Archaeology and alchemy applied: Experimental reproduction of Persian chromium crucible steel making

Abstract

In a recent publication, we presented the earliest archaeological evidence for high-carbon steel alloyed with c. 1–2 percent by weight chromium, dating to the early 2nd millennium CE, and postulated that this ‘Chahak process’ was based on the addition of dried organic matter and chromite mineral as part of the charge of a closed crucible which was then fired to c. 1350 °C to produce steel. Here, for the first time, we present experimental proof that the direct reduction of chromite sand in the presence of liquid steel using common organic matter as reductant can lead to the addition of several percent by weight of chromium to the steel. In contrast, manganese oxide remains largely unreduced in the slag, despite forming a much larger part of the total crucible charge. This is in accordance with the lower reducibility of this oxide. We report the necessary operating conditions and highlight some of the constraints limiting the success of the process, and document the composition of the resulting slag and chromium-containing high-carbon steel. Our controlled crucible steel making experiments for the scientific reconstruction of the Chahak process are based on the combination of alchemical (Biruni’s crucible steel making recipe), archaeological and analytical evidence. We found the results support the alchemical recipe ingredients, and closely match the archaeological production remains, and the composition of the only currently known early Islamic chromium-rich steel artefact.