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Discovering Forgeries of Modern Art by the 14c Bomb Peak

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Ion Beam Analysis and 14C Accelerator Mass Spectroscopy to Identify Ancient and Recent Art Forgeries §

Laboratoire de Mesure du Carbone xiv (LMC14), Laboratoire des Sciences du Climat et de 50'Environnement, Institute Pierre Simon Laplace (LSCE/IPSL), CEA-CNRS-UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France

§

This article is dedicated to the memory of dear missing colleagues Claire Berthier (CEA), Thierry Borel and Joseph Salomon (C2RMF).

Received: 24 Feb 2022 / Revised: 26 March 2022 / Accustomed: iv April 2022 / Published: 26 Apr 2022

Abstract

Forgeries exist in many fields. Money, appurtenances, and works of art have been imitated for centuries to deceive and make a profit. In the field of Cultural Heritage, nuclear techniques can be used to written report fine art forgeries. Ion axle analysis (IBA), as well as 14C accelerator mass spectrometry (AMS), are at present established techniques, and the purpose of this paper is to report on their capacity to provide information on ancient, equally well as modern, forgeries. Ii instance studies are presented: the production of silverish counterfeit coins in the 16th century and the detection of recent forgeries of 20th century paintings. For the counterfeit coins, two silvering processes were identified by IBA: mercury silvering (also called amalgam silvering or burn down silvering) and pure silver plating. The discovery of 14 mercury silvered coins is an important finding since at that place are very few known examples from before the 17th century. In the detection of recent forgeries, among the v paintings examined, 14C dating showed that three of them are definitely fakes, one is nigh likely a fake, and one remains undetermined. These results were obtained by using the bomb peak scale curve to date sail and paint samples.

ane. Introduction

Counterfeiting is the fraudulent imitation of a valuable product with the intention to deceive. This illegal practice is found in many fields: forgery of currency or documents, faux of items such as clothing, pharmaceuticals, motorcar and shipping parts, food, electronics, watches, and works of fine art. I of the main issues for experts is to identify or to engagement the material used in producing forgeries, as the appearance is mostly close to that of the 18-carat production. Carried out together with other expertise tools, scientific investigations are, thus, conducted to clarify the actuality of the product, based on composition and/or age determination.

When dealing with 18-carat or alleged cultural heritage objects, these investigations accept to exist as minimally invasive as possible. The nearly widespread methodology is mainly based on imaging techniques, such equally multispectral imaging, X-ray radiography, or microscopy, and on chemical analysis to await for anachronisms or contentious materials [one,2,3,iv,v]. In recent decades, the advent of nuclear techniques—derived from low-energy accelerators—for art and archaeological study has paved the mode for new methodologies to analyze and authenticate cultural heritage objects [6,vii,8,ix,10,11,12].

Ion beam analysis (IBA), as well as 14C accelerator mass spectrometry (AMS), are now established techniques [13,14], and the purpose of this paper is to study on their chapters to provide information on ancient and modernistic forgeries. Two case studies are presented: (one) the production of silver counterfeit coins in the 16th century Advert and (two) the detection of modern painting forgeries.

The results obtained here contribute to our knowledge of ancient counterfeiting and modern forgery practices.

2. Materials and Methods

2.1. Counterfeit Coin Characterization by Ion Beam Analysis

It is relatively rare to find apocryphal coins for many reasons. Get-go, counterfeit coins were not unremarkably kept as valuables by private individuals in the by and, when preserved, collectors have neglected them. Furthermore, the generally poor quality of the imitations, as well as corrosion, foreclose good preservation.

