This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
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.