crucible process

crucible process, technique for producing fine or tool steel. The earliest known use of the technique occurred in India and central Asia in the early 1st millennium ce. The steel was produced by heating wrought iron with materials rich in carbon, such as charcoal in closed vessels. It was known as wootz and later as Damascus steel. About 800 ce the crucible process appeared in northern Europe—likely as a result of trade contact with the Middle East—where it was used to make the high-quality Ulfbehrt swords used by the Vikings. The process was devised again in Britain about 1740 by Benjamin Huntsman, who heated small pieces of carbon steel in a closed fireclay crucible placed in a coke fire. The temperature he was able to achieve (1,600 °C [2,900 °F]) was high enough to permit melting steel for the first time, producing a homogeneous metal of uniform composition that he used to manufacture watch and clock springs. After 1870 the Siemens regenerative gas furnace replaced the coke-fire furnace; it produced even higher temperatures. The Siemens furnace had a number of combustion holes, each holding several crucibles, and heated as many as 100 crucibles at a time. All high-quality tool steel and high-speed steel was long made by the crucible process, but in the 20th century the electric furnace replaced it in countries where electric power was cheap.