The Structure of a Single-Edge Knife
Single-edged or single bevel knives are strongly associated with traditional Japanese knife culture. It's no exaggeration to say this style or versions of it have supported the Japanese culinary scene for hundreds of years.
This blade form, said to have been invented in the Edo period is now a symbol of Japanese knife culture, a culture that is loved all over the world.
This article will help you understand each part of a single-edged knife and why it's important, as well as introduce some Japanese terminology that you may want to remember.
What is "single-edged" (single bevel) ?
As the name somewhat suggests, a single-edged knife only has an edge on one side. A regular knife you may use at home would almost always be a double-edged or double-bevel knife which has the edge at the center. The single-edged structure opens up other possibilities, allowing you to easily fillet fish, cut sashimi and peel or shave vegetables by its sharpness without destroying the fiber of the food because one side of the blade is flat, so the flat side will not exert pressure on the food while cutting.
In particular, since sashimi is a dish made solely by cutting raw fish, the sharpness of the knife is crucial to making sashimi in a better way. These knives are an essential element of Japanese cuisine, a simple cooking culture that makes the most of the best ingredients.
Things to know about single-edged knives
Bar the exception of some more inexpensive items, most single-edged knives used in professional settings are created completely by hand. Unlike double-edged knives, which are normally shaped using a press and a roller amongst other machines, the flat surface and rounded edges of a single-edged knife are hammered, hardened and shaved by the hands of one or many craftspeople!
No matter how skilled a craftsperson is though, they are not a machine. In order to pursue the best hardness and sharpness levels possible, distortions and unevenness will occur - generally on the order of tenths of a millimetre. Keep in mind also that the sharpness, appearance, and lifespan of a single-edged knife will change greatly depending on whether or not you are aware of which parts are in contact with the whetstone and how.
Japanese knife part names and terminology
When sharpening a knife, even with terms like "top" and "right side" it can be hard to figure out where on the knife that refers to. For example, you may be thinking "What exactly is the "right" side of a knife?"
A sharpener might explain the condition of a knife using these terms also. They may say something like "The edge of the blade is dented about 3cm from the tip."
In order to help, let's take a closer look at a knife itself and summarise the points you should pay attention to when looking at a knife. In this instance we're looking at a yanagiba knife - a single-edged knife which is relatiely long, and is used for cutting sashimi.
While we've done our best to translate this into English, sometimes terms even in English borrow their Japanese counterpart - for example, you'll rarely hear side ridge, but you'll often hear shinogi line. Some of the other terms you'll likely hear the Japanese loanword for will be urasuki, uraoshi, and montan as their translations are less direct. This is beacuse some translations directly won't work. For example, umeoshi ans "back push", yet it refers to the flattening around the knife edge on the back of the blade, "pushed" down to remove the angle created by concaving the blade, which is urasuki. It's best to memorise the Japanese terms, and use the English ones solely as reference points. The Japanese romaji version of the words are in parentheses after each term. Sometimes, areas are simply referred to as their materials as well. Note that the muzzle can also just be called the water buffalo area, as that is often water buffalo horn that is used there.
Urasuki, Uraoshi and Shinogi are terms that are also used in English!
The urasuki (Back Concave)
Although a very important and more obviously noticed characteristic of single-edged knives is the angled "kireha (blade road)", the urasuki at the back side is also significant. For a bit of added context, ura (or 裏) means back, or opposite side in this context.
As the cross section demonstrates, the back side of the knife isn't completely flat. Infact, it has a concave!
This concave can be called urasuki or sometimes simply hi.
While it seems at first like you could make a double-edged blade with a sharper edge compared to a single-edge one, this isn't quite true. A double-edged blade needs a cutting angle on both sides, so when sharpening one of those sides, the steel at the cutting edge /kireha will thickens the other side. Also, this means that the entire blade must be made thinner due to a reduction in the cutting edge angle - resulting in a less structurally strong blade.
To finish a single-bevel knife, the back side is sharpened via a process called uraoshi. The back side is laid flat on the whetstone, and the edges are sharpened except for the part where the concave is. That is, only the outer periphery of the blade is sharpened and connects with the whetstone - the outside of ridge and the cutting edge. Even if more force is applied during sharpening, the structure of the blade does not allow the cutting edge to drift away from the whetstone, making it possible to create a blade with a very sharp angle.
Additionally, because the back of this type of knife is concave, the back side of the blade stays out from the pressure of the food. This contributes to the knife's exceptional sharpness. On the other hand, a double-edged knife will be interruped by the pressure of the food applied from both sides.
The back concave is an important part, but it is too plain to notice and requires effort to create. Therefore, some mass-produced Japanese knives don't have it. However, from the point of view of a knife maker, the meaning of having a single-edged knife seems to have been lost in that regard.
