Explainer: Japanese Style Blade Compositions

Explainer: Japanese Style Blade Compositions
Kyle Cooper

Japanese knives are made out of steel. Sometimes, they are made out of one piece of steel and other times they are made out of too many pieces of steel, though the latter is a very personal position. While there are plenty of opinions concerning which type of blade composition makes the best knife, we’re here to learn and have fun —not to argue; so we will focus on the types of Japanese blade compositions that we see a lot of at the shop, how you can identify them and how knowing more about them can factor into choosing the best option for you.

The hardness of knife-making steel is what allows the formation of clean, sharp edges that won’t decompose or flatten immediately with use. Keep in mind, this refers to the hardness of steel as a category of metal, encompassing a vast array of steel types1. The type of steel used to make paper clips is soft and would make a terrible knife, but it’s still steel. Conversely, Aogami 1 steel that some pretty incredible kitchen knives are made from would make a frail paperclip.

Along with several other characteristics, Japanese style knives have earned the reputation of being high performers by utilizing steels on the high end of the hardness scale2. Because the steel is so hard, special considerations need to be taken to avoid making the blades too fracture-prone to use. Lucky for us, generations of Japanese sword makers turned knife makers have already figured it out. 

What follows is not an exhaustive or even comprehensive list of composition types, nor is it meant to insinuate that Japanese style knives are the only knives with these compositions, but outside of the high-end custom knife making world, we see the most variation of compositions from Japanese makers so the focus will go there for now. 

In order to keep on topic, we’ll talk about the main composition types as mono-steels and laminated steels and speak to some subtypes of each. Please know that metallurgy and blade metallurgy specifically is an abyss of fine details and nuance that routinely contradicts itself9. Everything below is true but sometimes it’s not always true. We are going to stay at a depth that might get our socks wet.

Mono-Steels

Mono-steels utilize single pieces of one type of steel made into a usable blade. You’ll see mono-steels on the trash end of the quality spectrum as well as in the highest quality, most revered knives available. The differentiators are the processes and intention behind their builds. The majority of available entry level mono-steel –self-titled as mono-steel at this quality tier– are made from a single piece of softer steel that is hardened uniformly across the entire blade. Remember though, that they are hard enough to form and hold clean, sharp edges, but need to remain soft enough to absorb some applied force to keep from chipping and fracturing during regular use. These knives are generally cut from a manufactured sheet of steel then finished to shape and are typically a great way to get a good knife at an economical price. 

Factory Forged Mono-steel Blade     

On the other end of the mono-steel spectrum are honyaki knives. It is of the most desirable and most difficult to execute blade compositions with distinct roots in Japanese sword making. The high level of skill required limits their production to relatively few modern blacksmiths, thus limiting their production volume and availability as well —re: expensive.
 
Honyaki requires differentially heat treating a single piece of steel that’s capable of being functional at a very high hardness. In this technique, a blacksmith will coat the blade with clay during the tempering stage, applying a thicker layer to the spine and its surrounding area; maybe a third of the way up the spine towards the tip and a half inch to an inch down from the spine towards the edge. As the blade is quenched in either oil or water3, the area on the spine that was coated in the thicker layer of clay will cool to a higher temperature more slowly than the section of the blade with the thinner layer of clay. Because the temperature change of the steel at the spine is slower and less severe, this area is less compacted and becomes significantly softer than the rest of the blade. The soft spine acts as a dampener for an energy that is created during use that otherwise could lead to hairline cracks or even a complete fracture in the wrong conditions.

The trademark of honyaki style knives is the hamon. The hamon is a distinct visual contrast between the hard hira4 and the softer spine. Even though the blade is made up entirely of the same steel with the same molecular makeup, the difference in the compactness of grain structure between the hard and soft areas will appear visually different. Depending on the finish of a particular knife, the hamon will appear as a darker or hazier section or will have a distinct line where the two converge. Blacksmiths will often apply the clay coatings with an intentional patterning to give the hamon an extra bit of flair. 

Honyaki with Mt. Fuji Hamon

Honyaki knives are typically not ideal for first-time users of Japanese knives because they require a particular level of skill and technique in use to avoid damaging the very hard cutting edge. Despite the need for a disclaimer, they typically have higher edge performance in regards to their sharpenability5 and edge retention due to their very high hardness. Not to mention they are pretty stunning considering what it takes to create them.

Laminated Steels

At the shop, we see more laminated steel than mono-steel Japanese style knives. Laminated steel is exactly what it sounds like; at least two layers of different metal laminated together to form a single piece. Laminated steel for knives is either forge welded by the blacksmith or purchased in pre-laminated sheets of steel. Forge welding is the process of stacking the individual billets of steel and fusing them together through rounds of heating and hammering. Pre-laminated sheets are industrially made and the heated layers are generally bonded together by passing them through a series of rollers under tremendous amounts of heat and pressure. Forge welding is a more laborious process that requires more time and skill and usually leads to a more expensive product when done well. Pre-laminated steels are consistent but can be expensive to buy compared to raw billets, but the time saved usually offsets the cost. End-product quality differences between the two is a completely separate  and quite nuanced conversation.

