Real Steel: The Katana
The katana is a bigger-than-life weapon with a long and colorful history. It is surrounded by so much legend and mystery that it is sometimes difficult to get to the truth. In the previous article we discussed tamahagane, the steel from which traditional katana are made, and before that we talked in some detail about heat-treating. Now, let’s take a look at the making of a katana from forging to heat-treating and polishing—then perhaps a bit on use.
Keep in mind that I am not a traditional Japanese swordsmith, and this is not necessarily how I would go about making a katana. Techniques vary from smith to smith, but you’ll get the idea.
The Pre Form
The first step is forging the tamahagane billet into a pre form called a sonobe. The billet is drawn out to roughly the correct length, width, and thickness — with a little extra metal left in each dimension to allow for manipulation. This process is quite laborious when done by hand, and today even traditional smiths usually use a power hammer to get through this stage. This has no bearing on the quality; the modern labor saver allows the smith to devote more time to the final details including the final hand forging…
Forging the Blade
After the sonobe is complete, the blade can be forged closer to its final shape. The tip is usually forged first, by striking down and in at about 45° with strong hammer blows. Interspersed gentle hammer blows at varied angles make the tip, or kissaki, rounded rather than flat or chisel shaped. The bevels are usually forged next.
Katana have a single bevel on each side of the blade that runs from the spine of the blade all the way to the edge, as opposed to western blades, which generally have primary and secondary bevels. Although the blade is flipped over and worked on both sides, the anvil shapes the bottom of the blade as the hammer shapes the top.
As the bevels are forged, the blade is forced back, curving away from the edge and toward the spine. This curvature is called sori and must be corrected for by straightening the blade periodically during forging. (Although the katana is a curved sword, you’ll see shortly why it can’t have too much sori when it gets heat treated.) After the bevels are formed, the tang, or nakago, is forged.
When forging is complete, the blade is rough ground using coarse stones and files to remove the iron oxide forge scale as well as to enhance the shape.
As we discussed in the article on heat-treating, here’s where the magic happens. A mixture of clay and ash is skillfully applied to the blade and allowed to dry. The blade is heated to critical temperature and then quenched in warm water. This is a time of great angst for the smith since a lot of effort has been expended in forging this blade, and a high percentage of water quenches destroy the blade. As the blade is quenched, the smith will either feel a subtle vibration in the tongs as the blade sings to its maker that it is born or a nasty jerking accompanied by an audible ping: a sure sign that the blade has cracked and did not survive hardening.
In that fraction of a second, a lot happens in the quenching blade, but the one thing that can be seen is that the blade first curves down, and then up, resulting in the final curvature. The blade emerges from the quench with considerably more curvature than it entered with. Generally, an oil-quenched blade curves down and stays that way, so an exaggerated curvature has to be forged in before quench. Also, oil quenching is far less deadly to blades than water, but arguably also has a couple of disadvantages. Check out this video to see what happens during a water quench.
The clay coating protects the spine from rapid cooling and results in the spine being softer than the edge, yielding a tough and flexible blade with a hard edge capable of excellent edge retention and great sharpness. It also produces an amazing effect called a hamon. Ha means edge or blade, and mon translates to badge, so hamon is the badge of the blade. It is the physical border between the softer pearlite spine and the hard martensite edge. It is visually beautiful and can appear as clouds, mountains, the moon, or wisps of smoke. The creation of hamon is considered an art in and of itself.
After quenching, the blade is tempered. Traditional tempering is accomplished either by drawing the blade across a hot block of metal until it is sufficiently drawn back, or softened, or by placing the entire blade in the forge and quickly removing it until the desired temper is reached. The smith watches for the steel to change color as an indication of its state of temper. Oxides form on the surface of the steel and go through color phases: from yellow through orange, brown, blue, and purple as the metal becomes softer but tougher.
The last thing done before the blade is mounted is polishing. Although western smiths often sole author a katana, doing everything from forging (some even smelt their own steel) through mounting, a traditional Japanese smith will usually have the blade polished by a specialist. The process starts with relatively coarse bench stones and progresses through increasingly finer grits until the final stages are accomplished with very fine finger stones. Very small areas are worked with the finger stones pinched against the blade with the thumb and index finger. The polishing not only yields a mirror surface, enhancing the hamon, but also creates the final edge geometry and gives the blades its final edge.
It’s a Cutter
Depending on blade shape and geometry, swords can be primarily for thrusting or cutting. Cutting swords can be further divided into hackers and slashers. A gladius, for example, is a hacking and thrusting sword; the cuts are percussive. The katana is a slashing sword that is also pretty good at thrusting. Slashing cuts are also called draw cuts: the sword is pulled in through the cut. The highly polished surface of the blade helps it pass through the target, as does the subtle wedge-shaped geometry.
“I heard that Japanese soldiers during WWII disabled machine guns by cutting off the barrels with their katana.” Definitely didn’t happen. Although some steel blades can cut steel, it’s done with a band saw and takes some time. No sword has ever been made that can cut through a machine gun barrel. If your Great Grand Uncle Grumpas told you he saw this happen, with all due respect, he’s mistaken.
“I heard that katana were heat-treated by heating them and stabbing a slave or convict.” I can’t say for sure no one ever tried this, but I can tell you it won’t work. Organs and body fluids cannot carry the heat away fast enough to harden a blade.
“I heard katana were tested by cutting through corpses and received designations, such as one-body blade, two-body blade, etc.” True.
“I heard katana were tested by cutting through live convicts and received designations, such as one-body blade, two-body blade, etc.” Probably true.
“I heard katana are unbreakable.” Definitely not true. The katana is a delicate 3-ft.-long razor blade. Despite the clay hardening process that toughens it, repeated blade-to-blade and blade-to-armor contact quickly leads to chipping and eventually to breakage. This is why so few antique katana in decent condition exist today.