However, the hoard of Preuschdorf, found in 2005, offered the opportunity to explore the product of counterfeit coins in the 16th century [15,xvi]. Among 7527 coins found, 38 counterfeit coins were identified (Table 1), imitating contemporaneous official issues (mainly Pfennige) produced in the Holy Roman Empire between 1535 and ~1620 (Figure 1). Non-destructive techniques were used to investigate their manufacture: 10-ray radiography and Ion beam analysis (IBA) for the coins [xvi,17,xviii] and scanning electron microscopy (SEM) for fragments nerveless during restoration. Particle-induced Ten-ray emission (PIXE) and Rutherford backscattering spectrometry (RBS) were carried out simultaneously using the 3 MeV proton axle of the AGLAE accelerator at the Center de Recherche et de Restauration des Musées de French republic (Louvre Palace, Paris, France) [xix]. PIXE analysis provided elementary composition (mainly Ag, Cu, Au, Pb, Zn, Hg) and RBS—the depth profile of the major elements. The experiments were conducted with a setup combining two Si (Li) 10-ray detectors for PIXE and one surface barrier detector for RBS [20]. PIXE spectra were fitted by GUPIX or GUPIXWIN [21], and RBS spectra were simulated with SIMNRA [22]. Altogether, 140 coins were ion beam analyzed; only results on the counterfeit coins and their official counterparts are reported in this paper.

2.two. Painting Forgery Dating by xivC AMS

14C AMS was applied to the study of declared 19th–20th century paintings. Impressionist, Pointillist, Expressionist, Abstract, and Contemporary paintings were selected. Four of them were seized by law after the discovery of a workshop, and another one came from a private collection. For confidentiality reasons, details on the alleged artists cannot be disclosed.

Different materials were sampled (Table 2): forest from the stretchers, fibers from the canvases, and paint (Figure 2). Fibers were previously identified as natural fibers nether the microscope [12]. The paint sample was analyzed by X-ray diffraction (XRD), showing the presence of BaSOfour, ZnO, and CaCOthree-containing pigments in an organic binder.

For radiocarbon dating, fiber and wood samples were pretreated with acid-base-acid washes, and the paint sample was only pretreated with acid due to its small size. Samples were stale under vacuum at 60 °C and then placed in quartz tubes with excess CuO and Ag. The quartz tubes were sealed under vacuum (five × 10−six mbar) and heated at 850 °C for 5 h. COii gas was produced and separated from H2O using a dry-ice/alcohol trap (−78 °C) [23]. CO2 samples were then reduced to graphite targets past hydrogen over an atomic number 26 goad. Carbon isotopes were measured with the AMS LMC14/ARTEMIS facility (Saclay, French republic) [24]. The 14C contents were converted into calendar years with the OxCal calibration program [25], using the Intcal20 atmospheric curve [26] for pre-bomb ages (i.eastward., earlier 1950) and the Bomb13 NH1 post-bomb atmospheric curve [27] for the most recent decades (i.due east., after 1950).

3. Results

3.1. Counterfeit Coins

The compositions, in silver and copper, of the counterfeit coins and their official counterparts are presented in Figure 3. Ii main groups are observed. The official coins contain betwixt 20% and 42% of silverish, whereas the counterfeit coins show a low silvery content.

For the official coins, two major elements are present: silvery and copper. The hateful silver concentration for the coins issued by the city of Strasbourg is 37 ± 2%; this high content is in accordance with the official finesses of 375/chiliad gear up by the Regal minting ordinance (Reichsmünzordnung) of the Holy Roman Empire in 1559 [28]. In contrast, the coins issued by the bishopric of Chur in the Swiss Confederation have a low argent content (23 ± three%), reflecting a production of poor quality and known to exist struck for exportation [29]. The coins produced by the city of St. Gallen and the canton of Palatinate (Pfalz-Veldenz) show intermediate silver contents of 30 ± 3% and 27.5 ± one.eight%, respectively.

The apocryphal coins are characterized by their low content in silver (less than 8%) and loftier content in copper (from 76 to 98 wt%). The values are scattered due to the variable presence of other elements such as mercury, zinc, and tin. Mercury is detected in the apocryphal coins imitating those of the county of Palatinate (Pfalz-Veldenz, (catalogue number 104.F; see Table 1)), in one money copying that of the metropolis of Strasbourg (151.F), and in another copying that of the bishopric of Chur (188.F). Mercury is conspicuously correlated to silver (Figure 4), suggesting the presence of an Ag-Hg alloy at the surface of the coin. The mean ratio betwixt silvery and mercury is nearly 1, corresponding to a layer composed of fifty% Ag and 50% Hg. Withal, a large besprinkle is observed from one money to another, with ratios from 1.6 to 0.3, corresponding to compositions of ~62% Ag and 38% Hg to 25% Ag and 75% Hg.

