ART + Science 134 These include the Physics Institute at the University of Bern, the Laboratoire Archéomatériaux et Prévision de l’Altération du CEA in Saclay, ARCANE in Gradignan, and the Centre d’Innovation et de Recherche pour l’Analyse et le Marquage (CIRAM) in Martillac, all of which have worked to develop techniques for the authentication and dating of iron and of gold. Although these techniques are still experimental, they are nonetheless reliable, but for the moment their precision leaves something to be desired. THE NEW FAKES As one would imagine they might, forgers have access to scientific publications and their methods have unfortunately become increasingly sophisticated. Aware that the study of corrosion was becoming an important factor in the analysis of works, some began creating fakes combining old and modern elements (fig. 9). In order to identify such assemblages, it is necessary to X-ray the objects, and this can help ascertain whether they were only restored or result from this kind of pastiche. THE LEAD-210 TEST For more than twenty years, lead isotopes have been used for the seriation of metals containing copper. Measuring the amount of lead-210 isotope (210Pb) present makes it possible to glean chronological information, since its concentration diminishes by half every 22.3 years (its half- EXTREME CASES The study of corrosion provides conclusive information on the chronology of objects made of a variety of different metal alloys (including silver alloys, which we have not discussed in detail here), but this is not true of all metals. Iron, for instance, illustrates the limitations of such investigation. There are many reasons for this. First among these, iron corrodes very quickly and the corrosion products are limited to iron oxides and hydroxides—better known as rust. As such, it is difficult to differentiate between iron that is two hundred years old and iron that was buried for only a few years to produce a deceptive surface. This problem is compounded by the fact that it is not necessary to use acids to produce the latter since the natural humidity in the soil is sufficient to do the job. Since this is the case, only iron in its pure form can be studied and even then primarily through the nature of its internal inclusions or impurities. For example, the presence of manganese sulfides is an indication of modern metallurgy, or, at the oldest, the first half of the nineteenth century, when manganese began to be used to eliminate the sulfur naturally present in iron ore. Conversely, gold alloys corrode very little. Research methods used to determine gold’s age need to take into account its particular properties, but the metal is so complex that it deserves to be the subject of a separate article. Suffice to say here that in recent years, a few university and private laboratories have made progress. Certain preconceptions hinder the correct evaluation of works made of metal. These are a few of the primary ones: Aluminum can be found in old bronze! If one suspects the presence of bauxite (a naturally occurring ore containing aluminum and iron) in the clay core of a piece that is presumed to be old, and if it has diffused into the bronze, both iron and aluminum should be detectable as inclusions. If analysis reveals only aluminum in the metal, that directly implies that it dates from the middle of the nineteenth century at the earliest. See the example of the Benin plaque in fig. 10. Cleaning removes “patina,” making analysis impossible! No. Even if the products of corrosion—the “patina”—have been removed, the natural corrosion will have penetrated into the interior of the metal. The degree of the metal’s corrosion will still be definable and characteristic features observable, among them intergranular and transgranular networks of microfissures, as well as redeposited copper. The presence of cuprite on a bronze is proof of its age! Unfortunately not. Forgers’ techniques have evolved and they are capable of artificially creating red cuprite, green malachite, and blue azurite. Thus a surface analysis is not sufficient to establish a copper-alloy object’s age. Only a complete examination of a cross section using electronic microscopy is up to the task.
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