Quickly dipping a heated katana blade in a mixture of clay and charcoal at the mune and shinogi-ji (blade side) leaves a trail along the edge of the blade called Hamon.
This hardens the cutting edge, making it fearsomely sharp. The various schools of blacksmiths use typical Hamon by drawing the desired pattern in clay.
How does the Katana’s Hamon appear?
Hamon is created by thermal shock during partial/selective tempering. The purpose of selective tempering is to harden the katana at the bottom of the blade along the cutting edge (hasaki or ha). The rapid cooling of steel makes it hard and sharp, but also more brittle. So, to maintain flexibility and guarantee a blade that is both tough and sharp, clay is used to slow down the cooling of the steel at the back and side.
This technique is called selective tempering. As water is a better thermal conductor than the clay layer, the bottom of the blade cools suddenly, but not the top, thus retaining its flexibility and resistance to stress and impact. Steel thus has two distinct crystalline structures, whose demarcation is marked by the hamon.
What happens chemically during selective tempering?
Instead of transforming into cementite, the carbon (austenite) contained in the blade congeals into crystals that we call martensitic, thus modifying the structure of the crystal lattice. Martensite is very hard, but brittle under stress. The Hamon or hardening line marks the demarcation between the lower part of the blade (martensitic network) and the softer upper part.
How is a hamon made?
The Hamon is a distinctive pattern on the Japanese sword blades, such as katanas but also tantos and Wakizashi, resulting from a differentiated tempering process, it delineates the Yakiba, the result of tempering. Here are the four main steps involved in creating a Hamon:
- Coating
- Tempering
- Polishing and gentle heating
- Revealing
Coating: Before tempering, a thin layer of clay, usually mixed with charcoal, silica or sand and water, is applied to the blade. This layer is applied in such a way as to create a thermal contrast between the clay-covered part and the part that remains bare. The clay-coated areas cool more slowly during tempering, while the bare areas cool rapidly. The coating is then air-dried or gently dried without cracking.
Tempering (or rapid cooling): The blade is heated to a high temperature (depending on the type of steel) and then rapidly immersed in a cooling liquid, usually water, oil or a salt solution. The clay-coated parts of the blade cool more slowly, producing a different crystalline structure than the bare parts. This creates a distinct line of demarcation between soaked and unsoaked areas called Hamon. At this stage, however, it is barely visible.
Gentle heating: After tempering, the blade is heated and then cooled (without thermal shock) several times to a lower temperature than that used for tempering. This strengthens the blade and improves its flexibility, while preserving the Hamon pattern.
Revealing: Polishing the blade (Togi) followed by the following operations repeated 3 or 4 times will fully reveal the hamon.
- Immerse the blade in iron perchloride for one minute.
- Quickly rinse with water. Remove oxidation residues with steel wool (or a very fine abrasive, 1000 grit or more).
Finally, the blade must be thoroughly cleaned with steel wool and washing-up liquid. This product, being highly basic, neutralizes the residual effect of the perchloride on the metal.
Hamon shapes
There are an infinite number of Hamon shapes obtained by tempering. Not a single Hamon is identical, since the plaster is applied by hand. However, patterns formed by the laying of clay are frequently found. The most common are :
- Suguha: straight hamon parallel to the blade edge.
- Notare Hamon: wide hamon in the shape of a regular swell.
- Midare Hamon: irregularly patterned lines.
- Hitatsura: irregular hamon with holes.
- Choji: clove-shaped hamon.
- Gunome: Small, irregular, closely spaced waves.
- Kanesada: Hill shapes.
The hamon can also be more or less marked and milky, depending on whether or not small martensite crystals have accumulated along this line (nie (沸) or nioi (匂い)). A hamon composed of nioi (fine particles obtained at lower temperatures) and forming a milky trail is called nioi-deki.
What is a fake Hamon?
The use of acid or grinding can produce a false Hamon.
Stainless steel katanas, for example, have no carbon and therefore cannot be selectively tempered. A false Hamon is therefore drawn with acid or ground.