Modern tool steel or stainless steel is created with the addition of chromium and it is believed that this most important invention was made very recently - at the turn of the 19th and 20th centuries. Recently, however, scientists from University College London (UK) have received evidence that people learned to use chromium much earlier. Approximately 900 years ago, in the place of Chahak in the south of modern Iran.
In search of chromium steel, scientists were prompted by an old description of the process of forging it in non-standard crucibles for high temperatures. Source - manuscript "Al-Jamahir fi Marifa al-Jawahir", which can be translated as "Compendium for the knowledge of precious stones." Its author is Abu-Raikhan Biruni, a famous Persian polymath of the 11th century AD. The forging recipe is the only one for the entire book, which already emphasizes its importance - the author clearly wanted to preserve and pass on this information to descendants.
Deciphering the ancient description turned out to be hardly feasible due to the complexity of the language and the ancient terms incomprehensible to modern Iranians. Most of all, scientists were puzzled by the element called "rusakhtai", in which they eventually recognized the ore mineral chromite. It seemed incredible, but an archaeological expedition to Chahak, which is now an architectural monument, did find traces of chromium in an ancient smithy.
Analysis of a few droplets of metal recovered from the remains of crucibles and forge slag did show the presence of chromium impurities. There is very little of it there - 1-2% against modern standards of 8-13%, but the fact of artificial addition of chromium during steelmaking has been proven. Archaeologists also found traces of phosphorus, which coincides with the descriptions of ancient swords from Chahak - their blades had beautiful patterns, but the metal was too fragile. Probably, the ancient Persian metallurgists actively experimented with impurities in search of the optimal composition of steel, and as a result, it was the chrome alloy that was recognized as such.