Article on Katsumura Norikatsu blade metallurgy

[AlexiG]

As of late I have become fascinated with Norikatsu's work and while looking for information I found this article which I personally found very interesting so sharing here. I used google translate mostly so apologies for the poor translation. It seems that the construction is 3 plates (high carbon in the middle, and two composite plates on the side). At first I thought it was kobuse but does not seem to be.  Also fascinated to read the 4 levels of testing to demonstrate suitability for practical use.

 

Happy reading,

Alexi

 

Link: 水戸の豪刀・勝村徳勝の刀身構造

 

Quote

The turmoil of the Bakumatsu period awakened the Japanese sword from the long slumber of the peaceful Tokugawa era. It reminded us that the Japanese sword, which had become a decorative weapon for samurai, is originally a weapon for fighting. As is passed down from the famous Aratameshi of the Matsushiro domain, each domain established performance criteria for the Japanese swords they kept and conducted strict tests. One of them was the Mito domain, known for its strong emperor-respecting spirit. The standards for passing Japanese swords set by the Mito domain are as follows:

1. Stick Test (Strike both sides of the sword held in a standing stance with an oak stick about 5 cm in diameter. The spine is struck similarly. Since the sword is weak at the spine, if it is poorly forged or poorly heat-treated, it will break here. Next, the blade is struck diagonally to check for chipping or bending.)

2. Wrapped Straw Test (Cut wrapped green bamboo a few times to assess the sharpness of the blade.)

3. Deer Antler Test (Conducted several times, also includes cutting iron plates used in armor. If the blade's thickness is reduced, it cuts wrapped straw well, but when cutting hard objects, the edge chips. Thus, satisfying both conditions of the second and third tests becomes a difficult task.)

4. Water Test (Fill a large barrel with water and strike its surface with the flat of the sword dozens of times. Poorly made swords will bend or break after a few strikes.) Only the swords that passed these strict tests were adopted as the standard swords of the Mito domain. For swordsmiths accustomed to the swords of the peaceful era, this must have been quite bewildering. However, due to the demands of the times, each swordsmith bravely challenged themselves to create practical swords.

Among the Mito domain, Katsumura Norikatsu and his disciples are esteemed for their practical swords. Katsumura Norikatsu, born Katsumura Hikojuro to a samurai family of the Mito domain in 1809, initially learned from Sekiguchi Norimune  and inscribed his name "Norikazu 徳一." Later, in 1857, he was recommended to be a swordsmith for the Mito domain and established his residence in the Edo area of Kōishikawa, receiving guidance from Unju Korekazu and Hosokawa Masayoshi. He is also known for serving as a sword forging partner for the ninth lord of the Mito clan, Tokugawa Nariakira (Mito Reppō). His swords were cherished by the Mito domain samurai amid the rising momentum of the Sonno Joi (Revere the Emperor, Expel the Barbarians) movement and are famous for being the sword used to attack Tairo Ii at Sakurada Gate.

 

Recently, Takao Onoue, the president of Onoue High-Temperature Engineering, a prominent figure in the sword-making world of Seki City, analyzed the cutting blade of Katsumura Norikatsu.

 

Analysis of the cutting blade

Inscriptions: Made by Suifu resident Katsumura Norikatsu

Reverse inscription: August, 1867 (the year of the great restoration).

Blade length: 28 cm, tang length: 24.2 cm remaining, texture: Masame (straight grain).

 

Blade Structure * * Darker parts = high carbon (hard steel), lighter parts = low carbon (soft steel)

Analyst's observation: Demonstrates a complex forging technique and material combination to produce a wood grain pattern on the blade. The edge is slightly softened. Heat treatment with water quenching at low temperature. Overall, the upper forging appears simple and uses a combination of forged plates from the lower forging stage. Photomicrograph taken after applying a chemical etching solution (such as Nital). The dark colored areas represent high carbon regions, the light colored areas represent low carbon regions, the intermediate tone areas represent medium carbon regions, and the black areas represent non-metallic inclusions. The regularly arranged small black dots are indentations made by the Vickers hardness tester for hardness measurement.

