Many methods are used for joining materials. Wood is screwed or nailed, as well as being joined by glue. Metals might be welded, riveted or joined by nuts and bolts. Metal welding, as carried out by the blacksmith, is without doubt one of the most ancient of metal-working processes; yet, the modern technology of welding has undergone revolutionary change during the latter part of last century. In many instances where expensive riveting processes were once employed, steel sheets are now successfully joined by welding. The fabrication of the American ‘ Liberty ’ ships, during the Second World War, helped to establish this technique.