The Tacit Knowledge of Swordsmiths — The Physical Knowledge and Unwritable Skill Acquired Through 'Eye,' 'Ear,' and 'Hand'

What Is Tacit Knowledge? — Two Types of Knowledge in the World of Sword Smithing

The concept of "tacit knowledge," proposed by philosopher Michael Polanyi, refers to "knowledge and skill that cannot be conveyed in words." The famous example is that "you cannot teach someone to ride a bicycle through text alone" — and Japanese sword forging is a craft in which this tacit knowledge occupies an extraordinarily high proportion of the total skill required.

Various documentary means — secret transmission scrolls, oral teachings, and illustrated records — have long been used to pass down forging techniques (see The Oral Transmissions and Secret Documents of Sword Smiths for more detail). However, while written documents can convey certain aspects of the technique — primarily material selection, the sequence of processes, and summaries of empirical rules — they are in principle incapable of conveying the actual physical "sensation" of judgment itself.

At the core of the tacit knowledge a swordsmith must acquire are three sensory judgments: "hiiro" (fire color), "uchigoro" (the right moment to strike), and "ne" (sound). These are engraved into the body through years of training, eventually reaching a state known as "the craftsman's body," in which judgment fires reflexively without conscious calculation.

Judging Fire Color — The Trained Eye That Reads Temperature by Sight

Managing the temperature of iron during the forging process is one of the most fundamental yet most difficult skills involved. Modern metalworking relies on high-precision thermometers and infrared sensors, but in a traditional sword-smithing workshop, the smith estimates temperature by judging the color of the forge fire — "hiiro" — with the naked eye.

Stages of fire color: Iron glows in different colors depending on its temperature. At low temperatures it glows dark red (near-black red to red); at medium temperatures, orange to bright orange (roughly 700–900°C); and at high temperatures, yellow to white (900°C and above). The fire color a smith judges to be suitable for forging (roughly 800–1000°C) falls within the range from "yellow-orange" to "bright orange," and the actual temperature fluctuates due to multiple factors, including the temperature of one's wrist, the intensity of daylight, and the state of the charcoal in the forge.

The problem of lighting conditions: Judging fire color is heavily affected by the ambient lighting. Under bright midday sunlight, the forge's glow tends to look whiter, leading smiths to overestimate the temperature, while in the dim early morning it tends to look cooler than it actually is. For this reason, many traditional sword-smithing workshops deliberately keep windows small or maintain dim lighting to create a stable visual environment for judging fire color.

Mastering "the eye that reads fire color" is said to require at least several years of training. In the early stages of apprenticeship, a disciple begins by standing beside the master, absorbing into the body the sense that "this color means this temperature." Modern training methods that cross-reference a thermometer reading with visual observation are also attempted, but ultimately, the mark of a true craftsman is the ability to judge accurately "without looking at a thermometer."

The Sense of Uchigoro — The Tacit Knowledge of Timing a Hammer Strike

In "forging" (tanzo) — shaping iron by striking it — swordsmiths share a concept known as "uchigoro," the right moment to strike. Uchigoro is a sensory concept referring to the state in which heated iron is well suited to being struck: still soft enough for forging, yet still at an appropriate temperature.

What happens if uchigoro is missed: If the iron cools too much, it hardens, and striking it risks producing cracks ("kiretsu"). Conversely, striking iron too forcefully while it is overheated can cause it to melt, deform, or become contaminated with impurities. Correctly judging uchigoro is directly linked to efficient forging and the formation of high-quality steel structure.

Sensory indicators of uchigoro: The following sensory indicators, passed down among swordsmiths, are used to judge uchigoro:

• The state of the sparks: The volume, direction, and size of the sparks scattered by each hammer strike reflect the iron's temperature and condition. At the right temperature, sparks scatter finely and evenly; as the temperature drops, they become larger and more irregular.
• The "feel" (tegotae) of the iron: The sense of rebound and impact when the hammer strikes the iron conveys its softness. At the right uchigoro, the hammer sinks in with a soft, yielding sensation; when the iron is too cold, it rebounds with a hard, sharp resistance.
• Changes in sound (ne): The sound of the hammer strike changes according to temperature and the condition of the iron. A skilled swordsmith instantly judges, "that sound was a little cold," and decides the timing for returning the piece to the forge.

Judging the State of Iron by Sound — Sensory Knowledge of the Ear

The skill of judging the state of iron by its "sound" (ne) is considered especially difficult to put into words, even among the various forms of tacit knowledge a swordsmith must acquire.

The sound of the hammer striking during forging carries composite information reflecting multiple variables — the iron's temperature, the steel's carbon content, internal stress, and the thickness at the point of impact. A skilled swordsmith unconsciously analyzes this complex sound to determine the iron's current state and decide the next action, but explaining in words exactly how this is done is extremely difficult.

Oral teachings such as "when you hear this sound, return it to the forge" or "this sound is the sign that the iron is in good condition" do exist, but it is impossible to understand what "this sound" actually is without direct experience. The memory of sound is engraved into the body, and only becomes "embodied knowledge" after experiencing the same work thousands upon thousands of times alongside one's master.

How the Master-Disciple System Transmits Tacit Knowledge

The mechanism by which a swordsmith's tacit knowledge is passed down through the master-disciple system consists of three stages: observation, imitation, and experience.

The observation stage: In the early stages of apprenticeship, a disciple maximizes opportunities to observe the master's work at close range. The instruction "watch and learn" reflects the fact that, in acquiring tacit knowledge that cannot be verbalized, physical input through observation is of the utmost importance.

The imitation stage: The disciple attempts the motions the master performs. At this stage, rather than asking "why is it done this way," the disciple prioritizes the physical imitation of "how it is done." The craftsman's-world teaching that "the hands come before the theory" reflects the empirical rule that, in transmitting tacit knowledge, physical imitation is more effective than verbal explanation.

The accumulation of experience: Through repeated practice, individual judgments reach a state of automation in which they can be performed "without thinking." By repeating the same motions tens of thousands of times in the workshop, sensory judgment comes to fire in a manner close to a spinal reflex. This is the state referred to as having "acquired the craftsman's body."

Modern measurement technology and video recording have made it possible to visualize and quantify part of the tacit knowledge involved in Japanese sword making, but the essential fact remains unchanged: it must ultimately be acquired through the body. What the master-disciple system has passed down is not the "information" of the technique, but the "experience" of the technique itself.