Important points about the kissaki (Blade Point/Tip)
To check thickness of the blade tip, you can use your fingernail push it gently from the back (Please be careful not to cut yourself). It's good If it's slightly elastic. In some case, it may be too thin and bent when it is touched. In that case, continue to get used to sharpening it and over months you'll create your ideal blade. To sharpen the blade tip, please lift up the handle to make the surface near by the tip firmly touch the whetstone.
Feel free to read any of our sharpening guides if you're looking for more information.
Keep an eye on your hasaki / hasen (Blade Edge / Edge Line)
Hasaki or "edge" is the edge for cutting. This part will touch the food first and pierce into the food. Keeping it sharp is so important. It will become gradually rounded when it's used.
When checking the straighness of the blade, lift the handle point to yourself and point the blade tip away from you, look at it from heel to tip at a straight line as if you were looking into a telescope.
If you see a strong curve near the tip or notice a concave at the tip on closer inspection, you need to make sure you factor that into your sharpening.
Pay attention to the kireha (Blade Road)
Kireha or "blade road" is the part from Shinogi (side ridge) to hasaki (edge).
When checking the blade road, look at it at a straight line from the heel all the way to the tip, as if you were looking into a telescope.
You can check the shape of the blade edge by shining a light on the blade road and seeing the reflection while turning the blade slowly.
Ideally, you should sharpen in a way that creates a cutting edge line with as few irregularities as possible.
Tips for your shinogi line (Side Ridge)
While sharpening, you should check the condition of the curve of shinogi line regularly by pointing the blade tip away from yourself and looking at it from the handle side like a telescope. If it's dented in anyway, take that into account during your sharpening.
Ideally, try to sharpen your knife while maitaining the ratio of back to shinogi, then shinogi to blade edge.
Be wary of when you start your sharpening at first though. The whetstone connects really easily here, and if it does make contact with your shinogi the soft iron part might get dented.
Be mindful of your hazakai (Edge Border)
Sometimes you might see a gap between soft iron and core steel at the edge border. This is called an aike. You may see them more on Blue Steel knives compared to White Steel.
The gap created between soft iron and core steel at the back of the blade is called a kaisaki. Unlike an aike, originally this wasn't seen as a defect, but public opinion on this has changed. This makes sense, as it does look like an aike after all. It's only natural that people's opinion has generally swayed on this.
This gap can be prevented by significantly raising the temperature of the knife (way beyond the best temperature for steel). However, in that case the sharpness of the knife will be poorer. That small gap of a few millimeters likely won't hit the food, so some people also consider that the visibility of the kaisaki is a good indication that the knife has been forged at a lowe temperature - meaning that its forging focus was on sharpness.
Ultimately, it's up to you to decide if a knife is good or bad for you. The opinion of a knife and bevel's appearance change from person to person. If you don't like these blemishes though, be careful and inspect a knife before buying it.
Take a look at your hira (Flat)
The flat of your knife is the part that is not directly related to sharpness. It can have a mirror finish, hairline finish, engraving, etc.
It can be said that this part is like a face that determines the beauty of the knife. Getting a nice finish means there's more steps involved in the production, sometimes a significantly large amount more steps. This will inevitably increase the price of a knife as well due to the labor costs associated. Just make sure to take care of rust if you want to keep your flat looking beautiful. A mirror finished hira may also have increased rust resistance.
Inspect the machi (Neck)
"Machi" refers to the part from "heel" to the handle. Many chefs put their index finger here while using the knife. Since 2010, most Japanese knives in the range of medium to high price have their Machi polished, called "Machi-migaki" (Machi Polishing) in Japanese, to make users' finger feel more comfortable. That's quite important for chefs who have to use knives for a long time.
You may see some Japanese knives have step nearby the handle. This style is called "Machi-ari" (Machi exists). The length of the blade of this type is measured from the step to top. But some knives do not have step. We called "Machi-nashi" (Machi non-exist). In that case, we measure the length from the heel to tip, instead.
The interesting thing is that, whereas knives in Kanto region like Tokyo typically have a gap of 2-3 mm between step and handle, knives in Kansai region, like Osaka, typically have tang completely tucked in the handle so there is no gap.
To chefs in the Kansai region, a knife with a gap between step and handle looks like it isn't inserted all the way. In turn, a chef from Kanto region will think a fully inserted tang looks cramped.
This is really a matter of personal preference. If you're buying a knife from outside your normal area, pay attention to which style they're made.
