A katana (dao) is a single-edged Japanese sword that was invented between the Heian and Edo eras. A katana was fashioned by a swordsmith who melted Japanese sand iron, known as Tamahagane, and then folded the blade over and over to remove impurities. The result is a strong, lightweight sword with an almost indestructible core called tsuba (mounting). Modern katanas are made of steel. Katanas were commonly used in combat from 1180 to 1868, when they were finally replaced by the sword known as the shikomi. Today, katanas can be found in museums all over the world. Katanas are characterized by their sharp, curved blade which is usually between 30 and 50 centimeters in length. They vary in thickness from 1.5 to 3 millimeters at the edge, with the thicker part being near the hilt. The tip may be flat or rounded. Blades are usually black with a thin line of red along one side. The tang, which goes down into the handle, is usually made of steel or wood. The hilt usually has a pommel at one end and a guard at the other. It consists mainly of metal sheets wrapped around a core, like a watch spring. Wooden versions are also common. The katana's weight varies depending on the quality of the sword; high-end models can weigh up to 143 pounds (65 kg).