Aquatint is an intaglio printmaking technique, a variant of etching. Like this, Aquatint uses the application of acid to make the marks in the metal plate (usually copper or zinc). But the artist is here able to creating a tonal effect in different, nearly unlimited, variations.
different tonal effects made with aquatint technique combined with some etched lines
For creating this effect the artist applies powdered resin which is acid resistant to the surface of the plate.
The plate is then heated; the resin melts forming a fine and even coat.
After the plate is covered with an acid-safe varnish on all parts of surface outside of the shape, it is dipped in acid, producing an even and fine level of corrosion (the "bite") sufficient to hold ink.
Now the resin and protective coating must removed to get the bare metal plate which is ready for printing.
For preparing the printing procedure the ink will careful pushed on the surface of the plate to covered them completely.
Then the ink will wiped off again from the plain parts of the plate surface.
The printing process is the same like for etching prints.
If the plate were printed with no further biting, the paper would display a more or less tonal effect directly in between white (no ink) and black (full ink).
The artist then begins immersing the plate in the acid bath, progressively stopping out (protecting from acid) any areas that have achieved the designed tonality. These tones, combined with the limited line elements, give aquatints a distinctive, watery look.