A hamon is a visible line on the blade of a knife caused by different hardening as a result of the application of insulating clay on the blade, for example, which affects the way steel transforms with heat.
Chances are you’ve already seen and admired a hamon on a Japanese sword or custom knife at some knife show.
It is a visual demarcation that appears as a wavy line on the surface of a knife or the blade of a sword.
During incision, the acid devours more of the softer section, which appears darker, and the section of the harder edge appears lighter, creating a clear visual distinction.
In popular culture, hamon is most commonly found on the traditional blade of the Japanese katana.
Around 700 A.C.D., blacksmith Amakuni Yasutsuna is said to have developed both hamon and katana, establishing a blacksmith tradition that is still followed today.
Hamon is caused by differential hardening.
The cutting edge is made of hardened steel while the back is soft and flexible, making it less likely to break. The difference in hardness is the goal of the process while the aesthetic appearance is only a “side effect”.
However, the aesthetic qualities of the hamon are quite desirable, not only as evidence of the process of differential hardening, but also because the models can be quite complex.
The parts of the blade with and without clay/cementite/satanite harden differently, creating the hamon.
Steels suitable for the creation of the hamon
Most makers who create hamon choose carbon steels because they are “poorly hardened”.
The properties of high-carbon steel include very high strength, extreme hardness and wear resistance, and moderate ductility, a measure of a material’s ability to tolerate deformation without actually breaking,
Options include W1, W2, 5160, 01, L6 and the 10xx series as U10A.
10XX series steels (where XX or the last two digits represent the average or average carbon content of the quality) are simple carbon grades.
These steels are considered suitable for the needs of very fast hardening of hardening with clay coating.
You can also use stainless steels, but their composition does not produce a hamon that strikes is more like a straight line to make the knife more flexible on the back and less prone to breakage.
A hamon appears on the border between softer and hardened steel.
As a prelude to heat treatment, annealing and/or completely normalizing the blade must be carried out once a roughing is completed.
In terms of producing a clear hamon, this step prepares you for success.
The “coat” of refractory clay, satanite, etc.
In terms of “insulating clay”, there are some products that most knife makers use in addition to the traditional Japanese clay.
Refractory cement, which is a mortar used to coat furnaces and forges
Clay used to seal chimney bricks
Sealant for drains
A refractory material is a material capable of withstanding high temperatures for long periods without reacting chemically with other materials with which it is in contact.
Among the most common refractory materials there is refractory mortar, which is a type of material very resistant to high temperatures and is composed of clay, calcium and other additives.
It comes in powder form in packs ranging from 1 to 25 kg and must be mixed with water.
Once a homogeneous dough has been obtained, it is easy to process and high adhesion to the work surfaces.
Many makers use their own mixture of materials which includes baking clay, Holts Gungum and coal powder/hardwood ash but my advice is to buy some satanite.
Using your fingers, a brush or a spatula, cover both sides of the blade in the same place.
The shape you will give to the hamon depends on the way and form in which you apply it, it depends entirely on you.
You can do the traditional Japanese waves or you can create images for really interesting results.
The amount of clay:
If you apply too dense clay, the residual heat in the spine will spread to the edge and soften it.
Having too thin a layer will result in hardened thorns and edges, which will not produce a well-defined hamon.
You may want to experience the size, thickness, and placement of the pieces of clay used to make ashi, or the white smokiness that appears in the polished hamon.
At least 10 millimeters of steel should be exposed between the cutting edge and clay, since the hamon will not end exactly where the clay ends.
Let the clay air dry for a few hours, then put it in the oven at 275⁰F for 30 minutes, alternatively let it dry for 24 hours.
This phase is important because during heat treatment, if the clay is not dry, it boils and flakes and will affect the pattern, so some makers slightly heat the blade before applying the clay.
Types of hamon
Suguha hamon(straight tempered hardened line)
Clay insulation applied to obtain hamon will also insulate the blade during heat treatment.
When shooting for a hamon, you may want to increase the normal holding time as coated/coated steel needs a little more time to heat up evenly.
If you’re not using a furnace or hardening oven, use a magnet to check critical areas, being careful not to get burned.
