The hamon is there, lying within the reach of the knifemaker who is willing to take the time to thoroughly rub the steel to enhance the elusive line.
The hamon is a visual demarcation, showing up when etched as a wavy line across the surface of the steel. When etched, the acid eats away more at the softer section, and so shows up darker, and the harder edge section, showing up lighter, sometimes with a dark band where the two meet. This should not be confused with a “temper line”.
The hamon is caused by differential hardening (Yaki ire); the cutting edge, is hardened steel (martensite), while the spine is kept soft and tough (pearlite, ferrite, bainite, cementite), and so less prone to breakage.
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 where the patterns can be quite complex.
The Hamon (pronounced “huh-mown”) is a Japanese invention, in popular culture most widely recognized on the traditional Japanese Katana. The development of the hamon, along with the Katana itself is attributed in legend to the swordsmith Amakuni Yasutsuna, around 700 AD.
What To Look For in a Hamon
Discriminating knife buyers looking for a quality hamon should bear a few things in mind.
- A hamon can be wavy, undulating or any number of shapes. Whatever the shape, it must be well-formed, clear and unbroken.
- A good hamon will exhibit a sharp demarcation between the hard and soft areas.
- Watch out for a hamon line that dips too close to the cutting edge. If the hamon comes close to or reaches the edge of the blade, then a section of the edge is not hardened properly.
Steels for producing Hamons
Most makers who frequently produce a hamon opt for simple carbon steels because they are shallow hardening. The choices are W1, W2, 5160, 01, L6 and of course the 10xx series (1050, 1065, 1070, 1075, 1084* and 1095). These simple carbon steels (low manganese (.2%) and or no chromium) are conducive to the very fast quenching requirements of a clay-coated quench.
High-alloy / stainless steels can convert to hard martensite quite easily meaning they “through harden” and are not conducive to the creation of a very active hamon. What you get with the high-alloy steels, if anything, is more of a straight line.
Remember that the hamon appears at the boundary between hardened and softer steel. So once you have rough ground your blade, it’s best to anneal and or normalise (if you are forging this will reduce the stress and reduce the grain size) your blade thoroughly as a prelude to your heat treat. This step has a number of other benefits as well but in terms of producing a clear hamon, it definitely sets you up for success.
Wash the blade thoroughly to remove any oil or dirt and dry it off.
The “Clay” Coat (Tsuchioki)
As for the insulating clay, there are a couple of products that most knifemakers use (these are aside from traditional Japanese clays, usually a certain mix of clays and ash):
- Refractory cement, a mortar used for the most part to coat forges and furnaces.
- Ben Braven’s Fire Sealant, a clay used to seal fireplace bricks
- Exhaust sealant
- Some smiths use a special mix of their own creation (Holts Gungum, oven clay & hardwood ash/charcoal powder)
An almost runny but not watery mixture helps create a thin and even layer (think pancake batter not butter).
Apply a coat on the spine with your fingers, putty knife or brush coming down in the same place on both sides of the blade. As for the shape, you apply it in, it’s entirely up to you. The hamon shows up as a ghosty gray line, so flames or whisps are pretty cool. You can go with the traditional Japanese waves, just slather it on, or create images. You can get some very interesting results.
If your clay is applied too thick, residual heat in the spine spreads into the edge and softens it. If the layer is too thin, the spine, as well as the edge, hardens and you will not achieve a well-defined hamon. It’s worth experimenting with varying the size, thickness, and placement of the little bits of clay used to produce ashi. (See Kapp and Yoshihara’s Craft of the Japanese Sword for good examples of how Japanese smiths lay them out.) Different steels will respond differently.
The thickness varies, but aim for approximately the thickness of the spine where it covers, so if the spine is 1/4″, the layer of clay is 1/4″ as well. You can thin out the cement with a small paintbrush to wash around the edges of the already applied clay to increase the Ashi (White smokiness in the hamon when polished up).
Do not go all the way to the cutting edge. Try to keep at least a centimeter of steel exposed between the clay and the cutting edge as the hamon will not end exactly where the clay stops. However, make sure to include the most ricasso (the part near the edge to be left uncovered) area and some of your handle covered by the clay.
Let the clay air dry for a few hours (overnight), then place in the oven at 135⁰C for 30 minutes to dry completely through. If the clay is not dry, it will bubble and flake off during your heat treatment and affect your pattern.
