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What is a Hamon Line?

In the world of swordsmithing, the Hamon (which literally translates to ‘blade pattern’) is created on Katana as part of the differential hardening process. The Hamon indicates and outlines (yakiba) the hardened edge of the blade (ha). A sword which has been clay tempered, also know as differentially hardened are treated in this way in order to create a harder cutting edge and softer more flexible spine (mune). For example, a blade may be treated in order to create a cutting edge that is 58 HRC while the spine is 40 HRC, which is a substantial difference in hardness.

Differential hardening is achieved by applying a layer of clay to the blade before heating and cooling has been performed. Adding more clay to the spine and less or no clay to the edge will mean the edge will cool much faster than the spine once the blade has been quenched. The speed at which the metal cools creates different crystalline structures within the steel, a faster cool will create a tougher more rigid structure, but a longer cooling process allows for softer more flexible steel to be produced. Creating differences in flexibility and toughness allows for a blade that will remain sharp as well as being flexible enough to take a blow without snapping.

The Hamon line marks the transition between the tougher martensitic steel on the blades cutting edge and the more flexible pearlitic steel that makes up the swords spine. The objective of the process is to create this difference in hardness, the appearance of the Hamon line is only a side effect. However, the aesthetic appeal of the Hamon line should not be underestimated, not only as proof that the blade has undergone the clay tempering process but they are also artistically interesting. The complex patterns created within the Hamon are an appealing point for anyone considering purchasing a Katana.

Many modern Katana reproductions do not have Hamon line achieved through clay tempering process as the swords are produced from already hardened monosteel. Instead, the Hamon line may be reproduced through artificial means such as sandblasting, acid etching and low tech approaches such as wire brushing. Even if a modern day sword contains a genuinely produced Hamon, the effect may be further enhanced by acid etching to further emphasise the difference between the two types of steel.

A real Hamon can normally be identified by inspecting the Hamon line which will contain bright specks called ‘nioi’. The nioi is best viewed by looking along the plane of the blade and it cannot be currently be produced through artificial processes. If viewed through a microscope, the noio will appear as sparkly crystalline martensite grains surrounded by the darker in appearance pearlite.  All the swords we sell have a section that will tell you if the Hamon is artificially produced or the real thing.

History of the Hamon Line

According to the legend of the Hamon, a swordsmith called Amakuni Yasutsuna developed the process of differential hardening during the 8th century AD. The tale goes that when the emperor returned from battle with his soldiers in tow, Yasutsuna noticed that over half of the sword where broken. Amukuni with his son, Amakura, upon examining the broken weapons, vowed to create a weapon that would not break so easily.

Amakuni and Amakura then secluded themselves away for 30 days to work on the problem. When they next appeared, they had produced the legendary curved blade that would eventually evolve into the Katana. During the spring of the following year, another war was upon them. This time when the soldiers returned from battle not a single sword was broken. This feat of engineering carried Amakuni great favour with the emperor.

Even though it’s impossible to collaborate this legend and determine who exactly invented the clay tempering method, blades produced by Yasutsuna and date back to the 8th-century display Hamon lines, suggesting that even if he didn’t pioneer the methodology, he was at the forefront of its adoption.

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