The RBS spectrum of a coin containing mercury is presented in Figure 5a. The SIMNRA simulation [22] indicates that silver and mercury are together at the surface of the coin, forming a 1–two µm thick layer of 50 Hg wt% and 50 Ag wt%. A limerick of 27 Hg wt% and 73 Ag wt% was also found for another coin [20]. Both compositions are in agreement with the PIXE results and confirm the presence of a silver-mercury layer at the surface of some counterfeit coins, mostly the imitations from the county of Palatinate (Pfalz-Veldenz, 104.F). For comparison, Figure 5b shows the RBS spectrum of an official coin fabricated of a homogeneous argent-copper alloy.

None of the other 24 counterfeit coins contain a significant amount of mercury. These are the imitations of the city of St. Gallen (195.F), the bishopric of Speyer (133.F), the bishopric of Mainz (lx.F), the county of Stolberg (135.F), the county of Salm (118.F), the two other specimens of the bishopric of Chur (188.F), the other specimen of the city of Strasbourg (151.F), as well as the other identified (160.F, 146.F, 112.F) and unidentified coins. These coins were not well preserved, and observation under the microscope was necessary to interpret the depression content in silver (0.2 to 5.6 %). This result is due to a thin layer of pure silver coated at the surface of the coin, and the credible pct of silverish determined by PIXE is related to the thickness of the silver layer. The cores of these counterfeit coins are composed of copper or contumely (copper and zinc).

In summary, two counterfeiting processes were identified by IBA: (a) mercury silvering (besides called constructing silvering or fire silvering) of a copper core for fourteen coins, 12 of which come from the county of Palatinate (Pfalz-Veldenz, 104.F), equally well equally (b) a thin layer of pure silver coated on a brass, bronze, or copper cadre for 24 coins. The application of pure silverish can exist achieved by coating a thin silver foil or by electrochemical replacement. The latter procedure is a plating technique that uses the electro-differential between the solution and the metal to exist plated; copper blanks are immersed in a solution containing silver flakes, table salt (NaCl), and vino lees (potassium bitartrate, KC4H5Ohalf dozen) [30]. After a few hours, a thin layer of silver is deposited on the substrate [14], and the coins are set to be struck.

The discovery of 14 mercury silvered coins is an important finding since there are very few known examples from earlier the 17th century [31,32]. The primeval examples are the forged Iranian dirhem dating from the ninth to tenth centuries [33], four pennies of the 13th century [34], and one coin of the 15th century [35]. The simulated (135.F) of Ludwig II of Stolberg-Königstein (1535–1574) was wrongly attributed to the group of mercury-silvered coins in a previous publication [xviii]. As a event, the identification of one counterfeit coin (188.F) of Peter Ii Rascher (1581–1601), the bishop of Chur, and 12 apocryphal coins (104.F) of Johann Baronial of Palatinate-Lützelstein (1598 to 1611), the count of Pfalz-Veldenz, constitutes a significant contribution to our noesis of ancient counterfeiting practices [36] and the constructing silvering process.

3.2. Art Forgery Dating

Radiocarbon dating results, obtained for the wooden stretcher of four paintings, are presented in Table 3 and Effigy vi.

All the dates are between the 17th century and 1950, with variable distributions. The ranges are very large due to the shape of the calibration curve for these times (Figure 7). These dates, obtained for the wood, correspond to the time when the trees were still standing and incorporated 14C. To know the time when the trees were cut down, it is necessary to accept samples containing sapwood, which is not the instance here. As a outcome, the dated piece of wood can be older than the date of the tree felling (known as the "former wood effect"), making it difficult to approximate the stretcher manufacture. The most contempo year recorded by 14C for the stretcher wood is 1911, 1942, 1950, and 1938 for the Impressionist, Pointillist, Abstruse, and Contemporary paintings, respectively. These dates are coherent with the lifetime of the alleged artists. All the same, due to the wide distribution, every bit well equally the former wood effect, which also includes the unknown duration betwixt the felling of the trees and the stretcher manufacture, information technology is not possible to conclude the date at which the paintings were executed.

Radiocarbon dating results obtained for the canvas of four paintings and for the paint of one painting are presented in Table 4 and Effigy 8.

