescence analysis of archaeological finds and art objects: recognizing gold and gilding. Appl. Radiat. Isot. 7, 1420–1423. Vlachou, C., McDonnell, J.G., Janaway, R.C., 2002. Experimental investigation of silvering in late Roman coinage. MRS Proceedings, 712, (II9.2). Doi:10.1557/ PROC-712-II9.2.
Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF M. Hložek a, T. Trojek b,n a b
Methodical Centre of Conservation – Technical Museum in Brno, Purkyňova 105, 612 00 Brno, Czech Republic Czech Technical University in Prague, Department of Dosimetry and Application of Ionizing Radiation, Břehová 7, 115 19 Prague, Czech Republic
H I G H L I G H T S
Tiny remains of plating on the surface of coins were recognized with micro-XRF. More techniques of counterfeiting of coins were revealed. The determined low fluorescence ratio Kα/Kβ of copper was an evidence of plating.
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2015 Received in revised form 12 August 2016 Accepted 13 August 2016
Archaeological surveys and metal detector prospecting yield a great amount of coins from the medieval period. Naturally, some of these are counterfeit which an experienced numismatist can determine without using chemical methods. The production of counterfeit coins in the middle ages took place in castles, caves or other remote areas where waste from this activity can still be found today – copper sheets, technical ceramics and counterfeit coins. Until recently, it has been assumed that medieval counterfeit coins are made by silver-plating copper blanks using an amalgam. However, the performed analyses reveal that there are many more techniques of counterfeiting of coins. Other techniques were based on e.g. tin amalgam plating of the blanks or alloying so-called white metal with silver-like appearance from which the coins were minted. Current chemical analyses indicate that the coins were often tinned by hot dipping with no amalgamation. Micro-X-ray fluorescence analysis has been chosen as a suitable non-destructive method to identify present chemical elements in investigated artifacts and to quantify their concentrations. In addition, a quick technique telltale the plating was applied. This technique utilizes the detected fluorescence ratio Kα/Kβ of copper, which is the main ingredient of a lot of historical metallic materials. & 2016 Elsevier Ltd. All rights reserved.
Keywords: X-ray fluorescence Microanalysis Counterfeit coin Plating
1. Introduction Amalgam gold-plated metal artefacts are often objects of study in the field of cultural heritage. These artefacts are mostly found during archaeological digs or they occur as medieval or modern pieces of art. It is less known that amalgam silver-plating and tinplating was used as well (Vlachou et al., 2002). Amalgam silverand tin-plating can mostly be found in forgeries of medieval coins. In gold-plating (silver-, tin-plating) in fire, an amalgam of gold (silver, tin), i.e. metal dissolved in mercury, was used. Mercury evaporates by the heat and a thicker layer of gold, silver or tin forms on the surface of the metal-plated object. At present, this technique is not used due to toxicity of mercury. n
Corresponding author. E-mail address: tomas.trojek@fjfi.cvut.cz (T. Trojek).
Money counterfeiting workshops can mainly be found in castles, caves (the Koněprusy Caves), and possibly in other remote places in the Czech Republic. The workshop from about 1440 in the Skály Castle from where a large number of blanks and forgeries come from was the most preserved. In the second half of the 15th century, there was a workshop in the Křídlo Castle in eastern Moravia. Unique discoveries of waste from the equipment of a counterfeiting workshop in the Žampach Castle in eastern Bohemia were made also. A fake coin can be recognized by means of a combination of elemental and microstructural analyses (Bartoli et al., 2011). X-ray fluorescence (XRF) analysis is one of the most frequently used analytical methods in investigating old metallic artefacts (Ferretti, 2014). Besides its non-destructivity, the XRF measurements and data analyses are usually quick and easy to perform (Milazzo and Cicardi, 1997). Till today, it has been successfully applied for characterization of mint and issue period of silver Roman coins
http://dx.doi.org/10.1016/j.radphyschem.2016.08.013 0969-806X/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i
M. Hložek, T. Trojek / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
(Gorghinian et al., 2013) or ancient and modern copper based fakes were identified (Giumlia-Mair and Lucchini, 2005). It seems that the counterfeit and original coins differ in concentrations of major and minor elements or the counterfeit coins were manufactured by means of coating of a metaling substrate. In this paper, we are dealing with the recognition of the plating of copper or copper-based alloys using XRF analysis. There are several XRF methods/modalities available for coating identification. These include confocal XRF analysis, a method based on the internal X-ray ratios of present elements, and a depth-profiling technique based on at least two measurements performed under different experimental conditions. The depthprofiling with confocal XRF requires a special instrumentation including a micro-focus X-ray tube with focusing X-ray optics and collimating optics on the detector side of the confocal XRF setup. It is useful especially for analysis of paintings consisting of layers with different pigments (Kanngießer et al., 2004). An approximate depth distribution of an element can be also determined with its internal X-ray ratios (Kα/Kβ or Lα/Lβ) acquired with an X-ray detector (Cesareo et al., 2009). Thanks to different absorption of particular X-ray lines in a sample matrix, the deeper the element is located under the surface the more these ratios are affected. Last but not least, when two XRF measurements are performed under different conditions analysing different surface thicknesses, the coating can be identified also (Trojek et al., 2007) or (Trojek and Hložek, 2012).
