The evolution of the Japanese plane blade "Nikuoki" since the end of the Edo period

Before I get into the topic, I need to explain “肉置/Nikuoki”.　I was confused when I first saw this word in Tsuchida Noboru's article. "肉置/Nikuoki" is a word that is often used in the field of Japanese swords. I know very little about Japanese swords, I will not explain here what this word means in the field of Japanese katana swords.

There are two explanations for “肉置/Nikuoki” in the Japanese dictionary, and they are pronounced differently. The first meaning is pronounced as “Shishioki”, which means "The attachment condition of the flesh on the body." The second meaning is pronounced as “Nikuoki”, which means "Blade thickness.”

I asked Mr. Tsuchida Noboru about the specific meaning of this word in his article. Combining his answer with the meaning of the word in the article, I think “肉置/Nikuoki” has a simpler meaning of "Blade thickness" and another meaning of "Blade thickness distribution status" in certain situations.

In terms of the first simple meaning, the thickness of Japanese plane blades has been developing from thin to thick since the late Edo period, and the proportion of steel thickness in the entire blade thickness has gradually increased too.

Fig.1: Planer blades side view. Top: Modern blade, produced around the Showa period；Middle: Blades produced before the end of World War II；Bottom: Blades produced in the late Edo period, Around the end of the 19th century. 

What is the reason that drives the development of Japanese plane blades from thin to thick?

We first need to understand why plane blades were always made thin by blacksmiths before the late Edo period.　

1: The cost of raw materials. Until the middle of the Meiji era, the raw materials for Japanese tools were only “和鉄/Watetsu" and “和鋼/wako" produced in Japan. These two rough raw materials are not expensive, but using these two raw materials to make high-quality blades requires blacksmiths to have very rich experience in the preliminary material selection and spend a lot of physical effort and time to forge the “和鋼/wako" (the purpose is to adjust the carbon content according to the purpose of use, remove impurities and make the carbide structure of the steel more uniform). After the above-mentioned preliminary preparation work by the blacksmith, these two raw materials become particularly precious. Making thin blades can save the amount of these two raw materials.

• “和鋼/wako" currently also refers to modern steel used for blades (such as Yasugi steel)., but the “和鉄/Watetsu" and “和鋼/wako" mentioned here are raw materials obtained by traditional Japanese iron smelting methods. Please refer to the information below, the “Tamahagane” mentioned in the information is the high-quality raw material of “和鋼/wako"; Wari-tetsu or Hocho-tetsu is “和鉄/Watetsu”.
• “The large amount of ferrous composite produced by the Tatara operation is called Kera. Kera was carried into a workshop called Dohba, being crushed into smaller pieces, and was sorted based on quality. The hardest and finest quality steel of Kera that contained the least amount of carbon, was called “Tamahagane”. It was mainly used for crafting Japanese swords. The relatively low quality components were called Zuku and Bukera, which were forged into flat bars called Wari-tetsu (or Hocho-tetsu) after decarburization at the Oh-kajiba workshop. The Tanabe family’s historical records suggest that the total output of steel remained at approximately 20% of the entire production at Sugaya Tatara, with Zuku and Bukera making up the remaining 80%.” —— 「What is TATARA Ironworks?」https://unnan-tatara.jp/en/tatara_about.html
• Mr. Tsuchida's article contains a record of Ishido Teruhide (Kikuchi Seiichi) complaining about “和鉄/Watetsu”: "It is difficult to make money and survive by making “和鉄/Watetsu” planes, even if the price is five times that of “釜地/Kamaji” planes. I often spend a lot of money to buy a lot of “和鉄/Watetsu” to use as the soft iron part of the plane blades, but after I sort them out, I can't even find a piece of iron that can be used as blades." - "The History of Planes Seen by Shape" Tsuchida Noboru, "A Complete Collection of Plane Techniques and Famous Products" p72
  

Fig.2:A chisel made in the late Edo period using Tamahagan and Watetsu.

2: The characteristics of “和鋼/wako" during hardening (poor hardenability). If the carbon steel “和鋼/wako" or “玉鋼/tamagane" cannot be cooled quickly during the quenching stage, it will not achieve the desired hardness. The blade is made thinner to facilitate hardening. Due to this characteristic, the blade of the “和鋼/wako" or “玉鋼/tamagane" era was not only thinner overall, but the blacksmith also made more detailed processing on the thickness distribution of the blade. This involves the second meaning of “肉置/Nikuoki"：Blade thickness distribution status.

