arts

Article Between Fakes, Forgeries, and Illicit Artifacts—Authenticity Studies in a Heritage Science Laboratory

Stefan Simon 1,2,* ID and Stefan Röhrs 2 1 Global Cultural Heritage Initiatives, Yale University, New Haven, CT 06520, USA 2 Rathgen-Forschungslabor, Staatliche Museen zu Berlin, Stiftung Preußischer Kulturbesitz, 14059 Berlin, ; [email protected] * Correspondence: [email protected]; Tel.: +1-203-436-2544

 Received: 21 March 2018; Accepted: 22 May 2018; Published: 5 June 2018  Abstract: Since its inauguration in 1888, the Rathgen Research Laboratory of the National Museums in Berlin has been challenged by authenticity questions on cultural heritage objects. In the setting of an ever-growing market, often intertwined with the increasing global impact of illicit traffic, scientific investigations can contribute equally to art-historical, or archaeological expertise when solving questions of authenticity, and should therefore always be included when significant values are at stake. Looted or stolen artifacts, copies, fakes, and forgeries have been an intrinsic element of the market since ever, and only selectively addressed in a trans-disciplinary, more holistic way. This paper makes the case for a reliable, state-of-the-art analysis and illustrates the potential benefits of such a scientific approach to authenticity questions in selected examples: 1. the case of German art forger, Wolfgang Beltracchi; 2. brass objects of alleged Benin and Ife provenance.

Keywords: authenticity; painting; fake/forger; metal alloy; heritage science; conservation science

1. Introduction Over the past 150 years, the rapid progress of archaeological and historic sciences, in combination with increased mobility and logistics, significantly facilitated the growing transfer of cultural artifacts from so-called source countries into market countries (for definitions, see Mackenzie 2005), mainly Europe and the United States. This transfer soon emerged as what is discussed nowadays as illicit traffic (Yates 2016; Brodie and Renfrew 2005). In fact, the transport of works of art across state boundaries is much easier than the transfer of money (Guillotreau 1999). The links to international crime, and even terrorism, became undisputed with the ongoing growth of illicit traffic, and are addressed by law enforcement agencies internationally (Lacoursière and Talbot 2005). Several UN Security Council Resolutions (e.g., 1483/2003, 2199/2015, 2347/2017) and EU Council Regulations (e.g., 1332/2013, 1210/2003) have been dedicated to the struggle with illicit traffic, especially in the MENA region, Syria, and Iraq. Next to illicit traffic, forgeries, and fakes are another characteristic element of this market, one could consider it the other side of the same coin, ranging from antiquities to 21st century art. Already, Horace wrote, “He who knows a thousand works of art, knows a thousand forgeries” (Hoving 1997). Questions about authentication (dating and clarification of provenance) are usually addressed by heritage science laboratories. The Rathgen Research Laboratory, the guiding institution for conservation science, art technology, and archaeometry of the National Museums in Berlin, has been involved in the authentication of cultural artifacts since its founding in 1888 and has built up respective material databases (Simon 2007).

Arts 2018, 7, 20; doi:10.3390/arts7020020 www.mdpi.com/journal/arts Arts 2018, 7, 20 2 of 13

Conclusive answers to questions of authenticity can only result, if at all, through close collaboration between scientists, conservators, archaeologists, and art historians in a truly interdisciplinary effort. However, there are also ethical issues connected to such studies, which have been discussed in depth by Argyropoulos et al.(2011, 2014). There is an undisputed need for reliable, state-of-the-art analysis in authentication issues. However, by epistemological principle, it is impossible to provide evidence for originals, often to the disappointment of collectors and museum curators: as the making of a “perfect fake” cannot be excluded, at least theoretically, final evidence in the laboratory can only be obtained on copies, fakes, and forgeries. Some heritage science laboratories have developed policies on such analyses. The Rathgen Research Laboratory implemented the Berlin Resolution(2003) into its institutional policy for all cultural artifacts, whether they are modern or ancient. Since 2005, analytical services have been offered only on the premise that the rightful provenance of an object is ascertained. This policy, which was initially met with considerable resistance, even from within the institution, is anticipating respective federal legislation on the import of cultural property by more than a decade (Kulturgutschutzgesetz 2016). Consequently, the Rathgen Research Laboratory has focused on collaborating with law enforcement agencies and international bodies in order to contain the further growth of illicit trafficking of art and the detection of fakes and forgeries. In authenticity analysis, the presence of specific pigments may serve as “terminus post quem” for dating evidence. “Terminus post quem”—meaning “point of time after which“—implies that the use of a pigment is only possible after its invention and introduction into the market. The scientific knowledge in this field needs to be expanded continuously and new markers need to be identified, especially for the second half of the 20th century, in order to maintain an advantage over the forgers, who closely follow the scientific progress in the analytical approach used and constantly “improve” their work. Beyond the first use of a pigment and modifications in its production process, causing small changes in its properties moves it into the focus of analytical interest. Unfortunately, manufacturers and vendors are not overly enthusiastic about sharing information on their current products, and for older products, it is often not easy to retrieve this information from archival resources. Additionally, a good awareness of the respective detection limits and accuracies of the analytical technique applied remains a crucial element in the process, especially when the results are evaluated in a quantitative way. The use of complimentary methods is always an asset, as is a step-wise progress from non-destructive methods toward those requiring a sample. This paper illustrates an approach to scientific research in authenticity questions on two selected examples.

