Analysis of Metal BIBLIOGRAPHY Aucouturier, M.; M. Keddam; L. Robbiola; and H. Takenouti, 2003. “Les patines des alliages de cuivre : processus naturel ou oeuvre de l’homme.” Techné, no. 18, pp. 86–94. Bertholon, R., 2001. “Characterisation and Location of Original Surface of Corroded Metallic Archaeological Objects.” Surface Engineering, no. 17: 3, pp. 241–245. 135 Bobin, O. and H. Guegan, 2009. “A New Approach to the Authentication of Goldwork Using Combined Scanning Electron Microscope and External-beam PIXE.” ArchéoSciences, Revue d’Archéométrie, no. 33, pp. 341–347. Craddock, P., 2009. Scientific Investigation of Copies, Fakes, and Forgeries. Oxford: Butterworth-Heinemann. Eugster, O.; J. Kramers; and U. Krahenbülh, 2009. “Detecting Forgeries Among Ancient Gold Objects Using the U,Th – 4He Dating Method.” Archaeometry, no. 51: 4, pp. 672–681. Garenne-Marot, L.; C. Robion; and B. Mille, 2003. “Cuivre, alliages de cuivre et histoire de l’empire du Mali : à propos de trois figurines animales d’un tumulus du delta intérieur du Niger (Mali).” Techné, no. 18, pp. 74–83. Giumlia-Mair, A., 2005. “On Surface Analysis and Archaeometallurgy.” Nuclear Instruments and Methods in Physics Research, B-239, pp. 35–43. Ingo, G. M.; E. Angelini: T. de Caro; and G. Bultrini, 2004. “Combined Use of Surface and Micro-Analytical Techniques for the Study of Ancient Coins.” Applied Physics A, no. 79, pp. 171–176. Ingo, G. M.; S. Balbi; T. de Caro; I. Fragala; C. Riccucci; and G. Bultrini, 2006. “Microchemical Investigation of Greek and Roman Silver and Gold Plated Coins: Coating Techniques and Corrosion Mechanisms.” Applied Physics A, no. 83, pp. 623–629. Leroy, S.; E. Delque-Kolic; J.-P. Dumoulin; C. Moreau; and P. Dillmann, 2013. “Datation radiocarbone des alliages ferreux anciens,” Actes du colloque Sciences des matériaux du patrimoine culturel – 2, Paris, November 20 and 21, 2012, pp. 57–63. Magnusson, C.; D. Cottier-Angeli; and B. Duboscq, 2005. “Le calice de Belmont-sur-Lausanne.” Separatdruck aus Zeitschrift für Schweizerische Archäologie und Kunstgeschichte, no. 62, pp. 31–54. Mille, B . a nd E. R avaud, 2 000. “L’Art du m étal a u r oyaume du Bénin.” Techné, no. 11, pp. 89–97. Pernicka, E.; R. Schwab; N. Lockhoff; and M. Haustein, 2008. “Scientific Investigations of West African Metal Casting from a Collection in Bochum.” In E. Pernicka and S. von Berswordt-Wallrabe (eds.), Original-Copy-Fake? Examining the Authenticity of Ancient Works of Art–Focusing on African and Asian Bronzes and Terracottas, Proceedings of the International Symposium, Mainz, von Zabern, pp. 80–98. Plateau, J., 2003. “La naissance de l’aluminium.” Techné, no. 18, pp. 37–42. Robbiola, L. and R. Portier, 2006. “A Global Approach to the Authentication of Ancient Bronzes Based on the Characterization of the Alloy-Patina-Environment System.” Journal of Cultural Heritage, no. 7, pp. 1–12. Scott, D. A., 1991. Metallography and Microstructure of Ancient and Historic Metals. The Getty Conservation Institute. ———, 2002. Copper and Bronze in Art. The Getty Conservation Institute. Wanhill, R. J. H., 2003. “Brittle Archaeological Silver: A Fracture Mechanisms and Mechanics Assessment.” Archaeometry, no. 45: 4, pp. 625–636. ———, 2005. “Embrittlement of Ancient Silver.” Journal of Failure Analysis and Prevention, no. 51, pp. 41–54. Welter, J. M., 2003. “The Zinc Content of Brass: A Chronological Indicator?” Techné, no. 18, pp. 27–36. Willett, F. and E. V. Sayre, 2000. “The Elemental Composition of Benin Memorial Heads.” Archaeometry, no. 42: 1, pp. 159–188. life). It is generally accepted that after about 220 years, no 210Pb will be detectable in a bronze. Thus it will not be present in a bronze from antiquity or the Renaissance any more than it will be in one from the Djenne culture of Mali. The limitation of this technique lies in the fact that the absence of 210Pb