Forging Techniques & Processes
Born from Fire.
Techniques used to produce Japanese knives have origins as far back as the 14th century. Sword-smiths perfected forging methods using carbon steel, fire and water. Today, blade-smiths combine modern metallurgy with classic forging techniques. These methods include, Honyaki, Kasumiyaki, Sanmai-Awase, Warikomi-Awase and Suminagashi.
HONYAKI knives are forged using one type of steel for single- or double-bevel knives. Traditionally, made of high-carbon steel, now they sometimes are made of stainless alloys. These mono-steel knives need to be differentially heat treated, the same as applied to traditional Katana (Japanese swords). Differential heat treatment allows for the spine of the blade to cool at a slower rate. This produces a softer area which absorbs shock and provides structural integrity, while maintaining a very hard cutting edge. A special clay is used to insulate the spine and is applied in an artful manner, therefore producing the Hamon. It takes a very skilled smith to achieve this properly. The clarity of the Hamon line may be revealed by polishing or acid etching. Forging Honyaki blades and implementing the Hamon takes a tremendous depth of understanding. The blade-smith must fully know steel, temperature, timing, amount of clay used, tempering, quenching and hardening. It’s no wonder that knife-smiths claim the spirit of the blade lies within the Hamon. Since Honyaki knives have such high hardness, they are brittle and prone to chipping if misused. They do have incredible edge retention. It does take much more skill to sharpen as compared to Kasumi knives.
KASUMIYAKI is a technique of forge welding hard steel (Hagane) on the back side of the blade with a softer steel (Jigane), on the spine and face of single-bevel knives. It is commonly referred to as Kasumi and less commonly as Nimai (two layer). When further steps in tempering and heat treating are added, plus very high-quality steels are used in this cladding process, this higher-grade is referred to as Hongasumi. The soft steel provides structural integrity and shock absorption, while the hard steel performs as the cutting edge. The word Kasumi literally means “mist” in reference to the hazy appearance of the soft steel contrasted against the glossy hard steel. This wavy lamination line where the two steels meet is beautiful, somewhat resembling a Hamon line. Kasumi knives are easier to maintain and sharpen, but have shorter edge retention.
SANMAI-AWASE, is a method of cladding (applying one material over another). Sanmai literally translates to “three layers” and Awase is Japanese for “cladding”. Commonly called Sanmai, this technique is used for double-bevel knives. Its “sandwich” construction allows for a hard steel core to function as the cutting edge while the softer outer layers add structural integrity. This gives the knife better edge retention. What sets Sanmai apart from Warikomi is that the softer laminates are forge welded to the sides and not applied to the spine. Just as with Kasumiyaki, the lamination line is visible and resembles a Hamon.
WARIKOMI-AWASE is similar to Sanmai in that it has a hard steel core and a softer steel jacket. Likewise, this method is used for double-bevel knives. What is different with Warikomi, is the process. The hard steel core is inserted into a split soft steel, then forge welded, wrapping around the core. The hard steel does not extend fully to the spine. As mentioned with Sanmai, the hard steel core performs as the cutting edge while the softer jacket gives the knife structural integrity and better edge retention. Once again, the lamination line is visible almost resembling a Hamon.
SUMINAGASHI, globally known as Damascus, is the method of forge welding two or more parent steels. The blade will exhibit the attributes of all the components. Soft steels for flexibility and resilience. Harder steels for toughness and edge retention. Steel ingots form billets which are then hammered, forge welded, folded again and again, creating up to hundreds of layers. The flowing water-like pattern of Suminagashi received its name from the ancient Japanese art of paper marbling of the same name, literally translated to “floating ink”. The information here is simply an overview, since Damascus has a long history. Its origins came from India around 300 BC, and continue to evolve today. Modern Damascus is produced in a wide variety of patterns. In today's market it is common for Japanese knife-makers to use Suminagashi steels within Kasumiyaki, Sanmai and Warikomi techniques. For example, smiths clad a core of AUS-8, or VG-10 in Damascus. The wavy pattern can be accentuated by acid etching and polishing. Not only is this beautiful but it performs at a high-level.
HEAT-TREAT: NORMALIZING, ANNEALING, QUENCHING, & TEMPERING
After the steel has been forge-welded (or not, if it is Honyaki), stretched, hammered and brought to a rough shape, it is ready for heat-treatment and refining the shape.
NORMALIZING is a method of refining metal particles and making a knife less prone to warping when quenched. The unhardened blade is brought to a critical temperature in the forge. The smith eyeballs the color of the steel, then pulls the knife from the forge, cooling it to room temperature. This process is repeated a few times.
ANNEALING is important for bringing uniformity to the grain structure and components within the steel. Classically, the blade is heated to a relatively low temperature, then placed in straw ash to cool. Internal stresses are taken out of the blade, further strengthening it.
QUENCHING a blade is one of the most demanding and important stages of heat-treating. There is some debate about which quenching mediums are best. The reality is that there are pros and cons to each. A skilled blade-smith with a deep understanding will be able to balance the elements required to produce a knife from quality steel. Heating the blade, then quickly quenching (rapidly cooling) helps achieve hardness. This rapid cooling seizes the change in crystalline structure of the metal’s surface, hardening it. Oil is a popular medium for quenching because it is more forgiving. Conversely, since oil removes heat more slowly, it generates a lower hardness. Water quenching takes an extremely skilled blade-smith, since such rapid cooling puts more stress on a blade, potentially causing micro-cracks. Other mediums can be used, such as sand, salt or super-cooled liquid. Quenching ultimately improves the longevity of a blade's cutting edge, but makes the knife brittle.
TEMPERING is a method of bringing a blade to a specific temperature and time, then slowly cooling it to room temperature. This process can be repeated up to three times. Since quenching imparts stress within the steel, tempering helps relieve the internal strain. It also decreases hardness, but more importantly, decreases the brittleness of the steel, giving it the proper balance of strength and durability.
The knife is now ready for the final stages of sharpening, polishing, engraving, and handle assembly. Blade-smiths carefully watch each step along the way, ensuring confidence that their knives perform without fail. These traditional techniques and processes combined with passion and artistry, produce the world's finest cutlery.