The 14C Fraction Modern (FfourteenC) indicates the proportion of radiocarbon atoms in a sample as compared to samples that were modern in 1950. In the tardily 1950s and 1960s the xivC concentration in the atmosphere almost doubled due to the atmospheric nuclear weapon tests. Thus, a FxivC value college than 1 indicates "post-flop" samples. Later the atmospheric nuclear test ban treaty in 1963, the fourteenC content decreased due to its dilution in the atmosphere [37]. This event, including bogus 14C product and decrease, referred to equally the "bomb peak", is used as the calibration bend for radiocarbon dating of recent samples.

The fourteenC content (F14C) of all the fibers is higher than 1, showing, unambiguously, that the canvas fabrics originate from plants that grew at the fourth dimension when fourteenC was in excess in the atmosphere. After calibration, two solutions are determined for each radiocarbon consequence due to the shape of the calibration curve (Figure 8). The dates, corresponding to the plant harvesting, are the following: 1957 or 2000–2003, 1956–1957 or 2004–2010, 1957–1959 or 1987–1990, and 1955–1956 or 2012–2015 for the Impressionist, Pointillist, Expressionist, and Abstract paintings, respectively. For the first three paintings, both solutions are after the death of the alleged artists in the 1940s-early on 1950s. These results demonstrate that these paintings are forgeries performed in the years 1956–1957 or 2000–2010, for the Impressionist and Pointillist specimens, and in the years 1957–1959 or 1987–1990 for the Expressionist ane. For the Abstract painting, the estimation of the results—1955–1956 and 2012–2015—is less straightforward since the declared artist died in the 70s. The 2nd solution is subsequently the death of the artist, but the first one is 15–20 years earlier. It cannot be excluded that an artist may keep untouched canvases in his/her workshop for one or two decades, even if the typical storage elapsing is betwixt two and five years [38]. A like event was obtained for the paint sample (Table iv); however, information technology is less mutual to preserve a tube of paint for such a long fourth dimension. The latter event suggests that the Abstract painting is also a fake. For the Contemporary painting, the canvass dating failed due to the massive presence of glue still embedded in the fibers even subsequently a strong cleaning treatment.

Among the five paintings examined, 14C dating shows that three of them are definitely false, one is most probable fake, and one remains undetermined.

4. Conclusions

Ion beam analysis (IBA) and xivC accelerator mass spectrometry (AMS) are methods of selection to analyze ancient artefacts and works of art since they are minimally invasive. Both techniques tin can also contribute to the study of counterfeiting and fine art forgery [39]. In this paper, the metallurgical processes used to produce silver counterfeit coins in the 16th century were studied in detail, revealing the presence of xiv coins silvered with mercury. This discovery is an important finding since there are very few known examples before the 17th century. Five paintings were radiocarbon dated. Using the flop peak calibration curve, it was unambiguously demonstrated that 3 paintings, alleged to be of the beginning of the 20th century, are forgeries made after 1956. Information technology is suggested that another painting declared to be painted in the 1970s could be a more recent forgery.

Funding

This enquiry received no external funding.

Institutional Review Board Statement

Not applicative.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are bachelor on request from the corresponding author. The data are not publicly available due to restricted access to databases. The IBA archived dataset is managed past the AGLAE group at the Centre de Recherche et de Restauration des Musées de French republic (Paris, France). The 14C AMS archived dataset is managed by the LMC14 Laboratory (Saclay, France).

Acknowledgments

The author wishes to give thanks Dominique Robcis (C2RMF), Elise Alloin and Anaïs Vigneron (Archéologie Alsace), Ulrich Klein (numismatist) and the AGLAE group (C2RMF, Paris, France) for their invaluable contribution to the study of the Preuschdorf hoard and her current colleagues of the LMC14 laboratory for the painting sample preparation, 14C measurements and fruitful discussion. Many thanks also to Estelle Itié and Ilenia Cassan for their confidence in the study of one of the paintings presented hither. This is LSCE contribution number 7901.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Examples of counterfeit coins from the hoard of Preuschdorf (d–f) compared to their official counterparts (a–c). Details of their provenance are given in Table 1. (Photos: Anaïs Vigneron, Archéologie-Alsace).

