Shitaji Tanren and Sunobe — The First Forging Stages That Build the Skeleton of the Blade, and Standards for Material Selection

An Overview of Japanese Sword Forging — Where Does It Begin?

The forging process for a Japanese sword is conventionally described as a sequence: "orikaeshi tanren → sunobe → hi-zukuri → tsuchioki → yaki-ire → polishing." In practice, however, there is a preparatory stage called shitaji tanren (preparatory forging) that precedes the orikaeshi tanren, and it is this stage that lays the foundation for every subsequent process.

Sunobe is also the first stage at which the steel is "given shape" after orikaeshi tanren, and it is a critical juncture that determines the skeleton of the blade — the rough form of the nagasa and sori.

This article centers on these two stages and provides a detailed explanation of "how tōkō assess materials, and how they build the shape of the blade."

Shitaji Tanren — Preparing the "Rough Stage"

Starting with the Selection of Tamahagane

Before forging begins, the tōkō first selects the tamahagane that will serve as raw material. Tamahagane shipped from a tatara ironworks varies in carbon content, impurity level, and density from piece to piece. The tōkō strikes a test piece with a hammer, then examines the color of the fractured surface, the size of the crystal grains, and the presence of voids, classifying the tamahagane as follows.

Standards for Tamahagane Selection

• Jōtama (high-grade tamahagane): The fractured surface is close to silver-white, and the crystal grains are fine. Carbon content is high; used for kawagane (the high-carbon jacket layer at the edge).
• Nakatama (mid-grade tamahagane): Intermediate in character. Used according to the intended application.
• Getatama (low-grade tamahagane): Low in carbon, coarse in grain. Used for shingane (the low-carbon core layer) or as a supplementary structural material.

Neglecting this selection means that no matter how much skill is applied in later processes, the "poor quality of the material" will manifest in the quality of the blade. This is why it is said that "before forging, one must have the eye of an appraiser."

Tsumi-wakashi — Consolidating the Steel

Once the tamahagane has been selected, a process called tsumi-wakashi stacks small pieces of steel together to homogenize the iron.

Process Details

1. Stack approximately 10 to 20 selected pieces of tamahagane and weld them onto a metal rod (tsuchibo).
2. Heat to white-heat temperature in the forge.
3. Strike the whole with a large hammer to integrate them into one mass (at this stage, slag and impurities are expelled).
4. Sprinkle rice straw ash (warahai) on the surface to act as flux, dissolving the scale (oxidized film).

Performing this process with care advances the homogenization of the tamahagane and allows the subsequent orikaeshi tanren to achieve maximum effect.

Warigane and Oroshigane

In addition to tamahagane, tōkō sometimes use what is called warigane — old iron made by re-melting and refining used farm tools and iron implements. Its carbon content is low, but its homogeneity is high. Using it for shingane yields a core material with high toughness.

When the carbon content of the tamahagane is too high, it may also be mixed with iron as oroshigane to adjust the carbon level.

Shitaji Orikaeshi Tanren — Folding at the Rough Stage

After tsumi-wakashi is complete, the steel billet undergoes several rounds of shitaji orikaeshi (preparatory folding) before the full orikaeshi tanren begins.

Purpose and Procedure

Purpose

• Strike out and eliminate impurities (slag) within the steel.
• Homogenize the structure of the steel to a certain degree.
• Even out the overall carbon distribution through the "rough, large-scale folding" of the shitaji tanren stage.

Procedure

1. Extend the white-hot steel billet thinly with a hammer.
2. Fold it from one end to form two layers.
3. Return it to the forge, heat again, and strike to join the layers.
4. Repeat this approximately 4 to 8 times (this serves as a preliminary step for the full orikaeshi tanren).

At this stage, "organizing the material" is the purpose rather than precision folding, so the work is understood to be rougher than the main process. However, if the removal of impurities is insufficient, jikizu (surface flaws) may later appear within the hamon.

Transition to Hon-tanren — From Preparatory to Full Forging

Once shitaji tanren is complete, the steel is in a state prepared as "material for making a sword." In the full orikaeshi tanren (hon-tanren) that follows, finer folding is repeated to push the homogeneity of the steel to its limit, building the fine microstructure that gives rise to the "scenery of the hamon."

A Guide to the Number of Folds in Hon-tanren

• Typical uchigatana: approximately 10 to 15 times
• High-quality work: approximately 12 to 16 times
• Folding too many times burns off the carbon (shōson), and the benefit of folding is conversely lost.

Sunobe — The Process of Building the Blade's Skeleton

After shitaji tanren and hon-tanren are complete, the steel is extended into the basic form of the blade in the sunobe stage.

Purpose of Sunobe

The purpose of sunobe is to extend the small steel billet thinly and lengthwise to create "the rough skeleton of the blade." The following are determined at this stage:

• A rough setting of the nagasa (blade length)
• The rough direction of the sori (curvature)
• The broad outlines of the motohaba-to-sakihaba taper

The Technique of Sunobe — How Is the Steel Extended into "Sword Form"?

In sunobe, the direction in which the steel is extended is controlled by the manner, angle, and force with which the hammer is applied.

Extending in the ha (blade edge) direction: Striking from the ha direction (vertical direction) extends the blade thinly and broadly.

Extending from the mune direction: Striking from the mune (back of the blade) extends the blade long and narrow.

Diagonal striking: To create taper (the difference in width between motohaba and sakihaba), the steel is struck with gradually varying force from the base toward the tip.

The most difficult part is the work of "adding sori." At the sunobe stage, the steel is finished in a form that accounts in advance for the deformation that will occur during yaki-ire, so that "the sori will be correct after yaki-ire." This "prediction of the amount of sori before yaki-ire" is a craftsman's intuition grounded in experience, and it is considered one of the most important secrets transmitted orally from master to apprentice.

"Ara-kiri" After Sunobe

Once sunobe is complete, the excess ends of the steel are cut away in a process called ara-kiri (rough cutting) to rough out the general external form of the blade. The blade at this stage is called "ara-kiri sunobe," and it is now ready to advance to the precision-shaping processes of "uwase," "kozukuri," and others.

Why the Eye for Material Selection Is Tested

In the stages from shitaji tanren through sunobe, the tōkō's "eye for materials" is tested most of all.

• Using material with a higher-than-designed carbon content for kawagane makes the blade prone to cracking during yaki-ire.
• Using non-uniform material for shingane creates locally brittle areas.
• Failing to sufficiently remove impurities during shitaji tanren leaves flaws that cannot be corrected in later processes.

"Without a good shitaji, no masterpiece can be born" — this principle is why tōkō regard shitaji tanren and sunobe as "unglamorous but supremely important stages."

Conclusion — The Invisible Stages That Determine the Sword's Fate

Shitaji tanren and sunobe do not appear directly in the finished sword's outward appearance. Yet it is the material selection and forging precision at this stage that build the foundation for everything that follows — the beauty of the hamon, the density of the jigane, and the strength of the blade.

"A good sword comes from a good shitaji" — these words capture the essence of Japanese sword forging in a single phrase. More than half of the tōkō's work is, in fact, devoted to "preparation before giving shape."