Source: In common use
Nioi (匂) literally means "fragrance". In Japanese sword connoisseurship, it is the name of the misty effect of heat treatment at the blade's temperline. They consist of tiny martensite crystals that are too small to see individually. The temperline is thus also called the nioiguchi (匂口).
Hard but brittle martensite is formed when steel with a high carbon content is heated to a high temperature and then rapidly cooled, and carbon gets trapped in a crystal state.1
A nioi-based hamon on a 16th-century sword of the Bizen tradition.
1. For the science behind the martensite, see: www.metallurgyfordummies.com.
Nie & nioi
All temperlines consist of nioi, but sometimes also larger crystals are present which are referred to as nie (沸). The larger nie crystals are formed when the initial temperature is higher before cooling.
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.
The misty temperline on a Bizen sword in nioi-deki.
Made by Yosōzaemon jo Sukesada in 1523.
Sold by Mandarin Mansion in 2021.
The grainy nie particles on a Sōshū sword in nie-deki.
Attributed to Tametsugu, active in the 14th century.
Mandarin Mansion inventory 2022.
Aspects of the nioiguchi
The outline of the nioiguchi is called hamon (刃文), and there are hundreds of types, some of which typical for individual schools or smiths, others are in more widespread use.
The width of the nioiguchi can be described as:
Nioi-shimaru (匂締まる) "tight"
Nioi-fukai (匂深い) / Nioi-fukashi (匂匂深し) "wide"
The brightness of the crystals when the light bounces off them at an angle can be described as:
Yawarakai (柔らかい), "soft"
Akarui (明るい), “bright”
Saeru (冴える), “clear”. or “dull, subdued”
Shizumu (沈む), “dull, subdued”
Judging the nioiguchi
The nioiguchi is one of the most important elements in judging the skill of the smith and the quality of a Japanese sword. The Japanese sword is basically a canvas on which the smith "paints" his distinctive hamon. The shape of the hamon, and whether the nioiguchi it is tight or wide, bright or subdued, are matters of taste, not of quality.
To assess the quality of a nioiguchi, we check the consistency of the hamon from base (hamachi) to tip (kissaki), into the boshi. Whatever form the nioiguchi takes, it should appear with the same level of brightness and width over the entire blade.
The above terms appear in many sword publications, but to me, the most useful has been Markus Sesko; Encyclopedia of Japanese Swords. Lulu Enterprises, Inc. 2014. Available from Lulu.com. Also important for the study of Japanese sword are Sesko's authoritative Kantei Series where he goes more in-depth on the different features.
Another valuable resource is Kōkan Nagayama; The Connoisseurs Book Of Japanese Swords. Kodansha America, Inc. 1997.