The Technique of Etching
“This is an ‘indirect’ method of engraving, given that the incisions on the printing plate are obtained when the metal surface is eaten into by acid, which was once known as aqua fortis (hence the Italian name for this technique: acquaforte). The etching is obtain by first covering a metal plate with a substance that will not be eaten into by the acid; in the past, wax was used, but now a dark varnish that does not react with the acid. The engraver then traces his composition on the surface using a fine point, solely scratching the varnish, not incising the metal. The plate is then immersed in acid, which eats into the exposed areas where the varnish has been removed. The longer the plate is left in the acid, the deeper the incisions obtained.
When that process is complete, the varnish is removed using a solvent and the plate is ready
to be inked for printing (…)”

(Translated from P. Bellini. Dizionario della Stampa d’Arte, Garzanti Editore 1995).