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The role of archaeometallurgical characterization of ancient coins in forgery detection
Nuclear Inst. and Methods in Physics Research B 461 (2019) 247–255
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Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The role of archaeometallurgical characterization of ancient coins in forgery detection
T
Yussri Salem , Essam H. Mohamed ⁎
Conservation Department, Faculty of Archaeology, South Valley University, Qena, Egypt
ARTICLE INFO
ABSTRACT
Keywords: Ancient coins Authentication Elemental analysis SEM-EDX
The study of chemical composition and microstructural features of ancient coins provide researchers with invaluable and priceless information which can contribute to forgery detection. This study was conducted on nine copper and bronze coins preserved in a private collection in Egypt. Elemental analysis was performed to determine the major and trace elements of the coins as well as unalloyed inclusions in the metallic matrix. The microstructure was investigated using the optical and scanning electron microscopy. The results revealed that chemical composition could perfectly negate the authenticity of the coins through modernistic metal identification of major and minor elements. Also, the presence of common trace elements confirms likely the authentication and their absences preponderate counterfeit. Moreover, microstructural features related to ancient manufacturing process and internal damage can contribute to detect forgery.
1. Introduction Coins are important artifacts of the cultural heritage and have high artistic and cultural values. The writings, inscriptions, and figures on their surfaces can help to identify the age, city or country and the ruler of the coin's age. Thus, the coins are considered a record of lot of useful information about the minting period [1–6]. The authenticity of coins is increasingly important due to the increase in number of counterfeit coins with a similar look to authentic coins, the advances in replication technology, the coins are the most counterfeit objects of metallic artifacts, the ease in large numbers industry of counterfeit coins and to protect museums from internal theft and forgers replacement of original coins with false ones from museums is rare but possible. The authenticity of coins is increasingly important due to the increase in number of counterfeit coins with a similar look to the authentic ones. Due to the advances in replication technology, the coins are the most counterfeited objects of metallic artefacts. Provided by the massive industry of counterfeited coins, internal thefts may even happen in the museums, replacing the original coins with the fake ones [7]. The simulation and replication of the ancient coins are common because of many reasons as follows: The coins and their inscriptions are the easiest metallic artifacts in the simulation process. Many museums and individuals buy ancient coins. Therefore, the counterfeiters find a good market to sell their products, especially to
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those interested in buying and collecting ancient coins, but not having adequate experience to distinguish the original from the fake. Counterfeiting the archaeological coins started in Rome in the 18th century and became more widespread in the 19th century. The counterfeiters minted many coins of the Roman Empire. Such ancient counterfeit coins are more difficult than the contemporary ones in forgery detection because their distinction from the original requires examinations, analyses, and accurate observations. Even authenticators encounter difficulty in the forgery detection of these coins. That is, with advanced techniques, we are just discovering forgeries that were made a century or two ago [8,9]. The forged coins clearly increased in the 21st century due to the advances in replication technology [9–11] Therefore, authenticity is an important issue to protect the museums from counterfeit coins. Even the presence of coins in the museum is not strong evidence of their authenticity because they are obtained from many sources other than the archaeological excavations such as donations, confiscation, and buying from individuals [6,10]. These sources are unreliable and the presence of counterfeit coins in the museum is possible [7]. The authenticity of coins stands as a way of determining whether it is real (authentic) or not [12]. The experience of authentication experts was the first method to determine authenticity of coins. Microscopic and macroscopic details of the two surfaces of the coin are usually used for forgery detection of the counterfeit coins [4]. The counterfeiters pay
Corresponding author. E-mail address: [email protected] (Y. Salem).
https://doi.org/10.1016/j.nimb.2019.10.017 Received 14 September 2019; Received in revised form 13 October 2019; Accepted 15 October 2019 Available online 21 October 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
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attention to these details that are the fingerprints of the forgery [11]. Therefore, the basic principles of the authentication depend on the features and nature of the patina corrosion. The study of corrosion layer in ancient coins gives important data about the deterioration mechanisms that have occurred or are still occurring. This information can help to identify forged coins with an artificially grown patina [9]. However, this method is not useful with the clean coins [9]. Therefore, other methods from the metallic core besides the experience of the archaeologists and corrosion layer studies are required to reveal more information for forgery detection of such coins. The archaeometallurgical characterisation and elemental analysis studies of the metallic core can be useful for confirming or negating the authentication of the coins [9,13–15]. The coins were widely studied for archaeometry and the chemical composition [3]. Most of these studies were conducted to identify the mines of metal ores and the manufacturing technology [9,16]. The chemical composition of coins is useful in the authentication when there is an available database of real coins of the same type as the unknown ones [17,18]. In the absence of this database, some minor and trace elements can be used in supporting the authentication of coins. Platinum (Pt) is one of these elements. Pt is commonly bonded to silver in its ores. During the melting process in ancient metallurgical, Pt could not be totally separated due to the great difference in the melting temperatures between silver (700 °C) and platinum (1700 °C). Consequently, platinum remains as an impurity in silver [9]. However, contemporary techniques completely separate Pt from silver, and it does not appear in the fake coins. So, the presence of Pt is a strong evidence of authenticity. Also, the presence of lead (Pb) as an impurity in silver coins can be used for supporting their authentication as well as for general characterization of silver ores because the majority of silver used in the ancient times was extracted from argentiferous lead ores especifically cerussite [19,20]. The technology of silver production from lead ores might date back to the beginning or middle of the third millennium BC [20]. The silver metallurgy was developed from the lead melting technology, and metal silver was extracted from argentiferous lead ores [21–23]. Since the 19th century, an alloy named German or new silver has been commonly used for the production of fake silver coins and other objects. It was made of copper, zinc and nickel because nickel provides a color close to that of silver. The elemental analysis is sufficient to detect the forgery of these coins [7]. The microstructure features and elemental composition can be used in the authentication process; a few studies only addressed the authentication by the microstructure features. The present research aims to study the microstructure features and chemical composition of nine ancient coins to find and select some features that can be used in confirming or negating the authenticity.