Effigy i. Examples of counterfeit coins from the hoard of Preuschdorf (d–f) compared to their official counterparts (a–c). Details of their provenance are given in Table 1. (Photos: Anaïs Vigneron, Archéologie-Alsace).

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Figure 2. (Left): samples of wood, canvass, and paint. (Right): details of the pigment layer.

Effigy 2. (Left): samples of forest, canvas, and paint. (Right): details of the paint layer.

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Effigy 3. Silver vs. copper contents (in wt%) for selected coins of the Preuschdorf hoard coming from the cities of Strasbourg and St. Gallen, the county of Pfalz-Veldenz, and the bishopric of Chur. Open symbols represent to official coins, and filled symbols stand for to counterfeit coins.

Effigy three. Silvery vs. copper contents (in wt%) for selected coins of the Preuschdorf hoard coming from the cities of Strasbourg and St. Gallen, the canton of Pfalz-Veldenz, and the bishopric of Chur. Open symbols correspond to official coins, and filled symbols correspond to counterfeit coins.

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Figure 4. Silver vs. mercury contents (in wt%) for the counterfeit coins of the Preuschdorf hoard coming from the cities of Strasbourg (blue) and St. Gallen (green), the county of Pfalz-Veldenz (red), and the bishopric of Chur (orange).

Figure iv. Silverish vs. mercury contents (in wt%) for the counterfeit coins of the Preuschdorf hoard coming from the cities of Strasbourg (bluish) and St. Gallen (greenish), the county of Pfalz-Veldenz (red), and the bishopric of Chur (orangish).

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Effigy 5. The 3 MeV proton Rutherford backscattering spectrometry (RBS) spectrum of a counterfeit money (a) and that of an official money (b). For the counterfeit coin, RBS shows a silverish-mercury layer on a copper substrate, whereas for the official coin, RBS shows a homogeneous silver-copper alloy.

Figure 5. The iii MeV proton Rutherford backscattering spectrometry (RBS) spectrum of a counterfeit money (a) and that of an official money (b). For the counterfeit money, RBS shows a silver-mercury layer on a copper substrate, whereas for the official money, RBS shows a homogeneous silverish-copper alloy.

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Figure six. Radiocarbon calibrated dates (shown in calAD) of the wood samples, taken from the stretchers of the Impressionist (I), Pointillist (P), Abstract (A), and Gimmicky (C) paintings. The blackness bars indicate the death of the alleged artists. The radiocarbon calibrated dates were obtained using the atmospheric scale curve Intcal20 [26].

Effigy 6. Radiocarbon calibrated dates (shown in calAD) of the wood samples, taken from the stretchers of the Impressionist (I), Pointillist (P), Abstract (A), and Contemporary (C) paintings. The black bars indicate the death of the alleged artists. The radiocarbon calibrated dates were obtained using the atmospheric calibration curve Intcal20 [26].

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Figure 7. Calibration of the wood samples taken from the painting stretchers. The blue curve is the atmospheric calibration curve (Intcal20) used to calibrate xivC dates before 1950 [26].

Figure vii. Scale of the wood samples taken from the painting stretchers. The bluish bend is the atmospheric calibration curve (Intcal20) used to calibrate 14C dates before 1950 [26].

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Figure 8. Calibration of the fiber samples taken from the canvass of the Impressionist, Pointillist, Abstract, and Expressionist paintings. The bluish curve is the post-flop atmospheric calibration curve (Bomb13 NH1) used to calibrate xivC dates after 1950 [25,26,27]. See text for details.

Figure 8. Scale of the fiber samples taken from the canvas of the Impressionist, Pointillist, Abstruse, and Expressionist paintings. The blue curve is the post-bomb atmospheric calibration curve (Bomb13 NH1) used to calibrate 14C dates after 1950 [25,26,27]. See text for details.

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Tabular array i. Apocryphal coins identified in the hoard of Preuschdorf (Alsace, France) found in 2005. The design of the money imitates that of the official coin (Pfennig) produced past entities having the right to mint coins, such equally cities, bishoprics, counties, abbeys, and other small territories.

Table 1. Apocryphal coins identified in the hoard of Preuschdorf (Alsace, French republic) found in 2005. The design of the coin imitates that of the official coin (Pfennig) produced past entities having the right to mint coins, such equally cities, bishoprics, counties, abbeys, and other small territories.

