The report then proceeds to give the following details of a process of
photo-engraving, which was exhibited before the society by M. Garnier
in March last:
PHOTOGRAVURE.
In photo-engraving a distinction must always be made between the
reproduction of drawings in line and those with shaded tints.
A.--_Photo-engraving of Line-work._--A plate of copper is prepared by
covering it, either by flowing or with a roller, with a very thin
coating of a solution of:
Sugar 2 grammes.
Bichromate of ammonia 1 gramme.
Water 14 grammes.
This coating is equalized and quickly dried by means of an arrangement
which keeps it in rotation over a warm plate.
As soon as the plate is dry, a positive cliche of the drawing to be
reproduced is laid upon it, and the whole exposed to the sun for a
minute, or to the electric light for three minutes. The reaction
produced is the same as with the citrate of iron, but much quicker;
the exposed parts are no longer hygroscopic, but in the parts
protected by the lines of the drawing the sensitive coating has
retained its stickiness, and will hold any powder that may be passed
over it, thus producing a very clear image of the drawing. The coating
being excessively thin, the little moisture it holds and the powder
applied suffice to break its continuity, especially if the powder be
slightly alkaline.
Pages:
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155