microstructure can be seen. A microstructural investigation was conducted by the scanning electron microscope (SEM) with the following conditions: working distance 9.5 mm, current energy 30.00 kV, aperture size 414.00 μm and magnification of x1000. The optical microscope (OM) with polarized light was also used; BX51-P model manufactured by Olympus Company, eyes WH10X/22, the magnification of bottom lens 5, 10, 20, 50 and 100X, volt 100–120/220–240 and HZ: 50/60. The microscope is equipped with a Canon EOS Kiss X4 CCD camera. Examination was conducted after and before the etching. The first stage of the study is to identify the chemical composition and classification of the objects whether pure metal or alloy. The elemental analysis which was conducted before the etching was performed by an SEM (JEOL 840A and Zeiss EVO MA-15 at 20 kV) together with energy dispersive x-ray (EDX – LINK AN 9500 and INCA Energy 350). Also the elemental analyses of some cleaning coins were carried out by energy dispersive X-ray fluorescence system (EDX-RF) (JEOL JSX 3222 model) [24]. 3. Results and discussion 3.1. Description and status of the studied coins The study covered nine ancient corroded coins separated into two groups according to the deterioration state. The first group consisted of four coins (A1 to A4) in a quite good preservation state. The inscriptions and writings on the surfaces appeared clearly because their surface did not contain thick corrosion products; only aging patina which was mostly from oxides was formed (Fig. 1). Based on the inscriptions of the reverse and obverse, the coins A1 and A2 belong to the Hellenistic coins. The obverse depicts the head of the ruler with no legends. On the reverse, an inscription of a statue holding a stick in the left hand and pointing with the other hand as well as some inscription of a legend written in Greek. Since the lettering is readable, this coin can be characterized. The type of coins fit Hellenistic coins especially of the Seleukid emperor Antiochus IV Epiphanes (175–164 BCE), who was attacking Egypt in 170 BCE. The term “EPIPHANOS” does not correspond to Ptolomy V, since the reverse of its coins contains an eagle. Also some coins of Alexander contains the representation of Poseidon but the figure of Alexander always appears in the obverse of the coins with its representation “horned” “Zulcairn”. Some Bactrians coins of the II century BCE also have the image of Poseidon on the reverse. About Antiochus IV that the two coins belong to him, was the younger son of Antiochus III. He was sent to Rome as a hostage after the Peace of Apamea. He fought the Egyptians, destroyed the Temple of Jerusalem and won several victories over Ptolemy VI Philometor (180–145 BCE) The coins A3 and A4 are a common Ottoman coinage style. According to the inscriptions in Arabic, these coins date back to 1885 CE, saying 'minted in Constantinople in 1255 A.H. The obverse side shows common inscriptions of the Ottoman coins called Tughra; the signature style of the Ottoman rulers. Tughra of these two coins is a
2. Material and methods A cross-section was prepared to investigate the metallic structure and elemental analyses, small samples (2 × 2 mm) were cut from the coins by a jewelry saw. They were placed in a plastic cylindrical mold and were fixed in a way that would be better for metallographic investigation. The resin was mixed with the hardener and was directly poured in the mold. Then, the samples were polished with emery paper (800–4000 grit) and diamond paste of 1 and 3 μm. The polishing is the most important process for preparing a cross-section to obtain a smooth surface. In addition, the etching of the cross-section is very important to reveal some features of the microstructure that do not appear without this procedure. Nital solution was 10-ml sulfuric acid (98%), 100 ml potassium dichromate solution K2Cr2O7 dissolved in distilled water and 2 ml sodium chloride solution saturated in distilled water [24]. Two drops of the etching solution were applied on the coupons surface, each time for 5 s and etching was repeated 3 times. After every time, the solution was removed and the coupon was rinsed with distilled water and dried. After 10 s from the third etching, the details of the
Fig. 1. The coins of the first group A, the obverse side (up) and the reverse side (down). 248
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corrosion products mixed with deposits of the burial environment. However, the inscriptions below a corrosion layer could be slightly seen. According to these Arabic letters, the coin belonged to the Ottoman coins. 3.2. The characterization of the metallic microstructure The analysis of elemental composition is a fundamental tool in the study of ancient coins. It should be conducted on a cross-section to reveal the metallic core because some coins are covered with a layer of corrosion products or metal oxide [25]. The outer layer of the surface is sometimes exposed to the enrichment of the noble constituents due to corrosion processes [6,26]. The scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) is one of the most important analysis techniques of metallic artifacts. The elemental analysis using EDX of the microstructure during metallographic investigation and under high magnification gives very important data related to the technology, authenticity, phases of microstructure, insoluble inclusion, and chemical composition [16]. The elemental analysis of major, minor, and trace elements is a classical approach for ancient coins' authentication especially when the chemical composition of real coins similar to the analysed ones is available. Then, the chemical composition of the real coins is compared with the unknown coins [9,25,27]. The results of the elemental analysis of the studied coins were as follows:
Fig. 2. The coins of the second group B, the obverse side (left) and the reverse side (right).