Water shutdown against oil extinction
Increased hamon activity (a more widespread and cloud-like model) is achieved through shutdown in water but since this is a much faster extinction, there are greater chances of blade cracks and failures.
It is safer to use oil, as there is a lower risk of catastrophic failure of the blade with cracks and banana distortions and the model still has a well-defined hamon.
Some perform for the hardening a shutdown in water and oil, in the sense that once the blade is brought to temperature it is turned off for 3 or 4 seconds in water and then immediately immersed in oil
You can first dip the edge of the blade or the tip.
If the blade is moved back and forth, the vapor barrier/jacket is reduced, rapidly reducing the temperature.
Keep it in quench until it reaches room temperature.
Remove the residual clay from your hamon and clean the oil before just sanding just the edge of the blade to make the pattern more pronounced and see the result.
If it came out well at this point, you can also slightly smooth the blade to help you see the hamon then immediately in the oven for tempering.
It is used to determine the amount of dent/mark that a diamond tip can create in metal with a certain amount of weight.
Each steel has characteristics that allow it to achieve a maximum of Rockwell hardness that you can achieve.
Pulling out the Hamon
Removed the blade from the oven after tempering and cooled and you have obtained the desired hardness, smooth the blade bringing it to the final thickness and recover from the coarse grains up to the finest, first removing the black from the blade and then bringing the blade to grain 60, 120 and then 240 to rise up to 600 or 800/1000 depends.
A 600-grain abrasive should already “do the trick”, but pay attention to the results you want.
Be sure not to overheat the blade at this point in the sanding process, as this may ruin the hardening process performed and even your hamon.
Only after that perform the welding of the guard or the assembly of the guard.
Now it’s time for the acid incision.
Degrease the blade perfectly because oil or grease will block the acid.
Make sure the blade is very clean.
Then soak it.
While many people use a 4:1 ferric chloride acid but personally use 5:1 (one part ferric chloride and 5 parts water) and the blade is soaked and left for 4 to 5 minutes.
Repeat the process 3 times so as to allow the chloride to mark the blade more and more deeply.
Every 15/20 seconds or so, pull out the blade, clean the oxides with about 0000 of steel wool or an ultra-fine sandpaper and put it back in the acid.
Others use apple cider vinegar, boiling vinegar, lemon juice and even coffee!
Typically, a weaker acid will reveal more detail, but it will also require more cycles.
Repeat until the hamon is as clear as possible.
After removing the acid, it is necessary to neutralize and wash the blade with soap and water and eliminate all surface oxidation that has formed with acidation.
Now soak the blade in a solution of water and bicabonate so as to stop the action of ferric chloride.
Then it creates an oil / wax coating (it is recommended for carbon steels because they rust) so it is important that the blade is sprayed with oil and packaged overnight with a little paper and abundant oil.
After the night we proceed to polish and “let out” the hamon and for this first of all clean the blade well with a pass with a wheel of rags and very little abrasive paste to remove the bulk of the dirt and then clean better with alcohol and clean up paper scottex taking care to change the paper as soon as it gets dirty.
Final polishing is probably one of the most important steps.
In fact, Japanese sword makers often send their blades to master polishers to reveal the subtle beauty of their hamon.
You can achieve an outstanding result with metal polishes such as Autosol or Brasso or try a polish based on silicon carbide powder or aluminum oxide suspended in oil.
Magnetite is the black oxide that appears during polishing.
To bring out the subtle characteristics of the most complex hamon, the Japanese mix it with clove oil and rub it on the steel in the Sashikomi Nugui polishing process .
In repeated cycles, they use this oxide to enhance the subtle characteristics of the hamon.
For this point, however, I advise you to use paste for coachbuilders and a woolen cloth completing the polishing and you will observe on the blade the difference in porosity of the steel between the part that was covered by clay and the uncovered part.
The hamon is an aesthetic effect not only captivating that gives an effect to the blade really vintage but it is also a treatment that makes the blade in the area of the back more flexible and less prone to breakage and very hard in the area of the cutting edge.
The limit of the hamon is that it is limited to carbonaceous steels to obtain a very clear aesthetic effect but this does not mean that it cannot be done with other steels but this to obtain a different knife mechanics and not for aesthetic reasons.
In the coming months I will do some tests to show the differences!
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