① Make the middle part of the back of the blade thinner, as shown in the figure below, the back of the blade is concave. （The arrow in the lower right corner represents the camera shooting position）

Fig.3:A late Edo period blade made by a skilled blacksmith with a concave back.

② Make “裏すき/urasuki” on the steel part on the front of the blade, that the steel in the middle of the blade is thinner than the steel around it；

Fig.4:Blade by the third generation Kunihiro in the early 20th century. The red area is the "Urasuki"

Fig.5: The "Urasuki" concave as seen from the side of the Kunihiro blade.

In short, in addition to making thinner "Nikuoki" blades, blacksmiths in the "watetsu&wako" era also considered the distribution of blade thickness(the second meaning of Nikuoki) —— The blacksmith will reduce the thickness of the middle part of the plane blade where the temperature drops slowly, that the steel in the middle part can cool down at the same time as the surrounding steel during quenching to obtain uniform hardness.

In addition to reducing the thickness, the “裏すき/urasuki” part is roughened with a coarse whetstone to increase its surface area, which can also make the cooling speed faster during quenching. Before the quenching stage, the surface of the blade is coated with mud made of Japanese natural sharpening stone powder. Because when the high-temperature blade enters the water, the high-temperature blade will continuously vaporize the surrounding water, forming a gas cavity around the blade, which reduces the cooling speed. If the blade is coated with mud, the mud will absorb and evaporate water repeatedly and efficiently like a sponge, maintaining the cooling speed required for hardening.

The difficulty is that when a large plane blade made of carbon steel is quenched, even if the above treatment is done, it is difficult to quickly cool the steel in the middle part of the large blade. Mr. Tsuchida told me that Chiyozuru Korehide used running water when quenching the large plane blade, and Ishido Hidekazu added ice to the quenching pool.

By the way, this characteristic of carbon steel has caused most of the large plane blade in Japan to be made of blue steel (a kind of alloy steel) which is easier to harden. The chromium and tungsten elements in blue steel can effectively improve the hardenability of steel.

• “Increasing the steel hardenability and yield strength, which was accompanied by decreasing elongation and Charpy impact energy were observed in some steel alloys as a result of adding W” - R.L. Klueh, P.J. Maziasz, The microstructure of chromium-tungsten steels Metall. Trans. A (1989)

• The chart below shows the hardenability of three steels with a carbon content of 0.40% after the classic Jominy test.
  It comes from a blog on the Internet, the original address: https://www.harveyperformance.com/in-the-loupe/hardenability-of-steel/

• The carbon steel shown by the red line can only achieve the ideal hardness when cooled rapidly, while the alloy steel with other alloy elements added, especially the green line with high nickel and chromium content, can still achieve the ideal hardness even if the cooling rate is slower (within a certain range, the hardness also decreases beyond this range).
• Jominy Test:

• Jominy test video demonstration on the Internet: https://www.youtube.com/watch?v=qW0aUbTWtVM

From the user's perspective, thin blades (thin steel, thin wood body) have the following advantages and disadvantages:

Advantages:

1: The blade is easier to sharpen (As shown in the figure below, smaller bevel area to sharpen, smaller steel area to sharpen);

Fig.8: Comparison of the sharpening bevel of the blade in the late Edo period (left) and the Showa period (right).

2: The blade is more convenient to do “裏出し/Uradashi” (because the steel is thinner);

• For the meaning and practice of "Uradashi", please check this blog by Covingtonandsons——「Sharpening Japanese Tools Part 30 : Uradashi & Uraoshi」

3: It is more flexible to use. Before electric planers, carpenters needed to use the hand plane to process wood back and forth continuously. Thinner blades and thinner wood bodies make the entire plane lighter;

Disadvantages:

1: Thin blades (thin wood body) bend more when the steel is deformed by internal stress and the wood is deformed (wide blades and wide wood body bend more);

2: The steel part of thin blades is more easily worn (sharpening the ura without sufficient "Uradashi" or without "Uradashi", which often happens to carpenters with poor skills);

From thin blades to thick blades.

