The iron is made by burning high-quality charcoal and iron sand in a furnace at a maximum temperature of 1,500°C for three days and three nights without sleep or rest (one night), with a craftsman known as an "O-Kaji" playing a central role in the process. This is an ancient Japanese iron-making technique that is based entirely on the experience and intuition of the craftsman.

Japanese swords are made of tamahagane, a lump of iron made from iron sand that has been reduced to a low temperature. This tama-hagane is produced using an ancient Japanese iron-making technique called tatara-seitetsu. The entire lump of iron produced at this time is called "kera," and its quality is not stable because the carbon content varies from part to part. The lumber is split into small pieces, and then divided into fine grades ranging from hard to soft according to carbon content, size, and properties. In addition to tama-hagane, there are other materials called pig iron (zuku), knife iron (hocho-tetsu), large split iron (oawarishita), large blacksmith's iron (okajiya-yayou), mejiro (mejiro), copper (doshita), and others.
Each of the materials listed above has its own characteristics and can be transformed into various properties in the future processes.
Generally, people tend to think that the higher grades of tama-hagane are more expensive and better, however, even materials of lower grades or those with low carbon content can be transformed into interesting materials by the way they are processed.
The most important thing is to think about what kind of sword you want to make and how to select and process the materials to make it.

The tama-hagane is placed in a blacksmith's furnace called a hodo (fireplace). Pine charcoal is placed over the steel, and the steel is then heated to reddish color by blowing air through a bellows.
Reddened tama-hagane is crushed into a cracker-like thickness while dividing it into several pieces. Heat the entire piece to a temperature of about 800°C and then place it in water to cool rapidly and harden it. This process is called "mizubeshi."
Basically, if it does not contain carbon, it will not burn and the material will not harden. A material that does not contain carbon will not burn no matter how hot it is placed in water.
Blades must be hard in order to cut well, but if they are only hard, they are prone to spills and breakage. This is the reason why tama-hagane is said to be the most suitable material for Japanese swords, as it contains the most carbon of all steel materials and can be given the proper amount of tenacity through the subsequent forging process.
The tempered, cracker-shaped tama-hagane is hammered to split it, and any obvious impurities are removed. The cracked cross section is then sorted by looking at the crystals and color of the tissue, and divided into five to six levels of hardness.

Those with high carbon content, very hard, fairly hard, hard, and somewhat soft are used for blade iron (hagane) and skin iron (kawa-hagane).

The low carbon content and extremely soft nature of this material is used for shingan (core iron) and munegane (wing iron).

It has a very low carbon content, is very soft and contains impurities, and is used for shingane (core iron).

TAMA-HAGANE of similar hardness and stacking them on the end of a lever. The steel is then heated to a high temperature (around 1300°C) in a firebox. Slag, a liquid like a natural glue, is then drawn out of the steel and pounded into a lump. The slag is then beaten into a lump.
When tama-hagane is heated to a high temperature, the steel makes a sizzling sound. "Boiling" is the process of raising the temperature of the tama-hagane to the temperature at which it is ready to be forged, relying on the sound, the color of the flame, and the sound of the wind when it is sent from the bellows to feel the temperature and condition of the tama-hagane at the moment.

The leverage rod is used because the temperature of the steel is high, there are few places to grab it, and it is difficult to handle, and of course it is impossible to work with it directly by hand. The steel is once boiled on the tip of a bar called a lever bar to make it easier to work with.
The tip of the blade is then forged three to five times (see "Fold Forging") with a square piece of tamahagane, as it is swallowed by the sword material.
The leverage bar is cut off at the point where it is attached when the material has been extended to a certain length in the subsequent process of rough stretching.

After stretching the lump of material to a certain length in a square, cut a slit in the middle about two-thirds of the way through and fold it over. Then, the area is again boiled at around 1300°C and beaten to attach the folded joint. This is called "folding forging.
The process of attaching steel by heating it to a high temperature and beating it is called "forging" (tanchaku).
If the temperature is too high, carbon is rapidly released from the steel (decarburization) and the steel becomes unquenchable, or so-called “stupid".Conversely, if it is too low, not enough "slag" is produced and no amount of tapping will forge it.
If the forging is insufficient, the layers are not adhered to each other and will remain as "flaws" in the finished product.
Therefore, each turnaround workout requires concentration.
In the preliminary forging, it is essential to remove impurities and air from the tama-hagane, and it is at this stage that the quality of the material is determined.
The steel is then "under-forged" (approximately five times) to remove impurities from the steel and at the same time to equalize the amount of carbon in the steel and increase its strength.
The core of the sword, the shingane, is forged by folding and forging the sword 8 to 10 times while it is still in the lower forging stage. At the same time, the outer surface of the sword, the kawagane, is also made through a preliminary forging process.

