The Latin term intaligo which means incise applies to the printing techniques which result in the
image being created below the surface of the plate. The material used for forming plates up to
the early 19th century was copper sheet, this diminished after the the invention of the
more economic mild steel. The Etcher prepared the plate by covering it with a wax based 'ground'
and the image was drawn in the wax using an etching needle,this technique allowed the artist to produce
a much more free and expressive image. The edges and underside of the plate were
sealed with varnish for protection and the plate was them immersed in nitric acid. The acid would bite into
the plate where it was exposed by the needle producing an incised image on the metal.

After the plate was bitten to the satisfaction of the etcher, it was removed from the acid bath and
washed in hot water to remove all traces acid and the remaining wax. An ink charged 'dabber' was
then used to work the ink into the incised areas of the plate which was then wiped to remove any
surface ink. A piece of paper was thenlaid on top of the plate and then passed through a
press allowing the ink to be absorbed by the paper and produce the printed image.

Se an example of etching here.

See also Engraving Lithography and Chromolithography