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.
A punch with a circular extremity, for making round holes into cold sheet iron and other metals, is about 6 in. long, and made of an old round file, to avoid forging. The file is first thoroughly softened along its entire length, and one end is reduced until of a proper diameter to make the holes desired; this reducing is often done with a grindstone, while the file is soft, when forging cannot be effected, and the intended cutting extremity is ground until flat. When properly shaped, the tool is hardened by heating to redness about 3 in. of its length, and placing about 1 in. into water, moving it to and fro as for hardening other tools; as soon as the tool's extremity is cold, it is taken from the water and cleaned, during which time the heat slowly softens the end, and when a blue colour appears at 1/4 or 1/2 in. from the extremity, the hard part of the punch is cooled, but the remainder is allowed to cool as slowly as possible, that it may be quite soft.
Square punches and other angular punches for hand use are of the same length as round ones, and are made of properly softened round and square files. Punches are not merely required to make holes; they are useful for smoothing and polishing the boundaries of various recesses that cannot be filed, scraped, or ground. A punch for such work is held in one hand, and applied to the work while the head of the punch is hammered until the surface in contact is shaped. Tools of this class have shaping extremities of various forms, some being curved and convex, others are concave, some are provided with ridges, knobs, teeth, and other protuberances, the extremities of others are rectangular, triangular, and oval, having recesses of several forms. All such punches require a careful polishing, both previous to hardening and afterwards, and the better the polish given to the punch, the smoother will be the surface to be punched. The ends of such tools are specially tempered after hardening, to suit their respective shapes, those extremities which are broad, and consequently strong, being tempered to a brown, unless the steel happens to be a brittle cast steel, for which metal the temper denoted by blue is necessary.