1. The Wolfgang Beltracchi Case of forged paintings. 2. Brass objects of alleged Benin/ Ife provenance.

2. Results

2.1. The Wolfgang Beltracchi Case of Forged Paintings Between 2010 and 2011, the Rathgen Research Laboratory was tasked with investigating 7 out of 14 paintings that were at the core of the charges against the German group of forgers around Wolfgang Beltracchi, who had presumably sold over 50 faked art works attributed to , (Figures1 and2), Heinrich Campendonk, Andr é Derain, Fernand Léger, and to the art market. After the arrest of the forgers in September 2010, the lawsuit in finished with the verdict a year later in October 2011. Not only was the scale of the case and the damage caused to the art marked unprecedented, but so to was the effort of the group to produce convincing art works with a conclusive background story. For all these paintings, supposedly dating from the period between 1905 to 1927, old canvases and Arts 2018, 7, 20 3 of 13 stretchers had been used. Additionally, in the choice of the artist materials, the forgers were aiming at matching the materials available at the time. Building on a detailed technical survey, a wide range of analyticalArts methods 2018, 7, x FOR were PEER REVIEW used for investigating the paintings, which, in conclusion,3 of 13 were able to deliver solid evidence that allowed excluding a pre-1930’s date of the paintings and unmasking them as forgeries.deliver solid evidence that allowed excluding a pre-1930’s date of the paintings and unmasking them Artsas forgeries. 2018, 7, x FOR PEER REVIEW 3 of 13

deliver solid evidence that allowed excluding a pre-1930’s date of the paintings and unmasking them as forgeries.

Figure 1. Overview over the seven paintings of the Beltracchi case, with their attributions from upper Figure 1. Overviewleft clockwise: over “Liegender the seven weiblicher paintings Akt mit of Katze”, the Beltracchi Hermann Max case, Pechstein, with their1909; “Mattisse attributions from

upper left clockwise:peignant à Collioure”,“Liegender André weiblicher Derain, Akt1905; mit “Seinebrücke Katze”, Hermann mit Frachtkähnen”, Max Pechstein, Hermann 1909; Max “Mattisse Figure 1. Overview over the seven paintings of the Beltracchi case, with their attributions from upper peignant à Collioure”,Pechstein, 1908; Andr “Kubistischesé Derain, Stilllebe 1905;n”, “Seinebrücke Fernand Leger, 1913; mit Frachtkähnen”,“Landschaft mit Figuren Hermann und Vogel Max— Pechstein, leftgewidmet clockwise: Else Lasker “Liegender-Schüler”, weiblicher 1914; “La Akt Horde, mit Katze”, Max Ernst, Hermann 1927; “Collioure”, Max Pechstein, André 1909; Derain, “Mattisse 1905– 1908; “Kubistischespeignant1910 (photos: à Stillleben”, Collioure”, Christoph Schmidt) André Fernand Derain,. Leger, 1905; 1913; “Seinebrücke “Landschaft mit Frachtkähnen”, mit Figuren Hermann und Vogel—gewidmet Max Else Lasker-Schüler”,Pechstein, 1908; 1914; “Kubistisches “La Horde, Stilllebe Maxn”, Ernst,Fernand 1927; Leger, “Collioure”, 1913; “Landschaft Andr mit éFigurenDerain, und 1905–1910 Vogel— (photos: Christoph Schmidt).gewidmet Else Lasker-Schüler”, 1914; “La Horde, Max Ernst, 1927; “Collioure”, André Derain, 1905– 1910 (photos: Christoph Schmidt).

Figure 2. Painting allegedly attributed to Hermann Max Pechstein (1881–1955), “Liegender weiblicher Akt mit Katze” (1909); signature on upper right: “HMP 1909”; oil on canvas; stretcher frame, H 55.4

cm × B 60.2 cm (photo: Christoph Schmidt). Figure 2. Painting allegedly attributed to Hermann Max Pechstein (1881–1955), “Liegender weiblicher Figure 2. PaintingAkt mit Katze allegedly” (1909); attributed signature on to upper Hermann right: “HMP Max 1909 Pechstein”; oil on canvas; (1881–1955), stretcher “Liegenderframe, H 55.4 weiblicher Akt mit Katze”cm × B 60 (1909);.2 cm (photo: signature Christoph on Schmidt) upper. right: “HMP 1909”; oil on canvas; stretcher frame, H 55.4 cm × B 60.2 cm (photo: Christoph Schmidt).

