The term used to describe a decorative temper line created by a specific heat treatment process. The word itself is a loan word from Japanese, as most Japanese blades from even before the Heian Era were tempered this way. While in traditional tempering the blade edge is quenched in a tank of water or oil to reduce the temperature rapidly to form a straight temper line, the traditional Japanese method of tempering requires most of the blade to be first coated in a heat resistant clay. The clay, applied to the spine and flat of the blade increases the heat-up and cool down-time of the metal and the pattern is formed at the border where the bare steel blade is quenched faster than the area coated in clay. The Hamon effect may be enhanced with chemical etching using ferrochlauric acid.

Of course the temper is not just decorative, and functions just as a straight temper line would. Also, owing to it's highly decorative nature, there are many cheap immitations of this process including both chemical and lazer etched hamon patterns which are wholly superficial.