The tamahagane is heated and quenched many times - a process that alters the molecular form of the steel. Thanks to the clay slurry applied before heating, the different parts of the blade set into different crystal structures, with a particularly hard form of steel, known as martensite, at the edge of the blade, and a softer structure along the spine. The resulting difference in densities also causes the blade to curve, creating its iconic shape (Credit: Getty Images)