A genuine hamon is produced through a process called differential hardening, or tsuchioki in Japanese. The bladesmith applies a clay mixture along the spine and flat of the blade, leaving the cutting edge relatively bare. When the blade is heated and then quenched in water, the exposed edge cools rapidly and hardens, while the clay-insulated spine cools more slowly and remains comparatively tough. The boundary between these two zones becomes the hamon - a visual line of crystalline contrast visible on the polished steel surface. Because the clay application is done by hand and the quench introduces unpredictable variables, every hamon is unique. A hamon formed this way is fundamentally different from the acid-etched or wire-brushed lines found on lower-cost decorative swords, which are surface treatments rather than structural evidence of the forging process.
