Ko-zuchi and O-zuchi: How Swordsmiths and Hammermen Use Different Hammers in Forging
* Image is for illustrative purposes only.Table of Contents
The Fundamental Role of Hammers in Japanese Sword Forging
In the Japanese sword forging process, the hammer is the most fundamental tool for striking and stretching steel, creating forge marks, and forming the shape of the blade. Unlike modern machine processing, sword smithing is performed entirely through manual striking. Therefore, the choice of hammer and striking method are technical decisions that directly affect the quality of the jigane, the fineness of the grain, and the pattern of residual stress in the finished sword.
Two types of hammers are generally used in Japanese sword forging workshops: a small hammer operated by the sword smith themselves, and a large hammer wielded by a striking assistant (uchiko, an apprentice or helper). By coordinating their strikes, the two can continuously strike the hot steel (above 1200°C) pulled from the forge quickly and uniformly.
Characteristics of the Small Hammer and the Sword Smith's Role
The small hammer is a hammer directly held and operated by the sword smith, typically weighing between 800 grams and 1.5 kilograms. The shape of the head varies according to each sword smith's tradition, with flat striking surfaces or slightly rounded surfaces being common. The handle length is around 30 to 40 centimeters, designed to allow precise control through arm and elbow movement.
The main roles of the small hammer are divided into two types.
- Directing strikes (shiji-uchi): The sword smith lightly strikes a specific area of the steel with the small hammer to signal the striking assistant, "Strike here next." It serves as a sort of baton for conducting.
- Finishing strikes (shiage-uchi): In the precision striking process of fine-tuning the blade shape and creating fine forge marks, this handles the delicate control impossible with the large hammer.
Characteristics of the Large Hammer and the Striking Assistant's Role
The large hammer is a large hammer wielded by a striking assistant (apprentice) with both hands, typically weighing 2 to 4 kilograms, with some large ones exceeding 5 kilograms. Its striking force is several to ten times greater than the small hammer, and is used in the "rough striking (arauchi)" and "stretching" processes to quickly strike and stretch the steel to evenly thin the overall thickness.
A summary of the differences between the small and large hammers is as follows.
| Item | Small Hammer | Large Hammer |
|---|---|---|
| Operator | Sword smith | Striking assistant (apprentice) |
| Weight | Approximately 800 grams to 1.5 kilograms | Approximately 2 to 4 kilograms (large ones may exceed 5 kilograms) |
| Main role | Directing strikes, finishing strikes | Rough striking, stretching |
The technical role of the striking assistant is to swing the large hammer with accurate timing and force in response to the sword smith's small hammer signals. The striking assistant repeatedly performs "follow-up strikes (oiuchi)", striking the same location or an adjacent location after the sword smith's small hammer strikes. If this coordination breaks down, the steel can deform unevenly, or excessive strikes can concentrate on one area, causing internal cracks (forge cracks).
When there are multiple striking assistants, they alternate swinging the large hammer to apply many strikes before cooling begins, maximizing the time before the steel returns to the forge. This "multi-person striking (oohito-uchi)" is effective for large initial forging or the first cycle of fold forging.
The Influence of Hammer Material and Shape on Forge Marks
The head of the hammer is typically made of forged steel, but some sword smiths change the hardness of the striking surface according to the purpose, or change the shape of the striking surface (flat, convex, or curved). A flat striking surface provides uniform compression, making it suitable for the initial stages of fold forging. On the other hand, a slightly rounded striking surface (sometimes called "sugi-nari") uses the escape of force at the edges of the steel to create complex and fine forge marks.
The forged grain pattern (twisted pattern) of the jigane is created through the combined action of strike direction and hammer shape. Even with the same number of folds, the direction of the grain varies—itame, masame, or mokume—depending on hammer selection and striking method. When the sword smith intends to "stand the itame" or "align in the masame direction," conscious control of strike direction and hammer selection is necessary.
"Striking in Unison"—Words Born from the Forge
The coordination between the sword smith and the striking assistant has left traces in Japanese language. The expression "striking in unison (aidzuchi wo utsu)" (meaning to nod in agreement with someone's words) is said to derive from the rhythm of the forge where an apprentice strikes the hammer in response to the master's hammer strikes. The large hammer wielded by the striking assistant is also called the "facing hammer (mukouzuchi)," and the alternating sounds of the well-coordinated master and apprentice's hammers were long a symbol of smithing work. Just tracing the origin of the word shows how crucial the coordination of the two hammers was to the essence of forging.
Modern Sword Smiths' Hammer Selection and Craftsman Individuality
Among modern sword smiths, some continue to use the hammers they inherited from their masters, while others use hammers they have forged themselves. It is not uncommon for sword smiths to prepare several to a dozen or more custom hammers for each process, making hammer management itself a part of craftsmanship.
The weight and handle length of the hammer are adjusted to suit the craftsman's body size, arm length, and forging style. Even among sword smiths of the same school, hammer selection often differs between master and apprentice, and this is sometimes reflected in the finished sword blade as the "peculiarity of the jigane" that each individual sword smith possesses. In other words, the hammer is one tool that harbors the sword smith's individuality.
Points of Observation for Appreciators
The skill or lack thereof in hammer technique appears indirectly on the surface of the finished blade. When observing the jigane carefully, one can see whether the forged grain is uniformly packed or whether there are areas where the grain pattern is disturbed and sparse. Well-packed jigane is the result of accumulation through proper temperature management and uniform striking. Conversely, forge cracks or swelling defects in forging are sometimes traces of biased striking or poor temperature management. When viewing the blade through displays, catalogs, or enlarged images in online catalogs, turning your attention to the packing density of the jigane in addition to the hamon, you will be able to imagine the scene of the forging workshop where the hammer swings. Observing the quality of the jigane and the direction of the forge marks when appreciating sword blades is also to reflect on the hammer chosen by the sword smith and how it was used.