Imitation Type and Entity Catalogue Number Estimated Date Number of Coins Respective Figure
Pfalz-Veldenz, IAP–Johann Baronial of Palatinate-Lützelstein–Pfalz-Veldenz county 104.F 1598 to 1611 12 Figure 1d
St Gallen City 195.F 2d. half 16th c vii Effigy 1f
Chur PEC–Peter Two Rascher- Chur Bishopric 188.F 1581–1601 three
Strasbourg City 151.F 16th–begin. 17th c. 2 Figure 1e
Marquard von Hattstein–Speyer bishopric 133.F 1560–1581 2
Ludwig II of Stolberg-Königstein–County 135.F 1535–1574 one
Wolfgang von Dalberg–Mainz bishopric threescore.F 1582–1601 1
Otto von Salm-Kirburg–Salm county 118.F 1548–1607 1
Johann Seven. von Schönenberg 160.F 1581–1590 1
Other (Stolberg VLG, Zweibrücken) 146.F, 112.F 2
Unidentified six

Table 2. Paintings and materials selected for 14C dating: wood from stretcher (W), fiber from canvas (F), paint (P).

Table 2. Paintings and materials selected for 14C dating: woods from stretcher (W), cobweb from canvas (F), pigment (P).

Painting ane Estimated Date Samples
Impressionist Before 1945 W, F
Post-Impressionist Before 1940 W, F
Expressionist Before 1950 F
Abstract ~1970s W, F, P
Contemporary ~1990s W, F

Table 3. 14C accelerator mass spectrometry (AMS) dating results of the wood samples taken from the painting stretchers. Uncalibrated radiocarbon ages are reported in years Before Present (BP), i.east., in years earlier 1950. Calibrated dates are obtained by calibrating the radiocarbon ages using the Intcal20 atmospheric curve [26] (Figure 7).

Tabular array 3. 14C accelerator mass spectrometry (AMS) dating results of the woods samples taken from the painting stretchers. Uncalibrated radiocarbon ages are reported in years Before Nowadays (BP), i.east., in years before 1950. Calibrated dates are obtained by calibrating the radiocarbon ages using the Intcal20 atmospheric curve [26] (Effigy 7).

Painting Radiocarbon Age (BP) Calibrated Dates (95.iv%) AMS Laboratory Number
Impressionist thirty ± 23 1697–1724 (29.6%) SacA57262
1812–1836 (28.4%)
1880–1911 (37.v%)
Pointillist 135 ± 23 1675–1744 (26.5%) SacA57264
1750–1765 (4.2%)
1798–1942 (64.7%)
Expressionist No sample - -
Abstruse 220 ± 21 1644–1681 (42.ane%) SacA57267
1739–1753 (five.four%)
1762–1800 (45.5%)
Gimmicky 120 ± 21 1939–… (two.6%) SacA57269
1683–1735 (24.9%)
1803–1938 (70.6%)

Table 4. fourteenC Fraction Modern (F14C) results for the fibers taken from the painting canvases and for a sample of pigment taken from the white paint of the Abstract painting. F14C is the unit used for post-bomb samples. Radiocarbon dates were calibrated using the Bomb13 NH1 mail service-bomb atmospheric curve [27] (Figure 8). Run across text for details.

Table 4. 14C Fraction Mod (F14C) results for the fibers taken from the painting canvases and for a sample of paint taken from the white paint of the Abstruse painting. FxivC is the unit of measurement used for post-flop samples. Radiocarbon dates were calibrated using the Bomb13 NH1 postal service-bomb atmospheric bend [27] (Figure 8). Meet text for details.

Painting F14C Calibrated Dates (95.four%) AMS Lab Number
Impressionist 1.0859 ± 0.0027 1957 & 2000–2003 SacA57263
Pointillist i.0560 ± 0.0027 1956–1957 & 2004–2010 SacA57265
Expressionist one.1646 ± 0.0027 1957–1959 &1987–1990 SacA64025
Abstract, sail 1.0301 ± 0.0027 1955–1956 & 2012–2015 SacA57268
Abstruse, paint ane.0094 ± 0.0027 1954–1955 & 2017–… SacA57275
Gimmicky Failed - -

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