Temporal changes in Angkorian ironmaking technology in Northern Cambodia during the 10th-14th centuries

The Khmer people constructed numerous monuments from the 9th to 15th centuries (Angkor period) using iron tools. Ancient ironmaking sites have been identified as slag mounds near Khmer monuments. However, details of the ironmaking technology used during the Angkor period remain unknown. Herein, we show the temporal changes in ironmaking technology in northern Cambodia, which was a major iron-producing region of the Angkorian Empire. The chemical and Nd-Sr-Pb isotopic compositions of slag and ore samples indicate that Angkorian iron producers used diverse ores and employed at least two distinct ore formulations. From the late 10th to early 13th centuries, laterite and umber were used as iron ores with the addition of Mn oxide and sulphide ores, resulting in high iron yield. This technique may have evolved in response to increased iron demand during the peak of the Angkorian Empire. However, after the late 13th century, ironmaking became less efficient, relying solely on laterite and magnetite ores without additives, which coincided with the decline of the Angkorian Empire. The efficient ironmaking method was lost with only a part of the technique possibly being transferred to another area. These results provide new insight into how technological changes in iron production were linked to the socio-economic dynamics and eventual decline of the Angkorian Empire.