Source: In common use
Hamon (刃文) literally means "edge pattern". It is the demarcation of the hardened edge and the softer body of a blade, often a greyish, whitish, or lighting up in the right light. It consists of martensite crystals that were formed by sudden cooling of the high-carbon steel.
A Japanese sword made in the Mino tradition. The light gray wavy line along the edge is the hamon.
The primary function was of course to create a blade with a very hard edge on a tougher body. A hard edge can be polished sharper and retains its edge longer. However, it is also brittle, so it is not desirable to harden the whole blade like that. A tougher body ensures the sword will not snap easily.
In traditional Japanese bladesmithing, the hamon is achieved by applying clay to the blade, heating the blade, and then quenching it in water. The process is called yaki-ire (焼入れ). Those areas where clay is applied thicker will cool slower, while those coated with a thin layer or no clay at all will cool at a very fast rate, creating hard martensite and pearlite chrystals in the steel.
Nie & nioi
When the crystals are so small that the naked eye cannot make them out individually, and they appear like a whitish mist, it is called nioi (匂), literally "fragrance". Nioi is present to some extent on all blades, but when no or very little nie is present, we speak of nioi-deki (匂出来). When the work shows nie throughout, we call it nie deki (沸出来) where deki means workmanship or interpretation.
Left: The misty temperline on a Bizen sword in nioi-deki. Made by Yosōzaemon jo Sukesada in 1523.
Sold by Mandarin Mansion in 2021.
Right: The grainy nie particles on a Sōshū sword in nie-deki. Attributed to Tametsugu, active in the 14th century.
Mandarin Mansion inventory 2022.
Polishing the hamon
Japanese swords go through a basic polish called shitaji where the blade is ground to shape. In this stage, factes and bevels are shaped properly and the edge is sharpened. The second stage is called shiage where the details in the steel are brought out by polishing with very fine stones. The shiage stage is where the polisher makes specific choices on the finish of the blade. There are basically two types of finish: sashikomi and hadori.
The sashikomi is the more traditional of the two, also known as the "classical finish" and involves polishing the entire sword with a very fine grit stone. It shows all the details of the sword, but does not emphasize anything in particular and the effect can look a little subdued.1
With the hadori polish, the outline of the hamon is brightened using uchigumori polishing stone which is slightly coarser than the finishing stone. Today, it is the most common finish, not in the least because it makes the hamon much easier to photograph.
This kind of polish probably started with the Hon'ami family of sword polishers, most likely Hon’ami Ringa (本阿弥琳雅, 1859-1927) or Hon’ami Kōson (本阿弥光遜, 1879-1955). It marked an increased emphasis on the Japanese sword as an art form.2
Hadori polish is also known as keshō (化粧), meaning "cosmetic".3
An uchigatana blade made in the famous sword-making town Osafune, Bizen, 1523. Signed:
Bishū Osafune Sukesada
Daiei 3rd year 8th month (August 1523)
The photo shows the outline of the hamon from the side, whitened by a hadori polish.
When the same sword is viewed from a different angle it shows the true form of the hamon.
The light bouncing off the blade under this angle make the chrystals formed in the steel light up brightly.
Types of hamon
Over time, the method of applying the hamon turned into an art form with various regions, schools, or sometimes even individual smiths creating their own distinct hamon. Some of the most common types include the suguha hamon which is straight, gunome hamon which is wavy, midare hamon which is irregular, and choji hamon that looks like a row of cloves.4
1. Markus Sesko; Encyclopedia of Japanese Swords. Lulu, 2014. Page 91-92.
2. Markus Sesko; HON’AMI KŌSON (本阿弥光遜).
3. Markus Sesko; Encyclopedia of Japanese Swords. Lulu, 2014. Page 378.
4. Kōkan Nagayama; The Connoisseurs Book Of Japanese Swords. Kodansha America, Inc. 1997.