Etching is a collective term of gravure techniques produced with metal plates,
which are treated with acid. I use very fine ground French zinc plates and
nitric acid. It’s a question of definition, if also dry point should be regarded as
one of this group, as it is worked out only by engraving with a needle without
use of acid.

The real etching is made with a varnish covered plate. Drawing on it with a
needle you take away the varnish with each line. If you afterwards lay the
plate into the acid, this can attack the metal,  etching the drawing the deeper
into the plate the longer you let the acid work. After removing the varnish you
can rub color into this deepening. Covering the colored plate  with a wet
handmade paper you roll it through a print machine and get a mirrored print
of your drawing.

All other etching techniques are variations of this principle. The most
common is aquatint. Instead of a varnish you use asphalt or colophony
powder. The tiny grains are melted onto the plate and the acid can etch
between them. By this way you get a raster like surface. The longer you let the
acid attack the deeper become the raster holes and the darker is the color you
get on the print. By stopping the etching process covering some areas with
varnish you get nuances from very light to very dark tones. Normally etchers
work with five variations. Using a very thinned acid I normally get a variety of
12 or more nuances. The result is a more painting like effect.

Mezzotint is made on plates , which are deeply roughened before the shaping
process. The basic color will be very dark and by polishing you can produce
lighter areas.

Vernis mou is using a very soft varnish. You can lay flat things like leafs on a
vernis mou prepared plate and after rolling it through the printing machine
and taking the things away you get a fine structured surface, which now can
be etched.

Open etching means etching parts of the plate with very strong acid with the
effect that the entire part is deepened into the plate. The print later shows
light gray areas with dark zones at the borders.

Of course you can combine all techniques. For making the plates of my
Memento Mori  book I began with a normal etching, worked then with open
etching and thereon with aquatint and later with etching again.
Two states of a Memento Mori print.
left: print after etching and open etching     right: print after aquatint and polishing