signature of the Ottoman ruler Abdul Majeed I ibn Mahmoud II (1255–1277 A.H./1839/1861 A.D). On the reverse side, like all Ottoman coins, the date and place of minting were inscribed (in Constantinople in 1277 A.H.). The second group involves the coins (B1 to B5) (Fig. 2) in a bad state. They deeply suffered from green and red corrosion products. The corrosion on the surface was formed with a sub-millimeter thickness. The inscription on the surface is barely visible, but it is possible to define the age of each coin. All the coins of the second group were identified as examples of the Roman coinage system under the rule of Roman emperors, except for coin B5 which was Ottoman. The coins B1 and B2 were small. They were covered with a thick corrosion layer and reached a badly corroded state. The thickness and weight slightly increased due to the corrosion layer. This layer was a red copper corrosion as in the coin B2 or mixture of green corrosion products and reddish brown as in coin B1. The legend inscriptions on the reverse side were mineralized and their details became unclear. The ruler's head on the obverse side could be barely seen and its details were unclear. Therefore, it was not even possible to define their age. Some unclear letters on the obverse side of the coin B1 existed. The coins B3 and B4 belonged to the Roman imperial age; they had good metallic state and the details of the depicted head on obverse can be clearly seen because the surface corrosion layer is very thin. The figures on the reverse side were unclear and no legend inscriptions were present on both two sides. The coin B5 was completely covered with a thick green layer of
3.2.1. The coins A1 and A2 EDX analysis confirms that the coins A1 and A2 were made of zinc 71 wt% as main element, aluminium 20 wt%, and copper 6.7 wt% (Fig. 3). The inscriptions and writings on the reverse and obverse are strictly compatible with ancient coins of the imperial Roman age. Based on the results of elemental analysis, it is possible to confirm the forgery of these coins because the aluminium was not discovered before the mid-nineteenth century and zinc was often a secondary element in ancient alloys. This evidence is sufficient to prove the counterfeiting of these coins. The metallographic examination of the coins A1 and A2 by SEM and PM reveals dendritic structure (Fig. 4). This structure was one of the common types of ancient alloys, especially bronze alloys. This structure indicates that the coin was manufactured by the ancient traditional
Fig. 3. EDS spectra and SEM micrographs of the cross-section of coin A1. 249
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Fig. 4. OM (left) and SEM-EDX (right) of the metallic core of the coin A2 reveal the dendritic structure with a composition close to a eutectic point.
methods but this is not an evidence to prove the authenticity. The dendritic structure appeared as a result of cooling and casting processes and in most cases it is formed in alloys due to the different melting points a metal melts before another one. Therefore, a metal melts before another one. The shape and size of the dendritic structure depended on the rate of solidification and cooling [18]. EDX (Fig. 4) confirmed that the structure of this coin was formed as the dendrite arms of zinc and copper (gray) surrounded by the eutectic mixture of zinc, aluminum and copper, consisting of α and β phases as a fine interlaced mixture. The microstructure is clearly visible. It is concluded that the cooling was very slow as a simulation of the ancient manufacturing processes. It is evidence of the counterfeiters' attention to all details to obtain good simulated products. The phase diagram also reveals large dendrites of zinc and copper of α phase with smaller areas of the eutectic mixture of the α + β.
manufactured by cold-working and annealing that involved casting in the mold as a metal piece, putting in a hammering mold, and exposing to the striking process with multiple annealing. This information helps proving the authenticity and indicates that the coin was manufactured by ancient traditional methods. The examination reveals intergranular cuprite corrosion with red color. This kind of the corrosion is a highly reliable indicator of an extended period of burial and the long age of these coins. It is strong evidence of authenticity [1–3,5,12–15]. 3.2.3. The coin B1 The elemental analysis shows that coin B1 is a binary alloy of 91.1% copper and 8.2% lead as well as tin and iron as impurities Fig. 7. This alloy is uncommon because the main binary alloy of copper was with silver or tin. Some studies also argue that lead could have been mistaken for tin or lead was probably found with copper as a single mineral [22,25]. These assumptions are not commensurate with the high experience of the ancient craftsmen. With the concentration available, it is possible to confirm that lead was added intentionally to the copper to make a binary alloy. The addition of lead instead of silver or tin depends on the economic conditions of the minting period where lead was cheaper than silver or tin. In addition, there are other technological reasons related to the industry process such as the preference for liquidity and casting [2,6,29,30], decreasing the copper fraction and making the coin a little lighter. This result does not give any evidence or data of the authenticity or falsity, but it gives information about the economic status of the minting period. The metallographic investigation of coin B1 using SEM (Fig. 8) shows two phases of the metallic core [29]: the copper-rich phase which is the main phase and appears dark and the second phase which appears as grey areas containing a mixture of copper and lead. This distribution was confirmed by EDS, which also shows insoluble lead particles as another alloy phase, seen as white spots. Microcracks and surface corrosion appeared in lead-rich < beta > phase because the
3.2.2. The coins A3, A4 and B5 EDX-RF analysis of the clean surface of the Ottoman coins A3, A4 and B5 (Fig. 5) indicated that the coins were made of copper (97 wt%) with some impurities associated with its ores. The metallography examination of the ancient coins minted in a mold with the repetitive hammering and annealing reveals many important microstructural features such as recrystallized structure, twinned grains and slip lines [3,21,22,27,28]. An obvious example of these aspects is presented in the photomicrograph of coin A3 and A4. These features were formed due to the plastic deformation of metallic core through hammering and annealing. The coins A3, A4 and B5 had similar microstructures. The etching cross-sections of these coins (Fig. 6) showed many microstructure features such as: fully recrystallized, annealing twins and thin strain lines within the grains. A lot of slag inclusions were revealed as black spots. The elongation of inclusion spots was in one direction, indicating the hammering direction. These features confirmed that the two coins were
Fig. 5. A typical EDX-RF spectrum of the coin A3. 250
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Fig. 6. Metallographic investigation of the cross-section of coin 9, (a, b and c) OM shows a fully recrystallized structure, twinned grains, slag inclusions and straight twin lines within the grains. (d) SEM micrograph of the crystallized phases of an entrapped slag inclusion in the copper matrix.
