Traditional Japanese swords are made from a material called tamahagane. The process of making tamahagane hasn’t changed in over a thousand years. Satetsu or Sand Iron (Fe) is smelted in a large clay furnace called a tatara. More information about producing tamahagane can be found at Paul’s Martin’s excellent site ‘The Japanese Sword’ (http://www.thejapanesesword.com/tatara).
Both of my two pieces are second grade tamahagane (carbon content ranging from 0.5 to 1.2%). The range of colours found within them is amazing, from blues and purples to silvers and browns. I hope the photos give you a good indication of the compexity of the metal.