Source: In common use
Kōbuse (甲伏せ) literally means "armor covered" in Japanese. Gitae (鍛え) means "forging". It is used to describe a blade construction where an outer jacket of high-carbon steel called hagane (刃鉄) or "edge steel" is folded over a softer, lower carbon steel or iron core called shingane (心鉄), "core steel".
It is one of many ways to make a sword with a hard but brittle edge and a more resilient body.
Kōbuse construction method.
Two iterations of kōbuse.
Left the hagane (edge steel) ends in the shinogi-ji (surface above the ridge) in which case sometimes a welding line can be seen.
Right the hagane runs to the mune (spine). Often unseen, but sometimes one can make out the welds.
Another method to achieve basically the same was makuri-gitae (捲り鍛え), literally "rolling forging".
It is done by putting a plate of lower carbon core steel on a plate of edge steel, and folding it towards the core steel until it meets.
Pros and cons
Kōbuse is the most common construction for Japanese swords and, when done well, can make a very strong sword.
Pro: The high carbon outer jacket serves as a canvas on which the smith can "draw" his artistic hamon or "temperline". These heat-treating effects were raised to a high art form by the Japanese since medieval times.
Con: A downside of the construction is that with much polishing, the hard steel wears down until one eventually reaches the core steel. Other constructions like sanmai (三枚) do not have this problem.
Hamon on a Sōshū sword.
Mandarin Mansion inventory 2022.
Kōbuse and makuri-gitae are by far the most common construction method seen in antique Japanese swords. It is less labor intensive than the more elaborate constructions yet makes a fine sword with a hard, durable outside.
As mentioned, the high carbon steel jacket can form martensite crystals everywhere and thus works as an excellent canvas for the smith to create the most fantastic tempering effects in ha, hamon, and ji.
Commonly seen on the much coveted Bizen and Sōshu swords, schools both known and loved for spectacular heat treatment effects. In the case of Sōshu swords, the high carbon steel jacket itself would often consist of alternating layers with more and less carbon, which in quenching created lines with more or less nie particles in arrangements that resemble brushed sand called sunagashi (砂流し).
Sunagashi and ji-nie on a sword attributed to Den Tametsugu.
Mandarin Mansion inventory 2022.
Kōbuse and makuri-gitae became more common in other schools as well during the Sengoku period (1467–1615) when a constant state of civil war drove up demand for swords and called for efficient production methods.
Shin jūgomai kōbuse (真十五枚甲伏)
Around the Kanbun period of 1661-1673, some smiths started to mention a special 15-layer kōbuse construction, inscribed on the nakago (tang) of their swords. This seems to have started with Mito-master Ōmura Kaboku (大村加卜).1
The phrase found on these is:
shin jūgomai kōbuse saku
"True 15 sheet kōbuse made"
The 15 sheets probably refer to layers in forging, and Markus Sesko translates it as: "made in kōbuse technique by using 15-times folded steel". He believes the word shin may refer to the character 心 pronounced the same way, and that it's short for core steel.
The method was adapted by many of Ōmura Kaboku's students and remained in use by some smiths into the 19th century.
The most well-known late smith who sometimes made swords in this technique was Suishinshi Masahide (水心子正秀). He lived from 1750-1825 and was the driving force behind the Shinshinto era of sword making, marking an attempted revival of the styles of Heian and Kamakura period sword making.
He would also refer to 15 layers in a slightly different way:
kore shin jūgomai kōbuse-gitae
"... using 15 folded kōbuse forging" 2
1. Markus Sesko; Swordsmiths of Japan A-Z. Lulu Inc. 2015. (Available for purchase here.)
Glossary article: Shin jūgomai kōbuse (真十五枚甲伏)
Article: Construction methods of Japanese swords