Fig. 7. EDS spectra and SEM micrographs of the cross-section of coin B1. 251
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Fig. 8. SEM micrograph of the etching cross-section of coin B1 revealing two phases of the metallic structure.
Fig. 9. EDS spectra and SEM micrographs of the cross sections of coin B2.
cracks developed as a result of the striking process. The lead-rich phase is less hard than the copper-rich phase. Thus, the effect of the striking process appeared on the lead phase as microcracks and did not appear on the other phase.
The corrosion layer on the surface does not confirm the authenticity of this coin and EDX-SEM confirms the artificial formation of the layer. Many pieces of evidence suggest this formation. For example, the layer appeared as a deposited artificial layer on the surface and not as a corrosion layer resulting from interaction and reaction with the metallic core. The metallographic investigation reveals that the interaction between the metallic core and the corrosion layer does not exist in these coins and the dividing area was constantly found between the two layers. Also, the layer consists of a single corrosion product with one color and equal thickness.
3.2.4. The coin B2 EDX of three points of the coin B2 (Fig. 9) reveals that copper was 100% and no minor elements or impurities were found. This composition does not match the composition of archaeological coins which always contained natural impurities that are mixed with metal ores [2,4,30]. The ancient coins usually showed many trace elements e.g. (Ni, Fe, Mn, Sb, Co, Si Ti, Bi, Au, Pt, P, Al, Pb, and As), which were detected in ancient coins of Ag-Cu alloy [1,6,25,31,32]. These minor/ trace elements mainly originated from different ores or from varying manufacturing processes used in Ag production [11]. The coins of copper and bronze commonly contained additional elements such as Zn, Ag, Pb, As, Sb, S, and Fe [2,5,12,33]. They are present as impurities originating from copper ores [31]. The modern advanced technology used in the purification of copper does not transmit the impurities from the ore into the metal [8]. With this information, it is not possible to confirm that this coin is authentic.
3.2.5. The coins B3 The elemental analysis of the coin B3 shows three main elements: copper, tin and lead (Fig. 10). The average of these elements reports that the matrix of this coin was constituted by a ternary bronze alloy Cu-Sn-Pb with copper (78.12%, tin (6.50%) and lead (15.29%). The sample B3 reveals a microstructure similar to that of the coin B1. The main phase is copper-rich which appears as dark gray and contains many pores. The second phase is lead-rich phase which appears as whitish gray. The last phase involves chlorine-rich areas as internal corrosion product and it appears under SEM as smooth areas 252
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Fig. 10. EDS spectra and SEM micrographs of the cross-sections of coin B3 showing internal corrosion products corresponding to chlorine in EDS spectra.
with dark gray color. SEM-EDX confirm these phases and their distribution (Fig. 11). This type of structure proves the authenticity of this coin because the internal corrosion in the structure is due to long-term reactions and completely negates the forgery process.
4. Conclusion The archaeometallurgical characterisation of the ancient coins contributes to confirming or negating their authenticity, as follows: Some results of the elemental analysis can confirm the forgery as in the coins A1 and A2. Aluminum was revealed as a minor metal of the Roman coins although it was not discovered before the mid-nineteenth century. The results of elemental analysis of trace elements are sometimes evidence of the authenticity and provide researchers with information about the ores sources of the metals. The features of metallic structure which attributed to ancient manufacturing techniques, e.g., recrystallized structure, twinned grains, thin strain lines within the grains, and slag inclusions, help proving the authenticity of ancient coins. They are revealed in the coin A3 and indicate that the coin was manufactured by ancient traditional methods. The internal deterioration features of the metallic structure, including cracks, interlayer corrosion, intergranular corrosion and discontinuous precipitation of copper at the grain boundaries, are strong and conclusive evidence of the authenticity and completely deny the counterfeiting process. They are conclusive evidence of antiquity of this coin. The information obtained can be also useful in understanding the
3.2.6. The coin B4 The elemental analysis of the metallic core area (sample centre) reveals that the coin B4 consisted of copper (98.7%) in addition to tin and zinc as impurities (Fig. 12a–c). The analysis of the interlayer between the metallic core and the outer corrosion layer reveals high concentration of lead element in three points (4, 6, 11%) in addition to others elements attributed to corrosion products such as Cl and O (Fig. 12d–f). The presence of lead in the interlayer may be explained by the following reasons: manufacturing defect, burying the coin adjacent to or in contact with other objects of lead or its alloys, coating the coin with lead [34]. The official counterfeits of coins with a 0.2–0.5 mm another metals layer were all found with silver used as the coating layer [35]. The last possibility is the strongest because the lead layer appeared clearly along the cross section. The coating of ancient copper and bronze coins was a common practice in the Roman republican coinage and was performed by depositing a thin layer of silver or gold [36,37].
Fig. 11. SEM micrographs and the analysed area by EDS of the cross-section of coin B3 showing three regions: (a) copper-rich phase, (b) lead-rich phase (c) chlorinerich areas. 253
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Fig. 12. EDS spectra and SEM micrographs of the cross-sections of coin B4.
economic processes of coinage and answers specific questions related to chronology, ores provenance, and ancient technology.