The purpose of laminating knives is to give them ductility —in manufacture and in use— and in the case of stainless clad knives, added corrosion resistance. Cladding (jigane), refers to the layer(s) of soft steel or iron that’s on the outside of the knife with the core steel (hagane) in between. A knife is described as being clad when it’s made using laminated steel. A clad knife is able to have a very hard cutting core for high performance while not compromising the integrity of the blade.

Cladding makes it easier for the blacksmith to draw the laminated steel out to shape and if a blade becomes warped in the process of being made or being used, it’s possible to bend it back to straight where mono-steel knives will hold the bend. When a knife is clad with stainless steel rather than iron, the majority of the knife resists oxidation and corrosion and mitigates the amount of attention required for proper maintenance. While iron clad knives do not lend any help to prevent rusting, they are more easily thinned when the knife becomes thick or as the clad line is reached after several rounds of sharpening6.

The clad line is where the hagane stops and the hagane starts. In most cases, it’s fairly easy to tell a stainless clad knife from an iron clad knife. The clad line on a stainless clad knife shows as a clearly defined thin line that looks like it could have been drawn on. The clad line on an iron clad knife is more of a blending at the convergence point, more resembling a tide line7. There is still contrast between the two metals but it’s not as distinct as what we see with the stainless cladding.

Stainless Cladding (L) &  Iron Cladding (R)

There are a lot of configurations when it comes to laminating steel to make knife blades. Almost all of them are derived from sword making techniques when it was important to have a blade that was both sharp and difficult to break –re: dependable. While we don’t have representation of all the different types, we do see these usual few: Nimai, Sanmai, Suminagashi or Damascus. Warikomi and Gomai are less regular but show up from time to time and are worth discussing.

Nimai blades are made from two layers; one hard layer on the inside face (ura) and one soft layer on the outside face (omote). This is almost exclusively seen on single-bevel knives where durability is needed for toughness in the case of a deba or as support for the long blade of yanagi. Having only hard steel on the ura side cutting edge makes it easier to properly sharpen the knife. 

Sanmai blades are made from three layers; soft on both the ura and omote with hard hagane in between. Sanmai is the more prevalent of the types we see and is reserved for double bevel knives, made with stainless or iron cladding. When a knife is iron clad, it always has a carbon steel hagane. With stainless cladding, we will see both stainless clad stainless knives; with a very soft stainless jigane and a very hard stainless hagane, and also stainless clad carbon steel configurations. Sometimes, instead of having a hagane of a solid layer of hard steel, a piece of medium hardness steel is placed on the spine side of the blade behind the edge steel. When used in sword making, this gave added absorption of force and it could serve the same purpose in kitchen knives but now it is most often associated with lower manufacturing cost.

Warikomi is similar to sanmai in the sense that the jigane is soft and the hagane is hard but instead of three separate layers welded together, a notch is cut in the soft metal and the hard steel is placed in the opening. Think sandwich vs taco. This is a very traditional type of forging and is most often seen in high quality knives from small volume makers like elusive the Jiro Nakagawa.

 Jigane inside the notch in the Hanage on a Jiro Knife in progress..  

Suminagashi is the most recognizable of the laminated blade compositions and you may know it as damascus. While it is technically different by having multiple layers, functionally, it’s sanmai. The outer jigane is made from multiple layers of alternating metal types, then cut into two pieces and a piece of hard hagane placed in-between before it’s all forged back together. After the knife has been ground, it’s dipped in an acid bath that exposes the contrast between the alternating layers of metal showing the wavy-lined aesthetic. The design possibilities for suminagashi patterning is seemingly endless and can reach the point of spelling out words or pictures, but that’s for another time.

Hand Forged Suminagashi

There appears to be no rules dictating how many layers are enough or too many. While the modern existence of true damascus steel is up for debate,8 using the phrase to describe blades with wavy lines has been normalized, especially in marketing efforts. If the number of layers matters to you, know that in modern damascus style knives it really only refers to an aesthetic appearance and does not dictate performance. While you can find suminagashi knives in the highest quality tier, it’s there because of the skill of the maker, not because it’s damascus. There are lots of low quality damascus style knives that are made with substandard heat treatments and/or made from mono-steel that have had the wavy aesthetic applied on top. It’s generally best to avoid these.