I still remember that when I was learning Ming-style furniture making, I bought a 70mm Japanese plane made by Dogyu. In Japan, people like to call it "Ao dogyu”. The blade is heavy and thick. It is much heavier and thicker than the Chinese plane I was using at the time (the Chinese blade is very thin, about 5mm thick and about 50mm wide). The Japanese plane feels more stable when cutting than the Chinese one, but it takes longer to sharpen the Japanese blade.

Most Japanese plane blades currently on the market, especially those produced in large quantities in Niigata or Miki, have thick blade and thick steel. What is the reason for this situation?

1: The cost of raw materials for making blades has been reduced. Imported steel (industrial production, stable quality, uniform structure) and imported soft iron have been used since the Meiji era. Compared with the blacksmiths of the previous era who used “和鉄/Watetsu" and “和鋼/wako", the current blacksmiths can directly enter the forging process of soft iron and steel. The blacksmiths no longer have to forge the rough steel raw materials.

2: The popularization of materials with excellent hardenability among imported steels, even if the blade is made thick, it can obtain the ideal hardness after quenching.

3: By making the blade thicker (the steel part is thicker too), and the steel is not easily worn out even if it is used for a long time by a carpenter with poor skills.

But the above three points are not the most important reasons.

First, although imported steel can save blacksmiths a lot of preliminary preparation work, it must have been expensive when it first entered the Japanese market;

Second, from the Jominy test of the three types of steel mentioned above, we can know that although alloy steel with excellent hardenability is easier to harden than carbon steel, the experiment shows that when the cooling speed is too slow to exceed a certain range, the alloy steel still cannot obtain the ideal hardness. Therefore, for the steel of the plane blade, whether it is carbon steel or alloy steel, the one that can be cooled quickly during the quenching stage is always the best choice. This can explain why some plane blacksmiths still retain the "nikuoki design" of thin planer from the “Watetsu&Wako”era. 

Thirdly, for carpenters with poor skills, thicker blades and thicker steel means it is more difficult to do “Uradashi”.

From Mr. Tsuchida's point of view, the main reason for the thickening of blades is: 

4: Thick plane blades are the product of blacksmiths conform to modern production methods.

In the era of “Watetsu&Wako”, blacksmiths used manual production methods to make blades, and making plane blades was more difficult than imagined. The main difficulty was how to ensure that the entire piece of steel was evenly attached to the iron when using a hammer to forge the red-hot and softened thin steel sheet with the soft iron, and not to cause large fluctuations in the forging surface between the steel and the soft iron due to the difference in the force of each hammering. In addition, only two basic hand tools, files and sen, can be used in the blade shape finishing stage. In order to save time for this cold working, the blacksmith needs to make the blade shape as close to the finished product as possible during the forging stage, leaving only a small amount of processing allowance.

• Tsuchida's article records how blacksmiths with highly skilled hand forging techniques work.

  「上手な鍛冶屋ほど火造り終わりが正確で、その黒い酸化膜を薄く、ヤスリやセンで除去するだけで焼き入れ工程に移行できたものである」

  "The more skilled the blacksmith, the more accurate the forging process. They only need to remove the thin black fire scale with a file or a Sen before entering the quenching process.” --「History of Shape and Plane」Tsuchida Noboru, 『Comprehensive Collection of Plane Techniques and Inscriptions』 p75

Fig.10: This blade was made by the Kunihiro III in the early 20th century. It is obvious that the fluctuation of the forging line is larger than that in the above figure, and the steel is thin on the left and thick on the right.

Fig.11: The two blades in the picture above are both works from the era of hand forging. The one on the left is from the late Edo period and has more accurate forging; the right one is Kunihiro's work from the early 20th century, and the forging is rougher, there are many hammer marks on the back of the blade, and the hammer marks at the lower end have been smoothed out by the file (because this part is in contact with the wood body).

Entering the Meiji era, with the application of various electric grinding machines and spring hammers (first used in a small number of traditional blacksmith shops from the Meiji period to World War II, and quickly popularized after World War II), blacksmiths gradually relied on the power of machines instead of relying on human skills to make blades. Compared with the cautiousness of blacksmiths in the forging stage in previous eras, the working method of blacksmiths using machines gradually changed to using thicker iron and steel, using spring hammers to easily forge rough-shaped blades with sufficient cold work allowance, and then using various grinding machines to grind them into shape.

