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 cliché 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. If the rest of the surface were sufficiently
resisting, the plate might be bitten at once; but light alone is not
enough to produce complete impermeability: the action of heat must be
combined with it. The plate is, therefore, placed on a grating, with
wide openings, a large flame is applied underneath, and it is heated
till the borders where the copper is bare show iridescent colors. The
sugary coating thus becomes very hard in the exposed parts, but under
the powder it is broken, porous, and permeable to acids. The surface
is then covered with the biting fluid, which is a solution of
perchloride of iron at 45° Baumé, and after few minutes' contact the
plate is engraved. It only remains to clear off the bichromated sugary
coating which forms the reserve, and which, being hardened by the
heat, resists ordinary washing. It is removed perfectly by rubbing the
surface with a hard brush and warm potash lye; the plate is then ready
for printing. Sometimes it may be necessary to give several successive
bitings, or to use a resinous grain; in such cases the various methods
of the engraver's art are employed.
B.--_Photo Engraving for Half-Tones._--To reproduce by engraving the
image of any object, a portrait, or a landscape, the gradation of tint
is obtained by repeating three times in the following manner the
operation A, just described:
The copper plate being prepared as before, it is exposed to the light
under a positive, and given a long exposure, say four minutes, in the
electric light. The sugary coating hardens under the whites and the
lighter shades--it only remains tacky under the blacks. The positive
cliché is removed, the plate powdered, and bitten; the blacks alone
come out.
The plate is cleaned, then coated again with the sugary preparation,
and exposed a second time under the positive, care being taken to
preserve an accurate register, which may easily be done. The second
exposure is not so long as the first--say two minutes, and gives the
image of the middle tints and blacks. The plate is powdered and bitten
as before, bringing out the middle tints, and, at the same time,
giving greater depth to the shadows.
In the third operation, the plate is exposed still less to the
light--say one minute. The high-lights alone harden; the light shades,
middle tints, and the shadows remain permeable. After powdering and
biting, the plate is finished.
When necessary, after each operation, a resinous grain may be applied
in the manner usual with engravers.
It is important to note that M. Garnier affirms that in both cases the
engravings are untouched, and that this is one of the essential
characteristics of his process.
C.--_Engraving in Relief for Letter-Press._--In the case of drawings
in lines to be made into printing-blocks for letter-press printing,
the operation is conducted in its first phase absolutely in the same
manner as the foregoing, only, after exposure, instead of producing
the image with a slightly alkaline powder, powdered bitumen is used,
and the plate is slightly warmed, so that the powder may slightly fuse
and adhere to the metal, but not enough to make the bichromated sugar
become insoluble. The plate is then washed with water, and all the
sugary coating removed, leaving the surface of the copper bare, except
where it is protected by the bitumen forming the image. The plate is
then bitten with perchloride of iron, which gives a first biting,
leaving all the lines in relief. Further depth is obtained by
alternate inkings and bitings, as in the Gillotype method.
The above processes are very interesting, the use of the sugary
coating, the hardening it by heat, and the triple exposure and biting
are new--at any rate, have not, so far as I know, been published
before.
The report then goes on to describe a further application of the same
principle to obtaining photographic images recently invented by M.
Garnier, and called by him atmography.