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Archaeometallurgy

Archaeometallurgy is an important branch of Archaeometry dealing with the reconstruction of metallurgical and manufacturing processes of ancient metals from prehistory to modern times. Archaeometallurgical research is not only based on the characterization of metals themselves, but it also includes the investigation of all pyrotechnical debris such as slag and technical ceramics as well as the ore deposits. Systematic study of mining technology is a specialized field within archaeology. This is called mining archaeology, and its being strongly connected with archaeometallurgy.

Nearly all pyrometallurgical processes leave marks like metal de-bris, slag and technical ceramics like crucibles, moulds, furnace linings or tuyeres. It is therefore essential to identify the metallurgical process they belong to. These relicts can originate from a primary extraction process, the smelting, or from a manufacturing process like alloying, forging or casting.It is necessary to characterize deposits used in ancient times, by the analysis of ores and metallurgical debris in the surroundings, to get information about trading routes and relations. Therefore, semi-finished products like ingots or metallurgical debris such as slag may contain more valuable information about metallurgical processes and trading routes than the metal objects themselves. Indeed, such objects provide information about ancient fabrication techniques, which may be essentially different to modern processes. Joining techniques like brazing and welding, surface treatments like gilding, tinning or patination, or the processing of metals by casting or forging, and heat treatment are of particular interest.

Techniques applied for the chemical, structural and technological characterization of ancient materials are essentially based on methods used in materials and earth sciences, and research should include all metallurgical materials, the so called “chaine operatoire”. The application of a specific method always depends on the individual problem. Many questions can be answered by a single analysis, but more complex problems require a combination of different methods. Ancient metals are not pure as modern metals and contain a variety of impurities depending on the ore and the smelting process. Therefore, the determination of the chemical composition provides already some information about technology and provenance. Structurally, metals are no homogenous solids and contain various metallic and non-metallic phases. Some objects are also composites of different metals. Metallography, respectively materialography comprising non-metallic material as well, is an important tool to interpret chemical results correctly. Metallography means the description of complex, threedimensional microstructures of materials by identifying and quantifying the different phases.Isotope analysis is an important tool for provenance studies, because processes such as smelting do not alter the isotopic composition of some elements from ore to metal. Several isotope systems have been established in archaeometallurgical research in the last years, but application of lead isotope technique has a long tradition and is standardly used. The lead isotope composition can be determined for all metals containing traces of lead, thus providing information about the geological age of an ore deposit and therefore enabling the distinction between different deposits.

The application of science in the examination of rare or valuable objects of cultural heritage is generally limited; notably metal objects. Objects made of precious metals, as gold or silver are considered of higher value than the same objects made of a different materials. Taking of samples from such objects is usually not allowed, and non-destructive methods are required. The comparability of the objects’ surface composition with the core material is usually poor, as chemical processes during soil storage or usage affect the composition of the surface often leading to misinterpretation. Badly corroded archaeological metal objects can often not be analysed correctly, because of the drastic change of their composition. Also, some metals are totally converted to non-metallic compounds. The question of metal origin and approaches to determining this have always lead to a number of controversial discussions. There are important factors like recycling of metals, which has been used through the ages limiting the possibilities of determining provenance, but do not generally preclude any attempt. Generally, it should be clear, that any provenance study can only exclude ore sources. Even in cases of precise coincidence of chemical and isotope composition of artefacts with a particular ore source, it cannot be excluded, that there is another source with overlapping composition. Many old mines have been eradicated by modern mining, are no longer accessible or not yet identified.

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