Lines are bitten into the metal plate through the use of acid. To begin with, the plate is covered with a thin, acid-impervious coating called a ground which is smoked to a uniform black. Lines are drawn through the ground with a stylus, visibly baring the metal of the plate. Acid is then applied which eats into the exposed areas. The longer the plate is exposed to the acid, the deeper the bite and therefore the stronger the line. Different depths are achieved by covering some lines with acid-impervious varnish (stop-out) and biting others a second (or third) time. The ground is then removed and the plate inked and wiped. The appearance of etchings is usually free and spontaneous but the technique has occasionally been used to produce results almost as formal as engraving.
In all intaglio prints except mezzotint the design is produced from ink in lines or areas below the surface of the plate. The smooth surface is wiped of ink before printing, though some ink may purposely be left on the plate for tonal effects. Considerable pressure is used in the press to force the ink out of the lines and areas and, to an extent, to force the paper into them, so the final printed image will appear to be slightly raised above the surface of the uninked paper.