Multispectral imaging and micro X-ray fluorescence (µ-XRF) were used to examine the paintings in the first step. The multispectral imaging allowed for identifying restored and overpainted areas, ArtsArts2018 2018, 7,, 207, x FOR PEER REVIEW 4 of4 13 of 13

Multispectral imaging and micro X-ray fluorescence (µ-XRF) were used to examine the paintings andin distinguishingthe first step. The different multispectral paint formulationsimaging allowed that for sometimes identifying appear restored similar and overpainted to the human areas eye., Anand important distinguish elementing different of the paint analytical formulations protocol that has sometimes been a careful appear technologicalsimilar to the human investigation eye. An by microscopy,important which element revealed of thevery analytical specific protocol features has of the been fake a paintings. careful technological Further evidence investigation was obtained, by formicroscopy, example, by which dendrochronological revealed very specific investigation features of the of thestretcher fake paintings.frame. After Further the µ -XRF evidence analysis was for elementobtained detection,, for example samples, by weredendrochronological taken from selected investigation areas for of further the stretcher investigation frame. After by microchemical the µ-XRF spotana tests,lysis Fourierfor element transform detection, infrared samples (FT-IR), were andtaken Raman from selected spectroscopy. areas for A numberfurther investigation of pigments wereby identified;microchemical among spot them, tests, cinnabar, Fourier leadtransform chromate, infrared ultramarine, (FT-IR), and zinc Raman white, spectroscopy. titanium white, A number and copper of phthalocyaninepigments were blue. identified Most; ofamong the pigments them, cinnabar, found didlead notchromate contradict, ultramarine, the alleged zinc production white, titanium date of thewhite paintings., and copper Of critical phthalocyanine significance, blue. however, Most of is the pigments occurrence found of the did pigments not contradict titanium the whitealleged and production date of the paintings. Of critical significance, however, is the occurrence of the pigments copper phthalocyanine blue. titanium white and copper phthalocyanine blue. The presence of the latter has been proven by two alternative methods: microchemical spot test The presence of the latter has been proven by two alternative methods: microchemical spot test (Figure3) and Raman spectroscopy (Figure4). The microchemical test is based on a color reaction of the (Figure 3) and Raman spectroscopy (Figure 4). The microchemical test is based on a color reaction of pigment with sulfuric acid, which causes the color to change to yellow or yellow-green (De Keizer 1990; the pigment with sulfuric acid, which causes the color to change to yellow or yellow-green (De Keizer Kalsbeek1990; Kalsbeek 2005). A2005) characteristic. A characteristic feature feature of phthalocyanine of phthalocyanine blues blues is that is that they they regain regain their their blue blue color aftercolor the after addition the addition of water of (waterLutzenberger (Lutzenberger and Stege and Stege2009; 2009;Kühn Kühn and Thiemeand Thieme 2009 2009). ).

Figure 3. Images of the microchemical test on a blue paint sample. (a) The pigment before the test; (b) Figure 3. Images of the microchemical test on a blue paint sample. (a) The pigment before the test; paint particles in contact with sulfuric acid; (c) color of particles has changed to yellow-green; and (d) (b) paint particles in contact with sulfuric acid; (c) color of particles has changed to yellow-green; and after diluting the acid, the blue color returns. The scale bar is 250 µm. (d) after diluting the acid, the blue color returns. The scale bar is 250 µm. However, phthalocyanine blue, a synthetic organic pigment detected in blue and green paint layers,However, is unlikely phthalocyanine to be part of paint blue, layers a synthetic predating organic 1934 (Eastaugh pigment detectedet al. 2004; in Römpp blue and 1995; green Quillen paint layers,Lomax is unlikely2005). After to be A.V part Braun of paint and layersJ. Tcherniak predating isolated 1934 the (Eastaugh first metal et free al. 2004 phthalocyanine; Römpp 1995 as ;a Quillen side Lomaxproduct 2005 in). 1907 After (Braun A.V Braunand Tcherniak and J. Tcherniak 1907), the isolated first copper the firstphthalocyanine metal free phthalocyaninewas synthesized asby a H. side productde Diesbach in 1907 and (Braun E. von and der Tcherniac Weid in 19271907 ),(De the Diesbach first copper and Von phthalocyanine der Weid 1927). was Scottish synthesized Dye Works by H. de Diesbachpatented and copper E. von phthalocyanine der Weid in 1927 in ( 1929De Diesbach (British andPatent Weid 192 19278, cited). Scottish after Dalhen Dye Works 1935). patented The coppercommercial phthalocyanine use of this inpigment 1929 (British started Patentin 1935 1928 (Eastaugh, cited et after al. 2004).Dalhen 1935). The commercial use of this pigmentTitanium started dioxide, in 1935 known (Eastaugh as white et pigment al. 2004 ).titanium white, can also not be expected in painting technologyTitanium before dioxide, 1930. known Today as still, white it is pigment applied titaniumbroadly and white, forms can part also of not many be expectedoil colors, in often painting in technologysmall admixtures before 1930.due to Today its high still, efficiency. it is applied The introduction broadly and of forms titanium part white of many into oilthe colors, market often is indiscussed small admixtures extensively due in to the its literature, high efficiency. especially The by introduction Laver (1997). of An titanium early reference white into to “Blanc the market de Titan” appears 1924 in the Encyclopédie de Chimie Industrielle (Coffignier 1924). It refers to the is discussed extensively in the literature, especially by Laver(1997). An early reference to “Blanc discovery of titanium oxide (rutile) containing paints for ship bottoms or other surfaces exposed to de Titan” appears 1924 in the Encyclopédie de Chimie Industrielle (Coffignier 1924). It refers to the discovery of titanium oxide (rutile) containing paints for ship bottoms or other surfaces exposed to sea Arts 2018, 7, 20 5 of 13 Arts 2018, 7, x FOR PEER REVIEW 5 of 13 watersea by water John by W. JohnOverton W. inOverton the 1870s in the( British 1870s Patent(1893 (British Patent), in a 1893), mixture in witha mixture bituminous with bituminous compounds, hencecompounds, not as a white hence pigment. not as a white Earlier pigment. titanium Earlier green, titanium a titanium green, hexacyanoferrate, a titanium hexacyanoferrate was discovered, was by Frederickdiscovered Versmann by Frederick (British Versmann Patent 1861 (British). Titanium Patent is 1861). also aTitanium regular compound is also a regular of other compound earth paints, of suchother as ochres earth orpaints some, such greens, as ochres because or of some the presence greens, because of ilmenite. of the Therefore, presence elemental of ilmenite. analysis Therefore, alone (e.g.,elemental by XRF or analysis environmental alone (e.g. scanning, by XRF electron or environmental microscopy–energy scanning dispersive electron m X-rayicroscopy (ESEM/EDX)–energy analysis)dispersive cannot X-ray provide (ESEM/EDX sufficient) analysis evidence) cannot in authenticity provide sufficient questions evidence and needs in authenticity to be combined questions with methodsand needs for phase to be combined and compound with methods analysis. for phase and compound analysis.