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The role of archaeometallurgical characterization of ancient coins in forgery detection
T
Yussri Salem , Essam H. Mohamed ⁎
Conservation Department, Faculty of Archaeology, South Valley University, Qena, Egypt
ARTICLE INFO
ABSTRACT
Keywords: Ancient coins Authentication Elemental analysis SEM-EDX
The study of chemical composition and microstructural features of ancient coins provide researchers with invaluable and priceless information which can contribute to forgery detection. This study was conducted on nine copper and bronze coins preserved in a private collection in Egypt. Elemental analysis was performed to determine the major and trace elements of the coins as well as unalloyed inclusions in the metallic matrix. The microstructure was investigated using the optical and scanning electron microscopy. The results revealed that chemical composition could perfectly negate the authenticity of the coins through modernistic metal identification of major and minor elements. Also, the presence of common trace elements confirms likely the authentication and their absences preponderate counterfeit. Moreover, microstructural features related to ancient manufacturing process and internal damage can contribute to detect forgery.
1. Introduction Coins are important artifacts of the cultural heritage and have high artistic and cultural values. The writings, inscriptions, and figures on their surfaces can help to identify the age, city or country and the ruler of the coin's age. Thus, the coins are considered a record of lot of useful information about the minting period [1–6]. The authenticity of coins is increasingly important due to the increase in number of counterfeit coins with a similar look to authentic coins, the advances in replication technology, the coins are the most counterfeit objects of metallic artifacts, the ease in large numbers industry of counterfeit coins and to protect museums from internal theft and forgers replacement of original coins with false ones from museums is rare but possible. The authenticity of coins is increasingly important due to the increase in number of counterfeit coins with a similar look to the authentic ones. Due to the advances in replication technology, the coins are the most counterfeited objects of metallic artefacts. Provided by the massive industry of counterfeited coins, internal thefts may even happen in the museums, replacing the original coins with the fake ones [7]. The simulation and replication of the ancient coins are common because of many reasons as follows: The coins and their inscriptions are the easiest metallic artifacts in the simulation process. Many museums and individuals buy ancient coins. Therefore, the counterfeiters find a good market to sell their products, especially to
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those interested in buying and collecting ancient coins, but not having adequate experience to distinguish the original from the fake. Counterfeiting the archaeological coins started in Rome in the 18th century and became more widespread in the 19th century. The counterfeiters minted many coins of the Roman Empire. Such ancient counterfeit coins are more difficult than the contemporary ones in forgery detection because their distinction from the original requires examinations, analyses, and accurate observations. Even authenticators encounter difficulty in the forgery detection of these coins. That is, with advanced techniques, we are just discovering forgeries that were made a century or two ago [8,9]. The forged coins clearly increased in the 21st century due to the advances in replication technology [9–11] Therefore, authenticity is an important issue to protect the museums from counterfeit coins. Even the presence of coins in the museum is not strong evidence of their authenticity because they are obtained from many sources other than the archaeological excavations such as donations, confiscation, and buying from individuals [6,10]. These sources are unreliable and the presence of counterfeit coins in the museum is possible [7]. The authenticity of coins stands as a way of determining whether it is real (authentic) or not [12]. The experience of authentication experts was the first method to determine authenticity of coins. Microscopic and macroscopic details of the two surfaces of the coin are usually used for forgery detection of the counterfeit coins [4]. The counterfeiters pay
Corresponding author. E-mail address: [email protected] (Y. Salem).
https://doi.org/10.1016/j.nimb.2019.10.017 Received 14 September 2019; Received in revised form 13 October 2019; Accepted 15 October 2019 Available online 21 October 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
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attention to these details that are the fingerprints of the forgery [11]. Therefore, the basic principles of the authentication depend on the features and nature of the patina corrosion. The study of corrosion layer in ancient coins gives important data about the deterioration mechanisms that have occurred or are still occurring. This information can help to identify forged coins with an artificially grown patina [9]. However, this method is not useful with the clean coins [9]. Therefore, other methods from the metallic core besides the experience of the archaeologists and corrosion layer studies are required to reveal more information for forgery detection of such coins. The archaeometallurgical characterisation and elemental analysis studies of the metallic core can be useful for confirming or negating the authentication of the coins [9,13–15]. The coins were widely studied for archaeometry and the chemical composition [3]. Most of these studies were conducted to identify the mines of metal ores and the manufacturing technology [9,16]. The chemical composition of coins is useful in the authentication when there is an available database of real coins of the same type as the unknown ones [17,18]. In the absence of this database, some minor and trace elements can be used in supporting the authentication of coins. Platinum (Pt) is one of these elements. Pt is commonly bonded to silver in its ores. During the melting process in ancient metallurgical, Pt could not be totally separated due to the great difference in the melting temperatures between silver (700 °C) and platinum (1700 °C). Consequently, platinum remains as an impurity in silver [9]. However, contemporary techniques completely separate Pt from silver, and it does not appear in the fake coins. So, the presence of Pt is a strong evidence of authenticity. Also, the presence of lead (Pb) as an impurity in silver coins can be used for supporting their authentication as well as for general characterization of silver ores because the majority of silver used in the ancient times was extracted from argentiferous lead ores especifically cerussite [19,20]. The technology of silver production from lead ores might date back to the beginning or middle of the third millennium BC [20]. The silver metallurgy was developed from the lead melting technology, and metal silver was extracted from argentiferous lead ores [21–23]. Since the 19th century, an alloy named German or new silver has been commonly used for the production of fake silver coins and other objects. It was made of copper, zinc and nickel because nickel provides a color close to that of silver. The elemental analysis is sufficient to detect the forgery of these coins [7]. The microstructure features and elemental composition can be used in the authentication process; a few studies only addressed the authentication by the microstructure features. The present research aims to study the microstructure features and chemical composition of nine ancient coins to find and select some features that can be used in confirming or negating the authenticity.
