Forging the katana
The katana forge is a complex art. If everything of course was not invented in Japan (thus it is estimated that the Tamahagane would have been created at first in Korea, in the same way quenching was practiced in other civilizations), nevertheless the Japanese have been able to greatly improve the forging process. From their constant search for improvement and technicality, the katana has become the stabbing weapon that is probably the most famous around the world. These articles aim to shed light on some of the forging processes which moved the Japanese saber forward the nowadays legendary weapon.
Our Tamahagane : When Artkatana was created, we made our Tamahagane directly by ourselves in a blast furnace. Subsequently we ordered it outside, but after some disappointments, we resumed a full controlled in-house production in February 2018.
In fact the Nihontōs were forged with the traditional steel : the Tamahagane.
Usually, a good quality tamahagane has a shiny, silvery appearance, while poor quality looks greyish. We can sometimes observe orange, blue or red dots : these are not impurities but a natural result of oxidation when the “Kera” is cooled in air (see below). Therefore it doesn't affect the quality of the steel.
The Japanese steel, also called watetsu (the European steel is called oranda tetsu) is obtained from a ferruginous sand called satetsu. This sand contains much of molybdenum (preventing breakage) and comes from magnetite (Fe3O4), or from marcassite (Fe S2), or hematite (Fe2O3). But in addition to this positive component for the creation of a blade, this sand also contains not much phosphorus and sulfur (which prevents the breakage of the blade). There are actually two types, the rust colored akame, and the masa or masago, thicker, and black. It is the latter which contains the least impurities (phosphorus ...) and which is therefore the most recommended for the creation of blades (Akame phosphorus : 0.04% and for Masa : 0.01% - Akame sulfur : 0.061% and for Masa : 0.034% - these data about mass proportions from an Izumo vein). This sand was found in abundance in Hoki and Izumo, where it was of a very high quality. Therefore many Tatara furnaces were present in these regions at the time. The principle of these tatara furnaces, also called low furnaces, is to produce an oxidation- reduction phenomenon from the carbon coming from the charcoal, thus increasing the carbon content of the steel. The goal is therefore to extract iron from satetsu (which contains about 1% iron) and to produce a high carbon steel (actually steel means iron containing more than 0.03% carbon ). An incredible hardness is then obtained, which, thanks to the natural advantages of ferruginous sand, will not however be brittle. So, this steel can be sharpened very efficiently without risk of chipping (because a too low carbon rate produces a blade which would not be hard enough to be very sharp and which may get its edge worn very quickly). However, the blade stays flexible and absorbs shocks with a reasonable risk of breakage (because “logically” the problem is that the harder the steel is, the less it absorbs shock vibrations and the more it risks breaking). A paradox of hardness/sensitivity against breakage is therefore perfectly assumed by the Tamahagane steel. Today the blast furnaces have been resumed and they exist all over the world, but only one period furnace (tatara furnace) still exists. It was rebuilt by the NBTHK in 1976 at Shimane, Japan, in place of the Yasukuni Sanctuary where it is estimated that more than 8,000 traditional blades had been produced. But even for the Tatara furnaces, there were many different versions that evolved over time, from the ephemeral furnace to be rebuilt with each use, up to a semi-industrial and permanent structure during the Edo era. In fact, it takes 5 days for a reduction operation : 1 day for the construction of the sides of the furnace, then pine charcoal is burned, on which layer is deposited a bed of ferruginous sand 30 minutes later, then a new layer of charcoal is spread 30 minutes later. Everything burns for 3 hours, and the process is repeated. For three days the operation alternating layers goes on, allowing the iron of the satetsu to absorb carbon (contained in the charcoal). A temperature of 1200-1500 ° is reached. This allows the impurities to evaporate (reduction) and the carbon to attach. The last day is dedicated to extracting the steel and cooling it. For one reduction, 2 tons of Tamahagane is produced, with a consumption of 8 tons of coal and 13 tons of ore. When destroying the walls of the furnace, the “Kera” is recoved, that is to say the block produced by the reduction. Then the resulting block of materials is sorted out, because only half of this Kera possesses the tamahagane qualities (criterion of distinction : carbon content from 0.6% to 1.5%). In fact 2/3 of this part will actually have an optimal average rate of 1% to1.2% , the remainder (lower rate of 0.6%) being used for the manufacture of composite tamahagane blades. You have to know that the origin of the tatara furnace dates back to the 6th or 7th century, and that it was invented in Korea.
Japanese steel (watetsu) was not the only one to be used throughout Japanese history. Actually a period of massive importation of lower quality steel from Portugal and Holland (nanbantetsu or even hyotantetsu or konohatetsu) has occured during the Momoyama era (1573-1603).