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, armour, 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 centuries CE. A scanning electron microscope was used to analyse the slag samples, revealing traces of ore mineral chromite. Finally, an analysis of steel particles found in the slag suggests the Chahak crucible steel contained between 1% to 2% chromium by weight.
“The chromium crucible steel that was made in Chahak is the only known of its kind to contain chromium, an element known to us as important for the production of modern steel, such as tool steel and stainless steel,” explained Alipour. “Chahak chromium crucible steel would have been similar in terms of its properties to modern tool steel,” and the “chromium content would have increased the strength and hardenability, properties needed to make tools.”
A wealth of Persian crucible steel objects can be found in museums around the world, she said, and we already know that crucible steel was used to make edged weapons, armour, prestigious objects, and other tools. Chahak is also referenced in historical manuscripts as a place where crucible blades and swords were made, but the accounts “also mention that the blades were sold to a very high price, but they were brittle, so they lost their value.”
The phosphorus, which was also detected during the analysis, was added to reduce the melting point of the metal but also to reduce some toughness, which subsequently made the metal fragile.
Regardless, the discovery points to a specific Persian tradition of steel-making, which is in-and-of-itself quite important. To the best of the authors’ knowledge, the specific chromium content seen in the Chahak steel could be used to distinguish it from other artifacts.
“Previous crucible steel evidence, studied by scholars, belong to crucible steel production centres in India, Sri-Lanka, Turkmenistan and Uzbekistan,” said Alipour. “None of these show any trace of chromium. So, chromium as an essential ingredient of Chahak crucible steel production has not been identified in any other known crucible steel industry so far.” To which she added: “That is very important, as we can now look for this element in crucible steel objects and trace them back to their production centre or method.”
To that end, the researchers are hoping to work with museum experts to share their findings and to help with the dating and identification of objects with this unique chromium steel signature.