Sharpening Part 8 – Soft Iron 地金

The fissured and cracked jigane of a 70mm plane blade forged by Usui Kengo, another Niigata blacksmith (RIP). Notice the rod which retains the chipbreaker is non-existent, replaced by two short stubs. An elegant detail in this plane body by Ito-san (Soh 宗).

If you can’t explain it to a six year old, you don’t understand it yourself.

Albert Einstein

In the previous article about sharpening Japanese woodworking tool blades we looked primarily at the nature of the hard high-carbon steel used in making woodworking blades. In this post your humble servant will try to dispel some of the confusion that surrounds the other metal used in making most Japanese knives, axes and woodworking blades, namely the soft low-carbon/no-carbon steel called “Jigane” (地金). I hope this brief explanation will improve Beloved Customer’s understanding of some Japanese tools and aid your sharpening efforts.

Sources of Jigane

Most Japanese woodworking blades, and many knives, are comprised of a thin piece of hard high-carbon steel, discussed in your humble servant’s previous post, forge-weld laminated to a piece of softer low-carbon steel or wrought iron called “Jigane” (jee/gah/neh 地金) in Japanese, which translates directly to “ground metal.”

We will discuss this bi-metal lamination more in the next post in this series, but for now take my word that it is essential to the performance of many types of Japanese cutting tools nowadays, and for many centuries was also critical to manufacturing cutting tools in America and Europe as well.

The best jigane material for plane blades is said to be scrap iron salvaged from the boilers of old trains, boats, and factories, etc.. Such boiler tanks were subjected to thousands of heating and cooling cycles during their years in service driving out most of the carbon, indeed making the iron very soft to the point of weakness.

The most desirable jigane for plane blades is therefore called “tired” iron, named so because it is not only soft, but because it looks weak and exhibits a visible grain along with cracks and imperfections which those well-versed in Japanese plane blades covet.

A pile of jigane, probably old salvaged structural steel. Looks like boards of old wood, but it ain’t.

Wrought Iron Production

Nowadays, this very low-carbon steel, also known as “ wrought iron,” is not produced in any volume for several reasons. First, demand is just too low to make it worthwhile to manufacture. Hand-forged ornamental iron is the only commercial usage besides Japanese tools of which your humble servant is aware, relatively microscopic markets. In fact, a constant complaint from workers of ornamental iron is the difficulty of working the relatively hard material available to them nowadays.

The second reason is that steel production processes have changed drastically in the last 100 years. For instance, it used to be that steel began as iron ore, basically rocks, which were crushed, melted and refined into wrought iron, an intermediate product of steel production. Indeed, at the time this low-carbon product was much less expensive to produce than high-carbon steel and so was used for everything from the boilers, bridges, trains, ships and anchor chains mentioned above to axes, chisels, farming implements, machinery, what’s called “miscellaneous metals” in the construction industry, and of course plane blades. There are still a few surviving structures around that were made using this weaker material.

Nowadays, things are different. With high temperatures more easily attainable than they were prior to the 1850’s, manufacturing techniques have advanced to the point that carbon is incorporated into the steel automatically entirely eliminating the low-carbon wrought iron intermediate product.

Also, scrap metal has become critical to steel manufacturing processes nowadays. Remember what happened to steel prices worldwide when bloody-handed China was buying up huge volumes of scrap metal worldwide for its Olympic infrastructure building projects?

I think we can agree that this energy-efficient cost-reducing recycling of natural materials is a very good thing. But it does have a tiny downside, namely that most commercial scrap metal available in any useful volume today has been cycled through the modern steel-manufacturing process many times and already contains not only high levels of carbon, relatively speaking, but alloys such as chrome, molybdenum, and nickel from previous melting pots. Indeed, undesirable chemicals such as phosphorus, sulfur and silica tend to be high in typical scrap metal. Keeping the negative impacts of these unintended alloys and impurities under control is a serious challenge for manufacturers of tool steel.

In summary, wrought iron simply isn’t made anymore because it is neither an intermediate product nor a profitable one.

Japanese blacksmiths making high-quality plane blades nowadays mostly use wrought iron recycled from old anchor chains, old iron bridges, or other recycled structural components. If you see a hole in a plane blade, like the extra-wide plane blade pictured below, it once housed a rivet. Yes, structural steel was once connected with hot rivets instead of bolts. Hi-tensile modern bolts are decidedly better if less romantic.