Certain suminagashi patterns can be recognized as belonging to specific blacksmiths due to the consistent —but not identical— appearance created from their particular layering techniques. For instance, suminagashi from Satoshi Nakawaga looks a bit like a topographic map with winding steps and elongated circular peaks in-between while Tsukasa Hinoura’s suminagashi shows longer, narrower striations bunched between wider gapped lines, giving the illusion of water running between riverbanks. His River Jump knives are coveted for this aesthetic and because of his reputation as the expert’s expert. 

Gomai are blades made from five layers; the softest metal on the outside with layers of soft or medium hardness metal underneath and the hardest metal on the inside as the hagane. Gomai is used for added durability and because the different layers can create interesting aesthetic results. Sometimes metals like bronze and copper are used for added pizzazz. We don’t see many gomai blades, but at the time of this writing we have a very impressive hand forged gomai blade knife by Bay Area smith Nick Rolin.

Hand Forged Gomai

Applying all of the above to your next knife purchase can be as easy as, “it looks cool, I like it.” In terms of how it affects performance, it usually best to have blade composition fall outside of the top-three priority characteristics10 but there are some things to consider:
Mono-steel knives are typically pretty tough and affordable but can be found in a range of qualities within the same price tier. If you’re comfortable with carbon steel, there’s usually a bigger bang for the buck with mono-steel carbon knives.
Honyaki knives are incredible but experience is not preferred, it's required.
Sanmai knives are readily available in a wide price range with lots of aesthetic and material choices. If you’re an avid sharpener and like to do your own thinning work, an iron clad knife is beneficial as the hagane abrades easier. If you want an insurance policy against corrosion but still want a hard cutting edge, stainless clad is the way to go.
Warikomi is really impressive in terms of smithing skill and can be found in an accessible price range but the most coveted are coveted because they are both incredible performers and incredibly hard to get. 

Zknives, an incredible resource for blade steel info, has alluded to there being 6,000 types of steel. Give or take.
The Rockwell Hardness Scale is the most common rating of steel hardness used for knives. HRc or Rc  is the reading from the Rockwell “C” test that determines how much force is required to deform the surface of the steel to a determined depth. The more force required, the higher the number and the harder the steel. There is no set scale range but most of the knives we see range from 58 to 65 HRc. The HRc is one of many factors to consider when determining the potential performance of a knife and it can be misleading if used alone.
Metallurgy and its alloys can get wildly complicated. For example, some elements react differently in a certain amount when combined with a certain amount of another element but can react completely differently when added in the same amount to a different amount of that same other element. It’s accepted that without the aid of computer modeling, predicting exactly what an alloy makeup will result in is impossible and that’s just one of the ways it’s complicated.
Mizu Honyaki, or water quench honyaki makes for the hardest possible edge but due to the rapid heat loss during the quench, it has a high fracture rate even for experienced smiths. Abura Honyaki, or oil quench honyaki also makes for a very hard edge but the heat loss happens at a slower rate to a higher temperature resulting in lower failure rate but less hardness than mizu honyaki. Mizu honyaki is more difficult to pull off and generally makes it more expensive.
Hira refers to the flat section on the face of the knife below the spine and above the shinogi line (the shoulder where the first bevel angle starts on wide bevel knives). On convex ground knives, the hira extends all the way to the cutting edge. 
Sharpenability refers to how well a knife takes to being sharpened. It’s determined by how quickly and how clearly it forms new bevels. It’s a real word to us but spell check doesn’t agree.
It’s important to have exposed hagane for high performance cutting and effective sharpening instead of trying to use an edge that includes the soft jigane. The clad line is a great indicator for the need for thinning work because this is also usually when the knife becomes thick to the point where it starts to feel like it’s plowing through food rather than slicing through easily..
This hazier overlapping is a result of carbon migration. During the forging process carbon from the jigane starts to move into the hagane or vice versa. This doesn’t happen as readily in stainless clad knives due to the higher forging temperatures and could have something to do with the chromium layer that forms on the surface of the stainless steel.
True damascus steel was made from Wootz steel from iron ore mined in South Asia that was taken to the city of Damascus to be made into blades. The original Wootz alloy is a bit of a mystery because the technique used to produce the steel was lost as India and Sri Lanka were colonized by the British who banned the production of weapons. There are some people who are working to recreate Wootz steel and for all intents and purposes they have, but others argue that it’s hard to know if what they have created is exactly the same. Wootz’s naturally occurring characteristics took on a banded aesthetic that was vaguely similar to what we see in the modern damascus knives that subsequently piggybacked on the reputation of the original product. “Damascus” has been abused by companies of mass produced knives as a way to give the illusion of quality. Not all good knives are damascus, and certainly not all damascus knives are good.
Blade composition can be a big factor in how a knife performs, but I think edge geometry, jigane steel type & heat treatment quality should come first in prioritization and/or consideration.