The sugary coating hardens under the whites and the
lighter shades--it only remains tacky under the blacks. The positive
cliche 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.
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