Sharpening Japanese Chisels in 2023

[Ron Brese]

The theory behind sharpening any blade doesn't change with where the tool is made. The same actions that create a sharp edge will do so no matter where the tools is made. I think when people are teaching sharpening they should refer to the stones etc. as sharpening medium. It could be all manner of stones abrasive films, whatever. This way they could concentrate on teaching the principles of what makes things sharp in lieu of their specific way of doing it. This would serve people interested in refining their sharpening routine to a dependable system that yields sharp edges consistently.

Teaching methods that require the removal of less steel to yield a sharp edge certainly have their place and those options should be discussed. In the end with basic principles in place an individual can take that knowledge and make an informed decision as to how they wish to approach sharpening.

Ron

[Rafael Herrera]

My understanding of Japanese chisels is that they are hardened higher than western chisels, so require attention in the method used to sharpen. Their maintenance is more complex since one has to hammer flat the concave area behind the edge to restore the flat side of the chisel as it wears out.

As an aside, as with their writing system, their wood joinery technology was imported from China. They certainly evolved them, but it's worth pointing out that, for example, the construction techniques of all those amazing temples were not created in Japan.

[Warren Mickley]

The theory behind sharpening any blade doesn't change with where the tool is made.

Of course it does not matter where the chisel is made. What matters is how the chisel is made.

[Michael Bulatowicz]

The other reason for the backing is to act as a shock absorber - protection for the brittle, hard steel of the cutting edge.

There is a longstanding myth that this statement applies to a Japanese chisel in use; I suspect that this myth is simply a misinterpretation of the applicability of the statement (more below). It's not necessarily a myth of any consequence for use of the chisel, but it is quite simply technically inaccurate for the physics of a chisel in use. During a previous discussion you and I participated in on another forum, I offered to provide equations to back up the assertion that the jigane (soft backing) cannot act as a shock absorber in use: the offer remains open.

I have thought about the statement further since that discussion, prompted by comments David Weaver made during that same discussion. My conclusion is that this statement about the softer backing being a shock absorber appears to be a valid statement during the quench, at least for water-hardening steels such as Japanese white steel (Hitachi shirogami). So, it specifically applies during the process of making the chisel and not during its subsequent use.

I am not an expert in metal quenching, but here's my explanation.

Why is the "shock absorber" statement valid during the quench? Let's consider a piece of brittle rather than ductile steel (such as white steel upon quenching). As the steel cools, it changes in density. During the quench, the outside cools and "locks" in place with the center still much hotter. As the steel continues to cool, the steel in the center continues to change in density: the "locked in" outer portion of the steel and the still-cooling inner portion "fight" each other, and pressure (residual stress) increases until the steel reaches a uniform temperature. This build-up of pressure (residual stress) is dependent on a number of factors including the cooling rate and thickness of the steel: greater thickness means greater forces trying to tear the steel apart because the outside tends to cool at similar rates for thin versus thick steel, while the center stays hotter through a larger section for thick steel.

A thick cross-section of white steel may therefore very well crack or break apart during a quench: greater internal stresses increase the likelihood of cracking, particularly for brittle steel (which describes white steel quite well immediately following a quench). A thin cross-section (of hard steel) will, on the other hand, significantly mitigate this: the total pressure will be significantly less. If the material is instead ductile, once the pressure rises high enough the material will simply deform and absorb the energy--the process of deformation reduces the stresses.

Further, the laminated construction ensures that the "locked in" portion of the hard steel will only be on one face and two sides instead of fully surrounding the central portion, and the hard steel portion of the chisel will tend to warp as a primary result. The soft backing, made from a steel that remains ductile during this process, simply deforms to accommodate this warping (and its own) and relieve much of this internal stress with no significant consequences to the strength of the soft backing. So, the soft backing absorbs much of the rapid build-up of internal stress (the "shock") during the quench.

Hence, during the quench, the soft backing does act as a shock absorber and the statement is valid in the appropriate context.

Slower-quenching steels exhibit less stress (pressure) as a result of this effect because the temperature isn't quite so different from the outside to the center when the outside gets locked in--slower cooling of the outside surface leads to smaller temperature differences through the thickness. So, oil-quenching steels warp less from this effect and are less likely to crack, and air-quenching steels even less so.

Another function of the softer backing is that it adds significant stiffness compared to a hypothetical chisel composed solely of a thin piece of white steel.

[Jim Koepke]

Sharpening Japanese blades is as much ritual as it is practical.

Something taught as a ritual practice may be more memorable than teaching, "this is how it is done so do it this way."

Another function of the softer backing is that it adds significant stiffness compared to a hypothetical chisel composed solely of a thin piece of white steel.

