Sheets of metal have coin-shaped pieces cut from them. These are known as blanks, which will become coins once they are struck by dies. Blanks are usually round in shape, but an exception is the blank for Australia's 50 cent coin, which is dodecagonal (12 sided).
Blanks are rolled through a specially-shaped groove, which results in a raised edge. The raised metal assists in the coining operation by partially forming the rim so that the dies do not have to displace as much metal.
Blanks used for making proof coins undergo further treatment processes. Annealing, or controlled heating in a furnace, softens the proof blanks in preparation for striking. Pickling in a bath of hot acid cleans the blanks, and burnishing (spinning in a large drum with tiny ball bearings used as polishing material) smooths any surface blemishes.