The art of acid etching on steel and other metals is extremely ancient.
In its most basic form, the metal is covered with an acid-resistant coating – usually a wax – and the artist then scrapes away this coating with suitable tools to show the pattern he or she wishes to appear on the metal.
The metal is then dunked in a tank of strong acid for a suitable period, during which the acid will eat away the metal not protected by the wax.
After the acid has been washed away and the wax removed, the engraved pattern is revealed.
This is a simplification of the process which, in industrial applications, is much more sophisticated.
It is not unlike the process used to produce printing plates.
On some guns, the acid etch is further improved by a little hand work with traditional engravers’ tools.