FigureFigure 4. Raman4. Raman Spectra Spectra for for two two blue blue paint paint samples samples withwith referencereference (in red) Heliogen Heliogen Blue Blue (coppe (copperr phthalocyaninephthalocyanine blue). blue).

The first patents for the production of titanium whites date back to 1913 and are converted from NorwegianThe first patents patents forin the the following production years of titanium(Deutsche whites Patentschrift date back 1913 to, 1917a 1913, and1917b) are, as converted well as from from NorwegianU.S. patents patents (Deutsche in the Patentschrift following years 1917c (Deutsche, 1917d). Their Patentschrift introduction 1913 into, 1917a the, market1917b), was as well slow, as both from U.S.the patents paint (industryDeutsche and Patentschrift artists’ colo 1917cr makers, 1917d remained). Their skeptical introduction about into their the performance. market was The slow, early both theformulations paint industry of titanium and artists’ dioxide color pigments makers in remained the 1920s skeptical were composites, about their mainly performance. mixed with The barium early formulationssulphate, zinc of titanium oxide, or dioxide lithopone. pigments Its mos int early the 1920s use in were the pure composites, anatas modification mainly mixed can with be traced barium sulphate,back to zinc 1925 oxide,, according or lithopone. to (Laver Its1997) most. The early marketing use in theof titanium pure anatas white modification (on the basis canof the be more traced backstable to 1925, rutile according modification) to (Laver started 1997 even). Thelater marketing(U.S. Geological of titanium Survey white 2007). (on Hering the basis(2000) of describes the more stable1938 rutile as “terminus modification) post quem started” for the even first later use ( U.S.of titanium Geological white Survey in rutile 2007 modification). Hering(.2000 Figure) describes 5 gives 1938an as overview “terminus over post the quem development” for the first of use the of market titanium volume white, which in rutile really modification. started soar Figureing only5 gives after an overviewWorld overWar II. the development of the market volume, which really started soaring only after World War II. The evidence for the “terminus post quem” markers phthalcyanine blue and Rutile in original paintThe layers evidence helped, for the in “additionterminus to post further quem” evidencemarkers (e.g., phthalcyanine dendrochronological blue and Rutiledata on in the original stretcher paint layersframes helped, or a incomparative addition to analysis further of evidence the used (e.g., nails dendrochronological), to exclude the alleged data production on the stretcher dates of framesthese paintings. or a comparative analysis of the used nails), to exclude the alleged production dates of these paintings.

2.2.2.2. Brass Brass Objects Objects of Allegedof Alleged Benin/Ife Benin/Ife Provenance Provenance Metallic zinc was not available in Europe before the end of the 18th century. Therefore, the Metallic zinc was not available in Europe before the end of the 18th century. Therefore, production of brass alloys was performed by the cementation process, in which metallic copper was the production of brass alloys was performed by the cementation process, in which metallic copper heated with calamine, a zinc ore. For technological reasons, the cementation process reveals alloys was heated with calamine, a zinc ore. For technological reasons, the cementation process reveals alloys with a limited zinc content (Haedecke 1973). Because of the dependencies between the diffusion of with a limited zinc content (Haedecke 1973). Because of the dependencies between the diffusion of zinc gas in the solid copper matrix, the upper limit for zinc content ranges between 28–32%, and zincvaries gas in with the the solid process copper conditions matrix, the (Haedecke upper limit 1973; for Craddock zinc content 1995; ranges Pollard between and Heron 28–32%, 1996; andLambert varies with1997; the Riederer process conditions2004). (Haedecke 1973; Craddock 1995; Pollard and Heron 1996; Lambert 1997; Riederer 2004). Arts 2018, 7, 20 6 of 13 Arts 2018, 7, x FOR PEER REVIEW 6 of 13

FigureFigure 5. 5.Chronology Chronology of of import import andand exportexport of titanium dioxide dioxide pigments pigments for for the the United United States States (U.S. (U.S. GeologicalGeological Survey Survey 2007 2007).).