In Top Forging, we finish the material for the part used for the blade and the side of the blade, called hitetsu.
For the hitetsu, two to four types of pre-forged materials are thinly rolled into strips of 2 to 3 cm wide, 6 to 8 cm long, and 4 mm thick, respectively.
Strips of material of different types (hardness) that have been pre-forged are combined and re-stacked on the tip of the lever bar. After that, the pile is boiled and folded back and forged.
This process produces a beautiful pattern of fine layers of folded and tempered "Jigane" within the hitetsu, which is then sharpened. There are various ways of combining materials and stacking them to produce ground iron.

The blade, side, peak, and center of each piece of forged steel, each with a different hardness, are combined and gradually made into a single mass while raising the temperature to nearly 1,300°C. (It is just like Kintaro candy before it is stretched.)

It is gradually extended by beating with even force while repeatedly boiling.
Twisting the steel to the edge of the blade will cause the steel to come to the side of the blade, resulting in failure. Also, if the temperature is incorrect, cracks called "shinae" will occur.
Once this is in place, it cannot be modified.
When this process has extended to a certain length, it is separated from the leverage rod.

The shape of a long square bar is formed based on the exact number calculated backward from the dimensions of the finished sword when it is completed.

The shape of a Japanese sword is hammered out with a single hammer, leaving the line of Shimagi-line and thinly pounding out the blade and Shinagi-ji from a square corner.
At the end of the hizukuri process, a shallower warp is added than the finished product.

The surface of the finished firework leaves fine marks of hammering and combines with oxygen to form an oxide film, turning the surface black. It is then sharpened with a tool called a lathe and shaped with a file to bring out the original silver color of the steel and to check for scratches. (If there are major scratches at this stage, stop here.)

After the blade has been shaved and molded to a silver color (at this stage, the shape of the Japanese sword has already appeared), a special mixture of clay called yaki-hada-do is applied thinly to the areas to be burned and thickly to the areas not to be burned. If the border between the two is straight, it is called a "straight edge," if it is wavy, it is called a "curved edge," if it is irregular and rough, it is called a "midareba," and if it is shaped like the character "choji," it is called a “choji". This reveals the school of swordsmanship and the characteristics of the swordsmith. It is said that hardness increases more than five times before and after quenching. However, if the entire blade is tempered, the entire blade will break because it is too hard. Therefore, we intentionally create "hardened" and "un-hardened" areas to improve sharpness in the hardened areas, and to release impact and provide flexibility in the un-hardened areas.

After the Yaki-Ba-do clay is applied to the front and back sides of the blade, it is allowed to dry and the blade is then cooled to a uniform temperature of around 800°C. The blade is then placed in a long, narrow tub of water called a "yakibabune" to cool rapidly. By doing so, the thinly coated area (blade edge) is rapidly cooled and hardened (the hardness of the steel increases). However, temperature control is extremely difficult, and if the temperature is too low, the blade will not be tempered, resulting in an unsharpened "nagara-katana". If this blade were to break, it would never be repaired, and the 20-odd days of work that had gone into it would be lost.
Although the quenching process itself takes less than 10 minutes from the time the reddening process begins, it is a process that requires split-second judgment and concentration.

Immediately after the firing, the sword blade is slightly heated over a flame to bring the temperature to just under 200°C. It is then placed in a pail of water to slightly restore the firing. The tip of the blade expands due to the burning process. Tempering softens the force of expansion and restores the sword to an appropriate hardness, giving it tenacity and the ability to cut well without breaking or bending, which is the essence of Japanese swords.

After quenching, the back of the blade (the side without the blade) is heated with copper (called "akagane") to strengthen the warp and the shinogichi is hammered down to correct the warped areas.

The rasped (unburnished) areas are rasped and shaped. The entire blade is then sharpened with a whetstone. In this process, the so-called "blade pattern" can be seen, and the shape and condition of the blade pattern can be checked, as well as whether there are any scratches due to insufficient forging. If the shape of the blade pattern is not to your satisfaction, the blade is reddened to about 800°C, tempered completely, and then quenched again by applying tempered blade clay to the blade. If there is a scratch and it is beyond repair, we will stop production and start over from the beginning.

The part of the sword that is held is called "nakago," which is written as "stem" or "center. This part of the sword is where the swordsmith's work remains intact for future generations, and is not altered by other swordsmiths.
The sword smith takes the utmost care in the placement and shape of the blade, and uses a file or whetstone to shape the blade.

Once the metal collar has been sharpened, the stem of the blade is finished finely, and a special file is used to remove any unevenness and to create a pattern on the blade.

If the sword is satisfactory, the craftsman engraves his own "tohkoumei" (swordsmith's name) on it with a chisel.
Although nowadays they are produced as works of art, Japanese swords should be able to protect a family in times of emergency.
I will engrave an inscription with all my heart on a piece that I have judged to be a responsible one, based on its performance and durability as a weapon.
Of course, in my case, I would cut it as "fusa hiro saku".