1,000-Year-Old Precursor to Stainless Steel Found in Iran, Surprising Archaeologists

Chromium steel, commonly referred to as stainless steel, is thought to be a recent manufacturing innovation, but new evidence suggests ancient Persians stumbled upon an early version of this alloy some 1,000 years ago, in what is a surprise to archaeologists.

Ancient Persians were forging alloys made from chromium steel as early as the 11th century CE, according to new research published today in the Journal of Archaeological Science. This steel was likely used to produce swords, daggers, armor, and other items, but these metals also contained phosphorus, which made them fragile.

“This particular crucible steel made in Chahak contains around 1% to 2% chromium and 2% phosphorus,” Rahil Alipour, the lead author of the new study and an archaeologist at University College London, said in an email.

Archaeologists and historians were, up until this point, fairly certain that chromium steel (not to be confused with chrome—that’s something else) was a recent invention. And indeed, stainless steel as we know it today was developed in the 20th century and contains far more chromium than the steel produced by the ancient Persians. Alipour said the ancient Persian chromium steel “would not have been stainless.”

That said, the new paper “provides the earliest evidence for the consistent and intentional addition of a chromium mineral, most likely chromite, to the crucible steel charge—resulting in the intentional production of a low-chromium steel,” wrote the researchers in their study.

A translation of medieval Persian manuscripts led the research team to Chahak, an archaeological site in southern Iran. Chahak used to be an important hub for the production of steel, and it is the only archaeological site in Iran with evidence of crucible steel-making, in which iron is added to long tubular crucibles, along with other minerals and organic matter, which is then sealed and warmed in a furnace. After cooling down, an ingot is removed by breaking the crucible. This technique was vitally important among many cultures, including the Vikings.

“Crucible steel in general is a very high-quality steel,” Alipour said. “It does not contain impurities and is very ideal for production of arms and armour and other tools.”

A key manuscript used in the study was written by the Persian polymath Abu-Rayhan Biruni, which dates back to the 10th or 11th century CE. Titled “al-Jamahir fi Marifah al-Jawahir” (translated to “A Compendium to Know the Gems”), the manuscript offered instructions for forging crucible steel, but it included a mystery compound called rusakhtaj (meaning “the burnt”), which the researchers interpreted and subsequently identified as being a chromite sand.

Excavations at Chahak resulted in the discovery of residual charcoal in old crucible slag (waste matter that’s left over after the metal has been separated). Radiocarbon dating of this charcoal yielded a date range between the 10th and 12th…