I am currently in a materials science class and I am reading about how the strongest and least brittle steels are fabricated. It is actually far more complicated than I had previously imagined.
How did steel workers in medieval times, including those in Europe, Japan and China, know how to make strong and non-brittle steel? Did they understand the chemistry of what they were doing, or was it more of a trial and error trade?
In Europe they generally didn't use our modern methods of making strong non-brittle Steel. They used other methods to make Steel that made Steel of a lower quality but was much easier to do. The most common way this was done was through slack quenching (quenching the hot steel in something other than water, usually something warmer to make the cooling less extreme) and then not tempering the Steel. In other cases they would take armour and place it in a hot pile of charcoal and leave it there over night which would form a layer of steel along the outside of the armour but the core would still be iron. The only smiths who seemed to understand how to make the kind of strong non-brittle steel we see as standard now were the south German armoursmiths working in and around Innsbruck and Nuremburg in the mid Fifteenth Century and later. We honestly have no idea how these smiths figured out how to do it and it's impossible to get a really good grasp on what the common medieval artisan understood about the underlying principles of their work. In some cases they produced manuals but not all crafts did this. The only work on the subject I'm aware of is De Re Metallica by Georgius Agricola but that wasn't published until 1556.
The person to read if you're interested in this subject is Alan Williams. He has two massive books on the subject but both are very expensive so if you can't find them in a nearby library and you aren't staggeringly wealthy you're a bit out of luck. He has written a few articles on the subject though so you could look those up. They are:
Williams, Alan, ‘The Manufacture of Mail in Medieval Europe: A Technical Note’, Gladius, vol. 15, (1980). 105-134.
Williams, Alan, ‘Augsburg Craftsmen and the Metallurgy of Innsbruck Armour’, The Journal of the Arms and Armour Society, Vol. 9, No. 3, (1993). 121-146.
Williams, Alan, ‘Milanese Armour and its metallurgy’, Guy De Boe and Frans Verhaeghe (eds.), Military Studies in Medieval Europe Papers of the Medieval Europe Brugge 1997 Conference, (Zellik, 1997). 61-70
Williams, Alan, ‘Experiments with “Medieval Steel” Plates’, Historical Metallurgy, Vol. 32, No. 2, (1998). 82-86.
Williams, Alan, ‘The Metallurgy of Medieval Arms and Armour’, David Nicolle (Ed.), A Companion to Medieval Arms and Armour, (Woodbridge, 2002). 45-54.