R1: Characterization and description of minerals (Joint Session with The Gemmological Society of Japan)

[R1-P-04] Mineralogical features of smelting slags from the Early Iron Age Yashin Tepe site, northeastern Iraq

*Masanori KUROSAWA¹, Shin'ichi Nishiyama² (1. Univ. Tsukuba, 2. Chubu Univ.)

Keywords:slag, iron smelting, ironware, SEM-EDS, Yasin-Tepe site

Microstructures, materials, and chemical compositions of four metal-smelting slags and two specimens of corroded ironware from the Early Iron Age Yasin Tepe site in northeastern Iraq were analyzed with a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS) to examine the technical level of the Iron Age iron-smelting in the frontier area of West Asia. Two types of metal-smelting slag were identified in the slag samples: copper smelting slag and iron smelting slag. The surfaces of both types were covered with a thin altered film. The copper smelting slag was vitreous with a light greenish interior and consisted mainly of Ca-Al silicate glass and precipitated crystals of augite. The slag also contained tiny fragments of limestone in the process of melting reaction. The ironmaking slag was black vitreous inside and composed of dendritic crystals of wustite, Ca-Al silicate glass, and Ca-Al silicate mineral precipitates. The Ca-Al silicates were found to have precipitated at 1200-1300 degrees. Small pieces of limestone in the middle of the melting reaction were also observed. Since limestone is abundant around the site, it is thought that limestone was used as a slag-forming agent in metal smelting. The CaO-rich slag produced by limestone incorporation is favorable for the production of high-purity iron, suggesting that high-quality iron was produced in the early Iron Age. The ironware samples were almost completely corroded and consisted mainly of iron hydroxide and small amounts of iron oxides. However, some traces of metallic iron were also observed, and the metallic iron was of high quality with very few impurities. These results indicate that iron suitable for ironware making with high purity was produced in in the frontier area of West Asia from around 800 BCE using advanced smelting technology that utilizes high temperatures of over 1200 degrees and limestone as a slag-forming agent.