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, uraoshi means "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)
A very important and more obviously noticed characteristic of single-edged knives is the angled, dominant cutting side - but don't rule the back side out. The urasuki and uraoshi play important parts too. 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 cutting edge/kireha escapes to the other side, removing cutting power. 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 the cutting edge and the ridge. Even if more force is applied during sharpening, the structure of the blade does not allow the cutting edge to escape from the whetstone, making it possible to create a blade with a very sharp angle.
Also, the secret to this style of knife's sharpness is that its pressure is released through that back concave. A double-edged knife will have the pressure applied to both sides instead.
There are some mass-produced Japanese knives that don't have a back concave because of their more modest shape, which requires a lot of work in its own right. 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)
A couple of important points to ask yourself when it comes to the blade tip is mostly whether there is enough cutting steel near the tip. Alternatively, whether there is too much. Excessive steel protruding can sometimes also be called "widening your eyes" in Japanese kitchen knife culture.
A good way to check is to press the blade very lightly against a surface like your fingernail so you don't cut yourself (some people recommend your toe for this but injury is possible.) You're looking for a very slight amount of elasticity, but your tip may also be so thin that just doing this may change the shape of it so be very careful when checking.
If you find it too thin or thick, continue to get used to sharpening it and over months you'll create your ideal blade.
Feel free to read any of our sharpening guides if you're looking for more information as to what to do here.
Keep an eye on your hasaki/hasen (Blade Edge/Blade Curve)
When looking at your blade edge, look at it 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 (Cutting Edge)
When looking at your cutting edge, look at it at a straight line from the heel all the way to the tip, as if you were looking into a telescope.
Like the blade edge, 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.
You want to do your sharpening in a way that has as few irregularities as possible, with the cutting line having a clear arc.
Tips for your shinogi line (Side Ridge)
From the handle side, look at your knife again like a telescope. Sharpen while checking your shinogi's condition by looking at the curve of the knife. 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 or malformation between the edges of the blade at the edge border. This is called an ike (aike) and we want to avoid these. You'll see them more on Blue Steel knives compared to White Steel.
The gap created between the back (ura) and soft metal back (jiai) is called a groove (kaisaki.) Unlike an ike, originally this wasn't seen as a defect, but public opinion on this has changed. This makes sense, as it does look like an ike after all. It's only natural that people's opinion has generally swayed on this.
This gap can be removed, however. By significantly raising the temperature of the knife (way beyond the best temperature for steel) you can eliminate it. 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 visbility of the kaisaki is a good indication that the knife has been forged at a lower 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 can have a mirror finish, hairline finish, or a signature style that is not directly related to sharpness.
Many people believe that the face of the knife determines its beauty - especially in Japanese knife culture. Getting a nice finish means there's more steps involved in the production or repair process, 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 (Gusset)
Since the 2010s, most knives in the mid to high price range have had extra polishing done in the areas that are expected to come in contact with someone's fingers - like if they're holding the knife to cut with for example. For cooks that work for a long time using those knives, this means their fingers won't hurt and it will also be a knife that's easier to use. The difference between a knife with a polished gusset like that and one without is clear as day for any expert cook.
Not only polishing, but going all the way to a mirror finish or mirror-like shine will make a knife stand out from the ordinary, creating a luxurious and beautiful finish.
Where the gusset is depends on the region the knife is from. To chefs in the Kansai region such as Osaka, a knife with an empty gusset looks like it isn't inserted all the way. In turn, a chef from Kanto areas like Tokyo will think a fully inserted gusset looks stuffy or overcrowded.
This is really a matter of personal preference. If you're buying a knife from outside your normal area, pay attention to what gusset style or finish they're using.
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 pattern is a process where the steel in that area is shaved down layer by layer 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 (forged and hammered from the beginning without 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 front 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 (Muzzle and Tang)
As mentioned earlier, if the muzzle 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 muzzle 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 - in this case carbon.
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.
Ingredients → blade metal → base metal →hands
This is the general process of the feeling of these knives. It's a knife style that allows you to feel and control the sharpness more directly because it changes from ingredients to blade to hand through the above process.
However, handling of these knives becomes very tricky and it is also expensive to purchase and maintain. Think of it like a racing car, with a steering wheel that has absolutely no play in it. Wonderful, but fragile.
This type of knife is a dream come true 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. And this is also constantly evolving, too.
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 shap" 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!