Hamon (swordsmithing)

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 (iron) 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. In English, the terms "hamon" and "temper line" are sometimes used interchangeably, although subtle differences do exist, but both tend to refer to the entire line. In Japanese, however, "hamon" refers strictly to the pattern along the length of the blade, whereas the hamon at the tip () is called the . ==Types==

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" ( or ). 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 (e.g. , , , etc.), with many other differences sometimes added in for effect. resembled saw teeth, whereas resembled horse teeth. consists of multiple sizes and shapes of teeth mixed with areas of regularly sized and shaped teeth. appears as a combination of tooth and wave, resembling rows of plunging breaking waves. consists of waves with wide valleys and steep crests, and were mainly found on 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. originated with Osafune Kagemitsu and was carried on by Kunimitsu, whereas (very orderly and pointed peaks) 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 (as most ancient swords were) 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 , 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. ==Origins==

China was the first country to produce iron in Asia, around 1200 BC. The Chinese developed cast iron, and from this developed processes of making wrought iron, mild steel, and crucible steel. For a period of over 1000 years, from the first to the eleventh century, China was the world's largest producer and exporter of steel and iron. Nearly all metals then were imported into Japan from China, through Korea, including swords and weapons. Iron-making technology was typically a closely guarded secret, but was eventually imported into Japan from China around 600 or 700 AD, albeit with only a small amount of information to build on, so the ancient smiths began by trying to reverse engineer the Chinese methods, coming up with very different processes in the end. The Chinese swords had edges made of crucible steel similar to the metal found in Damascus swords, which were welded to a back of soft iron, to give both a hard and strong cutting edge but keeping the rest of the sword soft to prevent breakage. These produced a very hard and visible patterned-edge with a very visible transition at the weld, due to the different composition of the steels, despite the lack of any form of heat treatment. In tying to imitate the Chinese swords, the Japanese came up with unique processes and their own methods of creating a visible hardened edge, taking the Chinese methods and "refining them beyond recognition". The earliest swords forged in Japan ( and ) reflect the similarities between the Japanese and Chinese swords of the time. ==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 simpler 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 , which is a bright speckled line a few millimeters wide, following the length of the hamon. The is one of many features of Japanese swords that are sensitive to the viewing angle, seeming to appear and disappear when moved with respect to the light. Between the hardened edge and the hamon, the creates the actual boundary between the martensite and the pearlite, but is indistinguishable from the martensite in direct light. It is typically best viewed at long or grazing angles where it appears brighter than the hardened edge. The cannot be faked with etching or other methods. When viewed through a magnifying lens, the appears as a sparkly line, being made up of many bright martensite grains which are surrounded by darker, softer pearlite. A flame-hardened temper line is easily discernible from a true hamon, which produces a nioi that provides a very tough boundary between the martensite and the pearlite. Flame hardening lacks a nioi and instead produces a very brittle zone between the harder and softer metals, called the heat affected zone (HAZ), caused by internal stresses and extremely rapid cooling between the hot and cold areas. Therefore, while flame hardening and temper lines are common in hand-made knives, they are rarely found in swords or weapons where a lot of shear and impact forces may be encountered. == See also ==