Iron and steel pipes can be distinguished, the one from the other, very often by means of their appearance. Iron pipe is rough, and the scale on it is heavy, whereas the scale on steel pipe is light and has the appearance of small blisters or bubbles, underneath which the surface is smooth and somewhat white. Steel pipe seldom breaks when flattened, but if a fracture does occur, it will be noticed that the grain is very fine. Iron pipe, when subjected to this test, breaks easily and shows a coarse fracture, due to the long fibre of the material. The impression prevails with many that steel pipe is so exceptionally hard that it is threaded with difficulty and that the threads are easily broken off. In realty, steel pipe is soft and tough; threads made in it do not break, but they tear off, to avoid which it is necessary that the cutting die shall be sharp so as to cut above the center. Dies suitable for steel pipe can also be used on iron pipe, but blunt dies that work succesfully on iron pipe will tear the threads on steel pipe, owing to the softness of the metal.