In swordsmithing, Hamon is a visible effect created on the blade by the hardening process. The hamon is the outline of the hardened zone which contains the cutting edge (ha). Blades made in this manner are known as differentially hardened, with a harder cutting edge than spine (for example: spine 40 HRC vs edge 58 HRC). This difference in hardness results from clay being applied on the blade prior to the cooling process. Less or no clay allows the edge to cool faster, making it harder but more brittle, while more clay allows the center and spine to cool slower, thus retaining its resilience.

The hamon of a blade is created during the quenching process. During the differential heat treatment, the clay coating on the back of the sword reduces the cooling speed of the red-hot metal when it is plunged into the water and allows the steel to turn into pearlite, a soft structure consisting of cementite and ferrite laminations. On the other hand, the exposed edge cools very rapidly, changing into a phase called martensite, which is nearly as hard and brittle as glass.

The hamon outlines the transition between the region of harder martensitic steel at the blade's edge and the softer pearlitic steel at the center and 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 artistic value—and the patterns can be quite complex.

The shape of the hamon is affected by many factors, but is primarily controlled by the shape of the clay coating at the time of quenching. Although each school had its own methods of application, and kept secret the process, the exact mixture of the clay, the thickness of the coat, and even the temperature of the water, the clay was usually applied by painting it on in very thin layers, to help prevent shrinking, peeling, and cracking as it dried. Often, the clay is applied to the entire blade by piling up the layers very thickly over the entire sword, and then the clay was carefully cut away from the edge. However, in ancient times tempering was rarely used in Asia, and a fully exposed edge would cool too fast and become far too brittle, thus a thinner layer of clay was usually applied to the edge so as to achieve the correct hardness upon quenching without the need for tempering afterwards.

The smith shapes the hamon at the time of coating the blade. There are two basic styles, which are "straight edge" and "irregular pattern". Straight-edge hamons simply follow the edge of the sword with little deviation, except at the tip. This was by far the most popular style in every era and in every province, whereas the more complex patterns that were in themselves works of art tended to be reserved for the wealthy and elite. Straight patterns are usually classified by the width of the hardened zone, and divided into "wide", "medium", "narrow", and extremely narrow or "string" hamons.

Conversely, irregular hamons do not simply follow the edge, but deviate from it considerably in various ways. The two main groups are "undulating" or "wavy" and tooth-like or "zig-zag", and these are often classified by the wavelength or breadth of the irregularities. Sometimes hamons can consist of one style, a mixture of two, or all three, with many other differences sometimes added in for effect. Kataochi gunome resembled saw teeth, whereas uma-no-ha gunome resembled horse teeth. Fukushiki gunome consists of multiple sizes and shapes of teeth mixed with areas of regularly sized and shaped teeth. Koshi-no-hiraita midare consists of waves with wide valleys and steep crests, and were mainly found on Bizen swords of the Muromachi period.

The specific shape and style of the hamons were often unique and served as a sort of signature of the various swordsmithing schools or even for individual smiths that produced them. Kataochi gunome originated with Osafune Kagemitsu and was carried on by Kunimitsu, whereas togari gunome were mainly found on swords of the Sue-Seki school. On the most ancient swords, the hamon typically ended just before the sword guard, but on most later and contemporary swords the hamon extends far past the guard, under the handle, and ends with the tang, which provided added strength to the tang.

The shape of the hamon is affected by other factors as well. If a sword is made of a composite steel consisting of alternating layers of steel with different carbon contents, then the steel with higher hardenability will change into martensite deeper underneath the clay coating than the lower-carbon steel. This leaves a pattern of bright streaks that jut a short distance away from the hamon, called niye, which give it a wispy, misty, or foggy appearance. Likewise, complex swords that consist of sections of different steels welded together may show evidence of the welds near the hamon.

History of the Hamon

Like many other aspects of traditional Japanese swordsmithing, the exact origins of the hamon remain shrouded in mystery. However, legend has it that famous swordsmith Amakuni Yasutsuna -- who created the first single-edged sword with a curved blade -- pioneered this practice sometime around the 8th century. The story goes that Yasutsuna saw the emperor's army with badly damaged swords. In response, he and his son, Amakura, gathered the blades and began repairing them. After about a month, the father-son swordsmithing duo emerged with newly forged swords that featured differential hardening along with the characteristic hamon designs.

Modern reproductions

Many modern reproductions do not have natural hamon because they are thoroughly hardened monosteel; the appearance of a hamon is reproduced via various processes such as acid etching, sandblasting, or more crude ones such as wire brushing. Some modern reproductions with natural hamons are also subjected to acid etching to enhance their hamons' prominence. A true hamon can be easily discerned by the presence of a "nioi," which is a bright, speckled line a few millimeters wide, following the length of the hamon. The nioi is typically best viewed at long angles, and cannot be faked with etching or other methods. When viewed through a magnifying lens, the nioi appears as a sparkly line, being made up of many bright martensite grains, which are surrounded by darker, softer pearlite.

There are many types of Hamon with various shapes but the main shapes are the straight Hamon lines or the wavy Hamon lines.

Here are some patterns that exist

Suguha

This Hamon is in a straight line parallel to the edge of the sword. This type of Hamon is very old and is present on the most ancient Katanas, it has remained the most classical shape. There are variations of this Hamon because it has been much appreciated and used by many smiths like the hiro-suguha or chu-suguha or hoso-suguha.

Notare

Slightly wavy Hamon

Gunome

The Gunome represents a series of semi-circles referring to the stones used in the traditional game of GO, seen from the side. The name of this hamon is therefore directly inspired by the game of GO. Then, with the Kamakura period various hamon based on Gunome were produced like gunome choji, gunome kataochi or gunome kaku.

Midare

Notare

This Hamon looks like a light wave of water. This hamon started to appear at the end of Kamakura period, the founder of this hamon is the famous Masamune. This hamon became one of the most popular and influenced many blacksmiths, especially in Okinawa.

Ko-Midare

Ko-Midare Hamon is a hamon with small irregular and very complicated patterns. This Hamon line was often represented on swords forged at the end of the Heian period, it is meant to stand out from the traditional Hamon with few shapes.