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Japanese traditional sword making is bound to the use of certain types of steel which are the product of ancient manufacturing processes. Without the use of such steels, it wouldn’t be traditional japanese sword making anymore.
It is not possible to use those steels such as they come from their maker. In the old days there were no other steels available, and blacksmiths had to perform tanren 鍛錬 to turn the metal into useful stock. Tanren, along with quenching techniques, is what characterizes traditional japanese swords.
A swordsmith in the japanese tradition must learn to understand the nature and origin of the steels he is to use, and how to prepare them through tanren.
Steel coated in charred straw and clay, a traditional flux
Tanren implies something similar to kneading — stretching, folding, welding, stretching… and so on. In order to prevent the steel from oxidizing, it is coated in a flux made of charred straw and clay every time it’s heated to welding temperature. The straw gives carbon back to the steel, or at least prevents its losing some, and the clay prevents oxygen from reaching the steel’s surface and dissolves a certain amount of oxides. During heating, both charred straw and clay are first baked, then liquefied and run away just before the billet is taken out the forge.. if everything goes well, that is!
The following video shows the making of aku 灰汁, the charred straw used in the flux mixture along with clay during tanren.
The charring of straw when making aku
There are a number of tools without which tanren would be impossible, and some of them are presented below. The forge used for tanren must have a side tuyère for the liquified clay runs at the bottom and would jam any bottom tuyère in no time. The forge must also be able to accommodate enough charcoal to create a properly neutral, or at least non-oxidizing environment. The very size of each charcoal chunk allows to control the volume of the heating area by permitting or restraining the gases’ movement.
In order to facilitate manipulation of the steel during tanren, it is welded on one end of a tool called a teko, a tapered bar the extremity of which much be made of the same material as that to be forge-folded.
The te-boki is used to hold down hot work and brush away scale. Learn how to make one through this link!