Shitaji Tanren and Sunobe: The First Forging Stages That Establish the Skeletal Form of a Japanese Sword

The Full Picture of Japanese Sword Forging — Where It Begins

The forging process of a Japanese sword is conventionally explained as: "folding and hammering → rough drawing → fire forming → clay coating → quenching → polishing." However, in reality, there exists a preparatory stage called "ground forging (下地鍛え)" before entering the folding and hammering phase, and this lays the foundation for all subsequent processes.

Additionally, "rough drawing (素延べ)" is the first process of "shaping" that occurs after folding and hammering is complete, and it is a critical juncture where the skeletal structure of the blade — the rough form of edge length and curve — is determined.

This article focuses on these two processes to detail how the swordsmith discerns materials and creates the blade's shape.

Ground Forging — Refinement of the "Rough Stage"

Beginning with Tamahagane Selection

Before forging begins, the swordsmith selects tamahagane (玉鋼), which serves as the raw material. Tamahagane shipped from the tatara ironworks varies in carbon content, impurity levels, and density. The swordsmith observes the color of the fracture surface from a test fracture made with a hammer, the size of crystal grains, and the presence or absence of voids, and performs the following classification:

Tamahagane Selection Standards

• Jōtama (上玉)： The fracture surface is close to silvery-white, with fine crystal grains. High in carbon content, used for the skin iron (the high-carbon layer of the edge)
• Nakatama (中玉)： Intermediate properties. Selected for use depending on the application
• Getama (下玉)： Low in carbon, with coarse grains. Used for the core iron (the low-carbon layer of the core), or as auxiliary structural material

Neglecting this selection will cause the material's shortcomings to appear in the blade's quality no matter how skillfully later processes are executed. This is why it is said that "discernment is needed before forging."

Tsumiwakashi (積み沸かし) — Building the Foundation of Steel

After tamahagane is selected, the "tsumiwakashi" process is used to achieve steel homogeneity by stacking small steel pieces.

Process Details

1. Stack 10 to 20 pieces of selected tamahagane fragments and weld them to a metal rod (hammer handle)
2. Heat in a furnace to white-heat temperature
3. Strike with a large hammer to unify them (slag and impurities are expelled at this stage)
4. Sprinkle straw ash (wara-hai) on the surface as a flux to dissolve scale (oxidation film)

By performing tsumiwakashi carefully, the tamahagane becomes increasingly homogeneous, and the effect of the subsequent folding and hammering is maximized.

Wari-gane (割り鉄) and Oroshi-gane (卸し金)

In addition to tamahagane, the swordsmith may also use "wari-gane," old scrap iron. Old agricultural tools and iron products are remelted and refined, resulting in iron with low carbon content but high homogeneity. Using this for the core iron yields core material with high toughness.

When the carbon content of tamahagane is excessively high, it is mixed with iron as "oroshi-gane" to adjust the carbon content.

Ground-Stage Folding and Hammering — Folding in the Rough Phase

After tsumiwakashi is complete, the steel ingot undergoes "ground-stage folding" several times before formal folding and hammering begins.

Purpose and Procedure

Purpose

• To drive out and eliminate impurities (slag) within the steel
• To homogenize the steel's structure to some degree
• Through "rough, large-scale folding" as ground forging, to even out the overall carbon distribution

Procedure

1. Heat the steel ingot to white-heat and thin it with the hammer
2. Fold from the end to create two layers
3. Return to the furnace, heat, and strike to join
4. Repeat this 4 to 8 times (as "preparation" for formal folding and hammering)

During the ground forging stage, the objective is "material refinement" rather than precision in folding, so it may be done more roughly than formal folding and hammering. However, if impurity removal at this stage is insufficient, defects may later appear as "ground flaws (地疵)" within the hamon.

Transition to Formal Forging — From Ground to Finished Work

Upon completion of ground forging, the steel is now in a state prepared as "material for sword-making." In the subsequent formal folding and hammering (main forging), increasingly refined folding is repeated to bring steel homogeneity to the extreme, creating the fine microstructural organization that produces the "landscape" of the hamon.

Guide to the Number of Folds in Main Forging

• Standard blade swords: approximately 10 to 15 folds
• High-quality works: approximately 12 to 16 folds
• If too many folds are made, carbon will burn away (焼損), and the folding effect disappears

Rough Drawing — The Stage of Creating the Blade's Skeletal Structure

After ground forging and main forging are complete, the steel material is stretched into the basic blade shape during the "rough drawing (素延べ)" process.

Purpose of Rough Drawing

The purpose of rough drawing is to thin and lengthen a small steel ingot to create "the rough skeletal structure of the blade." At this stage, the following are determined:

• The approximate setting of edge length (nagasa)
• The rough direction of the curve
• The rough outline of the taper between heel and tip widths

The Technique of Rough Drawing — How to Stretch into "Blade Shape"

In rough drawing, the direction in which the steel is stretched is controlled by the angle, placement, and force of the hammer strikes.

Stretching in the edge direction: Striking the hammer from the edge direction (vertical direction) stretches the blade thin and wide Stretching in the spine direction: Striking from the spine (back) stretches the blade long and narrow Diagonal striking: To create taper (the difference in width between heel and tip), gradually vary the force while striking from heel to tip

Particularly challenging is the task of "imparting curvature." At the rough drawing stage, anticipating the deformation that will occur during quenching, the smith intentionally finishes the shape so that "after quenching it will have the correct curve." This "prediction of curve amount before quenching" is a craftsman's intuitive sense based on experience, one of the most important secrets passed down through oral tradition from master to apprentice.

"Ara-kiri" (粗切り) After Rough Drawing

After rough drawing is complete, the "ara-kiri" (rough cutting) process removes excess steel at the edges, finishing the rough outer shape of the blade. The blade at this stage is called "ara-kiri sunobe," and it is now ready to advance to the precise forming processes such as "uwase (上せ)" and "kozukuri (小造り)."

The Importance of Material Discernment

From ground forging through rough drawing, the swordsmith's "eye for materials" is most severely tested.

• If material with higher-than-design carbon content is used for the skin iron, cracks will occur during quenching
• If heterogeneous material is used for the core iron, locally brittle sections will develop
• If impurities are not sufficiently removed during ground forging, irreparable defects remain that cannot be corrected in later processes

"Poor foundation yields no masterpiece" — this principle is why swordsmiths regard ground forging and rough drawing as "unglamorous yet essential processes."

Summary — Unseen Processes Determine the Sword's Fate

Ground forging and rough drawing do not directly appear in the finished blade's appearance. However, the material selection and forging precision at this stage lay the foundation for everything that follows: the beauty of the hamon, the density of the jitetsu, and the blade's strength.

"A good sword comes from good foundation" — this phrase expresses the essence of Japanese sword forging in a single sentence. More than half of the swordsmith's work is actually spent in the "preparation before shaping."