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To etch the plate, I prepare a series of ferric chloride baths (usually
5), ranging from 45-Baume to 39-Baume (a measure of concentration
at 68 degrees Fahrenheit). The first etchant can penetrate only
the thinnest resit, corresponding to the shadow areas of the image.
A more dilute etchant, containing more water, soaks through the
thicker resist and starts etching the mid-tones and highlights in
succession. If the resist has been over-exposed, the etchant will
never get through the highlights. A correctly exposed resist enables
etching of all tones with full details. Ultraviolet-sensitive materials,
unlike standard photographic films, respond in a linear way to the
full range of light intensity, retaining tonal differentiation in
the near-whites as well as in the shadow. Since etching generally
speeds up as it progresses, etching of highlights at the end must
be brief in order to retain the light tones. At the right moment,
the etching is ended by immersing the copperplate in cold water.
Depending on the coverage of dust in the aquatint and the exposure
of the gelatin, the etch can range between 20 minutes to an hour.
The resist and the acquatint having done their work, they now can
be removed from the copperplate with a variety of strong chemicals.
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