 

Building side section

Central section

Blade side section 50x microscope photo: Since the area captured in high magnification microscope photos is small, the sword blade cross-section is generated by stitching together the photos of smaller areas.

 

Analyst's Viewpoint100X (times) Medium carbon materials within the burned blade organization. Distribution of carburized iron within MnS (sulfide), silica components. Is it possible to achieve such uniformity with short-term heating? *200X 50X Coarse needle-like malten site and inclusions in low carbon area. The diamond shape is the indentation trace of the hardness tester. 500X Western steel?

 

*Note from the author: The secret to creating a resilient sword is said to be found in low-temperature forging and completing the work in a short period of time. Please also refer to this regarding resilient blade structures.

Comprehensive Analysis Overview. What are the forging methods of the material and the rolling methods of the thin plates? Combined with the rolling direction of the layered forge welding, what are the expectations and designs of the skin? Has hot quenching not yet been passed? In quenching, it has also been treated at high temperatures for a short period, and the thin plates made of water-cooled iron (tatara iron) were used for the small plates of armor, but thicker items were not observed, which were simply layered for use. The fineness of the pearlite layer is nearly impossible with 'tatara iron.' The diffusion of carbon (Author's note: the transition of carbon between high and low carbon concentrations) is minimal, and hardly any interlayer movement is recognized. Further detailed investigation and research are necessary.

 

*Author's note: This time, the structure of the Norikatsu sword blade has been clarified. It has a significant feature of alternating forging thin plates of hard and soft steel and placing a thick composite material (a kind of hard and soft steel mixture) on the side of the blade (skin iron part?). A thin core-like iron is sandwiched in the central gap of the blade. The thin plate located in the center, judging by the carbon concentration distribution, seems to have used slightly harder steel. Although a viewpoint that this construction is a deformed version of the traditional three-layered structure has emerged, it may be difficult to apply it to the typical construction methods of new and new-new swords. The analyst commented on the 'complex forging techniques and material combinations to showcase a straight grain (masame) texture,' but it seems unlikely that Norikatsu aimed to bring out a forged texture while forging a durable practical sword. The reality may be that the surface texture accidentally exhibited a straight grain as a result of ingenuity in combining hard and soft steel to achieve toughness. It is believed that, especially after the Shinto period, the trend of constructions in the ShinShinto sword period was predominantly a skin-core iron structure, but swordsmiths aiming for strong swords pursued various unique innovations without being bound by fixed notions. It seems to have suited Tokugawa Nariakira (徳川齋昭), who was known to favor straight grain forging.

 

No analysis of the steel material was conducted this time. The analyst speculates from microscopic photographs of the pearlite layer that it does not use Japanese steel. It would also be desirable to know the steel used. Norikatsu was instructed by Ishido Unju Ichiyo and Hosokawa Masayoshi, so perhaps the blade structures of Ishido and Hosokawa were similar to these. An example similar to this blade structure exists, known as the "Zantetsuken," from swordsmith Kobayashi Yasuhiro.

 

*Note from the author: The HRC (Rockwell) hardness of the blade surface was measured. The hardness of the edge part (HS, Shore hardness) is 57 for the ancient sword Muramasa, 57 for the Manchurian Railways sword, and about 68 for the new sword Suishinshi Masahide. When converted to HRC hardness, the ancient sword equals 42.7, and the new sword is approximately 51. The surface hardness of Norikatsu’s sword was measured at 19 to 21 locations at 0.5 mm pitch from the edge. The area near the blade shows 53.5 to 43, the central part is 26 to 27, and the ridge shows 34 to scale over. The area near the blade is the hardest and harder than the Suishinshi Masahide sword. The central part is softer, and hardness increases from the ridged area to the top. The distribution of this blade hardness can be likened to the blade of Seki's Masamune, which arranged hard steel at the edge and ridge. This might be a coincidence resulting from striving for a practical sword across hundreds of years in time and space. Even from this distribution of surface hardness, it can be understood that it differs from the typical hon-sandai structure of new swords. This analytical material was obtained by Mitsuki Sobue, the secretary of the Modern Japanese Sword Research Association, and provided to the author.