The hamon is the visual, patterned effect created on the blade of a Japanese katana by the hardening process. It is a symbol of the swordsmith’s craftsmanship and adds significant aesthetic value to the weapon.
“The swordsmith was not a mere artisan but an inspired artist and his workshop a sanctuary. Daily he commenced his craft with prayer and purification, or, as the phrase was, ‘he committed his soul and spirit into the forging and tempering of the steel.’ Every swing of the sledge, every plunge into water, every friction on the grindstone . . .”
– Bushido, the Soul of Japan, by Inazo Nitobe
Tsuchioki: Creating the Hamon Design
After the completion of the cutting edge formation (shiage) the edge must be hardened. This requires are third type of steel (tamahagane) be introduced to the weapon. Kawagane and shingane are the types of tamahagane forged together to create the sturdy and functional blank base of the katana (sunobe). This additional steel is called martensite. It is extremely hard and can easily be sharpened. The downside to martensite is that it is too rigid and brittle to absorb and deflect an opponent’s blows without rendering the blade permanently damaged. The necessary flexibility and durability can be found in softer forms of steel called ferrite and pearlite. Over centuries Japanese craftsman have developed methods to transition the pearlite to martensite. The transition between the metals is called the habuchi and is clearly visible creating a pattern called the hamon. This is widely considered the most important aesthetic property of the katana.
Battle Sturdy Blade
The process to create the hamon begins with a clay mixture that is applied to the blade. The mixture called tsuchi-dori consists of riverbed clay, charcoal powder, pulverized sandstone and potentially other elements deemed necessary by the swordsmith. The tsuchi-dori is applied in varying thickness and acts as an insulator. The thinner the layer, the more martensite there will be in the microstructure. The thinnest portion is applied to the cutting edge, with a thicker portion for the upper part and back of the blade. This distribution is instrumental in determining the pattern of the hamon. As an additional precaution against damaging the fragile martensite the swordsmith applies multiple thin strips of tsuchi-dori to the surface of the blade causing veins of pearlite to form behind the martensite coated edge. These veins are called ashi. This will not stop cracking, but will instead allow any crack to disseminate through the martensite until it is absorbed by the soft pearlite, which will keep the sword functional during battle.
The process of hardening the blade is called “yaki-ire.” The tsuchi-dori must fully dry before the sword is heated. Once it is heated to a glowing red or orange it is quickly cooled in water. This technique is commonly performed at night so the swordsmith can see the true color of the heated metal. This process of heating and quinching is repeated several times.
Following the yaki-ire, the blade is drawn again through a low temperature forge and uniformly heated to approximately 160 degrees Celsius. This process is used to relieve some of the stress of the yaki-ire. Caution must be used during the yaki-modshi because even though it can result in a beautiful and complex hamon, it can also cause the hamon to fade or disappear. Once this is completed, the dry clay is removed and the blade is examined. A two percent nitric acid and ethanol solution is then applied to bring out the design and definition of the hamon.
The curvature of the katana was worked in to the sunobe stage of production, but during the yaki-ire process the cutting edge and back of the blade cool at different rates. This causes continued contraction of the metal, which can create up to half an inch of additional curvature. Adjustment of the curvature is called sorinaoshi and is accomplished by striking the back of the blade with a hammer.
With the completion of sorinaoshi the katana has taken its full shape and is ready to receive the finishing touches that complete the swordsmith’s masterpiece.