It cannot be generalized that thick blades made by blacksmiths using machines are not good. Many excellent blacksmiths put the energy and time saved by using machines into other processes, such as developing blades with multiple types of steel for selection; stricter quality control (using scientific methods to check the steel structure after hardening), etc. In addition, the introduction of mechanized production lines and the use of alloy steel with better hardenability performance allow blacksmiths to mass-produce blades with stable and good quality. Users can buy them at a lower price (for example, the “ao dogyu”blade has received many positive reviews from users and are inexpensive, making them very cost-effective blades).

The trend that needs to be alerted is that the manufacturers only consider their own perspectives, making it more and more convenient for themselves to produce, without considering the users, and abandoning the excellent designs that predecessors have accumulated on the plane blades.

Unfortunately, these blades that are easy to make but difficult to use are becoming more and more common. As the weaker party in the extreme information asymmetry, overseas buyers (including me in the past) often mistake these relatively crudely made blades for high-quality "handmade blades”.

Plane blades that rely too much on mechanical processing and focus on productivity have the following characteristics:

• “身が厚く、鋼も厚く、背中の肉抜きが不十分であり、ウラスキは深く、一言で表現すれば、重い鉋刃が、現在製作されているものの特長と言えます。”

  "The blade is thick, the steel is thick too, the back is not sufficiently cut out, the “Urasaki” is deep. In short, the blade is thick and heavy, which are the characteristics of the blades currently being made.” - Tsuchida Noboru, "About Planes: Part 2, Plane Blade “nikuoki”

The above-mentioned heavy blades have the following disadvantages:
1: A blade that is too thick affects its flexibility as a hand tool;
2: The blade is thick (the steel part is thick), which makes it take longer to sharpen. If a steel with better wear resistance and higher hardness is used at the same time, it will be more difficult to sharpen；
3: The thick steel of the blade makes it more difficult to do “Uradashi”；
4: The deeper "Urasuki" made with a grinder increases the difficulty of to do “Uradashi”；
5: The back is not sufficiently cut out (close to flat) or rough processing, which affects the adaptability of the blade to the wood body and increases the difficulty of adjusting the wood body.

Fig.12: A thick blade produced during the Showa era. The blade's bevel shows traces of "uradashi" made with a hammer. The steel on the right side of the blade is much thicker, which makes it more difficult to make "Uradashi".

Fig.13: A thick blade produced during the Showa era. The “Urasuki” part is made using a large grinding machine, which results in a deep middle part of the "Urasuki".

Fig.14&15: A thick blade made in the early 21st century that only considers productivity. The back of the blade is slightly concave(the back is not sufficiently cut out).

Fig.16: A thick blade made in the Showa era with a less precise back. The silvery area is where the carpenter trimmed it.

But thick blades are not without merit, especially for planer blades with a width of more than 90mm. Making them thicker can effectively reduce the bending of the planer blade after installation due to the bending of the wide wood body. Chiyozuru Korehide and first generation Yoshihiro used the traditional thin type when making common width blades, but made them thick for large plane blade.

It seems that the trend of plane blades becoming thicker and rougher is irreversible.This reminds me of Gregory Bateson's point:

• "There seems to be something like a Gresham’s law of cultural evolution according to which the oversimplified ideas will always displace the sophisticated and the vulgar and hateful will always displace the beautiful. And yet the beautiful persists.”　- Gregory Bateson, Mind and Nature: A Necessary Unity. New Jersey: Hampton Press, INC, 2002, p.5.

But as the last sentence of the above point said, “And yet the beautiful persists”. But as the last sentence above says, "Nevertheless, beauty remains". There are still younger generations of blacksmiths like Keizaburo, Ishikosho and Metsugu who continue the excellent blade "nikuoki design" of the previo us era's excellent blacksmiths and are still making blades that are light and delicate, more difficult to produce and easier to use.

Fig.17: Two classic thin type blades that were not forged by machine. The left one is made by Metsugu in the early 21st century, and the right one is made by Kunihiro in the early 20th century.

Fig.18: The Metsugu blade on the left is more finely finished near the top of the blade.

Fig.19: The Ideal shallow "Urasuki" of Metsugu blades is made using the hand tool "Sen". The middle part of urasaki tends to be flat. It is easier to achieve results when doing "Uradashi".