Aquatint is a form of etching perfected in the 1770s particularly for
the depiction of watercolour washes and tone.
The effect was created when a granulated resin (or powdered asphalt)
ground was applied to a copper plate with heat. As it cooled the particles
in the resin left a random pattern of exposed copper which when immersed
in an acid bath etched a network of lines between the grains of resin
producing a randomly speckled appearance to the image when printed.
Different tonalities were achieved by stopping out areas of the plate
or coating particular areas of the plate with a different grained ground.
Spirit-grounds (resin suspended in alcohol) where employed to produce
a reticulated pattern, with the coarser grains gravitating to the bottom,
if the plate was inclined, resulting in a more realistic landscape effect.
It was not uncommon for either engraved or etched lines to be incorporated
into the image.