Would it not also help by absorbing any shock waves emanating through the "hypothetical chisel" of thin steel?

jtk

[Michael Bulatowicz]

Would it not also help by absorbing any shock waves emanating through the "hypothetical chisel" of thin steel?

jtk

No, actually.

The greatest stress (pressure) by far is at the very thin steel right at/near the cutting edge. The stress can be mitigated somewhat by increasing the impact time, which would be accomplished better with a thin blade (less stiffness with regards to lengthwise compression). This effect, however, will be small compared to the impact-duration-increase effect of the handle, which has significantly lower stiffness in lengthwise compression.

Any shock wave from the wood one is chopping would necessarily start at and near the cutting edge and expand in spatial dimension as it passes from there into the body of the chisel, thereby decreasing in amplitude. If the amplitude of the wave is not large enough to hurt the cutting edge, how would it hurt anything in any portion of the body where it's necessarily much lower amplitude? From the other direction, any shock wave coming from the striking implement would start at the handle and increase in amplitude as it passes down the wedge of the bevel into the extreme cutting edge--again, it's at its highest amplitude at the cutting edge.

With regards to absorption of shock waves or any other form of mechanical energy, there's a negligible difference in thermoelastic energy dissipation between the hard steel and soft backer during any cutting operation, whether chopping or paring. The handle dissipates much more energy than either, as does the wood one is chopping. Ideally, the wood one is chopping will be the dominant source of energy dissipation.

An exception wherein the soft backer would still act as a shock absorber after the quenching process would be if you were to cause some kind of permanent deformation of the chisel, such as in the process known as ura-dashi wherein the chisel (or, more typically, plane blade) is bent in the direction of the ura (the hollow on the back) through judicious use of a hammer. In this case, the soft backer is definitely acting as a shock absorber--you can tell that it is because the hammer leaves dents and because the blade stays bent--but this isn't something done in the normal course of using a chisel and to my limited knowledge not something typically done with chisels at all but rather with plane blades. The extra energy dissipation, here, is all about the ductility and has nothing to do with the elastic properties--which will be similar for the hard steel and the soft backer.

In chopping, the least stiff part of any Japanese chisel I've ever seen is always the handle. Wood exhibits significantly greater thermoelastic dissipation than steel. It also flexes more when striking the chisel with, for example, a mallet, and therefore slows the impact more than any characteristics of the steel or soft iron backer. The handle, therefore, does more to absorb shock and slow impacts than any of the steel or the soft backer.

As evidence, I submit the following: using something metallic, lightly strike the metal of a Japanese chisel versus a Western chisel, and listen to how long the chisel "rings." A shorter "ring-down time" indicates greater dissipation of waves (of any sort). I just tried it with a couple of chisels in my workshop: a high quality Japanese oiirenomi purchased from Stan Covington versus an early-1900s Buck Bros chisel. If anything, my Japanese oiirenomi might ring slightly longer than my early-1900s Buck Brothers chisel, indicating less shockwave absorption rather than more.

[Cameron Wood]

I disagree with the "steel was expensive" theory.

With the wane of samurai, blacksmiths moved to making woodworking tools. Techniques for making samurai swords were certainly not based on saving some ryō.

As Derek Cohen points out above, thick sections of hard steel would be hard to sharpen (as well as other factors). Trying to get an edge on those old hard-as-hell Buck knives comes to mind.


"Carpentry was not the most important trade in feudal Japan, so carpenters made do."

Elaborate temples were being constructed for many hundreds of years leading to the feudal period, not suggestive of some unimportant carpenters making do with substandard tools.

[Joel Gelman]

Many thanks for the replies. In giving everything more thought, I guess if using waterstones, I would not seek to obtain a kana ban, and instead keep the waterstones flat with a Shapton lapping plate. My next step is to experiment with my lap-sharp on some chisels and planes. I found out that the company is out of business because the machines were too expensive to build to be profitable. When I purchased this system before 2017, I obtained a very large assortment of abrasives, more than I will use in a lifetime. I never really devoted the time to learning this system but will try. I purchased it back in the day to use at some point, and the time has come. I do not have super expensive chisels and planes at this time, and so I do not think I will ruin anything, especially if I am careful.

As for expensive chisels, that can be a separate thread.

[Rafael Herrera]

I disagree with the "steel was expensive" theory.

With the wane of samurai, blacksmiths moved to making woodworking tools. Techniques for making samurai swords were certainly not based on saving some ryō.

As Derek Cohen points out above, thick sections of hard steel would be hard to sharpen (as well as other factors). Trying to get an edge on those old hard-as-hell Buck knives comes to mind.