A plane blade with an old rivet hole in its face, probably from an old iron bridge that once stood in Yokohama and which was recycled many years ago.

Plane Blades

A plane blade by Ogata-san in his “Nami no Hana” series using a special version of Swedish Asaab K-120 steel. Notice not only the fissures and defects, but also the striations and grain typical of soft, tired “wrought iron.”

Mr. Takeo Nakano (see his photo below) makes our plane blades. He is a kind, unassuming man in the best tradition of Japanese craftsmen with the outward appearance of a sedentary grandfather, but when using hammer and tongs at his forge within his dark and smoky smithy, his posture and visage resemble that of an intense Vulcan reinforcing the steel gates barricading the world of light against a demon onslaught. Oh my!

Like nearly all the plane blacksmiths in Niigata, he uses scrap iron obtained in a single lot many years ago from an iron bridge that was dismantled in Yokohama Japan.

Mr. Nakano at home

I am told that most of the jigane used for plane blades in Hyogo Prefecture is old recycled anchor chains from a ship knacker.

The back of the same Usui plane blade. Notice the cracks and voids visible in this excellent jigane exposed at the polished bevel. Very wabi-sabi. This jigane was once part of an iron bridge in the city of Yokohama, Japan.

In the case of plane blades, structural strength is not critical, so laminating a thin layer of high-carbon steel forming the cutting edge to a soft iron body is adequate. Indeed, the thicker the hard steel layer, the more time and effort it takes to sharpen the blade, so in a high-quality blade the thicknesses of the high-carbon steel layer and the soft jigane body will be carefully balanced to ensure the blade’s bevel rides the sharpening stones nicely and can be quickly abraded.

More inexpensive plane blades are forged using the same strip jigane used for chisels, a material harder than the ideal for plane blades.

Chisel Blades

In the case of chisels, while ease of sharpening is still important, the body and neck must be harder/stiffer to prevent them from bending, so a different, stiffer variety of jigane with a higher carbon content and fewer defects is used, and the steel layer is typically made thicker.

The jigane used by our chisel blacksmiths is a commercial product not produced anymore (thank goodness they have stockpiles) called “gokunantetsu” 極軟鉄 which translates directly to “extremely soft iron.” With a carbon content of 0.04~0.07%, a better description would be “very low carbon steel.” When heated and quenched, it doesn’t harden significantly.

The adventure will continue in the next exciting episode where we will bring it all together into a blade. Don’t forget to have popcorn and jujubes on-hand!

YMHOS

If you have questions or would like to learn more about our tools, please click the “Pricelist” link here or at the top of the page and use the “Contact Us” form located immediately below.

Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google, fascist facebook, or thuggish Twitter and so won’t sell, share, or profitably “misplace” your information. If I lie may the fleas of a thousand camels infest my crotch.

Sharpening Part 5 – The Sharp Edge

The gem cannot be polished without friction, nor man perfected without trials.” 

Confucius

This post may not be as entertaining as previous ones in this series about sharpening Japanese woodworking tool blades: No swords or artwork or handsome Hollywood philosophers, I’m sorry to say. But with this post we will roll up our sleeves and get some work done.

Your humble servant prostrates and begs forgiveness if Beloved Customer’s tender sensibilities are in any way offended. Many Beloved Customers and Gentle Readers already know most of what is presented in this post, but it is my fervent hope that one or two useful gems may be discovered.

You know the difference between the quality of work a sharp edge performs compared to that of a dull edge. Cuts are clean and finished surfaces are smooth, maybe even shimmering. Your tools are happy, singing and chirping as they cut away. But have you given thought to what a sharp edge really is?

Since the purpose of sharpening is to produce this condition in a blade, a clear understanding is useful, so we will consider the basics in this post.

We shall also examine the naughty cutting edge that seems sharp but suddenly and unexpectedly dulls after just a little use. Would it be useful to know how to detect such a cutting edge before it fails wasting your time and money?

Let’s begin with bedrock.

The Basics

A cutting tool is essentially a wedge, with two flat sides meeting at an angle. Applying force causes it to wedge apart and sever materials, be it wood, metal, meat or mushrooms.

The geometry of this wedge is critical to its performance. At one extreme, the angle could be 90°. It won’t be sharp, it will be hard to push, and it will crush and tear wood instead of cutting it cleanly, but it will be durable.

