HAMON on Japanese swords

Jamon is a narrow hardening line, it is visually visible on the blade of a Japanese sword. This line (jamon) separates the hard edge of the saber from the softer one. To make such a saber, a process called differential hardening is used.

Essentially, starting with the forged and straight blade, the back of the handle is covered with a special clay mixture, so that when heated and hardened, the blade cools much faster than the rod, turning the steel in the jamon area into a crystalline structure. This structure is called martensite and in Japanese the large, easily distinguishable crystals are called Nie, and the smaller ones are called Nioi.

This process is a very important point in the hardening of the katana. The hardening process itself is a delicate jewelry work.  There's no other way to call it. There is always a risk of doing something wrong. For example, if a ringing sound appears during hardening, then this is already considered a defect and the blade cannot be used. That's why a katana with real hardening lines tends to cost more than a single temperature (i.e., the same hardness along the entire length) sword.

There are many types of hardening lines, but two main classifications are identified from them – Sugukha (straight) and Midare (wavy). One blade can combine several styles depending on the personal taste and skills of the master who creates it.

A well-made jamon that is visually appealing is a sure sign that the sabre itself is well made. The correct hardening line is characterized by clear crystalline shapes, clean lines, and lively "activity." And it is also desirable that the line is visible when light hits. Such a line can be very difficult to photograph. The line then appears and disappears depending on how the light enters it.

The process of creating a real hardening line is much more serious than just making it aesthetically beautiful. Martensite crystals make the tip much harder, but also somewhat brittle (about 60 HRC). Compared to perlite crystals, they are soft and absorb shocks to some extent (usually around 40 HRC).

This hardening, when properly performed, should run through the entire surface of the blade, and not just look beautiful on the surface.

The photo clearly shows how the blade looks after the test. You can see how the crystal structures of martensite penetrated through the entire blade.

One of the positive aspects of hardening a saber by this method is that it allows the blade to have a sharper tip, while avoiding excessive fragility of the entire blade or being broken in half when colliding with a harder target. The harder blade, even if it's not particularly sharp, also exhibits superior cutting ability on softer targets like bamboo, tatami mats, etc., and generally makes the blade better. You can literally feel this difference, since it is easier to apply cutting blows with such a saber, with a hardened blade.

But there are downsides. Because the blade has two different levels of hardness, it tends to remain bent at lateral stress (sideways). While the katana (at good hardening), with a mono temperature without jamon, will tend to return to its original form.

But this is not as big a problem as many people think, since the saber can be straightened again and this should not cause any real problems. If, of course, if the sabre is straightened several times, then the steel ties may weaken at the bending point.

Real Katanes and Fakes

Due to the fact that it is very expensive to make a katana with the right jamon: to find a sufficiently qualified master for manufacturing and minimize defects during manufacturing, fake, purely cosmetic hardening lines have appeared. Such sabers were not subjected to differential hardening (i.e. monotemperation). There are several methods how to fake jamon. Below is a video I shot on site in a forge in China that shows one of the popular methods.

While some fake jamons are fairly easy to spot, others are much more inconspicuous.

Some people think that one way to determine a real jamon is to try to polish it.  Thinking that the hardening line is an indicator and should be visible when polishing.

However, this is not actually the case. Polishing the blade is an art in itself, and many of them are so keen on acid to reveal details. But it is possible to completely hide the real hardening line using standard or less advanced polishing methods. To properly highlight the details, hybrid polishing is required.

Thus, for an inexperienced collector who does not know how to reliably check whether a line is real or not, the only method is to use the Rockwell hardness method. Check the relative hardness of the blade and butt, if they are the same, then the hardening line is definitely not real.