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Ancient iron production
Ancient iron production is different from ''modern industrial iron production'' in many aspects. The archaeological investigation of early iron production usually depends on the debris in the site to figure out the technology and working process. Slag, waste of iron-working processes such as smelting or smithing, is one of the powerful research objects. After the iron working process, the iron products were always moved and processed, while slag was left at the working site. The size, shape, chemical composition and microstructure of slag are related to the iron-working process. In addition, given its resistance to weathering, slag is usually well preserved in the archaeological site. Therefore, it could provide clues to the iron working technology and process.
== Overview ==
Ores used in ancient smelting process are rarely pure metal compounds and the worthless material has to be removed through slagging. Slag is the necessary material to collect impurities from ores, gangue, the furnace lining and charcoal ash, which can yield numerous aspects of information about smelting process (Bachmann 1982).〔Bachmann, H.G., 1982, ''The Identification of Slags from Archaeological Sites'', London: Institute of Archaeology〕 Slag is also direct evidence of ancient smelting activities. Through slag analysis, archaeologist can reconstruct ancient human activities concerned with metal and try to discuss the organization and specialization in metal production (Maldonado et al. 2009).〔Maldonado, B., Rehren, Th., 2009, Early Copper Smelting at Itziparátzico, Mexico, ''Journal of Archaeological Science'' 36, 1998-2006〕
In modern view, slag should have three functions. The first is to protect the melt from contamination. The second is to accept unwanted liquid and solid components. Finally, it can help to control the supply of refining media to the melt. In order to approach these objectives, a good slag should have lower melting temperature, lower gravity and better fluidity to ensure a liquid slag which can be separated well with metal melting. Moreover, slag should maintain the correct composition so that it can collect more impurities and be immiscible in the melt (Moore1990, 152).〔Moore, J.J., 1990, ''Chemical Metallurgy Second Edition'', Oxford: Butterworth-Heinemann Ltd〕 Yet this is not always the case for ancient slag. In the investigating of ancient metallurgy the intention and constraints of ancient people should be carefully considered.
There are several kinds of non-ferrous metal that have been used by ancient people. Slag analysis is usually applied in the research about smelting and refining processes of copper, tin, lead and zinc. Slag can be created in many steps of metal production. Based on the mechanism of creation, slags can be catalogued as furnace slag, tapping slag, crucible slag, etc. In a smelting furnace, as many as four different phases could co-exist. From top to bottom, they are slag, matte, speiss, and liquid metal (Thornton 2009).〔Thornton, C.P., Rehren, Th., Pigott, V.C., 2009, The production of speiss (iron arsenide) during the Early Bronze in Iran, ''Journal of Archaeological Science'' 36, 308-316〕 The separation between these phases would not be perfect and therefore, three other phases can usually be observed in the slag if they ever existed. Through chemical and mineralogical analysis of the slag, things can be learnt like the identity of metal smelted, kinds of ore used and technical parameters such as working temperature, gas atmosphere, slag viscosity, etc.
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