The oldest known art forms are stone carvings. Initially, they were often directly carved in the rock. Before the discovery of steel, all stone carvings were made using an abrasion technique, which consisted of roughly cutting a block of stone with a hammer. The main reason is that bronze, which was the hardest metal available until the arrival of steel, was not strong enough. The development of iron made possible the creation of tools intended for stone carving, such as chisels, drills and saws made from steel, which could be hardened to become hard enough to cut stone without deform.