This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Forging metal consists in raising it to a high temperature and hammering it into any form that may be required. Good wrought iron may be seriously injured by want of care or skill in forging it to different shapes, Repeated heating and reworking increases the strength of the iron up to a certain point; but overheating may ruin it; the iron should therefore be brought to the required shape as quickly as possible. The form given to forgings is also important; there should be no sudden change in the dimensions - angles should be avoided - the larger and thicker parts of a forging should gradually merge by curves into the smaller parts. Experiments have shown that the continuity of the fibres near the surface should be as little interrupted as possible; in other words, that the fibres near the surface should lie in layers parallel to the surface. If wrought iron be "burnt," i. e. raised to too high a temperature, its tensile strength and ductility are both seriously reduced. These qualities may, however, be to a great extent restored by carefully reheating and rerolling the iron. Forging steel requires still more care in order to avoid overheating. Each variety of steel differs as to the heat to which it can safely be raised.