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Japanese Blades: Technical Notes


As a weapon of war, a sword had to be efficient, reliable, and practical. A good sword should cut well, and not break, bend or chip. Despite the many advances in metallurgy since the first iron blade was pounded out, the major challenges in forging swords remain the same to present times. Steel has been universally favoured as the blade material of choice because it can be hard or soft or somewhere in between, when the correct heat treatments are applied. A sword crafted entirely of very hard steel would hold an edge well and cut superbly but would be too brittle to make a good weapon. At the other extreme, a sword crafted from dead-soft steel would be effectively unbreakable but make a poor cutting instrument.

The bladesmith's challenge thus, was to craft a blade with a very hard cutting edge which could cut through light armour, married to a softer, flexible body which would resist the impact of the blow. How a smith combined these two conflicting attributes - hardness and ductility - defined the quality of his blades.

Possibly the simplest method was to forge-weld a hard strip of steel onto a blade of iron. The strip of steel formed a hard edge while the softer iron body cushioned the impact of blows. While relatively quick and easy, swords made using this method often bent in hard use, their soft iron bodies being flexible to the point of unbreakability. Having an unbreakable sword was advantageous but the necessity of needing to stomp one's sword straight after every other stroke no doubt caused the wielder many problems.

Through much trial and error, the Japanese smiths eventually learnt to combine edge-holding and flexibility by wrapping a core of soft, low-carbon steel in a jacket of hard, high-carbon steel. When cut open, the cross-section of traditional Japanese blades display this composite contruction method clearly: a dark core surrounded by a lighter jacket of hardened steel. To further heighten the cutting ability of the blade, the ancient Japanese smiths hardened the edge of the blade by heat treating it. There are several variations of this method and each school of blademaking has its own techniques and formulas.

While a well-forged Japanese blade is beautiful to examine, its steel also hints at the skill of its maker. The tempered line* or hamon is often specially polished to enhance its aesthetic appeal, but it also tells the knowledgable eye that the smith has indeed hardened the edge in the traditional clay-hardening method used on fighting blades. Other attractive features of the metal - its colour, texture, and tight welds - similarly testify to the skill of the smith.

* Incidently this feature has been misnamed for what seems like ages. It should technically be called the hardening line since it is produced by the effect of quenching (hardening), not tempering. Tempering comes after hardening, and is intended to relieve stresses in the cross-section of the blade caused by the quenching process. Tempering adds ductility to the blade, increasing its flexibility.

 

Fire & Steel: Forging The Blade
The traditional raw material for Japanese blades is tama-hagane, a steel made from ferruginous sand smelted in a traditional Japanese smelter called a tatara. Each firing of the tatara produces rough chunks of smelted ore with varying carbon levels. These are broken up into smaller pieces for ease of sorting and transporting. Experieced smiths can grade the chunks according to the carbon content by their colour and texture fairly accurately. The most useful portions for forging have a carbon content in the range of 0.5% to 1.5%. Chunks of steel outside this range can be rendered more useful by further smelting either to increase or decrease the carbon levels, but are generally discarded.

Chunks of tama-hagane must first be worked to create a uniform material, free of porosity. This is done by heating the chunks of steel to the forging temperature and compressing them into rough bars by repeated folding and hammer-welding. The end result is a thick bar of steel that folds smoothly, without visible air pockets that would otherwise result in a flawed blade. With skilled hammering, the bladesmith flattens and lengthens the bar, carefully shaping it to the rough shape of a sword blade. When done by a experienced bladesmith, this process appears deceptively simple. In truth, forging efficiently requires the bladesmith to vary the strength of each blow to produce just the right amount of expansion in the glowing steel. Beginners however will find out that it is anything but that; for the glowing steel seems to fight you at every blow, twisting and bending and scorning all attempts to correct the errors.

The forging work has to be done quickly as carbon loss during the heating is inevitable. If too much carbon is 'burnt' by heating, the blade would respond poorly to hardening and would have to be discarded. Japanese bladesmiths refine their techniques towards producing a final rough blade with a carbon content of 0.6% - 0.7%. When the main forging is complete, the bladesmith ends up with a rough blade which is within 90% of the finished length. This is to allow for warpage and expansion during heat treatment. Next, the bladesmith clamps the rough blade to a vise and goes over the flats with a two-handed drawknife called a sen. Using the drawknife, the flats are trued up and smoothened. The blade is now ready for quenching. It is a little known fact that traditional Japanese blades are forged straight and it is the quenching process that imparts the slow graceful curve.

Quenching or yaki-ire exploits a unique property of steel - when heated and quicky cooled in water, it hardens. To harden only the edge, Japanese bladesmiths used layers of clay, from very thin on the edge tapering to very thick on the back or spine. This allows the edge to experience the full effect of the quenching while insulating the spine from the cooling water. The clay coating is typically done to traditional patterns. The shape of the hamon on the finished blade is determined by the pattern of the clay applied by the bladesmith, following the border between the thin coat on the edge and the thick coat of clay on the sides. Once he is satisfied with the clay coating, the bladesmith begins heating the blade in a coal fire until it is at the correct temperature, about 1,400 F. Judging the correct temperature is done entirely by the bladesmith's experienced eye and thus the quenching is often done at night or at least in a darkened forge area.

When the blade reaches the correct temperature, the bladesmith plunges it hissing into the water trough. The blade first begins to curve downwards (like a hawksbill blade) before reversing and curving upwards into the familiar form. This unusual process occurs due to the change of crystaline structure within the edge steel. When first quenched, the steel shrinks on the cutting edge, drawing the blade down into a hawksbill shape. As the structure of the edge changes from austenite to martensite, it occupies more area, forcing the blade to recurve upwards. The final product is a blade with a hardened martensitic edge married to a softer back of pearlite. This particular phenomenon is unique to blades made of simple carbon steels.

The quenched blade is then lightly ground on a water-cooled grinding wheel to remove the clay and examined carefully for warpage or cracks as a result of stresses in the steel caused by the quenching. Typically, some adjustment of the blade shape is required and this is done carefully by the bladesmith to avoid damaging the almost completed blade. When he is finally satisfied the bladesmith usually goes over the blade with a set of polishing stones to refine the shape and give it a rough polish before sending it to an expert sword polisher for finish polishing.

 

Sword Polishing
Polishing a Japanese blade is regarded as a separate craft by itself, since a sword polisher may spend close to 100 hours to do a good job on a katana blade. While the bladesmith may have forged the sword and given it rough lines and shape, it is the duty of the sword polisher to bring out the details of the steel, the texture of the grain and the beauty of the hamon. This is done by using a set of progressively finer and finer stones to abrade and smoothen the surface of the steel. This way, each stone removes the scratches left by the previous one until the final scratches are too small to be visible except under magnification. In times past, while the sword was still functioned as a weapon, it was also the job of the sword polisher to resharpen and repair minor damage to blades by grinding and repolishing to remove small nicks and blemishes.


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