Until the early part of the present century copper was practically the only metal used for engraving.  Only a limited number of impressions can be taken from a copper plate because it wears rapidly, and it is not suited to such work as the production of postage stamps.  About 1830 the way was found to make steel of sufficient softness and fineness of grain to be available for engraving.  To-day annealed steel is almost exclusively used for this purpose.  Annealed steel is steel which has been softened without being decarbonized.  The surface is carefully ground and polished to a mirror-like brightness.  Any work which is to be reproduced many times, such as postage stamps and parts of bank-notes, is made on small pies of steel called dies.

If the design to be used is in the shape of a drawing or engraving, a sheet of gelatin may be laid over it and the outlines traced with a sharp-pointed instrument.  More often a photograph is taken on a ferrotype plate and the outlines scratched into the plate.  These outlines are filled with vermilion.  A piece of paper is then laid on the plate and the two passed through a hand-press.  This is called “pulling” an impression.  While the ink of the impression is still moist it is sprinkled with powdered vermilion to strengthen the lines.  The block of steel is then covered with an etching ground (a composition of asphaltum, wax, resin and ether) and the impression is transferred to this.  The outlines are cut through the etching ground and bitten into the steel with acid.  The coating is then removed from the block and the artist proceeds with the engraving.  The mechanical details and various methods of engraving are highly interesting but time will not permit their discussion.

An engraver is seldom expert in more than one style of work.  Each makes a specialty of some branch, portraiture, lettering, scroll-work, etc.  For this reason several engravers are usually employed on each die for a postage stamp.  And in this inability of one individual to do all styles of work equally well lies one of the great securities against counterfeiting.

In the course of making a die, proofs are usually taken and these are much prized by collectors.

The die being finished, it is placed in a bath of cyanide of potassium and heated until the vessel containing it is red hot.  This process occupies from fifteen minutes to half an hour for dies but may take as much as an hour for a large plate.  The die is then transferred to a bath of oil, to cool and temper it.  By this process it is thoroughly hardened.

[Illustration:  From “The Popular Science Monthly,” Vol.  XLVI, No. 5.  Copyright, 1895, by D. Appleton & Co.]