The rise in popularity of katana by samurai is believed to have been due to the changing nature of close-combat warfare. The quicker draw of the sword was well suited to combat where victory depended heavily on fast response times. The katana further facilitated this by being worn thrust through a belt-like sash (obi) with the sharpened edge facing up. Ideally, samurai could draw the sword and strike the enemy in a single motion. Previously, the curved tachi had been worn with the edge of the blade facing down and suspended from a belt.
At this stage it is only slightly curved or may have no curve at all. The gentle curvature of a katana is attained by a process of differential hardening or differential quenching: the smith coats the blade with several layers of a wet clay slurry which is a special concoction unique to each sword maker, but generally composed of clay, water and any or none of ash, grinding stone powder, or rust. The edge of the blade is coated with a thinner layer than the sides and spine of the sword, heated, and then quenched in water (some sword makers use oil to quench the blade). The slurry causes only the blade's edge to be hardened and also causes the blade to curve due to the difference in densities of the micro-structures in the steel