During this period, the tradition of Japanese swordsmithing developed using laminated or piled steel, a technique dating back over 2,000 years in the Mediterranean and Europe of combining layers of soft and hard steel to produce a blade with a very hard (but brittle) edge, capable of being highly sharpened, supported by a softer, tougher, more flexible spine. The Japanese swordsmiths refined this technique by using multiple layers of steel of varying composition, together with differential heat treatment, or tempering, of the finished blade, achieved by protecting part of it with a layer of clay while quenching (as explained in the article on Japanese swordsmithing). The craft was perfected in the 14th century by the great swordsmith Masamune. The Japanese sword ("tachi" and "katana") became renowned around the world for its sharpness and resistance to breaking. Many swords made using these techniques were exported across the East China Sea, a few making their way as far as India.