A knife's texture in its montan and jiai (Crest Area and Soft Metal Back)
Some high-quality knives have a wave pattern on the back of the blade. This part is soft iron called "jiai" (soft iron back). This wave pattern is called "Montan" which only few craftsmen in Sakai can create it. They shave down the steel to improve its connection to the soft iron. In the finishing stages, the polisher will use a special natural whetstone powder on each piece by hand to bring out a wonderful color at the end.
Not all knives have this jiai. Stainless steel Japanese knives (which forged and hammered from pre-layered steel without the need of forge welding) and honyaki (single layer) knives are missing this.
Note: With honyaki carbon steel knives a blade pattern will appear, but this is caused by the temperature differences in the quenching process and is different from the montan/jiai.
The knife's mine is important too (Spine)
In recent years, we've seen knives that have been slightly shaved and rounded to adjust the balance and weight between the index finger and the grip. These can sometimes simply be called shaved back knives, or shaved spine knives. Take a look at the spine to make sure it either is or isn't one of those depending on your preference.
To check for straightness here, once again look at the knife from the handle forward, as if you're peering into a telescope. We want to make sure the handle and knife have been connected together correctly and on the right angle, while keeping mind of the straightness of the knife, noting that "the straightness of the back" and "the straightness of the blade edge" of the knife are independent. Making sure there is balance with the blade side is the most crucial step.
A Knife's e and its connection to knife health (Handle)
A wide variety of handles are used, including materials such as magnolia wood, walnut, ebony, and yew. The shape and balance will also be affected by this handle choice, so try to get into a knife store and hold the knife you're interested in if possible.
Furthermore, Japanese knives are made with the assumption that the handle can be replaced even though they are used for a long period of time. The handles are in effect, designed to eventually be replaced as doing so isn't destructive to the knife, unlike a Western knife where it can be.
That said, if you see the handle of muzzle bulging (think like a bulging battery) then immediately take it to a knife store for an inspection. This could be because the core/tang has rusted and swollen. You can still save the knife if it's swollen, but if it corrodes away the life of your knife ends there. So be sure to quickly get the handle replaced if you need - good care should make it last many years though. Make sure there's no way for water to get into the handle, and keep it dry.
A kuchiwa's condition showcases a healthy nakago (Bolster and Tang)
As mentioned earlier, if the bolster breaks, it is a sign that the lifespan of the core is decreasing - possibly due to rust. Conversely, as long as the core remains, a Japanese knife made by forge welding or honyaki can be used no matter how small it becomes.
Note: In rare cases, cheaper knives may not have the proper internals to be sharpened endlessly.
There are still cases where even major, well-known brands do not put adhesive or putty in their handle after inserting the tang into it, but we don't recommend this as it's dangerous and allows moisture to enter the handle's interior, greatly shortening the lifespan of the knife.
In pursuit of the best sharpness possible, some people choose not to add adhesive and detach the handle every time they sharpen their blade, but it's important to understand that only those who can manage these risks appropriately are doing this. For general use, again we don't recommend this.
The difference between forge welded knives and honyaki knives
Forge welded knife
The illustrations used above are for a double-layered, forge welded knife made of soft iron and blade steel.
A forge-welded knife is a knife forged from a composite material that combines two types of soft iron and harder blade carbon.
Basic Japanese knives are mostly made with this structure and are called awase, kasumi, and wakashi-tsuke knives.
Honyaki
Some Japanese knives are made by forging and hardening only the blade metal itself, which is called "honyaki."
The origin of honyaki is that the quenching process is similar to that of Japanese swords, which is why it was named honyaki (hon = real, yaki = burn).
When you cut something by general knives, your hand will feel the sensation of cutting from food via soft iron. The feeling of cutting is conveyed in the following order :
food → blade metal → base metal →hands
On the contrary, honyaki knife will allows you to feel the sharpness and control the cutting more directly, like food → blade metal → hands, because your hand can feel the sensation of cutting from food more directly.
However, handling of these knives becomes very tricky and it is also expensive to purchase and maintain.
This type of knife is a dream knife for Japanese chefs and has only a handful of makers in the world that can properly do it. In our store at least, it's the pride of our shopfront and impresses people every day, with customers from overseas flocking to buy honyaki knives. We also have a selection of them on our website, but our in-store range is significantly larger.
As mentioned above, the structure of a Japanese knife itself has a lot of intentional design choices and even if you look at it one part at a time, many processes are put into it to improve the perfection.
As a knife shop, when it comes to owning a knife like this and caring for it, we recommend "maintenance to reduce the size of the knife while keeping it in its original shape" as this will extend its lifespan. Most knives will stay sharp as long as they still can have a cutting edge applied to them.
However, there is nothing more satisfying than just plain using the knife. So be sure to use it, and enjoy it!