Clay Tempering Methods
Clay tempering is the traditional method used to produce Japanese swords, or "katanas." Clay is wrapped around the blade when it is "quenched," or cooled in a water bath. Clay tempering allows the forger to achieve the perfect balance between hardness and flexibility. The sword's blade remains hard while its spine is flexible, allowing the blade to cut with ease and the spine to rebound easily from each blow.
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Clay Wrapping
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The blade is wrapped in clay to prevent it from cooling too quickly when quenched. The clay is thinnest at the blade's edge and thickest at its spine (the opposite edge from the blade). This way, the blade cools the fastest, creating an extra hard steel, and the spine cools more slowly, resulting in a softer and more flexible metal. Some forgers leave the blade completely exposed during quenching for maximum hardness.
Quenching Solution
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Some debate exists over what water temperature to use for quenching. The colder the water, the faster the blade cools, which can result in a super-hard yet brittle blade and a stronger but less flexible spine. Some smiths use hot or even boiling water, which increases spine flexibility and avoids blade nicking or chipping. Some forgers will add other elements to their quenching water, such as salt. Others cool their blades in oil. One historical forger was even said to have cooled his blades in fine sake.
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Quenching Time
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The sword's curve does not develop until it is quenched. The traditional method is to lower the sword into the quenching bath blade first. When the clay-wrapped blade is first plunged into the water, it curves downward. Then, as it cools, the blade curves upward. At this stage, the forger must watch carefully to remove the blade when it has achieved the perfect curve. A sword may be wasted if it is removed too soon or too late from the quenching bath. A curved sword is desirable for slashing attacks, which are faster than thrust attacks in battle.
Temper Line
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The temper line, or "hamon," is the white line running along the length of the blade. A true hamon is the telltale sign that a blade has been clay tempered. The temper line indicates where the thin layer of clay on the blade ends and the thicker-wrapped spine begins. It is said that a sword's forger can be identified by the temper line. The forger can control the appearance of the hamon with how the clay is edged before quenching. It is recommended not to leave the clay edge straight, as a wavier edge creates a stronger blade.
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