A hamon line is a patterned hardened edge that appears on a blade. It’s created through differential heat treating aka yakiire.
A hamon is produced when the cutting edge of a blade is differentially hardened, while the spine remains soft (pearlite, ferrite, cementite) to minimize breakage.
Applying the Clay
A hamon line on a sword is created by applying clay to the blade before it is heated. This allows a master sword smith to control the speed at which different areas change temperature during the heat and cool process of tempering.
The result is a differentially hardened blade, where the edge of the sword is harder than the spine, or mune (the back part of the blade). This is due to the different hardnesses of the two parts of the steel.
To achieve this, the steel is shaped as much as possible in its softer state and then coated with a thin layer of clay along the edge and a thicker coating on the body and spine. The difference in cooling rates caused by the clay creates two different hardness zones in the metal, where the edges cool faster and transform into a hard steel structure called martensite.
In feudal Japan, swordsmiths learned to use clay to achieve this by allowing the edges to cool more quickly and the spine to cool more slowly. This technique was referred to as differential heat treatment, and is still used by modern sword smiths.
Heat Treating
In a differentially hardened blade, the cutting edge is harder (martensite) than the spine, which is softer (pearlite, ferrite, bainite or cementite). The difference in hardness helps with edge retention and reduces the likelihood of breakage.
Differential heat treating is a common way to achieve this result, using an oxygen acetylene torch to selectively heat only the bevel area of a knife blade. This prevents the spine from reaching critical temperature while still allowing the blade to take a lot of abuse without breaking.
After heating the blade to a non-magnetic point, quench it in oil. This ensures that the blade does not oxidize and that the spine remains cooler than the rest of the blade during the tempering process.
Once the knife is cooled to room temperature, it can be etched to reveal the hamon line. This is a visual demarcation of the steel, showing up as a wavy line across the surface.
Polishing
A hamon line is one of the most important characteristics of a blade. It is caused by differential hardening, where the cutting edge is harder than the spine, thereby creating a unique pattern in the steel.
Differential hardening is a process that allows the knife to hold a higher temperature for longer, which helps to make it more durable and reduces the risk of breakage during use. It also gives the knife a distinct appearance.
After heat treating the blade, it is time to polish it. This can be done using different methods, including mechanical and chemical polishing.
The main aim of the polishing process is to provide a smooth surface. This finish helps to enhance the appearance of the product while preventing corrosion, improving cleanability, and reducing adhesion. In addition, the polished surface is often reflective.
Finishing
The hamon line is one of the most visually impressive elements on a blade and requires careful attention to detail. This includes tempering the steel to a desired hardness and using a proper buffing compound to keep it shiny.
You have to be careful not to over-temper your blade as this can dull the hamon. The best way to do this is to start at a low hardness and work your way up, but be sure to keep an eye on your progress. You might be surprised by the results. A good rule of thumb is that a blade around 60-61 HRC should be about right for chopping, slicing or both. A higher hardness makes the blade more prone to cracking or breaking. This is especially true if you have a blade with a more complex hamon pattern, such as a hadori. Lastly, remember to use a good protective oil to ensure your blade stays looking great.