microstructure can be seen. A microstructural investigation was conducted by the scanning electron microscope (SEM) with the following conditions: working distance 9.5 mm, current energy 30.00 kV, aperture size 414.00 μm and magnification of x1000. The optical microscope (OM) with polarized light was also used; BX51-P model manufactured by Olympus Company, eyes WH10X/22, the magnification of bottom lens 5, 10, 20, 50 and 100X, volt 100–120/220–240 and HZ: 50/60. The microscope is equipped with a Canon EOS Kiss X4 CCD camera. Examination was conducted after and before the etching. The first stage of the study is to identify the chemical composition and classification of the objects whether pure metal or alloy. The elemental analysis which was conducted before the etching was performed by an SEM (JEOL 840A and Zeiss EVO MA-15 at 20 kV) together with energy dispersive x-ray (EDX – LINK AN 9500 and INCA Energy 350). Also the elemental analyses of some cleaning coins were carried out by energy dispersive X-ray fluorescence system (EDX-RF) (JEOL JSX 3222 model) [24]. 3. Results and discussion 3.1. Description and status of the studied coins The study covered nine ancient corroded coins separated into two groups according to the deterioration state. The first group consisted of four coins (A1 to A4) in a quite good preservation state. The inscriptions and writings on the surfaces appeared clearly because their surface did not contain thick corrosion products; only aging patina which was mostly from oxides was formed (Fig. 1). Based on the inscriptions of the reverse and obverse, the coins A1 and A2 belong to the Hellenistic coins. The obverse depicts the head of the ruler with no legends. On the reverse, an inscription of a statue holding a stick in the left hand and pointing with the other hand as well as some inscription of a legend written in Greek. Since the lettering is readable, this coin can be characterized. The type of coins fit Hellenistic coins especially of the Seleukid emperor Antiochus IV Epiphanes (175–164 BCE), who was attacking Egypt in 170 BCE. The term “EPIPHANOS” does not correspond to Ptolomy V, since the reverse of its coins contains an eagle. Also some coins of Alexander contains the representation of Poseidon but the figure of Alexander always appears in the obverse of the coins with its representation “horned” “Zulcairn”. Some Bactrians coins of the II century BCE also have the image of Poseidon on the reverse. About Antiochus IV that the two coins belong to him, was the younger son of Antiochus III. He was sent to Rome as a hostage after the Peace of Apamea. He fought the Egyptians, destroyed the Temple of Jerusalem and won several victories over Ptolemy VI Philometor (180–145 BCE) The coins A3 and A4 are a common Ottoman coinage style. According to the inscriptions in Arabic, these coins date back to 1885 CE, saying 'minted in Constantinople in 1255 A.H. The obverse side shows common inscriptions of the Ottoman coins called Tughra; the signature style of the Ottoman rulers. Tughra of these two coins is a
2. Material and methods A cross-section was prepared to investigate the metallic structure and elemental analyses, small samples (2 × 2 mm) were cut from the coins by a jewelry saw. They were placed in a plastic cylindrical mold and were fixed in a way that would be better for metallographic investigation. The resin was mixed with the hardener and was directly poured in the mold. Then, the samples were polished with emery paper (800–4000 grit) and diamond paste of 1 and 3 μm. The polishing is the most important process for preparing a cross-section to obtain a smooth surface. In addition, the etching of the cross-section is very important to reveal some features of the microstructure that do not appear without this procedure. Nital solution was 10-ml sulfuric acid (98%), 100 ml potassium dichromate solution K2Cr2O7 dissolved in distilled water and 2 ml sodium chloride solution saturated in distilled water [24]. Two drops of the etching solution were applied on the coupons surface, each time for 5 s and etching was repeated 3 times. After every time, the solution was removed and the coupon was rinsed with distilled water and dried. After 10 s from the third etching, the details of the
Fig. 1. The coins of the first group A, the obverse side (up) and the reverse side (down). 248
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corrosion products mixed with deposits of the burial environment. However, the inscriptions below a corrosion layer could be slightly seen. According to these Arabic letters, the coin belonged to the Ottoman coins. 3.2. The characterization of the metallic microstructure The analysis of elemental composition is a fundamental tool in the study of ancient coins. It should be conducted on a cross-section to reveal the metallic core because some coins are covered with a layer of corrosion products or metal oxide [25]. The outer layer of the surface is sometimes exposed to the enrichment of the noble constituents due to corrosion processes [6,26]. The scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) is one of the most important analysis techniques of metallic artifacts. The elemental analysis using EDX of the microstructure during metallographic investigation and under high magnification gives very important data related to the technology, authenticity, phases of microstructure, insoluble inclusion, and chemical composition [16]. The elemental analysis of major, minor, and trace elements is a classical approach for ancient coins' authentication especially when the chemical composition of real coins similar to the analysed ones is available. Then, the chemical composition of the real coins is compared with the unknown coins [9,25,27]. The results of the elemental analysis of the studied coins were as follows:
Fig. 2. The coins of the second group B, the obverse side (left) and the reverse side (right).
