The Structure of a Single-Edge Knife

Like mentioned above, the kireha or "cutting edge" is the beveled part of the knife that extends from the shinogi line (side ridge) to the hasaki.

When checking the kireha, look at it the same way you would inspect the hasaki too - 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 kireha by shining a light on it 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. The less irregularities, the smoother the experience of cutting will be for you.

While sharpening, you should check the condition of the curve of shinogi line regularly by pointing the blade tip away from yourself like before and looking at it from the handle side like a telescope. If it's dented in anyway, take that into account during your sharpening to correct it. Again, you want to remove irregularities.

Ideally, try to sharpen your knife while maitaining the ratio of back to shinogi, then shinogi to kireha.

Be wary of when you start your sharpening at first though. The whetstone connects to the knife really easily here, and if it does make contact with the shinogi itself the soft iron part of your knife might get dented, which can require a reshaping to fix.

Sometimes you might see a gap between soft iron and core steel at the hazakai or edge border. This gap is called an aike. You may see them more on Blue Steel knives compared to White Steel ones.

The gap created between soft iron and core steel at the back of the blade in turn 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 sacrificed in turn. 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 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.

The hira or 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. If you're curious as to the labor steps behind what you can do on a hira, read our guide about making a mirror finish on a knife!

The machi ("joint" or "neck") refers to the part of the knife between the end of the ago ("heel" or "chin") to the handle. In the example above, you cannot actually see the machi which we will get into later.

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 ago polished, referred to as machi-migaki (Machi Polishing), to make a users' fingers feel more comfortable. That's quite important for chefs who have to use their knives for a long time.

You may see some Japanese knives also have a small step which looks almost like a second heel. This is located very close to the handle and often a few millimeters long at most. That step is a little bit of the knife's tang intentionally not hammered all the way into the handle, and thus left visible. This style is called machi-ari (lit. machi exists). When measuring a blade length, this visible tang piece is not factored in, and the measurement is done from the beginning of the machi to the kissaki. But some knives in turn do not have this step. This is called machi-nashi (lit. machi does not exist). In that case, we measure the blade length from the heel to the kissaki instead. Effectively, if you were to measure a 300mm machi-ari and machi-nashi knife side by side, even though that tang being visible visually adds a few millimeters to the blade, they would both be classicied as 300mm knives as the measurement starts from the same point of the knife.

Whether a machi exists on a knife or not is interestingly enough, more of a regional preference. Knives in the Kanto region like Tokyo typically have a gap of 2-3mm between the step and handle. However, knives in the Kansai region like Osaka typically have the knife's tang completely tucked into the handle so there is no visible gap. Our visual example above is a machi-nashi knife.

To chefs in the Kansai region, a knife with a gap between step and handle or machi-ari knife looks like it isn't inserted all the way. In turn, a chef from the Kanto region will think a fully inserted tang or machi-nashi knife looks quite 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. Our own knives tend to be machi-nashi knives, but of course we can change handles in store to fit someone's preferences.

Some high-quality knives have a wave pattern on the back of the blade. This part of soft iron called jiai (soft iron back). This wave pattern itself is called montan which only a few craftspeople in Sakai can make, thus its visibility is usually a sign of high craftsmanship. Craftspeople shave down the steel to improve its connection to the soft iron. In the finishing stages, the knife's 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 are 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.

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. Remember to note that that "the straightness of the mine" and "the straightness of the kireha" of the knife are independent. Making sure there is balance with the blade side is the most crucial step.

A wide variety of handles are used, including materials such as magnolia wood, walnut, ebony, and yew. The shape and balance of the knife 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 be a consumable item as replacing them isn't destructive to the knife, unlike a Western knife where it can be. And these knife handles can be repurposed into other things once removed from a knife if they aren't cracked - for example a tiny flower pot.

That said, if you see the handle of the muzzle bulging (like the way a battery bulges) then immediately take it to a knife store for an inspection. This could be because the tang has rusted and swollen up. You can still save the knife if this has happened, but if it corrodes away more 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.

As mentioned earlier, if the handle or even bolster of the handle breaks, it's 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.

If you can keep your handle nice and healthy and dry, that means it as well as the tang inside it are more free from corrosion.

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. Most people when they are looking at traditional, single-edged Japanese knives are looking at forge welded ones.

Basic Japanese knives are mostly made with this structure and can be 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" or either 本焼 / 本焼き
The origin of honyaki is that the quenching process is more similar to that of Japanese swords or nihontou, which is why it was named honyaki (hon = true or real and yaki = burn).
Note: This is the same "yaki" that you see in words like takoyaki and yakisoba!

When you cut something with more 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, a honyaki knife allows you to "feel" the sharpness and control the cutting more directly, like food → blade metal → hands. There's less changes of material in the way, and thus you feel it more. This is a core essence of kireaji, which is in effect the "flavor of cutting", but can also mean sharpness.

However, the handling of these knives becomes very tricky and it is also very expensive to purchase and maintain them. Sharpening a honyaki knife should be left to an expert.

This type of knife is a dream knife for Japanese chefs and there are only a handful of makers in the world that can properly make a honyaki knife. In our store at least, it's one of the true prides of our shopfront and these knives impress people every day, with customers from overseas flocking to look at or buy them to take home. 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!