An international team of archaeologists from University College London, the Cyprus Institute and the University of Cambridge has analyzed finds from the 11th-century CE archaeological site of Chahak in southern Iran and found evidence for the intentional and regular addition of the chromium mineral chromite to the crucible charge, resulting in steel containing around 1 wt% chromium.
Crucible slag adhering to the interior of a crucible sherd. Image credit: Alipour et al, doi: 10.1016/j.jas.2020.105224.
For more than a century, evidence for the production of crucible steel in Central and Southern Asia, prior to the European Industrial Revolution, has fascinated and challenged material scientists, historians and archaeologists.
At the same time, chromium-alloyed stainless steel was developed in the early 20th century, building upon 19th century experiments with low chromium steel.
“Our research provides the first evidence of the deliberate addition of a chromium mineral within steel production. We believe this was a Persian phenomenon,” said lead author Dr. Rahil Alipour, a researcher in the Institute of Archaeology at University College London.
“This research not only delivers the earliest known evidence for the production of chromium steel dating back as early as the 11th century CE, but also provides a chemical tracer that could aid the identification of crucible steel artifacts in museums or archaeological collections back to their origin in Chahak, or the Chahak tradition.”
Chahak is described in a number of historical manuscripts dating from the 12th to 19th century CE as a once famous steel production center, and is the only known archaeological site within Iran’s borders with evidence of crucible steel making.
While Chahak is registered as a site of archaeological importance, the exact location of crucible steel production in Iran remained a mystery and difficult to locate today, given numerous villages in Iran are named Chahak.
The manuscript ‘Kitab al-Jamahir fi Marifah al-Jawahir’ (‘The Book Most Comprehensive in Knowledge on Precious Stones’, 10th-11th-century CE) written by the Persian polymath Abu-Rayhan Biruni, was of particular importance to the scientists given it provided the only known crucible steel making recipe.
Dr. Alipour and colleagues argue that the mysterious compound ‘rusakhtaj’ from Biruni’s recipe refers to the mineral chromite.
They also used radiocarbon dating of a number of charcoal pieces retrieved from within a crucible slag and a smithing slag found at the archaeological site of Chahak in southern Iran to date the industry to the 11th to 12th century CE.
Crucially, scanning electron microscopy technique enabled them to identify remains of chromite.
They also detected 1-2 wt% of chromium in steel particles preserved in the crucible slags, demonstrating that the chromite ore did form chromium steel alloy — a process that we do not see used again until the late 19th and early 20th century.
“In a 13th century Persian manuscript, Chahak steel was noted for its fine and exquisite patterns, but its swords were also brittle, hence they lost their market value,” said Professor Thilo Rehren, an archaeologist in the Institute of Archaeology at University College London and the Cyprus Institute.
“Today the site is a small modest village, which prior to being identified as a site of archaeological interest, was only known for its agriculture.”
The researchers believe it marks a distinct Persian crucible steel-making tradition — separate from the more widely known Central Asian methods in Uzbekistan and Turkmenistan — for the production of low-chromium (around 1 wt%) steel.
“The process of identification can be quite long and complicated and this is for several reasons,” said Professor Marcos Martinon-Torres, an archaeologist in the Department of Archaeology at the University of Cambridge.
“Firstly, the language and the terms used to record technological processes or materials may not be used anymore, or their meaning and attribution may be different from those used in the modern science.”
“Additionally, writing was restricted to social elites, rather than the individual that actually carried out the craft, which may have led to errors or omissions in the text.”
The findings were published online in the Journal of Archaeological Science.
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Rahil Alipour et al. Chromium crucible steel was first made in Persia. Journal of Archaeological Science, published online September 23, 2020; doi: 10.1016/j.jas.2020.105224