Fig. 1. Photograph of coin #1 – forgery of the coin of Ferdinand I of Habsburg.
2. Experimental setup and data analysis The coins were analyzed with a micro-XRF system because some of them were damaged and only tiny remains of their plating were found. The micro-XRF device consists of a molybdenum anode X-ray tube (XOS, Power Flux PF) with polycapillary focusing optics (minimal FWHM 15 mm for Mo-Kα line) and an SDD detector (Amptek, 25 mm2 0.5 mm). The X-ray tube was operated at a maximum voltage of 50 kV and a maximum current of 1 mA. The detector was located approximately 10 mm from a sample. The geometry arrangement was 90°/45°. The acquisition time was 30 s for individual analyses and only 2 s per spot in the case of XRF scanning. The goal of the XRF analysis was to identify present elements and to identify the coating. The approximate depth distribution of an element can be determined with its internal X-ray ratios acquired with an X-ray detector, e.g. Kα/Kβ. In this particular case, the Kα/Kβ ratio of copper, the major element in all specimens, was evaluated. This ratio is about 6–7 for most thick homogeneous copper alloys. Its value depends on attenuation of these copper K-lines in a matrix of a certain alloy. If such alloy or pure copper is plated, the ratio is reduced in most of cases. The X-ray fluorescence spectra were evaluated with the code WinAXIL that made it possible for us to determine the net peak areas of both Kα and Kβ lines independently of each other. The code MCNPX version 2.4.0 (MCNPX™ 2.4.0 user's manual, 2002) was used for calculating the Cu Kα/Kβ ratio for the given experimental conditions and the specimen composition.
Fig. 2. XRF spectrum of coin #1; analyzed area marked in Fig. 1.
Fig. 3. Photograph of coin #2 – forgery of the pfennig of Ottokar IV of Styria.
3. Results Several dozen old coins suspected to be counterfeits were analyzed with the micro-XRF system at the Czech Technical University in Prague but only three of them are discussed in detail in this article. These three coin specimens and their XRF spectra are shown in Figs. 1–6. These coins are supposed to be forgeries of the following coins:
Fig. 4. XRF spectrum of coin #2; analyzed area marked in Fig. 3.
Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i
M. Hložek, T. Trojek / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 5. Photograph of coin #3 – forgery of the coin of Franz Joseph I of Habsburg.
Fig. 6. XRF spectrum of coin #3; analyzed area marked in Fig. 5.
#1: coin of Ferdinand I of Habsburg (1526–1564) #2: pfennig of Ottokar IV of Styria (1164–1192) #3: coin of Franz Joseph I of Habsburg (1869–1916) For each coin, at least 2 areas with a diameter of approximately 100–200 mm were analyzed. That should reduce the effect of sample heterogeneity. The plating identification based on the copper Kα/Kβ ratio is demonstrated in Table 1 which summarizes the XRF analyses of these counterfeit coins and three standard reference materials (SRM). The brass SRM was used as a comparative specimen of known composition for quantitative analysis and Monte Carlo simulations. The applied Monte Carlo method requires at least one SRM with the same element(s) which would be determined in analyzed samples (Trojek and Čechák, 2007). Two other SRM were measured and simulated to validate the Monte Carlo calculation method. The acquired XRF spectra were evaluated with the code WinAXIL and the Cu Kα/Kβ ratio was determined (third column in Table 1). The uncertainty of this ratio is derived from the fitting of
an individual spectrum. The Cu Kα/Kβ ratio is quite low for coins #1 and #2 and thus it indicates plating of copper-based alloys. On the other hand, the Cu Kα/Kβ ratio of the third coin is much higher than for the brass SRM. The following procedure was performed to prove the hypothesis about the plating of the two coins and to explain the high value of the Cu Kα/Kβ ratio for the last one. The elemental concentrations (second column in Table 1) were calculated with the code WinAxil using fundamental parameter method with normalization of concentrations. The specimens were considered to be thick and homogeneous, i.e. without plating. For the composition determined in this manner the corresponding Cu Kα/ Kβ ratio was calculated with the MCNPX code (forth column in Table 1). The uncertainty of this ratio is derived from the uncertainties of the Monte Carlo simulation and the measurement of the calibration brass SRM. The measured Cu Kα/Kβ ratio is considerably lower than the calculated ratio for the homogeneous material for those two coins. Thus, the results prove the plating. The last coin #3 has a bright hue but noble metals are absent. Since the calculated and measured ratios are almost identical, it indicates homogeneous distribution of copper, the coin is not plated, and the element concentrations in the second column are correct. This is an evidence of a forgery made of Cu-Zn-Ni alloy. Such high value of Cu Kα/Kβ ratio (in comparison with brass SRM) is caused by the high concentration of nickel which has the X-ray absorption edge just between the energies of Kα and the Kβ lines of copper, i.e. the Kβ line of copper is much more absorbed than the line Kα in a matrix with nickel. If a copper-zinc alloy is plated with nickel, the Cu Kα/Kβ ratio would be even higher. Since the measured and calculated Cu Kα/Kβ ratios are almost identical, the coin #3 has obviously homogeneous composition and the determined elemental concentrations imply that it is an example of modern forgery. The last coin #4 described in this article is a forgery from the counterfeiting workshop in the Starý Světlov Castle (15th century). Its whole surface was scanned with our micro-XRF system with a step of 200 mm. It is made of copper substrate and traces of silver and mercury were found in 2 areas on its surface. The copper Kα/ Kβ ratio decreases in those places where the remains of the coating have been preserved, see Fig. 7. Its value shows also small statistical fluctuations outside these areas because of short acquisition time (2 s per spot). The results demonstrate that medieval silver coins were counterfeited by means of several techniques. The most widespread technique of counterfeiting workshops was metal-plating of copper blanks using an amalgam of silver and tin. Archaeological finds of production waste are the main signs of coin counterfeiting in castles. These are first of all remains of copper sheets, which blanks and blanks with traces of metal-plating were made from. Analyses of coins, where surface traces of silver and mercury were identified by XRF spectrometer, could be the evidence of silver-plating using an amalgam. Other techniques used the principle of so called white metal preparation (Richtera et al., 2014); see the examples of modern-times coins. Chemical analyses show that fire metal-plating (tin-plating) without the use of an
Table 1 Measured and calculated ratios of copper Kα/Kβ lines for analyzed specimens and SRM; their difference indicates plating. Specimen
SRM SRM SRM Coin Coin Coin
– Brass – Pure copper – Silver #1 #2 #3
Element concentrations [%] (assuming homogeneity)
3
Cu Kα/Kβ ratio measured Cu Kα/Kβ ratio calculated with MCNPX (assuming homogeneity)
Certified: Cu 78.8, Zn 14.5, Sn 1.9, Pb 1.9, Ni 1.1, Fe 0.5 6.83 70.03 Cu 100 6.777 0.03 Ag 92.5 Cu 7.5 6.02 70.04 Ag 61.6, Cu 8.9, Hg 29.5 5.40 70.06 Cu 24.6, Sn 51.9, Pb 23.5 4.85 70.07 Cu 66.7, Zn 22.8, Ni 10.5 8.32 70.08
– 6.727 0.04 5.90 70.04 6.157 0.04 5.96 70.04 8.50 70.04
Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i
M. Hložek, T. Trojek / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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forgeries. We were able to distinguish between authentic coins and forgeries very quickly and effectively. Based on the results of the analyses, in forgeries, we are able to state if the blank was metal-plated with an amalgam or using fire-technique. Using micro-XRF, it was further proved that it is possible to identify the metal-plating traces even if the plates are covered with a layer of green-coloured corrosion products. The evaluation of the copper Kα/Kβ ratio provided us with the approximate information on depth distribution of this element and thus, the plating of a copper-based material can be recognized.
References
Fig. 7. Kα/Kβ ratio of copper obtained with XRF scanning of the coin #4. Two circles show the areas with remains of silver-plating. Scanned area: 15 10 mm2.
amalgam, when copper blanks were metal-plated in a copper kettle in molten tin, was very common.
4. Conclusions In XRF analyses of coin forgeries, there are certain complications regarding interpretations of the obtained analytical data. When using a commercial XRF spectrometer, which does not enable a selection of the microscopic spot to be analyzed, it is impossible to find out whether a blank is metal-plated. In this case, average information of copper and tin (possibly silver) contents in the examined area will be obtained. If all the areas representing the remains of metal-plating are sufficiently small and locally damaged, the results of the analyses of copper- and tin-plated blanks are the same. Application of micro-XRF approach, which enables selective spot analysis or elemental mapping of a larger area of a coin blank, is recommendable. At common silver coinages, there are also certain difficulties regarding misinterpretation due to the effect of technology or corrosion. Silver coin blanks were relatively often whitened with wine stone before minting. Whitening of coins increased the impression and acted as corrosion protection at the same time. The obtained results of all the above mentioned methods applied on well preserved coins are always overvalued. Usually, more than 90% of Ag is detected, which does not correspond to the real purity of the coins (Richtera and Zmrzlý, 2013). The performed analyses showed that the micro-XRF analysis can be successfully used for non-destructive examination of coin
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Please cite this article as: Hložek, M., Trojek, T., Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF. Radiat. Phys. Chem. (2016), http://dx.doi.org/10.1016/j.radphyschem.2016.08.013i