Tamahagane Grading and Selection: Criteria for Sorting Steel into Kawagane and Shingane After Tatara Smelting

What Is Tamahagane? — The Special Nature of Japanese Sword Materials

Tamahagane (玉鋼) refers to high-quality steel blocks selected from among the non-uniform carbon-content steel ingots produced through tatara smelting that are suitable for Japanese sword production. Its name means "steel like a jewel" and reflects the appreciation for the beauty of its fracture surface and luster.

In industrial modern steelmaking, uniform-quality steel with precisely controlled carbon content can be mass-produced. In contrast, tamahagane is a non-uniform material whose carbon content varies widely from 0.5 to 1.5% depending on location, and this non-uniformity enables sword smiths to select suitable pieces for kawagane and shingane, contributing to the formation of the unique jihada and hamon characteristic of Japanese swords.

Kera Removal and Initial Classification of Tamahagane

When tatara smelting operations end, the work of "kera removal" begins, in which the furnace (takedono) is dismantled and the steel ingot "kera" is extracted. Kera is a several-ton steel ingot that accumulated at the bottom of the furnace, and its cross-section is composed of multiple layers.

The upper part of the kera (near the center of the furnace) has high carbon content (approximately 1 to 1.5%) and tends to become the hardest steel. This is the part that is classified as the highest-quality tamahagane. The middle section of the kera has medium carbon content and is often utilized as shingane. The bottom of the kera and areas near the furnace walls have low carbon content (0.1 to 0.5%) and are used as soft "zuku" or iron for agricultural and industrial tools.

This initial classification is performed by an experienced murage (the overall supervisor of tatara smelting) during the dismantling and removal of the furnace.

The Technique of Sword Smith Selection of Tamahagane

Tamahagane distributed from Nihon Touken Hozonkai tatara undergoes further precise selection by the sword smith himself when it arrives at the sword smith's workshop. The sword smith's eye for quality—that is, the ability to discern the quality of tamahagane through visual, tactile, and experiential judgment—is an essential skill that forms the foundation of the sword smith's craft.

Observation of Fracture Surfaces

One of the most important criteria in tamahagane selection is the observation of "fracture surfaces" (waguichi). By observing the color, luster, grain size, and structure of the cross-section exposed when tamahagane is struck with a hammer, one can estimate the carbon content and the state of the iron's crystalline structure.

The fracture surface of high-carbon tamahagane (for kawagane) shows a fine granular crystalline structure with a silvery-white luster. This reflects the fine microstructure of martensite and pearlite unique to high-carbon steel. Low-carbon tamahagane (for shingane) tends to show a grayish coarse granular cross-section. "Zuku" with excessively high carbon content (over 1.5%) exhibits a whitish brittle fracture surface and requires special processing because it is difficult to handle in folding and forging.

Judgment by Color and Luster

The color of the fracture surface (from silvery-white to gray to black) and the intensity of the luster serve as indicators of carbon content and the amount of impurities (slag) present. Tamahagane with heavy slag inclusion shows black streaks and voids in the fracture surface and must be expelled through repeated forging. High-quality tamahagane is characterized by uniform luster and minimal slag.

Classification into Kawagane and Shingane

The selected tamahagane is classified as either "kawagane" or "shingane" based on its carbon content and quality. Kawagane is a high-carbon steel used for the outer skin of the blade (including the edge) and is involved in hardness and hamon formation. Shingane is a low-carbon soft steel used for the core of the blade and provides toughness.

This composite structure of "high-carbon outer skin and low-carbon core" is the essence of the design that simultaneously realizes the three characteristics of Japanese swords: "does not break, does not bend, and cuts well."

Coordination with Folding and Forging

Selected and classified tamahagane undergoes the processes of "tumi-wakashi" and "ori-kaeshi tanren" to equalize carbon content and expel slag. The sword smith adjusts the number of folds to bring the final carbon content of the kawagane close to the target value (approximately 0.7 to 1.2%).

If the carbon content of the kawagane is too high, the blade becomes brittle and prone to edge chipping; if too low, it becomes soft and loses cutting ability. The sword smith accomplishes this invisible work of "adjusting to the proper carbon content" by empirically controlling three variables: the number of folds, heating temperature, and forging intensity. The higher the precision of tamahagane selection, the easier the adjustment work becomes, and the more stable the final quality of the jihada and hamon.

The quality of Japanese swords is already decisively influenced at the stage of "tamahagane selection" before the blade is made. The eye for quality of an excellent sword smith is truly the comprehensive knowledge of sword smithing, a unity of the ability to assess materials and technical expertise.