Sharpening Part 9 – Hard Steel & Soft Iron 鍛接

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A piece of hot high-carbon steel, which will become the cutting edge, has been placed on the orange-hot low-carbon steel body of a knife. An acidic flux powder has been placed in-between and on the metals in preparation for forge laminating them together into a single blade.

“It is our choices, Harry, that show what we truly are, far more than our abilities.” 

J.K. Rowling, Harry Potter and the Chamber of Secrets

If you are reading this, it’s safe to assume you are interested in sharpening woodworking blades. You may have little experience with Japanese tools, and even then you may not be aware of some of their important details. In this post we will try to remedy that by examining some simple historical points common to woodworking blades around the world, as well as some details that make Japanese blades unique.

I believe an understanding of these basic facts will you aid your sharpening efforts, or will at least tickle your interest in Japanese blades. Please comment and let me know your thoughts.

Laminated Bi-Metal Construction

As discussed in previous posts in this series, before technological advances in the 1800’s steel was difficult to make and expensive. Consequently, it was standard practice not only in Japan, but everywhere including Europe and the United States, to reduce costs by minimizing the amount of precious steel used to make axe, scythe, plane and chisel etc. blades by laminating smallish pieces of high-carbon steel to softer and much cheaper wrought-iron bodies through a process called “forge welding.”

Most chisel and plane blade blacksmiths in Japan continue to employ this lamination technique even today, not out of some navel-gazing preference for the archaic, but because it has serious advantages.

A 30mm Hidarino Ichihiro Atsunomi, approximately 12″ OAL.

The best Japanese plane and chisel blades are generally comprised of a layer of very hard high-carbon steel called “hagane” (鋼) in Japanese, forge-welded to a softer low-carbon (ideally no-carbon) iron body called “jigane” (地金). We discussed both of these metals in the previous two posts in the series. Here and here.

Why go to so much trouble? One advantage of this construction is that it allows the cutting edge to be made much harder, and therefore cut effectively longer than a blade of uniform hardness. For instance, a blade made entirely of steel hardened to HRC65 might cut very well, but it would break or shatter in use. And even if it did not break, it would be time consuming and irritating to sharpen such a wide expanse of hard steel. Remember, the harder a piece of steel is the more work it takes to abrade it.

A 42mm Hidarino Ichihiro Oiirenomi

By combining a thin layer of this very hard steel with a thicker layer of soft low-carbon steel or wrought iron the blade can be made thick, rigid, resistant to breaking, and will hold a sharp edge relatively longer while still being easy to sharpen. This once-common ancient structure is clearly superior to all other structural systems for planes and chisels at least.

Laminated Blades in the West

If you have examined antique plane blades with wooden bodies you may have noticed many have blades stamped ” Warranted Cast Steel”

Despite being designated “cast steel” in England and America in past centuries, unlike Conan’s Daddy’s sword, or the orc blades made by in Isengard, plane, chisel and saw blades with this mark were not “cast” by pouring molten metal into a mold to form a blade. Rather the process to make the steel involved melting steel in a crucible and pouring it into molds “casting” a piece of high-carbon steel which is then forged to make the blade, hence the name.

This technology was widely used in the United States and Europe through the 1860’s. In fact, one steel mill is said to have been producing crucible steel until the 1960’s. Toolmanblog has an interesting summary on cast steel.

With few exceptions, these plane blades have a thin piece of high-carbon steel forge-welded to a soft wrought iron body, very similar to Japanese plane blades. I have used a couple of these antique blades to make Krenovian planes and testify of their excellent cutting ability.

Chisels were also once made in Europe using this same lamination technique, although fewer examples remain extant.

Axes, hatchets, and many farming implements were also mass-produced up until the 1920’s in the US using a variation of this same technique with a “bit” of steel forming the cutting edge laminated to or sandwiched inside a body of low-carbon steel or wrought iron. Axes are still made this way in Japan. It’s a proven technique with a lot of advantages, but it does require a skilled blacksmith to pull off successfully.

