There are two classes of welding - forge or pressure welding, and autogenous welding. Forge or pressure welding is applied to two pieces of metal heated to a plastic state, which are forced together by pressure or hammering. The familiar example of this is the weld which the blacksmith makes by heating two pieces of steel or iron in the forge fire and then hammering the ends together on the anvil. Another form of forge or pressure welding is seen when two pieces of metal are heated by an electric current, and then forced together. This latter process is known as butt or spot welding.