Well of course it depends on the blade. But if we’re talking about Japanese swords, we have an excellent source with both historic and scientific information.
Just to make things interesting, let me translate an account by another Japanese swordsman. This passage appears in a book by ISHIGAKI Yasuzô, whose father and grandfather both were headmasters of one of the main lineages of the Jikishinkageryû. This tradition rose to prominence during the late Tokugawa period, and many of its students became involved in the duels, revolutionary and counter-revolutionary activities of that time. As a result, members of that school emphasized the importance of owning a properly fitted sword. Ishikagi’s book is titled: Jikishinkageryû gokui denkai (Revealing the Pivotal Points of the Jikishinkage Style of Swordsmanship; 1992, revised 2001).
Again, this translation is NOT presented as the gospel truth. I do not accept Ishigaki one hundred per cent. Nonetheless, since views similar to his are commonly voiced in Japan, I believe they are worth our consideration and comment.
This concludes my explanation of the gokui (pivotal points) and kuden (oral initiations) regarding the Jikishinkageryû’s “Tachi seisaku kokoroe” (Sword Fabrication Conventions), none of which have ever before been revealed to the public.
Before ending this section, though, I must emphasize that no matter how much care one might exercise in assembling one’s sword furniture, no matter how excellent the quality of the sword furniture used, it is totally pointless unless one fits them to a reliable sword blade that is free from flaws. A trained swordsman must select a wazamono (good cutting sword) that is balanced, free from defects, and properly forged from quality steel so that it will not easily bend or break.
Avoid striking with the back of the blade (mune uchi).
In chanbara (sword play) movies and television shows frequently depict scenes in which the hero flips his sword around and strikes with the back of the blade. Supposedly he does so to indicate that he has no intention of cutting (i.e., killing) human beings, but [in reality] he could never have sufficient numbers of blades to do so. Because swords are weapons designed for cutting, they are forged so that their strength lies in the cutting edge of the blades. The backs of sword blades are their weakest points.
The book Ten buyôron (Treatise on Swords for Military Applications) by the famous shin-shintô (New-New Sword Period; i.e., post ca. 1781) swordsmith, Suishinshi Masahide, describes numerous incidents when a sword broke from being struck on its back. I will briefly mention a few of them.
[Note: Suishinshi Masahide (a.k.a. Kawabe Masahide, 1750–1825) almost single-handedly began the New New Sword movement through his exhaustive investigation of and revival of Old Sword manufacturing techniques. He trained over a hundred disciples, many of whom became noted smiths.—W.B.]
(1) There was a man named Terada who lived in the same Akimoto Domain as Suishinshi. One night while at home Terada used the back of his sword blade to strike a burglar. The sword broke in half and the tip flew into the next-door neighbor’s house. It was a sword made by Mizuda Kunishige with a very wavy (ô midare) edge pattern.
(2) An underling in the same Akimoto Domain struck a dog with the back of his wakizashi. The wakizashi broke in half and the dog ran away. The sword was an unsigned blade with a very wavy edge pattern.
(3) One of Suishinshi’s disciples, a man named Kobayashi Masaoki, was talking to a retainer (i.e., samurai) named Motoyama from the Awa Domain when he happened to mention that long shintô (New Swords; i.e., blades manufactured between ca. 1570 and 1780) break very easily. Thereupon, Motoyama said, “If that is so, then here, take my katana and try to break it,” and presented Kobayashi with his sword. When Kobayashi struck the back of the blade against one of the stones in the garden, it snapped like an icicle.
(4) The Awa Domain retainer named Motoyama had tested the cutting ability of swords produced by various smiths, but he had never tested them for durability. Acting under orders from his lord, he proceeded to test a wide variety of swords, including ones that had been made by Inoue Kunisada, Echigo-no-kami Kanesada, Osafune Sukesada, Suishinshi Masahide, and many others. When the back of blades were struck against one another (i.e., mune to mune), all of the long swords broke and almost all of the short swords cracked along their cutting edges.
(5) A porter in the Okayama Domain used the back of his sword to strike a thief. The sword blade broke in half. Thereupon, the porter picked up a bamboo pole and used it to knock the thief off of his feet. The porter tied up the thief, and inspected him for injuries. The thief had a big bruise where he had been struck by the bamboo, but no mark could be seen where he had been hit by the back of the sword blade.
(6) A warrior named Nagai in Kôzuke Province tried to test his sword by striking the back of the blade against the side of his house. With the first blow the sword broke into three pieces. It was a long sword made by Kawauch-no-kami Kunisuke.
The above incidents relate eye-witness accounts of swords breaking when struck on their backs. In addition, there is the well known story of Araki Mataemon of the Yagyû Shinkageryû. In 1634 when Araki and his brother-in-law, Watanabe Kazuma, were attacked by Kawai Matagorô and Sakurai Hanbei, Araki’s sword broke in half as a result of being struck on its backside. The sword was a wazamono that measured 2.77 shaku (about 84 cm) made by Rai Kanemichi.
During the war years durability tests were conducted on military swords. In 1943 I witnessed a public demonstration of these tests outside a department store near Ueno Park in Tokyo. The test consisted of dropping a iron ball about 50 cm in diameter and weighing about three kanme (about 11 kg) onto the center of a sword blade to see if it would bend or break. The results of this test showed that if the weight was dropped on the cutting edge of the sword, hardly any damage could be observed. But the sword broke easily when the weight was dropped on its back even if only from a height of 15 cm. It demonstrated just how weak the back of a sword blade is.
Students of kenjutsu should know that even if they select a wazamono that is balanced, free from defects, and properly forged from quality steel so that it will not easily bend or break, that the back of the blade still is its weakest point and that, therefore, they must not imitate the actors in movies and television shows.
The end of Ishigaki’s remarks.
Dept. of Asian Languages & Cultures