"Carpentry was not the most important trade in feudal Japan, so carpenters made do."

Elaborate temples were being constructed for many hundreds of years leading to the feudal period, not suggestive of some unimportant carpenters making do with substandard tools.

Iron metallurgy in Japan developed in the 6th century to the point of being able to produce steel, but in very small quantities. They used Iron sand, an ore with a very small mount of iron in it. The process was very resource intensive and their yields small. They figured methods of maximizing the use of their steel, producing their katanas, and whatever else needed an edge.

If one thinks about it for a moment, steel welded to wrought iron is a brilliant solution, not because it was easy to sharpen, but because they couldn't fabricate the whole thing out of steel, which would have been wasteful, even absurd to consider.

There are today professional katana sharpeners that spend a very very long time sharpening those swords, it's not unreasonable to assume the same happened in the past. Ease of sharpenning was not the reason for laminating steel to wrought iron.

Over the centuries they may have had access to higher quality ore, but steel was not something in abundance there, nor anywhere else in the world until the after the industrial revolution was well under way.

https://en.wikipedia.org/wiki/Early_...ing_techniques

This video shows traditional japanese blacksmiths using iron sand to produce steel.

[Cameron Wood]

Iron metallurgy in Japan developed in the 6th century to the point of being able to produce steel, but in very small quantities. They used Iron sand, an ore with a very small mount of iron in it. The process was very resource intensive and their yields small. They figured methods of maximizing the use of their steel, producing their katanas, and whatever else needed an edge.

If one thinks about it for a moment, steel welded to wrought iron is a brilliant solution, not because it was easy to sharpen, but because they couldn't fabricate the whole thing out of steel, which would have been wasteful, even absurd to consider.

There are today professional katana sharpeners that spend a very very long time sharpening those swords, it's not unreasonable to assume the same happened in the past. Ease of sharpenning was not the reason for laminating steel to wrought iron.

Over the centuries they may have had access to higher quality ore, but steel was not something in abundance there, nor anywhere else in the world until the after the industrial revolution was well under way.

https://en.wikipedia.org/wiki/Early_...ing_techniques

This video shows traditional japanese blacksmiths using iron sand to produce steel.

Very interesting video. But in this case, the whole knife is made from the rare and laboriously produced steel, so doesn't help the point above...

[Rafael Herrera]

Very interesting video. But in this case, the whole knife is made from the rare and laboriously produced steel, so doesn't help the point above...

Darn! How about: the video illustrates traditional steel production, what they do with it is another story?

[Keegan Shields]

Techniques for making samurai swords were certainly not based on saving some ryō.

Let me try to be more clear about the two points.

1. Carpentry was a less important vocation than making war in feudal Japan, therefore making something like a katana with the available steel/iron would have taken precedence over a metal smoothing plane or timber framing chisel. Given the scarcity of iron ore, its no wonder Japanese carpenters used wooden plane bodies and developed chisels with laminated blades.

2. The fact that katanas are traditionally made with many layers of folded steel is the practical solution to the problem of impurities in the steel that was used causing a weak spot. This is my attempt at showing another example outside of woodworking where a bug is turned into a feature over time. All that extra work to fix a problem turns into some mythical benefit to admire. The point has nothing to do with material savings.

Hopefully that helps clear up what I was trying to convey.

[Rafael Herrera]

its no wonder Japanese carpenters used wooden plane bodies

I'm not sure if you're implying that if there had been more iron resources, they would have used metal bodied planes; there were no metal bodied planes before the 19th century.

Also, certainly carpentry tools were being manufactured throughout Japanese history, they have a centuries old woodworking tradition.

The katanas were mentioned as an example of the manufacture of a cutting instrument forge welding irons with different carbon content. The same technology applies to more mundane devices like chisels, plane irons, etc.

[Tom M King]

I always wondered exactly how hard the "hard" steel was anyway, since it's so easy to tap the back out when the edge intrudes on the hollow.

[Cameron Wood]

Let me try to be more clear about the two points.

1. Carpentry was a less important vocation than making war in feudal Japan, therefore making something like a katana with the available steel/iron would have taken precedence over a metal smoothing plane or timber framing chisel. Given the scarcity of iron ore, its no wonder Japanese carpenters used wooden plane bodies and developed chisels with laminated blades.

2. The fact that katanas are traditionally made with many layers of folded steel is the practical solution to the problem of impurities in the steel that was used causing a weak spot. This is my attempt at showing another example outside of woodworking where a bug is turned into a feature over time. All that extra work to fix a problem turns into some mythical benefit to admire. The point has nothing to do with material savings.

Hopefully that helps clear up what I was trying to convey.

Yes, good points.

Jay