A real hamon is the visible boundary line between the hardened edge and the softer spine of a differentially tempered blade. It forms during the clay-tempering process, where clay is applied to the spine before quenching, causing the uncoated edge to cool faster and harden into martensite while the spine remains in a softer pearlitic state. The hamon appears as a misty or cloud-like band running parallel to the edge, often with distinct activity - nie (granular crystalline structure visible to the naked eye) or nioi (a finer, mist-like boundary) - that shifts visibly under raking light. The key distinction from a cosmetic or acid-etched hamon is that a real hamon is three-dimensional and embedded in the steel structure, not a surface-applied pattern. When you tilt a clay-tempered T10 or 1095 blade under a directional light source and the hamon changes in character - appearing brighter, sharper, or more detailed depending on angle - that is the clearest indicator of authentic differential hardening. Acid-etched lines tend to look flat and consistent regardless of lighting angle.
