The Potential and Limitations of the Microanalysis of Copper Alloys Metal is a mediumthat is less used than wood in tribal art, but it remains a vital one in the expression of the myths, beliefs, and customs of many traditional societies, especially in Africa, where bronze has existed for some 4,500 years. It is a particularly interesting and intriguing 132 material to the chemist and the physicist because it is almost impossible to date and because it is so varied and changeable. In fact, in this context it is not especially instructive to speak of metal as a general subject since there are so many differences between bronze (a copper-tin alloy), brass (a copper-zinc alloy), cast iron, silver, and gold. Even within these, it is more appropriate to speak, for example, of bronzes with a high-tin or high-arsenic content, copper, or tumbaga (a gold alloy with a high copper content used in Pre-Columbian art). Of course, the analysis of metal is also of great interest to curators and art aficionados. Rigorous examination of the medium can be a source of valuable information that can shed light on the nature of an object and the techniques used to create it, and it can contribute to the identification of its provenance and its chronology. This article will present the techniques of analysis used for studying copper alloys and will discuss the kind of information that can be gleaned from such study as well as its limitations. ANALYSIS TECHNIQUES FOR METAL Carbon-14 dating is one of the analysis techniques that most often comes to mind when art and science are mentioned together. While this can be useful in the study of textiles or works made of wood or other organic materials, it is completely unsuitable for the analysis of metal, save for one exception that is discussed in the “Preconceptions” section below. Another well-known technique is thermoluminescence (TL), which is widely known and used in the art market. Measuring the amount of radiation retained in crystalline materials, in certain specific instances, can yield chronological information relevant to metal; however, the successful use of this method requires in-depth theoretical knowledge in order to avoid the pitfalls presented by certain realities of nuclear physics. As no scientific technique is universal, TL is relevant only for metal objects that still retain the clay core around which they were cast. This is most often the case for objects made using the lost-wax technique (fig. 1). Scientifically incontrovertible results can be obtained only from closed forms whose clay cores are unlikely to have been contaminated or intentionally inserted at a later date. As such, the performance of a TL test on an open form, such as a Benin royal plaque or hollow head, can be seen as hearsay for the simple reason that the core, which is accessible from the exterior, may have been contaminated, thus implying that the TL results could be unreliable. Worse, a modern core with unfired clay may have been introduced, which can provide a deceptively old date to the untrained eye. For that reason, in such cases TL is best seen as a complementary technique. Composition and corrosion are the most pertinent approaches for metals and provide the most compelling results. The analyses of the chemical composition of a metal and of its corrosion (or patina) are made with an optical microscope and with a scanning electron microscope (SEM), which provides contrasting chemical image and analysis of elements. These studies require a small sample some several millimeters in size, which can be taken from the object with a jeweler’s saw. CLUES FROM ALLOY COMPOSITION Microscopic examination can yield important preliminary information about an object’s microstructure. A good understanding of this can yield clues about the techniques that were used to make the piece. The presence of dendrites, for example, is characteristic of an object produced by casting (fig. 2). Flattened and aligned inclu- FIG. 1: Rooster scepter. Ife, Nigeria. 12th–16th century. H: 65 cm. Ex Bernard Dulon. Analysis showed that the shaft is iron covered with bronze and that the rooster is made of a high-tin-content bronze using the lost-wax technique. A TL test done on the core indicates an approximate age of 525 years +/-25. FIG. 2 (right top): Cross section as seen through a scanning electron microscope (backscattered electron image x 500) of a tin-rich Zhou Dynasty Chinese bronze. This view shows the microstructure typical of a cast bronze. The darker areas are richer in copper (so-called dendrites) and the lighter ones in tin. Trefoil-shaped copper sulfite inclusions can also be seen. FIG. 3 (right center): Cross section as seen through a scanning electron microscope (backscattered electron image x 500) of an object made of silver-and-copper alloy. The perfect orientation of the copper inclusions (black) indicates that the metal was laminated. ART + Science The Analysis of Metal in Tribal Art By Olivier Bobin and Richard Cheret
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