signature of the Ottoman ruler Abdul Majeed I ibn Mahmoud II (1255–1277 A.H./1839/1861 A.D). On the reverse side, like all Ottoman coins, the date and place of minting were inscribed (in Constantinople in 1277 A.H.). The second group involves the coins (B1 to B5) (Fig. 2) in a bad state. They deeply suffered from green and red corrosion products. The corrosion on the surface was formed with a sub-millimeter thickness. The inscription on the surface is barely visible, but it is possible to define the age of each coin. All the coins of the second group were identified as examples of the Roman coinage system under the rule of Roman emperors, except for coin B5 which was Ottoman. The coins B1 and B2 were small. They were covered with a thick corrosion layer and reached a badly corroded state. The thickness and weight slightly increased due to the corrosion layer. This layer was a red copper corrosion as in the coin B2 or mixture of green corrosion products and reddish brown as in coin B1. The legend inscriptions on the reverse side were mineralized and their details became unclear. The ruler's head on the obverse side could be barely seen and its details were unclear. Therefore, it was not even possible to define their age. Some unclear letters on the obverse side of the coin B1 existed. The coins B3 and B4 belonged to the Roman imperial age; they had good metallic state and the details of the depicted head on obverse can be clearly seen because the surface corrosion layer is very thin. The figures on the reverse side were unclear and no legend inscriptions were present on both two sides. The coin B5 was completely covered with a thick green layer of
3.2.1. The coins A1 and A2 EDX analysis confirms that the coins A1 and A2 were made of zinc 71 wt% as main element, aluminium 20 wt%, and copper 6.7 wt% (Fig. 3). The inscriptions and writings on the reverse and obverse are strictly compatible with ancient coins of the imperial Roman age. Based on the results of elemental analysis, it is possible to confirm the forgery of these coins because the aluminium was not discovered before the mid-nineteenth century and zinc was often a secondary element in ancient alloys. This evidence is sufficient to prove the counterfeiting of these coins. The metallographic examination of the coins A1 and A2 by SEM and PM reveals dendritic structure (Fig. 4). This structure was one of the common types of ancient alloys, especially bronze alloys. This structure indicates that the coin was manufactured by the ancient traditional
Fig. 3. EDS spectra and SEM micrographs of the cross-section of coin A1. 249
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Fig. 4. OM (left) and SEM-EDX (right) of the metallic core of the coin A2 reveal the dendritic structure with a composition close to a eutectic point.
methods but this is not an evidence to prove the authenticity. The dendritic structure appeared as a result of cooling and casting processes and in most cases it is formed in alloys due to the different melting points a metal melts before another one. Therefore, a metal melts before another one. The shape and size of the dendritic structure depended on the rate of solidification and cooling [18]. EDX (Fig. 4) confirmed that the structure of this coin was formed as the dendrite arms of zinc and copper (gray) surrounded by the eutectic mixture of zinc, aluminum and copper, consisting of α and β phases as a fine interlaced mixture. The microstructure is clearly visible. It is concluded that the cooling was very slow as a simulation of the ancient manufacturing processes. It is evidence of the counterfeiters' attention to all details to obtain good simulated products. The phase diagram also reveals large dendrites of zinc and copper of α phase with smaller areas of the eutectic mixture of the α + β.
manufactured by cold-working and annealing that involved casting in the mold as a metal piece, putting in a hammering mold, and exposing to the striking process with multiple annealing. This information helps proving the authenticity and indicates that the coin was manufactured by ancient traditional methods. The examination reveals intergranular cuprite corrosion with red color. This kind of the corrosion is a highly reliable indicator of an extended period of burial and the long age of these coins. It is strong evidence of authenticity [1–3,5,12–15]. 3.2.3. The coin B1 The elemental analysis shows that coin B1 is a binary alloy of 91.1% copper and 8.2% lead as well as tin and iron as impurities Fig. 7. This alloy is uncommon because the main binary alloy of copper was with silver or tin. Some studies also argue that lead could have been mistaken for tin or lead was probably found with copper as a single mineral [22,25]. These assumptions are not commensurate with the high experience of the ancient craftsmen. With the concentration available, it is possible to confirm that lead was added intentionally to the copper to make a binary alloy. The addition of lead instead of silver or tin depends on the economic conditions of the minting period where lead was cheaper than silver or tin. In addition, there are other technological reasons related to the industry process such as the preference for liquidity and casting [2,6,29,30], decreasing the copper fraction and making the coin a little lighter. This result does not give any evidence or data of the authenticity or falsity, but it gives information about the economic status of the minting period. The metallographic investigation of coin B1 using SEM (Fig. 8) shows two phases of the metallic core [29]: the copper-rich phase which is the main phase and appears dark and the second phase which appears as grey areas containing a mixture of copper and lead. This distribution was confirmed by EDS, which also shows insoluble lead particles as another alloy phase, seen as white spots. Microcracks and surface corrosion appeared in lead-rich < beta > phase because the
3.2.2. The coins A3, A4 and B5 EDX-RF analysis of the clean surface of the Ottoman coins A3, A4 and B5 (Fig. 5) indicated that the coins were made of copper (97 wt%) with some impurities associated with its ores. The metallography examination of the ancient coins minted in a mold with the repetitive hammering and annealing reveals many important microstructural features such as recrystallized structure, twinned grains and slip lines [3,21,22,27,28]. An obvious example of these aspects is presented in the photomicrograph of coin A3 and A4. These features were formed due to the plastic deformation of metallic core through hammering and annealing. The coins A3, A4 and B5 had similar microstructures. The etching cross-sections of these coins (Fig. 6) showed many microstructure features such as: fully recrystallized, annealing twins and thin strain lines within the grains. A lot of slag inclusions were revealed as black spots. The elongation of inclusion spots was in one direction, indicating the hammering direction. These features confirmed that the two coins were
Fig. 5. A typical EDX-RF spectrum of the coin A3. 250
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Fig. 6. Metallographic investigation of the cross-section of coin 9, (a, b and c) OM shows a fully recrystallized structure, twinned grains, slag inclusions and straight twin lines within the grains. (d) SEM micrograph of the crystallized phases of an entrapped slag inclusion in the copper matrix.
