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Tamahagane, the original steel used by the Japanese swordsmiths
Property Type: Japanese Sword Forging
Steel is iron that contains carbon, so the more carbon a steel has , the harder it is. Most of the Japanese sword smiths are working with the traditional steel that is better known as tamahagane. In Japan there’s only 1 official smelter left. This Japanese style smelter is called a tatara. The tatara relies on the propensity of very hot iron to combine with carbon in its vicinity to produce steel.
In the tatara, it is the burning charcoal that supplies the carbon. These typical Japanese smelter is made of clay, about 5 feet wide, 48 inches tall and 15 feet long, where it’s walls are 10 inches thick.
One operating cycle of the tatara takes 5 days. One day to build the walls (clay and sand), 3 day’s to smelt and 1 day to remove the iron out of the tatara. In this 5 days process , they need about 13 tons of charcoal and 8 tons of satetsu (black sand) to produce 2 tons of iron and steel, which is called Kera.
About half of this Kera is composed of steel ranging from 0.6 to 1.5% carbon and it is this portion that is called tamahagane. Only Two thirds of the tamahagane is of a good quality. The rest of the Kera can be used for forging swords if another separated forging operation is done, this process is called Oroshigane, where carbon is added or reduced.
Every time Tamahagane need to be made, the tatra needs to be rebuild. Its walls must be build out of clay bricks. The mixture that is used to make the bricks does contain a large amount of sand (silicon oxide) which makes the tatra resistant to fire and melting.
Since the forging process of a sword produces a continues loss of carbon, most of the smiths like to start with tamahagane that has an carbon content of 1.0 to 1.5% to forge the kawagane (jacket steel).
A billet of tamahagane t from the tatara furnace is very big (as you can see in the video) and includes various qualities of steel. The big ‘rock’ of Tamahagane is broken into many small pieces to check the quality.
From here on the smith can choose the right pieces and start the forging process of this katana.
This very nice video is showing the process of making Tamahagane