To sensitize the
paper, it is dipped for a couple of minutes in a solution of potassium
bichromate (1 in 25), then taken out and dried in the dark.
The paper is now placed beneath the drawing in a copying-frame, and
exposed for several minutes to the light; it is afterward laid in cold
water in order to remove all excess of chromate. A copy of the original
drawing now exists in relief on the swollen gelatine, and, in order to
make this relief sticky, the paper is next dipped for a short time in
water, at a temperature of about 28 deg. or 30 deg. C. It is then laid on a
smooth glass plate, superficially dried by means of blotting-paper, and
lamp-black or soot evenly dusted on over the whole surface by means of
a fine sieve. Although lamp-black is so inexpensive and so easily
obtained, as material it answers the present purpose better than any
other black coloring substance. If now the color be evenly distributed
with a broad brush, the whole surface of the paper will appear to be
thoroughly black. In order to fix the color on the tacky parts of the
gelatine, the paper must next be dried by artificial heat--say, by
placing it near a stove--and this has the advantage of still further
increasing the stickiness of the gelatine in the parts which have not
been acted upon by light, so that the coloring matter adheres even more
firmly to the gelatine. When the paper is thoroughly dry, place it in
water, and let it be played on by a strong jet; this removes all the
color from the parts which have been exposed to the light, and so
develops the picture.
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