Alter talking about bloomery steel

European armor in the Middle Ages was typically made of steeliv, defined as iron that contains carbon in any amount up to about 1.7 percent. Iron purified and worked in a charcoal-fueled fire is bound to have some carbon content however small, therefore in a medieval context the terms iron and steel will be used interchangeably.10 Sections II and III will establish a basis to understand the technologies and limitations of iron working in medieval times, including the bloomery furnace that was used for production of raw iron from the early Iron Age through the Middle Ages.
Bloomery iron through the mid 13th C. was a heterogeneous material, with varying carbon content,hardness, and impurities within the product of a single production run. Impurities (slag) made the metal
brittle, certainly an undesirable feature for armor worn in battle. Bloomeries were also limited in the quantity of metal produced. With natural air circulation, or even hand-pumped bellows, the output was typically less than 10 kg. With a limited supply of relatively poor quality material, the most common form of helmet through the early 1300s was pieced.11
Knowing these limitations it is natural for one to jump forward to the iconic 15th C. suits of polished plate armor that we see in museums, many of which are identified as hardened steel.v Surely this is the answer - that some great improvement in the working of steel led to the ability to shape these works of art. Surprisingly, this is not the answer. It is certain that techniques for hardening steel were known and did create metal with superior toughness, but the process of hardening steel occurs after the finished piece is shaped. The earliest known piece of fully hardened steel armor is a bascinet dated to 1330/4012, yet one-piece helmets of unhardened steel are common from the same time period and later. Williams (2003)13 lists 19 items of 14th C. German armor identified as “Bascinets and other one-piece items,” of which only one is hardened. The rest were produced from bloomery-quality steel, not substantially different from the steel seen in earlier armor. Even into the 15th C. unhardened armor was the norm, with the more costly hardened pieces going to those who could pay for the additional processing. It is interesting to note that after 1510, even the high end Italian armor suits were made of unhardened steel, as the aristocratic elite chose decoration over toughness.