The point I am trying to make is that blades made using forge-welded laminated technology were the very best available in Europe and the United States for many centuries. It is sad that this superior technology has been discarded and forgotten except in Japan, but wars and economics change everything while people remain the same.

Here is a link to a blog post by Paul Sellers where he praises the old chisels and laments the new.

U-Channel Construction

A closeup of the 42mm Hidarino Ichihiro Oiirenomi showing the lamination line between the steel cutting layer and low-carbon steel body of the blade
The same 42mm Hidarino Ichihiro Oiirenomi. Notice the hard-steel lamination wrapped up the blade’s sides to add rigidity.
A 30mm Hidarino Ichihiro Atsunomi, approximately 12″ OAL. Notice the hard steel lamination forming the cutting edge at the bevel. This is a beautiful lamination.
A beautiful hand-filed shoulder detail typical of Yamazaki-san’s work

The shape of the hard steel cutting layer laminated to the softer low-carbon steel (or wrought iron) body was historically a simple flat plate in Western blades. This is also the case for Japanese plane blades, axes, and farming implements. But if you imagine Japanese blacksmiths would be satisfied with such a simple design for all applications, you don’t know the Japanese mind well.

Notice the lighter-colored hard steel lamination wrapped up the chisel’s sides in the four photographs above forming a “U channel” of hardened steel adding necessary rigidity and strength. This is a critical detail for Japanese chisels intended to be struck with a hammer. Interestingly, carving chisels are not typically made this way.

Plane blades are not subjected to the high loads chisels experience and so would not benefit from this structural detail.

The Ura

Japanese chisel and plane blades, among others, typically have a hollow-ground depression called the “Ura” (pronounced “ooh-rah”) which translates to “ocean” or “bay,” located at what is called the “flat” on Western blades. Notice the polished hard steel lamination extending from the cutting edge to several millimeters up the neck. The black area surrounded by the shiny lands is the same hard metal, but has been ground to form a hollow called the “ura.”

This clever and effective design detail is unique to Japanese tools to the best of my knowledge. We will look at this design detail more in the next post in this series.

The Point

What does any of this have to do with sharpening? Glad you asked. This design has some potential disadvantages that have been cleverly turned into distinct advantages you need to understand when sharpening Japanese woodworking blades.

For instance, the layer of high-carbon steel laminated into our chisels and planes is usually 64~65 HRc in hardness. The typical Western blade is made much softer at 50~55 HRc to avoid breakage. This extra hardness makes the blade stay sharper longer, an important benefit if your time is worth anything. This is good.

But if the entire blade were made of a solid piece of this extra-hard steel, it would a royal pain in the tukus to sharpen, I guarantee you. It would also break. That would be bad.

The softer low-carbon/no-carbon steel or iron body, however, is much softer and easily abraded making it possible to keep the hard steel layer thin, and therefore easily abraded, while protecting it from breaking. This is good.

Unlike the blade’s bevel, however, the ura is all one-piece of hard steel. Without the ura depression, you would need to abrade all that hard steel to initially flatten and regularly sharpen the blade, a necessity I guarantee would ruin your mellow mood without massive quantities of controlled substances. But with the addition of the ura detail, we only need to abrade the perimeter planar lands (the shiny areas in the photos above) around the ura. This is exceedingly good.

The ura depression makes it easier and quicker to not only sharpen the blade, but also to to keep the “flat” planar (in a single plane). Without the ura, such a hard blade would be difficult to maintain planar and frustrating to sharpen. With the addition of the ura, the blade is genius.

An important skill to learn when sharpening Japanese blades is how to maintain the lamination and ura effectively. We will discuss this important subject more in future posts.

Conclusion

If you didn’t learn at least three new things from this post then you are either very smart or weren’t paying attention. ¯\_(ツ)_/¯

In the next installment in this bodice-ripping tale of romance and derring-do we will examine the hollow-ground “Ura” in more detail. It’s important enough to deserve a special post.

YMHOS

© 2019 Stanley Covington All Rights Reserved

Please share your insights and comments with everyone in the comments section below. If you have questions or would like to learn more about our tools, please use the question form located immediately below.

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