ナイフの生産プロセス
Forging 1 (Preparations)
The first step is to prepare mild steel bar stock for forge welding it with carbon steel. Coke is used to quickly raise the temperature of the furnace to 1000 degrees.
The red-hot bar stock is then hammered to the appropriate cross section, usually a rectangular shape with thickness and width depending on the knife to be made.
Forging 2 (Forge Welding)
Forge welding is a traditional Sakai technique used to join a base or jacketing layer of mild steel to a thin cutting layer of carbon steel. The steels are stacked on top of each other, heated to 1000 degrees and hammered together, making them bond with each other on a molecular level.
To ensure complete adhesion agents such as Boric acid, borax, iron oxide, etc. are used. This prevents oxidation of the steel surfaces to be forge welded.
Forging 3 (Shaping)
Once the mild steel base and the carbon steel layer are safely welded together, a belt hammer is used to gradually bring the still rectangular bar closer to the shape of a knife.
At this time, if the temperature of the knife is raised too high, carbon, which is an important element for sharpness, will escape and the finished knife will not perform as well as it could. This is why temperature control is paramount throughout the forging process.
Forging 4 (Resting)
After it has been hammered to shape the craftsmen let the knife rest for a while to allow the steel’s grain structure to solidify and stabilize.
Once cooled down the knife is checked for any flaws in the forge weld or other defects.
Cutting and Grinding
A flat steel template of a finished knife is used to mark off the final outline on the blank. The excess material is then cut off using shears and grinders.
The knives are carefully cut by hand, without the use of heavy machinery.
Cold Forging
Cold hammering the mild steel front layer of the knife will solidify and compress the grain structure, making the knife stiffer and less prone to bending.
At the same time, a hollow is put on the back side by using patterned hammers that stretch and enlarge the surface of the mild steel on the front side of the knife.
Hardening 1 (Mud Coating)
For the hardening process, the lifeblood of any knife, a thin layer of mud is first applied to the entire blade. The mud dries to form a thin layer of insulation, which ensures that the temperature is evenly distributed throughout the blade and protects it from local heat spikes.
It also has the purpose of speeding up the cooling process by suppressing air bubbles that form on the blade’s surface when cooling with water or oil.
Hardening 2 (Quenching)
The coated blade is heated to around 800 degrees and rapidly cooled by sticking it in a water bath right next to the furnace. This changes the crystalline structure of the carbon steel, causing it to harden. Note that the mild steel layer remains soft because it contains no carbon.
Hardening is a process that requires a great deal of concentration and is carried out at dusk or dawn. The light of the blue hour allows the smith to see the color of the steel in greater detail than daylight would. As temperature control is not mechanized, this step requires both skill and experience.
Another characteristic of Sakai cutlery is the use of pine charcoal as opposed to coke for hardening, as it allows to raise the temperature evenly and reduces the risk of overheating. After the knife has been tempered the heat treatment process is done.
Straightening
After hardening, the knife has reached the final stage of the blacksmithing process. This process involves checking for and correcting slight distortions and scratches before handing the blade over to a sharpener.
Rough Grinding
In this first step of sharpening, the knife blank is mounted onto a tool called a sharpening stick. That way the sharpener can lean into the rotating whetstone and put pressure on the knife to remove material relatively quickly. Because the stones cut fast it is important to carefully keep checking the geometry of the knife while working.
Main Sharpening
With rough grinding done, the next step is to grind the blade thinner to make it actually cut.
A knife that has been sharpened too much cannot be brought back to its original state, so this is the process that tests the craftsman's skill the most.
Buffing
In order to remove grind marks from rough grinding and to give the blade a shine, a series of emery wheels and buffing wheels is applied. Buffing starts at low grits, gradually becomes finer, and in the end, the knife is finished with a beautiful shine.
Bokashi
Powder of different Japanese natural whetstones is blended together with water to form a sticky clay, which is then applied to the blade road of the knife and rubbed on using a piece of rubber or wood. This process turns the soft layer of the knife a hazy gray, while the hard carbon steel layer stays a shiny silver. Bokashi as a term refers to the application of this gray finish, which makes the forge welded layers of the knife apparent in the finished product.
Finishing
The final step is to hand-sharpen the knife after the Bokashi process.
The process requires a different type of whetstone for each job, which makes it a labor-intensive and time-consuming process.
Finish sharpening
Once the knife has been forged and sharpened, the knife is completed by attaching the handle.
Sakai cutlery is traditionally made in a system of shared labor, and the highest quality knives are created through improvement in each individual field.
Make sure you try a Sakai Takayuki knife, made by the people at the forefront of a 600-year-old tradition of Sakai cutlery.