At the other extreme, the wedge might be made more acute, say 3°. It could be extremely sharp indeed, but it would be too fragile to cut anything but whip cream for long. The point is that the sharp edge is a compromise, acute enough to cut well, but not so acute that cutting pressure and friction will make it dent, roll, wear away, crack or chip easily.

The effective blade must have a bevel angle that will cut the intended material well, but at the same time supports the thin cutting edge from premature wear and damage for a relatively long time. The words “well” and “long” in the previous sentence are where the magic lies. We will examine these important points in future posts in this series.

Wood Shaving’s Eye View

Ideally, the extreme edge of the ideal metal tool’s extreme cutting should be perfectly smooth and only a single molecule thick. In the real world, cutting edges are rougher and wider, but still manage to cut pretty well.

Examine a sharp cutting edge under a microscope, and you will see imperfections. A dull blade will look even worse of course, showing dents, rips, and even cracks. 

knife edge_microscope800
The edge created by an 800 grit stone
Still sharp but starting to wear
A dulled and dented knife blade

Using a blade wears away and damages the cutting edge rounding and flattening it, destroying the geometry that makes it an effective wedge. Sharpening is the process of (1) restoring the intended wedge geometry; and (2) removing defects from the meeting of the wedge’s sides by abrading metal from one or both sides down past any damage, leaving a relatively clean, uniform wedge with minimal defects. This is the sharp edge. It is what the wood experiences. It requires effort to achieve, but it ain’t rocket surgery.

The most difficult part of achieving the two objectives listed above is making nothing from something, in a place that cannot be seen. Now that’s a Zen koan if I ever heard one.

Building confidence in one’s ability to achieve results at the microscopic level is not easy. The key is to understand the goal, and to consistently follow reliable procedures. I will describe those goals and procedures in future posts in this series.

Edge Failure

The ideal cutting edge is uniformly sharp, but few edges in the real world meet these severe criteria at the microscopic level where it matters most. A blade may be sharp in some places, and dull in others. Likewise, a blade may cut well for a while and then dull quickly and suddenly. We have all experienced these irritating failures.

One common cause of these inconsistencies and failures is that the edge is sharp only because it has a defect called a burr. Burrs by themselves can be sharp indeed, but they are thin, irregular, fragile and can bend, roll over, or break off at the root suddenly and unpredictably creating a nasty dull edge in an instant. A truly sharp edge will not just feel sharp, but will stay sharp for a relatively long time because it is properly shaped and well supported, instead of being only temporarily sharp because of an irregular and fragile burr.

I call burrs a “defect” because they are, but creating a burr is an important step in making a sharp edge. The trick is to continue to refine the wedge after the burr is created until the burr melts away on the stones and the edge is as perfect as we can reasonably expect to make it. Stop the refinement work too soon, or fail to do it completely, and all or part of that unreliable burr may survive to plop a floater into your punchbowl.

So how does one tell if an edge is properly sharp and free of deceptive burrs without using a scanning electron microscope?

Do you remember ‘Nando’s philosophy described in my previous post? One must reverse the latin lover’s logic. Don’t rely on mahvelous appearance. Don’t rely on bar room stunts like shaving arm hair or telemarketing tricks like cutting strips of paper. Develop skills and train your senses other than eyesight to detect the shape of steel at the microscopic level. This may sound strange but it is possible because your nerve endings are microscopic and can sense the difference between a burr and a truly sharp edge.

I will save the explanation of detailed techniques for a future post, but for now, here are two essential techniques for sensing things too small to see: Use your fingerprints and the exquisitely fine nerves connected to them to detect the presence and size of burrs; Use your fingernails to check the condition of the burr and determine when the blade is ready to move onto the next stone in the sharpening process. Please don’t cut yourself.

In the meantime, let’s have some pleasure before pain. Prepare to be amazed, Ladies and Germs, because in Part 6, coming soon, The Mystery of Steel will unfold before your very eyes! There will be marble relief carvings, bronze statues, oil paintings, gods and demons, death and destruction, and even a pagan soap opera about forbidden love. Oh my! We’re in negotiations for the movie rights now ♫꒰・‿・๑꒱ and need someone to play Vulcan. If anyone knows Spiderman’s agent, please have his people contact my people right away.

YMHOS

If you have questions or would like to learn more about our tools, please click the “Pricelist” link here or at the top of the page and use the “Contact Us” form located immediately below.

Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google, fascist facebook, or thuggish Twitter and so won’t sell, share, or profitably “misplace” your information. If I lie may all my donuts be infested with lawyers.

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