Why a Japanese Knife Is the Best Choice for Chefs: Buying Guide

A knife renowned for its beauty and high quality starts its life as unremarkable bars of steel and iron. The blacksmith’s job is to hammer these two metals together. The harder hagane steel forms the cutting edge, while the softer jigane iron overlaps and supports it, enhancing durability. A certified traditional craftsman stands in front of an open furnace, then uses a belt hammer to beat red-hot bars of hagane and jigane into what will become a blade. He works on a few sets in succession, amid the din of the rhythmically beating machine. From the steel bar he cuts off the part he has hammered into a rough shape. He then picks up a soft-iron bar, applies flux powder to its end, and places the cut steel on top. Next, he heats the two layers in the furnace before beating them with a hammer.

“I make the flux powder by mixing boric acid, borax, and iron oxide,” master blacksmith Ikeda explains. “It’s not an adhesive, by the way—when the two metals are heated to about 1,000 degrees Celsius, the powder combines with oxides and other impurities on the welding surface before flowing out of the joint. That way, it helps to make a stronger weld.” Known as forge welding, this method utilizes the dual forces of heat and pressure. As Ikeda repeatedly heats and hammers the metals together, sparks of iron oxide fly in all directions. All the while as he hammers, he is shaping the desired form. When the two metals are completely joined, he covers the welded piece with straw, which immediately flares into flames. In so doing, he is helping the blade to cool at a desired rate. “The burning straw slows down the cooling process, and so prevents the blade from hardening too fast. Otherwise, it might crack. Tempering is done later, at the next step of heating and quenching.”

The higher the carbon content of the steel, the longer Ikeda allows for it to cool. Of the different types of steel shown on page 8, white carbon steel #1 requires more time to cool than #2, and #2 more than #3. The delicate blue carbon steel #2 is given still more time to cool than white carbon #1. “With its high chromium and tungsten content, blue steel does not wear out easily. Many chefs will swear that blue steel is ‘hard,’ but actually white steel #2 and blue steel #2 are not so very different in hardness. It’s more accurate to say that blue steel’s makeup is such that it slides more readily.” While blue-steel knives cut smoothly through ingredients without catching, they are very difficult to sharpen. Nevertheless, once properly sharpened that edge lasts long, as it has high resistance to wear. These considerations, and more, go into the welding process. If not allowed to cool slowly enough, the two metals will not join properly. The imperfect weld then shows up on the blade, such as in a discoloration, when its edge is being formed. The flaw may be so slight as to be indiscernible to any but the most experienced eye of a polisher, but still the knife will not be fit for sale. All of this is why the cooling step demands the utmost concentration. Once the cooled blade is shaped into a desired form by further grinding, hammering, and trimming of burrs from its edges, it begins to look like a proper knife. Now the forging process moves to its red-hot quenching climax.

In the quenching process, the knife is reheated to 780 to 800 degrees Celsius and then plunged in water to cool rapidly. This step effectively gives the steel its cutting edge. Before heating, Ikeda first coats the blade with clay and lets it dry. “The coat of clay is applied to suppress water vaporization on the knife surface, and thus quicken the speed of cooling. Without the clay coating, the surface would form iron-oxide layers, impeding the progress of hardening. Artisans of olden days must have discovered the use of clay by trial and error. Swordsmiths in the 12th century probably used the same method,” Ikeda explains. When placed in the center of the furnace, the knife gradually takes on reddish tints. Ikeda keeps the room dark so he can clearly see subtle changes in the knife’s color. When it gets to just the right hue, he removes it from the fire. He uses no thermometer. “When the knife becomes the color of sunset, it’s time to take it out,” he says matter-of-factly. “More precisely, a sunset at the moment just before the sun sinks beneath the horizon. People used to call this the color of ripe persimmons. As its color begins to approximate that of the setting sun, the red-hot knife looks more and more majestic—like a torch silhouetted against a darkened sky. The transformation happening in the deep recess of the furnace seems like a sacred ritual, some mystical and very ancient rite of passage. Ikeda pulls out the knife and plunges it into a water bath. A mist of vapor rises with a hiss. What alchemic change occurs in the steel when it is cooled so rapidly? Austenite, a nonmagnetic allotrope of carbon and iron found in steel heated to high temperature, transforms into martensite, a steel crystalline structure. This, in chemical terms, is what hardens the material. As it is still too brittle at this stage, the knife must undergo further tempering by means of another heat treatment. It depends on the item, but Ikeda typically reheats knives to 170 to 180 degrees Celsius in an electric oven for 30 minutes to an hour. Taking out the tempered knife, he lightly hammers out slight warps caused by the heat. With these adjustments done, the knife is now ready to leave Ikeda’s hands and undergo the next process.

“You can tell a lot about a chef just by the knives he’ll send in for repair. You can even tell how good his cooking is,” asserts Shinpei Ino, a certified traditional craftsman and a Sakai polisher with a loyal following of patrons and fellow artisans. When a nascent knife arrives at Ino’s workshop from the smithy, his work begins with rough grinding to craft an edge. This involves grinding the jigane soft iron on the front side of the knife to expose the steel underneath, which is welded to the back. “At this stage I pay special attention to the shinogi [the ridge between the flat and cutting edge], to thickness, and to the angle of the blade,” he says. The grinding wheel sings fiercely as Ino sets the knife to it. In almost no time at all, a cutting edge emerges. To Ino’s knowing eyes, the smith’s workmanship and idiosyncrasies are revealed, too. “If a smith were to turn in poor work, no amount of skilled polishing could remedy it,” Ito explains. But as a matter of course he does make a certain degree of adjustments in the act of polishing. With the knife roughly shaped, Ino holds it up to the light to spot small pits or other distortions, then hammers them out. Years of experience lie behind the keen eyes and seasoned hands that tell him exactly where to beat and how much force to apply. Using a tool called kojibo to correct the largest warps, he gradually brings the knife closer to its ideal form. As he polishes the backside, making the blade thinner and sharper, a rough-looking knife appears. Ino next polishes the tool on a buff wheel, a rotary sharpener coated with an abrasive emery powder. “Its surface is like sandpaper,” he says. He crafts a slightly different shape that hones and keeps the edge sharp for longer. As sparks fly, the knife takes on the fine luster of a razor-sharp blade, but Ino’s work is not completed yet. He next works the blade on a wooden wheel to give its spine, face, and backside an elegant matte finish. “It’s a final touch, toning down the glossiness and honing the blade one last time,” he says.

Hafting is the last process the blade must undergo to function as a cutting tool. A skilled artisan makes minute adjustments to determine the best angle at which to insert the blade into its handle. One of the characteristics of Japanese knives is the way the blade is affixed to the handle. A Western knife is secured to its handle by the use of rivets, while a Japanese one is fitted via the nakago, the sticklike shank that extends from the blade. It is also known as a tang. “If you force the tang into the handle, the handle will crack. First, you heat the tang of another knife and insert it into the handle to enlarge the hollow,” explains Aoki, the owner of Knife System. He takes the knife to be hafted and drives its tang firmly into the heated handle.

When the handle cools, there is no way of pulling the knife out again. Despite the ultimate simplicity of the structure, once fitted securely in place, the knife will not budge. Choosing a handle for your knife adds pleasure to your purchase.