1. What Is the Difference Between an All-Steel Knife and a Welded Knife?
Look at the cutting edge of a Japanese kitchen knife and you will find hard steel, which is mostly iron with carbon, and sometimes other elements, mixed in. A honyaki knife is forged from one kind of steel, so people also call it "all-steel." A kasumi knife has steel only along the edge. The rest is soft iron, and the two metals are joined by hammering them together while hot [1].
The city of Sakai, one of Japan's best-known knife-making towns, describes the same idea for its forged blades. The hard steel is called hagane and the soft iron jigane. The two are heated red, hammered together, and joined. The city says this gives a blade that is both strong and tough [3]. Not every Sakai knife is a welded one, though. A maker there says that most Japanese kitchen knives are the welded kind, and that the all-steel kind is the rarer exception [2].
From the outside, the two can look almost the same [1]. The difference is inside, and it shows up in price, in sharpening, and in what happens if you drop one. That brings us to the real puzzle: if steel alone is risky, why make it?
2. Why Would a Maker Risk Cracking the Blade by Using Only Steel?
A maker of all-steel knives in Sakai gives the main reason as sharpness that lasts. Because there is no welding step, the steel can be forged at a comparatively low temperature. Forging it again and again from low heat makes its fine internal grain more even, and the maker says this gives a firm, keen edge [2]. A second Sakai seller makes a related point: heating steel for a long time at high temperature can make the grains coarse, and an all-steel knife avoids that [4].
There is a smaller benefit too. A welded blade joins two metals that behave differently, and the same maker says it can slowly curve, because the soft iron gets pulled toward the hard steel. An all-steel blade is less likely to warp [2].
The price of these benefits is risk. Because the blade is one piece of hard steel, it can crack during forging, during hardening, or even while the edge is first ground. That maker says that to finish one knife, a craftsperson sometimes makes two or three extra. The finished knives cost nearly three times as much as welded ones made from the same steel [2]. These are the makers' own descriptions. This article could not find independent tests that measure how much longer an all-steel edge lasts.
3. How Does a Coat of Clay Keep a Hard Steel Knife From Breaking?
If the whole knife were as hard as it could be, it would shatter at the first knock. So makers do not harden it evenly.
Steel gets hard when it is heated to a temperature that suits its type and then cooled very fast. One Sakai workshop heats its blades to around 800°C (about 1,470°F) and cools them quickly in water or oil, then reheats them gently to about 200°C (about 390°F) so the steel is less brittle. Temperatures and cooling methods differ between workshops and kinds of steel [5].
For an all-steel knife, the maker first paints a thick layer of clay on the flat sides of the blade, leaving the cutting edge bare. When the red-hot blade goes into the cooling bath, the clay-covered part cools more slowly and ends up a little less hard. The edge, with no clay, cools fastest and becomes the hardest. So one dunking produces both a hard edge and a slightly softer, more springy body that acts as a cushion against shocks [2].
This is the answer to the puzzle. The all-steel knife is not simply "all hard." It is steel that has been hardened unevenly on purpose, using clay to steer where the hardness goes. The cost is that the job is delicate, and makers say a hardening bath that cools too violently is a common moment for the blade to crack [2].
4. What Does This Have to Do With Samurai Swords?
The word honyaki comes from a likeness to the way swords were made. Makers describe the clay-and-quench method as similar to the hardening of a Japanese sword, and that is where the name is said to come from [1][2].
The Nagoya Sword Museum explains the sword problem in the same terms. People say a good sword "does not break, does not bend, and cuts well," but those goals pull against each other: softer metal helps a blade survive a blow without snapping, while harder metal resists bending and keeps an edge. Swordsmiths forged hard steel for the outer skin and softer steel for the core, then joined them. When they hardened the blade, they painted clay on the body to control how deeply the hardening went. If the whole sword were made hard, it would break [6][7].
So the two kitchen knives line up with two sword tricks. The kasumi knife uses the hard-skin, soft-core idea, and the honyaki knife uses the clay idea. A simple way to picture it is that each knife is its own answer to the same old problem: how to get hardness without brittleness. The sources do not say whether today's knife makers think of swords when they work, so this is only a comparison, not a claim about their intentions.
5. Why Is the Welded Knife Called "Haze"?
A Sakai seller says the name comes from the finish. The steel in the middle of the blade is polished to a shine, while the soft-iron part is often given a cloudy, hazy finish. The cloudy look gives the knife its name [8]. So when you look closely at a welded knife, you can often see a line where the shiny part meets the hazy part.
One Sakai workshop describes how the metals are joined. The soft iron is heated to over 1,000°C (about 1,830°F), and steel is stuck to it with the help of a flux, which is a powder such as borax that keeps the surfaces clean. The workers then heat the pair again and hammer them until they bond. They watch the temperature closely, because overheating can burn off the carbon that gives steel its cutting power [5].
6. Which One Is Better for Everyday Use?
Neither wins every time. A well-known knife maker explains the trade-off like this. An all-steel knife has a hard surface to grind, so it is very difficult to re-sharpen, and a careless twist or a knock might snap it in two. A welded knife has mostly soft iron on the grinding surface, which is easier to sharpen, and it is less likely to chip badly or break in half [1]. A Sakai seller adds that all-steel knives can chip more easily and are harder to handle for people who are new to knives [4].
The same maker also says that forging has improved so much that not every all-steel knife is better than a welded one anymore, and that even professionals may choose a welded knife for everyday jobs and keep an all-steel knife for special ones, such as slicing raw fish [1].
So the answer to the opening question is not "all-steel is the best." Makers choose it because of the edge it can hold and because craftspeople value the challenge, and they accept more cost and more risk to get it.
7. How Can You Spot the Difference Yourself?
With a grown-up, ask to look at a kitchen knife, and only look. Do not touch the edge. Near the cutting edge, see whether there is a shiny part and a cloudy part, and whether a line runs between them. Many home knives are stainless steel and are made differently from the knives in this article, so you may not find what you expect.
If there is a knife shop nearby, you can ask an adult to take you and ask the staff whether a Japanese knife is all-steel or welded. The question worth taking along: does the price, the way it is sharpened, and how it is cared for match what you read here?