of refining the steel by forge-folding—the actual tanren—a process similar to kneading dough. These pieces will eventually be expertly combined into a single chunk of steel that possesses carefully controlled qualities. This enables the smith to select steel for particular applications. For example, steel with higher carbon content is better suited for the jacket of a sword. Steel with lower carbon content would serve better within the core of a blade. Groupings all depend on the carbon content, crystalline structures, amount of impurities, and degree of consistency.
Each steel grouping will have a role to play in the finished product. Shingane, the soft core-steel (forge-folded only four to six times), serves the important function of keeping the sword from becoming too brittle. Steel with high carbon content will be very dense and rigid. This hard steel will become a jacket in which softer steel will nest. Called kawagane, the jacket steel will be folded between ten and fifteen times depending on the construction method used. In due course, these two types of steel will be welded together seamlessly.
Tanren, the process of folding and refining the sword steel described above, represents the most critical aspect of the swordsmith’s work, along with yaki-ire, the quench-hardening of a blade. It is a common misconception that swords are folded thousands of times. In reality, the jacket steel is folded ten to fifteen times. This results in multiple layers being doubled with each fold. The end result is a surface-steel displaying a grain, or jihada, that is the result of approximately 1,024–32,768 layers—not folds. In most blades, these layers can be seen by the naked eye when a blade has been well polished. The slight variances within the carbon-content of the jacket steel make the folds visible. The jihada layers appear as fascinating and intricate patterns.
The type of grain pattern, or hada, can be manipulated depending on the effect the swordsmith wants. Although grain is affected by many subtle factors such as the amount of carbon, the amount of impurities, and so on, the primary factor is the direction and number of folds the billet receives. Folding the steel, either lengthwise or perpendicularly, in relation to the final direction in which the sword will be lengthened, creates differing grain patterns. One of these, masame, resembles an elongated wood grain pattern, which runs the length of the sword. A common form of steel structure is itame, which looks like a swirling and knotty wood grain. Both itame and masame can be created in varying degrees and sizes. Others such as ayasugi (undulating grain) or mōkume (burly grain) can also be seen.
Mino sword smith Mr. Kanemichi uses a power hammer to work the steel with speed, efficiency and precision. Today, many smiths use a power hammer because young people are decreasingly interested in the traditional arts and trades.
These are two of the several methods of combining core and jacket steels.
Assembling a Sword’s Component Steels
The first steps of making a sword involve refining and preparing the raw materials in order to make them usable. Next, during the tsūkūri-kōmi process the billet is assembled combining carefully prepared steels—each with differing carbon content. There are several construction methods for combining the hard jacket steel with the tough core steel. Some use two pieces, some use up to five. Here, we shall limit ourselves to a brief explanation.
In the kōbūse construction method, the jacket steel is hammered into a “U” shape, which will contain the softer core steel inside.1 The core steel is enclosed within the folded jacket steel in the same way that a hot-dog is placed inside its bun. The two are welded together to create the soft-core/harder-jacket combination (called kōbūse-gitae). That is one of the features that make Japanese swords so unique, distinguishing them from their celebrated Toledo or Damascus counterparts. Four and five-piece billet assemblies are another option. Such blades integrate steel of low, medium, and high hardness. Some believe that these more complex billet designs create a superior blade.2
The billet is elongated until it becomes a sūnōbe—a flat blank without a profile.
The blank has been hammered into a rough blade profile—a process called hi-zūkūri.
Visible here is the clay slurry that has been painted on the blade to insulate the steel during the heating and quench-hardening process. Clay application for three common temperline patterns are shown.
Elongating the Billet
Assisted by his apprentices, or for those working alone, using a power hammer, the smith hammers out the billet and it gradually becomes elongated. The smith pounds a cadence with his mallet and turns the billet underneath the apprentices’ hammers. A steady tempo of hammer strikes creates a rhythm that helps create a smooth collaboration between the smiths and his apprentices. If hammering were to accidentally push through the kawagane to the shingane, exposing the core steel, it would ruin the blade. So the hammers pound steadily and carefully, in a mesmerizing rhythm to create the sword blank, or sūnōbe, with precision and care. The geometry of the sūnōbe is tapered and rectangular. At this point the metal is long and more or less rectangular when viewed as a cross-section, as if it were cut from a sheet of steel. At this stage the sūnōbe appears to be one solid and seamless bar.
Creating the Profile
The process of hammering the sword blank into a profiled rough blade is called hi-zūkūri. The swordsmith works to shape the various surfaces of the blade a few inches at a time. Any mistake at this point would be difficult to correct. So as the sword progresses, there is less and less room for mistakes. When this stage is completed, the makings of what will become the distinctly Japanese sword shape emerge. When hi-zūkūri is completed the blade is still straight.
The shape and geometry of the blade are further refined using waterstones, files and drawknives made from hardened steel. Although the edge is still a blunt 2–3 mm thick, the filing and cutting away of inconsistencies leaves the sword looking quite exact in its geometry—a critical prerequisite for the next step. When all the lines are correct the smith is ready to proceed with creating both the curve and performing the heat treatment: he can proceed the yaki-ire process.
Quench-Hardening the Steel
When steel is heated above a certain temperature and then quickly cooled, its carbon atoms don’t have the time to travel out of the unstable molecular structure that higher temperatures allow, and are locked within, giving way to a highly stressed and hardened material. In the craft of sword making this phenomenon is used in the selective hardening of a sword edge. This process in sword making is called yaki-ire. The difference in hardness between the edge and the body of the blade allows the creation of the hardening mark, or hamon. The carbon content of the steel along with the rate at which the steel cools dictates the hardness that it will attain; the higher the carbon, and the faster it cools, the harder the steel. By allowing the steel at the edge of the sword to cool more quickly than the steel on the sides and back of the blade, a differential is created. The edge steel is extremely hard, whereas the jacket/sides remain only moderately rigid. Meanwhile the soft core prevents the blade from becoming brittle.
The creation of a hamon is a delicate and complex task that must be approached in an exacting manner. Due to its beauty and mysterious flowing appearance, the hamon is one of the most distinctive elements of nihōntō. Volumes have been written on the subtle effects that have been created by different smiths over the millennia. The names of such