Some knifemakers heat the blade up slightly before applying the clay.
The clay insulation we’ve applied to achieve the hamon will also insulate the blade during heat treatment. You may want to increase your normal holding time when shooting for a hamon as the coated/covered steel needs a bit longer to heat through so that the blade is a uniform temperature.
If you’re not using a furnace/oven, use a magnet to check for critical on the cutting edge. Please don’t burn yourself.
Water vs Oil Quenching (Yaki Ire):
The biggest difference in hamon activity is in the quenching medium, such as water or oil.
A greater hamon activity (more cloudlike, more diffuse) is achieved through a water quench but as this is a far more rapid quench, there is far more potential for cracking and blade failure. If you are going the safer route and using oil (more willing to follow the clay pattern and is more defined on its boundaries), there is less chance of catastrophic blade failure.”
Submerging your blade tip first (vertical tank) or edge (horizontal) both work (edge quench just ensures that the Spine does not cool too fast. Moving the blade back and forth will reduce the vapor Barrier/Jacket, to ensure quick temp reduction. Leave it in the quenchant until room Temperature.
Interrupted quench? Some makers quickly quench in hot water (to lessen the thermal shock to the steel) and then finish in oil. The water gives increased activity in the hamon and the oil prevents the blade from cracking (ping!).
Check for you hamon by scraping off the remaining clay and wipe off the oil before lightly grinding the blade edge with your last belt grit to show the Hamon in the right light. This is my favourite part of the process because you get to see if all your work paid off. You can lightly etch the blade to help you see the hamon at this stage.
If your edge did not fully harden or the hamon isn’t what you want…. Don’t throw the blade at the wall. Go back up to the Thermal Cycling step and start over.
If you have the Hamon you like, it is important to get your blade tempered. Right now it is very brittle and you need to bring the Hardness down. Different steels quenched in different quenchants need to be tempered at different temperatures. Do your research and follow a reputable resource.
You can adjust your HRC for the intended purpose of your blade. A blade around 60-61 HRC is great for a slicer. Choppers should be softer due to the impact they have to endure.
FYI: Over-tempering will fade a hamon, so precise control over your temperatures is necessary.
Bringing Out Your Hamon
After you have tempered your blade to your desired hardness. Clean up the entire blade on the grinder getting it to final thickness (roughly #400 grit but your belt progression may be different). Be very careful not to overheat your blade in the grinding process now. You can ruin your temper and even your hamon if you are not very careful.
The closer you can get to your final, final geometry the better, as you’ll be only polishing thereafter. Granted it is a risk vs reward exercise as too thin and you risk cracking your blade.
Hand Finish / Rough Polishing (Kaji-Togi):
Now the fun part… Hand Sanding, #2000 grit here we come!!!
Now time for the grand reveal of the acid etch (Ferric Chloride Etchant 4:1). Some people prefer lemon juice, apple cider vinegar, boiling vinegar or even coffee if you’re just experimenting for what works for you. In general, a weaker acid will bring out more detail but will require more cycles.
Make sure the blade is very clean (grease or oil will block the acid), and submerge in the acid. Every 10 seconds or a few minutes, take the blade out and clean off the built-up oxides with some 0000 steel wool (or your final grit paper), then place it in the acid, until the hamon is about as clear as you can get it. Once you’re satisfied, clean the acid off and neutralize and wash. Carbon steels do rust so a coating of oil/wax is recommended.
Hamon Polishing (Togi):
Probably one of the most important steps is the final polishing. Japanese sword makers send their blades to master polishers for this step so that the subtle beauty (Ashi) can be brought out and revealed. For the humble knife makers, it’s time to apply some metal polish (Brasso, Autosol, Turtle Wax Green Line etc).
You can go into extremely high powdered grit aluminum oxide/silicon carbide suspended in oil polishes (with a soft backing) at this point if you so desire. Traditionally fine stones and baking soda (to combat rust) were used.
FYI: The black oxide that forms during etching is Magnetite (Fe3O4). Which is what the Japanese mix with Clove Oil and rub into the steel in the Sashikomi Nugui process to bring out the subtle features of more complex hamons. It is this oxide that they use in repeated cycles to enhance the effect of the hamons subtle features.
Good reference: The art of Japanese sword polishing by Setsuo Takaiwa
Then apply another liberal coat of protective Oil on the entire blade and complete your knife!