What Is Zenko (All-Steel)? The Fully Forged Knives of Vini Yasu
“Japanese knife.”
“Hand-forged.”
“Carbon steel.”
When searching for Japanese knives, you will often come across these words.
But what does “forged” actually mean?
And are all forged knives made in the same way?
The answer is not that simple.
The performance and character of a knife are not determined by whether it is forged or stamped alone. The type of steel, heat treatment, blade geometry, grinding and finishing, and the way the blacksmith works with the steel all contribute to the final character of the knife.
Vinicius “Vini” Yasu, the first Master Blacksmith selected by Oak Bay Knives, specializes in Zenko (全鋼)—an all-steel construction in which the entire blade is forged from a single type of steel.
In this article, we explore what “Zenko” means in Japanese knife making, and why this demanding construction places such high demands on the blacksmith’s skill and experience.
First, What Does “Forging” Mean?
Forging is a process in which steel is heated and shaped by hammering it with tools such as a hammer and anvil.
A piece of steel is heated and repeatedly hammered on an anvil, gradually transforming it into the desired shape.
In traditional Japanese blacksmithing, the blacksmith carefully observes the temperature, color, and condition of the steel while shaping it step by step with hammer and anvil.
Through forging, the blacksmith can begin shaping the blade’s thickness and overall geometry—from the heel toward the tip—at the forging stage itself.
However, forged does not automatically mean better.
The final cutting performance and edge retention depend greatly on the steel itself, heat treatment, and the blade geometry created through grinding and finishing.
That is why Oak Bay Knives does not use “hand-forged” as a selling point by itself.
What we look at is:
Who forged it, what steel they used, how they forged it, how they heat-treated it, and what kind of knife they ultimately created.
That work itself is what matters.
Japanese Knives Can Combine Different Metals
In traditional Japanese knife making, different metals are often combined to create a single blade.
One well-known example is Kasumi (霞).
In a typical Kasumi construction, a hard cutting steel, known as hagane, is forge-welded to a softer jigane, often soft iron.
The harder steel forms the cutting edge, while the softer material supports much of the blade.
By combining metals with different properties, this construction can provide a hard, sharp edge while also making the blade easier to forge, grind, and sharpen.
Zenko is different.
There is no forge-welded combination of dissimilar metals. The blade body is made from a single type of steel.
What Is Zenko (All-Steel)?
At Oak Bay Knives, we use the term Zenko (全鋼) to describe a knife in which the entire blade body is made from a single type of steel, without forge-welding a dissimilar metal such as soft iron.
A single piece of steel is heated, forged, shaped, heat-treated, and ultimately finished into a complete knife.
In other words:
The entire blade—from the heel to the spine—is made from the same steel material.
This is one of the defining characteristics of Zenko.
But the real challenge of Zenko is not simply making a knife from one piece of steel.
Zenko: A Test of the Blacksmith’s Skill
When different metals are combined, each material can be given a specific role.
With Zenko, however, the steel itself must perform all of those roles.
Which steel should be selected?
To what temperature should it be heated?
How should the hammer be used?
How far should the steel be forged and shaped?
And how should it be heat-treated?
Every one of these decisions directly influences the character of the finished knife.
Heating the steel excessively can potentially affect its properties.
On the other hand, if the steel is not heated sufficiently, it becomes difficult to forge and shape as intended.
Leaving too much thickness can affect the knife’s handling and cutting performance.
Making it too thin can make it more difficult to maintain the right balance of strength and cutting ability.
And when it comes to heat treatment, hardness is not the only goal.
A skilled blacksmith must consider the balance between sharpness, edge retention, toughness, and ease of sharpening.
This is why Zenko is more than simply a “single-steel” knife.
Creating an entire knife from a single steel requires the blacksmith to understand the characteristics of that steel and develop the skill to bring out its potential throughout the entire process.
Zenko is therefore a demanding form of craftsmanship—one that tests the blacksmith’s technical ability and experience.
And that expertise is not built through years of forging alone.
Before establishing his own forge, Vini studied steel professionally and worked at a research institute where he was involved in research related to metallurgy and surface treatment.
That experience informs the way he approaches heat treatment, with an understanding of the properties and behavior of steel.
Vini is not simply pursuing hardness.
His goal is to achieve a balance that brings out the hardness and sharp cutting performance characteristic of Zenko, while keeping the knife practical and comfortable to use in a working kitchen.
Achieving that balance requires an understanding of the steel itself and a consistent approach to the entire process—from forging and heat treatment to grinding and finishing.
It’s Not Just About “Forged.” It’s About Who Forged It.
The words “Hand-Forged” certainly have a strong appeal.
But at Oak Bay Knives, those words alone are not what we value.
Even when the same steel is used, the finished knife can be very different depending on who forges it, how they work the steel, and how they approach heat treatment.
That is why we look at:
Not only the steel, but the blacksmith.
Not only the process, but the blacksmith’s skill.
And ultimately, we look at the knife that emerges from that skill.
This is also why Oak Bay Knives selected Vini Yasu as our first Master Blacksmith.
He works directly with a single piece of steel, seeking to understand its potential and transform it into a tool that can be used by chefs in a real working kitchen.
Zenko reflects not only the character of the steel, but also the skill and experience of the blacksmith who shapes it.
Is a Zenko Knife Right for You?
A Zenko knife is not necessarily the answer for everyone.
Some cooks prioritize the convenience and ease of maintenance offered by stainless steel.
Some chefs prefer the characteristics of a Kasumi construction.
Others enjoy working with carbon steel and taking care of a knife while experiencing the character of the steel itself.
Choosing Zenko is not simply about choosing a “better knife.”
It is a choice about what kind of steel you want to use, how you want your knife to be made, and what kind of tool you want to cook with.
At Oak Bay Knives, we work with Japanese Master Blacksmiths such as Vini Yasu to create knives for individual chefs and craftspeople.
Every knife is made to order.
The blade length, shape, thickness, weight, balance, and handle can all be considered and designed around your hand and the work you do.