In swordsmithing, hamon (刃文 hamon) (from Japanese, literally "blade pattern") is a visual effect created on the blade by the hardening process. Blades made in this manner are known as differentially hardened, with a harder ha than mune (for example: mune 40 HRC vs ha 58 HRC) as a result of clay applied on the blade during the cooling process. Less or no clay allows the ha to cool faster, making it harder but more brittle; more clay allows the mune to cool more slowly and retain its resilience.
The hamon is the transition between the region of harder martensitic steel of the blade edge and the softer pearlitic steel of the back of the sword. This difference in hardness is the objective of the process; the appearance is purely a side effect. However, the aesthetic qualities of the hamon are quite valuable—not only as proof of the differential hardening treatment but also in its own right—and the patterns can be quite complex.
Many modern reproductions do not have natural hamon because they are thoroughly hardened monosteel; their hamon is faked via various processes such as acid etch, or more crude ones such as wire brushing. Some modern reproductions with natural hamon are also subjected to acid etching to enhance that hamon's prominence.
According to legend, Amakuni Yasutsuna developed the process of differentially hardening the blades around the 8th century AD. The emperor was returning from battle with his soldiers when Yasutsuna noticed that half of the swords were broken:
"Amakuni and his son, Amakura, gathered up the broken blades and examined them. They were determined to create a blade that would not break in combat and locked themselves away in seclusion for 30 days. When they reappeared, they carried with them the curved blade. The following spring there was another war. Again the soldiers returned, only this time all the swords were intact and the emperor smiled on Amakuni."[1]
Although impossible to ascertain who actually invented the technique, surviving blades by Yasutsuna from around 749-811 AD suggest that at the very least Yasutsuna helped establish the tradition of differentially hardening the blades.[2]