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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 found at the site to figure out the technology and working process used. Slag, or the waste created as a byproduct of iron-working processes such as smelting or smithing, is a powerful research object. 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 archaeological sites and can provide valuable clues to the iron-working technology and process.

Ores used in ancient smelting process are rarely pure metal compounds and the impurities have 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). Slag is also direct evidence of ancient smelting activities. Through slag analysis, archaeologists can reconstruct ancient human activities concerned with metal and try to discuss the organization and specialization in metal production (Maldonado et al. 2009).

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 (Moore 1990, 152) although this is not always the case for ancient slag. In the investigation of ancient metallurgy, the intention and constraints of ancient people should be carefully considered.


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