After the smith hammers all slag from the tamahagane, he heats the hard, high-carbon steel and shapes it into a long, U-shaped channel. He then hammers the tough, low-carbon steel, which he has shaped so it will make a snug fit into the channel and forges the two metals together. Both types of tamahagane are now exactly where they need to be: the hard steel forms the sword's outer shell and deadly blade, while the tough steel serves as the katana's core. This perfect balance of properties is what made the katana the samurai's most durable and prized weapon.