The brass founder, Nehemiah Champion, registered Patent No 454 in 1723 for a better The brass founder, Nehemiah Champion, registered Patent No 454 in 1723 for a better cementation cementation process, based on a finer granulation of the copper metal (Day and Tylecote 1991). This process,allows basedfor a faster ona diffusion finer granulation of zinc vapor of in the the copper copper metal matrix ( Dayand raises and Tylecotethe maximum 1991). zinc This content allows forto a approximately faster diffusion 33. of3%. zinc This vapor seems into thebe the copper maximum matrix zinc and concentration raises the maximumreachable in zinc brass content by the to approximatelycementation process. 33.3%. This In experimental seems to be tests the maximum of this process, zinc concentrationthe composition reachable of the obtained in brass brass by the cementationalloys shows process. between In 13 experimental–31% zinc (Doridot tests of et this al. process,2006). Some the compositionstudies, however, of the predict obtained zinc brass contents alloys showsup to between 40% (Welter 13–31% 2003; zinc Ullwer (Doridot 2001; Zwic et al.ker 2006 et ).al. Some 1985). studies, Only through however, addition predict of metallic zinc contents zinc to up tothe 40% melting (Welter process 2003; —Ullwera technology 2001; Zwicker not known et al.in 1985 Europe). Only, but known through in additionsome parts of metallicof Asia— zinczinc to thecontent meltings of process—a 39–45% could technology be achieved, not knownfor example in Europe,, as early but as known1601 in inbrass some astrolabia parts of in Asia—zinc Lahore contents(Newbur ofy 39–45% et al. 2006) could. be achieved, for example, as early as 1601 in brass astrolabia in Lahore (NewburyAlthough et al. 2006 metallic). zinc appears in Europe around 1700, and zinc-rich brasses appear more often afterAlthough 1800, zinc metallic-rich brasses zinc appears with contentsin Europe exceeding around 1700, 30% and could zinc-rich be found brasses at times appear in more Europe often. afterAccording 1800, zinc-rich to Riederer brasses (2004) with, this contents was a result exceeding of us 30%ed raw could materials, be found such at as times metallic in Europe. zinc, imported According to viaRiederer the East(2004 India), this Company was a result(zinc distillation of used raw was materials, known in such India as as metallic early as zinc, the 15th imported century) via or the from East Indiasmelting Company sites (zinc in C distillationentral Germany was,known such as in Indiathe Harz, as early where as themetallic 15th century)zinc was or obtained from smelting by sitescondensation in Central on Germany, colder parts such of as the the oven Harz, during where the metallicmetallurgic zinc processing was obtained of zinc by ores. condensation This might on explain the use of zinc-rich brasses for musical instruments in the 16–17th century. colder parts of the oven during the metallurgic processing of zinc ores. This might explain the use of The evaluation of trace elements, such as cadmium, indium, silver, or iron, is difficult. Potential zinc-rich brasses for musical instruments in the 16–17th century. re-use of older alloys in later production processes can have an enormous impact on whether these The evaluation of trace elements, such as cadmium, indium, silver, or iron, is difficult. Potential data can be used for establishing a context for provenance and trade of raw materials (Pollard and re-use of older alloys in later production processes can have an enormous impact on whether these Heron 1996). data can be used for establishing a context for provenance and trade of raw materials (Pollard and A study by Manescu et al. (2008) on brass elements in organ pipes shows that before 1750 in HeronEurope 1996, these). always contain lead. The average lead content then slowly decreases from 7–8% (1624) to Aca. study 2% in bythe Manescu mid-18th etcentury. al.(2008 The) on first brass “lead elements-free” parts in organ appear pipes around shows 1750. that After before 1820, 1750 lead in Europe,completely these disappears always contain from brass lead. alloys. The average This is probably lead content a consequence then slowly of decreasesthe fact that from lead 7–8%increa (1624)ses tothe ca. brittleness 2% in the of mid-18th the alloy century. and hence The may first disturb “lead-free” the production parts appear process. around Manescu 1750. et Afteral. (2008) 1820, also lead completelydescribe the disappears increase of from zinc brass contents alloys. to ove Thisr 30% is probably after 1750 a consequence. of the fact that lead increases the brittlenessIn conclusion of the, zinc alloy contents and hence significantly may disturb above the 30% production can be attributed process. toManescu production et al. in( 2008the 19th) also describeor later the 18th increase century. of Brasses zinc contents of recent to overorigin 30% are afternot only 1750. characterized by high zinc contents, but alsoIn by conclusion, significant zinc cadmium contents contents. significantly According above to 30% the compositional can be attributed analysis to production data collected in the in 19th the or laterRathgen 18th century. Research Brasses Laboratory of recent database, origin the are cadmium not only content characterized of brasses by highproduced zinc contents, by the traditional but also by significantcementation cadmium process contents. is always According below 0.002%. to the Highercompositional cadmium analysis contents data may collected indicate in the the use Rathgen of Researchmetallic Laboratory zinc in the database,smelting process. the cadmium content of brasses produced by the traditional cementation processThe is always Berlin belowdatabase 0.002%. of metal Higher analysis cadmium and cultural contents artifacts may indicateat the Rathgen the use Research of metallic Laboratory zinc in the smeltingcomprises process. more than 17,000 objects, among them approximately 500 are attributed to Benin Arts 2018, 7, 20 7 of 13

