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133 sions are evidence of hammering, but if the alignment of these is too perfect, it indicates the modern process of lamination (fig. 3). Examination of the internal structure can provide other technological indicators as well and, by extension, contribute information relating to the date of the object’s manufacture. This is especially true for alloy composition. While analysis of the concentrations of copper, tin, zinc, lead, etc. may not suffice to determine an object’s specific age, this can be very useful when it establishes that the sample is modern. The example of brass objects from the Kingdom of Benin is instructive in this respect. While often referred to as “bronzes,” these are actually brass objects because they are made of a copper-zinc alloy. Over the course of time, the concentrations of the metals in the alloy used to make them changed significantly. Between the seventeenth and nineteenth centuries, the amount of zinc increased from two to thirty percent, while the amount of tin dropped from six percent to zero. The proportion of trace elements (alloy components present in very small quantities, of the order of 0.01 percent) also changed. To give but one example, the quantity of antimony was reduced by a factor of five during this period. This type of analysis should in no way be considered to be a method of dating. One must be careful to avoid simplifications that might lead to a conclusion such as an object containing ten percent zinc and two percent tin, for example, being considered to be of seventeenth-century origin based on this fact alone. Nonetheless, this does not preclude the use of compositional analysis in helping to establish chronological reference points. While zinc may not represent a precise chronological factor, the presence of aluminum, phosphorus, chrome, or manganese (beginning with concentrations of about 0.2 to 0.3 percent) are indicators of modern manufacture. These elements are naturally occurring and have always been in the environment, but their deliberate use in the manufacture of metal alloys dates only to the nineteenth century or even to the beginning of the twentieth where phosphorus is concerned. Examination of the works published by the international scientific community reveals that these elements are never found in old alloys except in trace amounts of the order of about 0.01 percent. To those—obviously non-scientists— who maintain that aluminum can be present in antique alloys due to contamination from the core, we reply that such contamination results in inclusions that are distinct from the alloy since they are not completely Analysis of Metal dissolved. Moreover, aluminum is never alone in a core but is always associated with other elements: silicon in kaolinite and sodium, potassium, or calcium in feldspars. Thus, if 0.5 percent aluminum is detected in brass, silicon would also need to be present if it represents this kind of contamination from the core—although this is never the case. The presence of aluminum is effectively tantamount to proof of the use of modern metals. The same reasoning applies to phosphorus, manganese, and chromium. EXAMINATION OF METAL CORROSION, OR PATINA The most important part of the examination of a metallic object is the analysis of its corrosion, generally referred to as its patina. While that term implies its surface, the study of corrosion involves not only the products of superficial corrosion (the patina, properly speaking) but also the process of corrosive change over time that takes place inside the metal. A copper or silver alloy that is several hundred years old, for example, will have undergone environmental attacks such as humidity, temperature variations, and the development of microorganisms, to name just three. These events lead to degradation of the metal, the analysis of which can either provide indicators about the age of the object or point to its being of modern origin. In copper alloys, the most characteristic indications of natural corrosion that has occurred over a period of centuries and is considered consistent with the presumed date of an antique work include a profound alteration of the metal (fig. 5); degradation that especially affects areas with high copper concentrations, which are the most fragile; the presence of multiple corrosive products, such as cuprite, azurite, malachite, atacamite, nantokite, tin oxide, etc. (fig. 6); layers of associated sediments; and absence of recurring chlorine and/or sulfur. On the other hand, if the corrosion is superficial and parallel to the surface of the object and the metal is homogeneously attacked by it (fig. 7), and if there is detectable chlorine in the corrosive products, the alteration is artificial and modern, that is to say, fake. Finally, it is important to remember that even if an object has been thoroughly cleaned and its patina has been removed, microscopic study of it remains possible and useful. The processes of corrosion penetrate into the material and leave traces and indicators that even vigorous abrasion does not remove (fig. 8). FIG. 4: Figure. Gan, Burkina Faso/Northern Ghana. 18th century or earlier. Copper alloy. H: 39.5 cm. © Pace Primitive, New York. Photo: Oren Eckhaus. Analyses showed that the metal is copper with arsenic and that it is naturally corroded.


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