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Apart from engraving techniques of direct
incision a large range based on chemical
incision exists, in which the metal plate is
bitten by means of a mordent.
Etching is the most important technique in this
group and at the roots of all others. The artist
carves into the desired areas using a nitric
acid solution, commonly known as aqua-fortis.
Nowadays etching is a general term not only for
engravings carried out with nitric acid, but
indistinctly applied to any etching done with,
for instance, iron perchloride or hydrochloric
acid.
The metal plate must be protected from overall
mordent action, and is therefore covered with an
acid-resist or ground which is sufficiently
ductile to allow drawing and to expose the metal
under the artist’s traces, while being resistant
to the mordent. This protective ground normally
contains a resin or wax base which makes it
tougher and more resistant, while Jew’s pitch is
added to achieve a dark tone that helps to
distinguish the artist’s traces and adds further
protection against the mordent. After the
printing plate has been cleaned of greasy or
oily impurities and the ground applied, the
image may be traced upon it with a stylus,
exposing the metal accordingly. After the image
has been engraved, the plate is bathed in a
mordent solution to bite the drawing. Bathing
time depends on various factors: the desired
trace depth, the mordent degree or acid/salt
concentration used, the working temperature,
whether the acids have been employed previously
etc.. The degree of concentration is measured on
the Beaumé scale with a muriatic acid
densimetre. With a copper plate the most
adequate acid to be used is Dutch mordant, a
solution of hydrochloric acid, potassium
chlorate and water. This mordent is slow but
remains closest to the etcher’s traces and gives
excellent results for aquatint and fine
textures. Nitric acid is as often used as Dutch
mordant but whereas its reaction time is much
faster, the results are less accurate. It is
mostly applied on zinc plates, which in turn are
used frequently. Another oft-employed mordent on
copper plates is iron perchloride, a nontoxic
salt which does not cause fumes or burns but
does leave aggressive stains. Although it is as
slow as Dutch acid, it creates clear and
delicate traces.
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