The Berlin database of metal analysis and cultural artifacts at the Rathgen Research Laboratory comprises more than 17,000 objects, among them approximately 500 are attributed to Benin provenance. Technological features—as well as the receding limitations in the zinc content in brass over the course of the 19th century—and the compositional difference between well-provenanced objects in museum collections, compared with objects originating from the art market (collectors), help indicate fakes and facilitate dating. Generally, the majority of those objects derived from the art market differ significantly in their elemental composition from those with better provenance. Six objects of alleged Nigerian (Benin and Ife) and Mali provenance, attributed to the Paul Garn Collection Dresden (allegedly purchased 1920 or 1920–1940 (Object 1)) and dated from ca. 1430 to 1650 AD, were subject to a technological study in 2012 (Figure6). Figure7 shows the example of a documentation of the core sample for an object. The results of the atomic absorption spectrometry (AAS) analysis are summarized in Table1. Objects 1, 2, and 6 show an increased zinc content, which contradicts the alleged dating in the 15th or 17th century, respectively. Objects 1, 2, 3, 5, and 6 show an increased cadmium content, which is an indication for post-1900 production. Contents of iron and nickel are surprisingly high in object 4. Figure8 shows the zinc/tin ratios in Mass%, determined by AAS, for objects of Benin provenance in the database of the Rathgen Research Laboratory. The ratios differ significantly for privately owned (PRIV) from those in the public collections of Berlin (B), Dresden (D), Cologne (K), Leipzig (L), Munich (M), and Stuttgart (S). Objects in private possession usually show a higher zinc and tin content, which reveals them as modern copies. All but object 4 are well fitted in this group. In Figure9, the respective zinc/lead ratios of the Benin complex are shown. Two separate clusters of alloys are visible: one high in Zn, but low in Pb- content; the other with a rather constant Zn/Pb ratio. Most privately-owned objects fall into the first group, with lower lead concentrations, a trend that points toward a more recent production date, according to Manescu et al.(2008).

Table 1. Results of atomic absorption spectrometry (AAS)-analysis (in Mass%, Cu = 100% − Σ% all other elements) (concentrations contradicting alleged production date are indicated in yellow).

Object Cu Sn Pb Zn Fe Ni Ag Sb As Bi Co Au Cd 1 62.50 1.34 3.34 32.19 0.253 0.277 0.025 0.078 <0.10 <0.025 <0.01 <0.02 0.005 2 62.88 1.40 3.12 31.81 0.446 0.323 0.024 <0.05 <0.10 <0.025 <0.01 <0.02 0.005 3 64.36 1.26 3.36 30.32 0.397 0.285 0.016 <0.05 <0.10 <0.025 <0.01 <0.02 0.004 4 75.18 0.81 2.00 19.36 0.904 1.720 0.020 <0.05 <0.10 <0.025 <0.01 <0.02 0.002 5 63.49 1.62 5.55 28.41 0.313 0.312 0.023 0.153 0.132 <0.025 <0.01 <0.02 0.004 6 60.55 0.90 3.58 33.62 0.892 0.314 0.031 <0.05 0.103 <0.025 <0.01 <0.02 0.003

Table2 presents compositional data obtained by ESEM/EDX for object 4. There is sufficient correlation in the results obtained by ESEM/EDX and AAS for the main elements. Discrepancies for lead, iron, and nickel contents can be attributed to their inhomogeneous allocation in the alloy texture. Furthermore, objects 2 and 4 contain aluminum, phosphor, and silicon in significant concentrations—all elements of modern alloys—which indicates post-19th/20th century production. According to Schwab et al.(2007), aluminum, known since 1827, is added to increase the oxidation and wear resistance of the alloy and is a common constituent in modern brasses in the range of 0.2 to 0.7%. In conclusion, the alleged dating between 1430 and 1650 could be excluded for all investigated objects.

Table 2. Results of environmental scanning electron microscopy–energy dispersive X-ray analysis (ESEM/EDX) (object 4) (concentrations contradicting alleged production date are indicated in yellow).

