The Metallurgy Behind Jihada — How Itame, Mokume, and Ayasugi Patterns Form

What Is Jihada?

The jihada (surface texture) visible on a polished nihonto blade — particularly on the upper half toward the spine — is the steel's internal crystalline layer structure made visible through polishing. Different periods, schools, and forging approaches produce distinctly different patterns, making jihada one of the most reliable indicators of a sword's origin.

Major Jihada Pattern Types

Itame (plank grain): The most common type — irregular flowing streaks resembling wood plank grain. Standard result of folded forging; found across Bizen, Soshu, and Mino traditions.

Mokume (burl grain): Dense, close-set swirling patterns resembling wood burl. Common in Yamashiro-tradition swords, particularly Rai and Awataguchi schools.

Masame (straight grain): Parallel straight lines, rare in long swords but found in certain Yamato school pieces.

O-itame (large plank grain): Coarser, larger-scale version of itame; common in Nanbokucho-period large swords.

Ayasugi-hada (cedar-grain pattern): Unique diagonal herringbone stripes, virtually synonymous with the Gassan school. The living Gassan tradition preserves this technique today.

Nagare-hada (flowing grain): Unidirectional flowing streaks; associated with Satsuma's Naminohira school and blades made from continental iron.

The Metallurgical Mechanism

Jihada patterns form from differences in carbon content, crystal grain size, and layer orientation in the folded steel. Polishing selectively reveals these microstructural differences: high-carbon zones (pearlite) reflect light differently than low-carbon zones (ferrite); fine-grained versus coarse-grained regions; layer orientation from the fold direction. The distinct patterns of different schools reflect their characteristic forging temperatures, fold counts, and material choices.

Appreciation Guidance

Optimal jihada viewing requires raking light from natural window light or a quality LED spot, varying the blade angle to catch different surface areas. Evaluation criteria include fineness and density of pattern, uniformity across the blade, presence of ji-nie (glittering nie particles in the surface), and chikei (bright activity lines between hamon and surface). The table of school-to-jihada correspondences provides a starting framework, but reading jihada fluently requires examining many genuine examples firsthand.