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.