Measurement Spot Cu Sn Pb Zn Fe Ni Al Si P 1 (M%) 74.4 1.2 0.7 19.3 1.1 2.0 1.3 0.1 0.03 2 (M%) 72.4 0.9 0.1 22.3 1.7 1.1 1.5 0.2 0.01 3 (M%) 73.1 1.2 1.0 19.6 1.5 1.8 1.3 0.3 0.21 Mean (M%) 73.3 1.1 0.6 20.4 1.4 1.6 1.3 0.2 0.08 ArtsArtsArts2018 20182018, 7,,, 7 207,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof8 1313 of 13

((1a)) ((2b)) ((3c))

(4d)) ((5e)) ((6f)

FigureFigure 6.6. VariousVarious brassbrass objectsobjects ofof allegedalleged BeninBenin andand IfIfee provenanceprovenance (photo:(photo: AndreasAndreas Schwabe).Schwabe). Top,Top, Figure 6. Various brass objects of alleged Benin and Ife provenance (photo: Andreas Schwabe). Top, fromfrom leftleft toto rightright ((1a)) (93)(93) Oni (Ife)(Ife) head,head, ca. 1450;1450; ((2b)) (89.2)(89.2) QueenQueen MotherMother (Benin),(Benin), ca.ca. 1650;1650; ((3c)) (89.3)(89.3) from left to right (1) (93) Oni (Ife) head, ca. 1450; (2) (89.2) Queen Mother (Benin), ca. 1650; (3) (89.3) headhead with coral hood (Benin),(Benin), ca. 1600; (4d)) (100)(100) IfeIfe head,head, ca.ca. 1430;1430; ((5e)) (58.2)(58.2) reliefrelief withwith animalanimal sacrificesacrifice head with coral hood (Benin), ca. 1600; (4) (100) Ife head, ca. 1430; (5) (58.2) relief with animal sacrifice (Benin),(Benin), ca.ca. 1620;1620; ((6f) (39) mask (Mali?),(Mali?), ca. 1650.1650. (Benin), ca. 1620; (6) (39) mask (Mali?), ca. 1650.

((a)) ((b))

FigureFigure 7.7. DocumentationDocumentation ofof samplesample locationlocation (object(object 1),1), OniOni Head,Head, allegedalleged IfeIfe provenance,provenance, thethe photophoto Figure 7. Documentation of sample location (object 1), Oni Head, alleged Ife provenance, the photo ((aa)) onon thethe leftleft isis aa macroscopicmacroscopic detaildetail ofof thethe overviewoverview photophoto ((bb)) onon thethe right.right. (a) on the left is a macroscopic detail of the overview photo (b) on the right. Arts 2018, 7, 20 9 of 13 Arts 2018, 7, x FOR PEER REVIEW 9 of 13 Arts 2018, 7, x FOR PEER REVIEW 9 of 13

Prov Prov 12 12 ? ? 10 B 10 BD 8 DK 8 KL 6 Sn LLKA 6 Sn LKAM 4 M 4 Priv Priv 2 S 2 S 0 0 0 10 20 30 40 0 10 20 30 40 Zn Zn

Figure 8. Tin content (M%) as a function of zinc content (M%) determined by atomic absorption Figure 8. Tin content (M%) as a function of zinc content (M%) determined by atomic absorption Figurespectrometry 8. Tin content(AAS) for(M%) bronze as a andfunction brass of objects zinc content of presumable (M%) determined and certain by atomic Benin provenance absorption spectrometrys(Rathgenpectrometry Research (AAS) (AAS for ) Laboratoryfor bronze bronze and Database and brass brass objects. B,D,K,L,M,S: objects of presumable of presumable public and museum certain and certain Benin collections, provenance Benin Priv: provenance private (Rathgen Research(Rathgencollectors, Laboratory ResearchLKA: group Database. Laboratory of investigated B,D,K,L,M,S: Database objects). B,D,K,L,M,S: public. museum public collections, museum Priv: collections, private collectors, Priv: private LKA: groupcollectors, of investigated LKA: group objects). of investigated objects).

Prov Prov 40 40 ? 35 ?B 35 B 30 D 30 DK 25 K 25 L 20 Zn LLKA 20 Zn 15 LKAM 15 M 10 Priv 10 PrivS 5 S 5 0 0 0 5 10 15 20 0 5 10 15 20 Pb Pb