Fig. 7. EDS spectra and SEM micrographs of the cross-section of coin B1. 251
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Fig. 8. SEM micrograph of the etching cross-section of coin B1 revealing two phases of the metallic structure.
Fig. 9. EDS spectra and SEM micrographs of the cross sections of coin B2.
cracks developed as a result of the striking process. The lead-rich phase is less hard than the copper-rich phase. Thus, the effect of the striking process appeared on the lead phase as microcracks and did not appear on the other phase.
The corrosion layer on the surface does not confirm the authenticity of this coin and EDX-SEM confirms the artificial formation of the layer. Many pieces of evidence suggest this formation. For example, the layer appeared as a deposited artificial layer on the surface and not as a corrosion layer resulting from interaction and reaction with the metallic core. The metallographic investigation reveals that the interaction between the metallic core and the corrosion layer does not exist in these coins and the dividing area was constantly found between the two layers. Also, the layer consists of a single corrosion product with one color and equal thickness.
3.2.4. The coin B2 EDX of three points of the coin B2 (Fig. 9) reveals that copper was 100% and no minor elements or impurities were found. This composition does not match the composition of archaeological coins which always contained natural impurities that are mixed with metal ores [2,4,30]. The ancient coins usually showed many trace elements e.g. (Ni, Fe, Mn, Sb, Co, Si Ti, Bi, Au, Pt, P, Al, Pb, and As), which were detected in ancient coins of Ag-Cu alloy [1,6,25,31,32]. These minor/ trace elements mainly originated from different ores or from varying manufacturing processes used in Ag production [11]. The coins of copper and bronze commonly contained additional elements such as Zn, Ag, Pb, As, Sb, S, and Fe [2,5,12,33]. They are present as impurities originating from copper ores [31]. The modern advanced technology used in the purification of copper does not transmit the impurities from the ore into the metal [8]. With this information, it is not possible to confirm that this coin is authentic.
3.2.5. The coins B3 The elemental analysis of the coin B3 shows three main elements: copper, tin and lead (Fig. 10). The average of these elements reports that the matrix of this coin was constituted by a ternary bronze alloy Cu-Sn-Pb with copper (78.12%, tin (6.50%) and lead (15.29%). The sample B3 reveals a microstructure similar to that of the coin B1. The main phase is copper-rich which appears as dark gray and contains many pores. The second phase is lead-rich phase which appears as whitish gray. The last phase involves chlorine-rich areas as internal corrosion product and it appears under SEM as smooth areas 252
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Fig. 10. EDS spectra and SEM micrographs of the cross-sections of coin B3 showing internal corrosion products corresponding to chlorine in EDS spectra.
with dark gray color. SEM-EDX confirm these phases and their distribution (Fig. 11). This type of structure proves the authenticity of this coin because the internal corrosion in the structure is due to long-term reactions and completely negates the forgery process.
4. Conclusion The archaeometallurgical characterisation of the ancient coins contributes to confirming or negating their authenticity, as follows: Some results of the elemental analysis can confirm the forgery as in the coins A1 and A2. Aluminum was revealed as a minor metal of the Roman coins although it was not discovered before the mid-nineteenth century. The results of elemental analysis of trace elements are sometimes evidence of the authenticity and provide researchers with information about the ores sources of the metals. The features of metallic structure which attributed to ancient manufacturing techniques, e.g., recrystallized structure, twinned grains, thin strain lines within the grains, and slag inclusions, help proving the authenticity of ancient coins. They are revealed in the coin A3 and indicate that the coin was manufactured by ancient traditional methods. The internal deterioration features of the metallic structure, including cracks, interlayer corrosion, intergranular corrosion and discontinuous precipitation of copper at the grain boundaries, are strong and conclusive evidence of the authenticity and completely deny the counterfeiting process. They are conclusive evidence of antiquity of this coin. The information obtained can be also useful in understanding the
3.2.6. The coin B4 The elemental analysis of the metallic core area (sample centre) reveals that the coin B4 consisted of copper (98.7%) in addition to tin and zinc as impurities (Fig. 12a–c). The analysis of the interlayer between the metallic core and the outer corrosion layer reveals high concentration of lead element in three points (4, 6, 11%) in addition to others elements attributed to corrosion products such as Cl and O (Fig. 12d–f). The presence of lead in the interlayer may be explained by the following reasons: manufacturing defect, burying the coin adjacent to or in contact with other objects of lead or its alloys, coating the coin with lead [34]. The official counterfeits of coins with a 0.2–0.5 mm another metals layer were all found with silver used as the coating layer [35]. The last possibility is the strongest because the lead layer appeared clearly along the cross section. The coating of ancient copper and bronze coins was a common practice in the Roman republican coinage and was performed by depositing a thin layer of silver or gold [36,37].
Fig. 11. SEM micrographs and the analysed area by EDS of the cross-section of coin B3 showing three regions: (a) copper-rich phase, (b) lead-rich phase (c) chlorinerich areas. 253
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Fig. 12. EDS spectra and SEM micrographs of the cross-sections of coin B4.
economic processes of coinage and answers specific questions related to chronology, ores provenance, and ancient technology.
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