Half-tone lines, or the marks
of a red pencil on the original, are not reproduced in the nigrographic
copy.
"Anthrakotype" is a kind of dusting-on process. It was invented by Dr.
Sobacchi, in the year 1879, and has been lately more fully described by
Captain Pizzighelli. This process--called also "Photanthrakography"--is
founded on the property of chromated gelatine which has not been acted
on by light to swell up in lukewarm water, and to become tacky, so that
in this condition it can retain powdered color which had been dusted
on it. Wherever, however, the chromated gelatine has been acted on by
light, the surface becomes horny, undergoes no change in warm water, and
loses all sign of tackiness. In this process absolute opacity in
the lines of the original drawing is by no means necessary, for it
reproduces gray, half-tone lines just as well as it does black ones.
Pencil drawings can also be copied, and in this lies one great advantage
of the process over other photo-tracing methods, for, to a certain
extent, even half-tones can be produced.
For the paper for anthrakotype an ordinary strong, well-sized paper must
be selected. This must be coated with a gelatine solution (gelatine 1,
water 30 parts), either by floating the paper on the solution, or by
flowing the solution over the paper. In the latter case the paper is
softened by soaking in water, is then pressed on to a glass plate placed
in a horizontal position, the edges are turned up, and the gelatine
solution is poured into the trough thus formed.
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