Figure 9. Zinc content (M%) as a function of lead content (M%) determined by AAS for bronze and Figurebrass objects 9. Zinc of content presumable (M%) andas a certainfunction Benin of lead provenance content (M%) (Rathgen determined Research by Laboratory AAS for bronze Database and. Figure 9. Zinc content (M%) as a function of lead content (M%) determined by AAS for bronze and brassB,D,K,L,M,S: objects ofpublic presumable museum and collections, certain Benin Priv: privateprovenance collectors, (Rathgen LKA: Research group of Laboratory investigated Database objects).. brass objects of presumable and certain Benin provenance (Rathgen Research Laboratory Database. B,D,K,L,M,S: public museum collections, Priv: private collectors, LKA: group of investigated objects). B,D,K,L,M,S: public museum collections, Priv: private collectors, LKA: group of investigated objects). 3. Discussion 3. Discussion 3. DiscussionIn the setting of an ever-growing market, deeply intertwined with the increasing global impact of illicitIn the traffic settingking ,of scientific an ever -investigationsgrowing market, may deeply contribute intertwined equally withto other the professionalincreasing global expertise impact for ofsolving Inillicit the trafficquestions settingking of ,related anscientific ever-growing to investigationsthe authenticity market, may deeplyof culturalcontribute intertwined artifacts equally. withThey to other theshould increasingprofessional, therefore global expertise, always impact forbe of illicitsolvingincluded trafficking, questions in the scientificdue related diligence investigationsto the process authenticity when may significant of contribute cultural values artifacts equally are. Theyat to stake. other should professional, therefore, expertisealways be for solvingincluded questions in the due related diligence to the process authenticity when significant of cultural values artifacts. are at They stake. should, therefore, always be included in the due diligence process when significant values are at stake. Arts 2018, 7, 20 10 of 13

While looted or stolen artifacts, copies, fakes, and forgeries have been an intrinsic element of the market since a long time ago, the related questions have only been selectively addressed in a trans-disciplinary, more holistic way. In general, once a work of art is accepted to the laboratory, anamnesis—the investigation of its history and provenance—is the first step to be taken, and is of key importance to the process as a whole. This step should be followed by a whole cascade of analytical methods, starting from non-destructive imaging technologies, to micro-sampling and subsequent chemical analysis. The aim is to look for inconsistencies, such as unusual metal components in alloys or pigments with a “terminus post quem” in paintings. The two cases discussed in this study illustrate how scientific laboratories with their respective expertise are well positioned to provide crucial information and help assemble evidence for legal cases in the accelerating race between various stakeholders of the art black market on the one side, and law enforcement agencies on the other. The analytical technologies and scientific landscape are continuously changing and developing. Thus, on the one hand—and this works to the advantage of law enforcement—tomorrow we shall have better tools, and more sensitive and innovative methods at our disposal. Meanwhile, the forgers, once their products have left their hands, cannot interfere with them anymore. On the other hand, the scientific knowledge in this field needs to be expanded continuously and new markers need to be identified, especially for the second half of the 20th century, in order to maintain an advantage over the forgers, who closely follow the scientific progress in the analyses used and constantly “improve” their work. Most importantly, this requires an increased public understanding of the need to act in a more collaborative effort across a broad variety of disciplines, which include provenance research, criminology, and heritage science. This understanding will hopefully lead to a renewed public commitment to invest in research and education, respective staff positions, and innovative infrastructure and equipment across market and source countries, all essential conditions for not losing this race in the realm of cultural heritage preservation.

4. Materials and Methods

4.1. Raman Spectrometry Raman spectra were measured with a Horiba XploRa Raman-microscope, equipped with a 532, 638, and 785 nm laser. Laser powers were at 25 mW (532 nm), 24 mW (638 nm), and 90 mW (785 nm). For measurements of the samples, a filter with 10% or 1% transmission was used, mostly in combination with a 50x objective with a long working distance.

4.2. Atomic Absorption Spectrometry (AAS) Atomic absorption spectrometry was performed on a PU 9100 (Philips) in an air/acethylen flame in continuous mode, with background compensation (deuterium bulb); the detection limit for specific elements varies with wavelength and input. Twenty milligram bore chips (1 mm drill bit) (Figure7) were dissolved in nitric and hydrochloric acid and diluted in water to 20 mL. From this solution, 12 elements were measured, the difference from 100% was expressed as the copper content. The drill holes were filled afterwards with a patina/wax mixture to blend in with their surroundings.

4.3. Environmental Scanning Electron Microscopy–Energy Dispersive X-ray Analysis (ESEM/EDX) Selected bore chips were embedded for cross sections in order to allow for the analysis of elements and their local distribution in the alloy. The investigation of these surfaces, combined with selective element analysis, was accomplished by scanning electron microscope Quanta 200 (Fei) in environmental mode. EDX for semi-quantitative analysis involved X-ray analyzer XFlash 4010 (Bruker axs). Arts 2018, 7, 20 11 of 13

Author Contributions: S.S. drafted the manuscript and S.R. revised it critically. Both authors read and approved the final manuscript. Acknowledgments: Collaboration and support of René Allonge, Markus Schönfelder and Thomas Walter from the art squad of the Berlin State Office of Criminal Investigation (LKA) is gratefully acknowledged. Special thanks go to Cristina Lopes Aibéo, Regine-Ricarda Pausewein, Joseph Riederer †, Andreas Schwabe and Sabine Schwerdtfeger at the Rathgen Research Laboratory for their close collaboration and always fruitful discussions, as well as to Christoph Schmidt from the Gemäldegalerie, National Museums Berlin for photography and Petra Winter from the Central Archives, National Museums Berlin for leading the provenance research. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References and Notes

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