Tamahagane Selection and Splitting: Raw Material Evaluation and Processing Techniques Performed by the Tōkō

What is Tamahagane?

Tamahagane is a uniquely Japanese iron-making product, produced by tatara (traditional smelting), in which iron sand is smelted in a furnace at ground level. From ancient times through the Edo period, Japanese tōkō (swordsmiths) used tamahagane as their primary raw material. Tamahagane takes its name from its roughly spherical shape. A single piece of tamahagane typically weighs anywhere from a few kilograms to several tens of kilograms.

The defining characteristic of tamahagane is its inhomogeneity. Because tatara involves extremely complex chemical reactions, even pieces of tamahagane produced in the same furnace will differ in carbon content, type and quantity of impurities, and crystal structure depending on the location within the mass. This very inhomogeneity was both the greatest challenge and the greatest opportunity tamahagane presented to the tōkō.

Tamahagane Selection Criteria Used by the Tōkō

The criteria a tōkō applies when selecting tamahagane can be organized into the following three points.

• Visual inspection: The tōkō closely examines the color and luster of the tamahagane's surface, checking for scratches and cracks. High-quality tamahagane has a distinctive surface luster and fine granular texture distributed uniformly. Inferior tamahagane often shows numerous cavities and large cracks.
• Ori shiken (folding test): The tōkō breaks a piece of tamahagane as a test in order to observe its internal structure. From the color, luster, and crystal size of the fractured cross-section, the tōkō estimates the carbon content and level of impurities. High-carbon tamahagane tends to have a whitish luster, while low-carbon tamahagane tends to appear dark. This empirical judgment is an exceptionally advanced artisan skill.
• Sense of weight: Even among pieces of tamahagane of the same size, some have higher density and others lower. Tamahagane that feels dense and has a substantial heft tends to have a sound internal structure. This is a subtle difference the tōkō can judge by the feel of the piece in hand.

The Process of Splitting Tamahagane

The process in which the tōkō splits (saku) tamahagane is extremely dangerous and requires advanced skill. The process proceeds in roughly the following sequence.

1. Heat the tamahagane to red heat (annealing). By heating to the appropriate temperature, internal stress is relieved and the piece becomes easier to split. Temperature control is extremely difficult: if too low the piece will not split, and if too high it will deform.
2. Mark the splitting point. While estimating the tamahagane's internal structure, the tōkō selects the position judged to be most effective for splitting. Past experience and intuition (kan) play a large role in this judgment.
3. Place a chisel at the marked point and strike carefully with a hammer. One must not attempt to split the piece with a single blow. By striking multiple times, cracks are gradually introduced. Throughout this process it is necessary to sense the properties of the tamahagane and make fine adjustments to the force applied.
4. Judge the progress of the split from the distinctive sound produced as the piece splits. One must not split it so much that it crumbles to pieces. Equally, a piece that has only partially split cannot be used. Work proceeds carefully until the piece is fully separated.

Evaluation and Grading of Split Tamahagane

After splitting the tamahagane, the tōkō evaluates it in detail once more. By splitting it, the internal structure is exposed, and its full picture can be seen for the first time. From the split cross-section, the tōkō observes the distribution of carbon content and the quantity and distribution of impurities (such as inclusions of iron sand).

Forging and Use of Split Tamahagane

The graded tamahagane proceeds next to the forging (tanzō) process. The tōkō reheats the split tamahagane in the furnace and shapes it by hammering while it is soft. In doing so, impurities are removed as much as possible and the iron crystals are aligned.

• High-carbon tamahagane: Because of its high hardness, it is suited for the cutting edge.
• Low-carbon tamahagane: Because of its greater flexibility, it is suited for the mune (back of the blade).

By combining multiple different types of tamahagane, the tōkō produces a sword that balances hardness and flexibility. This technique of "composite forging" is the secret that gives the nihontō its strength and beauty.

The Artisan Skill and Experience of Tamahagane Selection

Tamahagane selection and splitting are advanced skills that can only be acquired through decades of training. Only after seeing, handling, and splitting thousands of pieces of tamahagane does a tōkō come to understand their properties intuitively. It is not textbook knowledge but experience ingrained in the body that matters most.

The ability to distinguish good tamahagane greatly determines a tōkō's worth. Even among tamahagane produced in the same place and period, the quality of the resulting sword varies greatly depending on how it is selected and used. This is one of the reasons why, even using the same materials, the quality of the work differs from one tōkō to another.

The Transmission of Tamahagane Selection in the Modern Era

Japanese sword craftsmen continue to this day to carry out tamahagane selection and splitting. They procure tamahagane from traditional tatara smelters, apply their own skill to select the best pieces, and produce swords. Many elements of this process cannot be mechanized, and the craftsman's experience and intuition are indispensable.

Tamahagane selection is the most fundamental part of nihontō production. No outstanding sword is born without the selection of quality materials. Today's tōkō produce swords for the new era while preserving the selection techniques passed down from their predecessors. The toukenza online catalog allows visitors to discover swords that are the crystallization of such artisan skill. Encountering a masterpiece at the end of the long production process that begins with tamahagane selection will bring a deep understanding of nihontō culture.