Honyaki: The Kitchen Katana

Most Japanese knives are made like a sandwich — a very hard steel core sandwiched on each side by a softer steel. The hard steel is what makes a Japanese knife feel sharper and keep its edge longer than other knives. The soft steel acts like a shock absorber to protect the harder steel from damage (ie: dropping on floor). This is the common manner of making a knife, but there is an older and more romantic manner of knife construction, called honyaki. Fewer than 1% of the knives in Japan are honyaki forged.

Having a great relationship with Sakai Takayuki (a well-known knife maker from the city of Sakai), we are lucky enough to be able to make special requests. We are extremely pleased to introduce the first fruits of our special requests: the 210mm honyaki gyuto. The blade is masterfully forged by master blacksmith Kenji Togashi-san and sharpened/polished by master Koji Tosa-san.

Here's what makes these knives so incredibly special & rare: Honyaki-style blades are hard to come by these days and rarely in these smaller sizes, which I think are perfect for home use. Usually, honyaki gyutos are 240 mm or longer and the yanagiba are at least 300mm. When we requested these knives, we also specifically asked to use shirogami #2 steel and water quenching — you’ll find out why soon.

Togashi-san, the blacksmith behind these gorgeous blades, is one of only 25 certified traditional craftsmen in that region. He started forging blades at the age of 19, in 1967. That’s over 50 years of experience! The sharpener & polisher, Tosa-san, is also a certified traditional craftsman in the region who has sharpened blades since 1962. It’s safe to say these men really know what they are doing.

Making a honyaki knife is just like making a katana, Japan’s famed samurai sword. The knife is made from one piece of steel and heat treated in such a way that the edge is very hard and stays sharp for a very long time. The spine of the knife, though the same piece of steel, is softer and protects the blade from breaking.

Things are about to get really intense and scientific, so I hand you over to Naoto Fujimoto, Knifewear’s Japanese Cultural Ambassador and resident steel super-nerd.

As Chris started above, close to 90% of Japanese knives are made with a hard steel core surrounded by a  softer steel laminating either one or both sides. This softer steel can be very soft carbon steel (C<0.2%), soft stainless steel (like SUS410), or softer layered steel (either carbon or stainless) that we colloquially call 'damascus'. These outer layers of steel will act like bubble wrap protecting a porcelain vase. So, while the exposed hard edge steel can be chipped or damaged when dropped or abused, the whole knife will not shatter like a glass. These layers are called sanmai, warikomi, kasumi, awase, or suminagashi depending on who you ask.

Another 9% of knives are made with one piece of relatively soft steel such as AUS8, AUS10, SUS440C, and so on. Generally, those knives are controlled to be hardened and tempered to approximately HRC 58-59, which makes a tougher and less fragile knife. They don’t retain their edge as long as other Japanese knives but still stay sharp longer than most Western knives. These knives are generally called zenko, or mono-steel in English.

Honyaki knives start with a solid piece of high carbon steel, something that can attain more than 61 HRC. At this hardness a knife is extremely fragile and dropping on a tiled floor would be devastating. What makes honyaki knives different from zenko knives is how they are heat treated, especially when they are quenched. Quenching is when a blacksmith heats up steel to a precise temperature and dunks the hot blade in either water or oil, which hardens the steel. When they quench 90% (awase, kasumi, sanmai, etc…) of knives, blacksmiths will evenly coat the blade with clay mud, as an insulator, for even heat distribution. When they quench honyaki knives, they do it differently.

Instead of evenly coating the blade with clay, the blacksmiths put a thin layer close to the edge and a thicker layer of clay on the spine. This causes the edge to achieve the high temperature necessary to quench before the spine does. Called differential heat treating, this process leaves the spine softer than the edge and able to act as a cushion the way the lamination does in other knives. A line forms in the steel where the clay thickness differs and is called the hamon (see the photo above). Because this technique was used by swordsmiths, kitchen knife makers named the process honyaki from the words honmono (genuine) and yakiire (quench).

The difference between water quenching and oil quenching depends on the steel that a blacksmith chooses to work with. White carbon steel will get the best results from water quenching because white carbon steel can get hardened at lower temperatures when quenched in water than oil. To achieve a hardness of HRC 65+ you only need to heat white #2 carbon steel to 775℃ when quenching in water, but have to increase the temperature to 850℃ if using oil. The higher heat increases the size of the steel particles resulting in the knife being more fragile and there is the potential that the steel could lose carbon, which would make the blade softer than it should be. On the other hand, blue carbon steel has better quenchability (Read more about this in the book The Knifenerd Guide to Japanese Knives) so it can attain a hardness of HRC 64 by heating to 800℃ and quenching in oil.

Most honyaki knives are 3-4 times more expensive than awase knives. The reason is that differential heat quenching is much more difficult to perform and perfect. Even experienced master blacksmiths, like Togashi-san from Sakai, will have at least 1 in 10 blades crack or break after this style of heat treatment. Many blacksmiths have a much higher failure rate. I assume that when austenite changes into martensite, the steel will expand, but the part of the knife protected by thicker clay does not expand as much as the part with a thinner coating. This different expansion rate in one piece of steel can cause a blade to crack but when it is finished properly and the hamon is exposed, honyaki are perhaps the most beautiful, elegant knives you will ever see.

Honyaki knives appeal to people for a variety of reasons. Knives produced this way aren’t prone to bending or warping as steel ages and contracts. The finished knife is incredibly hard; making it sharper and with better edge retention than most other knives, even those made of similar materials. The trade-off comes when it is time to sharpen, and in durability. Ultra-hard steels can take longer to sharpen and are more fragile than others. When it comes to sharpen a honyaki knife, take your time and be careful. There’s no need to rush.

Getting a honyaki gyuto from Sakai, a place known for the more traditional single-bevel shapes, was no small feat. We specially requested the material and quenching method, blacksmith, and sharpener. There really are no other knives like these ones and they are a great addition to any serious collector’s kitchen. Just because they’re fancy doesn’t mean you shouldn’t use them.