could be due to old alloys being reused to fabricate a modern object. We hope that this brief presentation has demonstrated that certain scientific methods less well known than frequently used ones like C-14 and thermoluminescence have great potential for helping to answer many archaeological questions in general and those of the art market in particular. Above and beyond simple age tests, these analyses also trace technological changes and evolution, both how raw materials were used and the way in which objects were preserved over time. A repatinated object should be considered a fake! From a scientific point of view, not so. Even then it is possible for microscopic analysis to detect the presence of natural, long-term corrosion—one compatible with a temporally distant date of manufacture—in an object that has been given an aesthetic patina at a later date. In a case like this, science has nothing to add, and it remains up to the art expert to determine what the consequences of this modern surface treatment may be. Metal cannot be dated! Yes and no. Copper alloys cannot be dated per se, but there are alternative solutions, as has been discussed. In recent years, researchers have developed a technique for dating gold based on a measurement of helium atoms. This makes it possible to determine the most recent time at which the gold was melted, but only with a fifty percent probability. Moreover, other laboratories have attempted to date iron alloys using Carbon-14. In order to do this, the carbon must be extracted from the iron alloy (a very complex procedure) after which a standard radiocarbon test is performed. Far left, top to bottom FIG. 5: Cross section as seen through a scanning electron microscope (backscattered electron image x 500) of a Greek bronze. The metal is deeply corroded. Both intergranular (dark areas) and transgranular (dark grid patterns) corrosion can be observed. FIG. 6: Cross section seen through an optical microscope (dark field x 100) of a high-copper-content Asian bronze dating to before 1000 BC. The red (cuprite), green (malachite), and grey (tin oxides) areas are different corrosive byproducts that together constitute the patina. The natural metal appears yellow. FIG. 7: Cross section as seen through a scanning electron microscope (backscattered electron image x 800) of a brass object that has undergone chemical treatment. The dark layers at the surface are an artificial patina, which runs parallel to the surface. FIG. 8: Cross section as seen through a scanning electron microscope (backscattered electron image x 700) of an Egyptian bronze from the Ptolemaic period. No superficial patina can be detected, but the processes of corrosion (darker areas) that have developed inside the metal are apparent. FIG. 9: X-ray of a metal drum. Both an assemblage of modern pieces with well-defined cuts and older parts (granular texture) can be observed. FIG. 10 (left): Relief plaque. Kingdom of Benin. Presumed to be 16th–18th century. Private collection. The analyses showed that the object is made of a brass with a 2% aluminum content, which underwent chemical treatment intended to replicate an old patina. For non-believers who remain convinced that the presence of aluminum does not prove that an object is modern, this finding is supported by the fact that the brass in this piece is only superficially altered and that its patina is artificial. Even if some may be skeptical, the study of the corrosion irrefutably reveals that this plaque is modern. It should be noted that all of the objects we analyzed that contained aluminum also turned out to have faked patinas. FIG. 11 (above): Helmet. Proto-Senufo, Côte d’Ivoire. Bronze. H: 17 cm. Courtesy of Galerie Alain Bovis. Analyses showed that the object is made of naturally corroded brass (14% zinc content).
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