Tamahagane steel is made by building and firing a Tatara, the traditional Japanese sword-steel smelter. This charcoal-fired furnace produces a very pure steel from iron sand, and this steel “Kera” or bloom can be broken and separated into high- and low-carbon pieces, which respectively form the “skin” steel and “core” steel of the blade. The skin steel is forged and folded repeatedly, to remove slag inclusions and voids and is then wrapped around the core steel before the resulting billet is forged into a blade. Careful heat treating, shaping and polishing reveals the tight “Hada” or layer pattern of the blade and the white particles of the “Hamon” or temper line. While this process results in the aesthetic qualities much admired by collectors it also produces a very functional blade, as the high carbon content of the skin steel makes a very hard edge possible while the softer core steel gives the blade its resilience and ability to absorb shock.
1) The steel is smelted in the traditional Tatara furnace creating a 'Kera' or bloom of raw steel. This bloom is broken into pieces by hammering and the pieces are separated visually by an accomplished smith, who can determine carbon content of each piece by its appearance.
2) Pieces of high carbon steel are selected for forming a billet of skin (outer layer) steel while pieces of lower carbon steel are selected for forming a core (inner layer) steel billet. The high carbon skin steel, when differentially quenched, will form a very hard edge while the lower carbon core steel will be softer, providing resilience to the blade and supporting the hard edge.
3) The pieces of high- and low-carbon steel are flattened and formed into separate stacks, ready for forming into billets.
4) Each stack is then covered in burnt straw; which aids in the forging process, and then forged into individual high- and lower- carbon billets.
5) The outer skin steel is folded multiple times to remove slag inclusions and voids. The repeated folding produces a fine Hada (grain pattern) when the blade is finally polished.
6) The folded skin steel is then wrapped around the softer core steel billet and forged into a blade. The blade is then ready to be differentially quenched, using the traditional clay method, polished, and mounted.