9.—Differentiation of the printing processes.
Typographic prints are distinguished by the lines, figures, etc., being somewhat pressed into the paper, and show raised up on the back, which is technically termed “impression.” When this “impression” is very plain, that is to say when the printing has gone pretty deeply into the paper, it is advisable before reproduction to remove this by strong pressure.
The second method of printing is exactly the opposite of the first, and is founded on the principle of printing from intaglio. The lines and points which should print are cut mechanically (by engraving or piercing) or chemically (by etching) into the stone. The etched or engraved intaglio plate represents a negative, or a reversed wood-cut, only with the difference that the lines do not {13} lay in the same plane. In order to obtain an impression the ink is pressed into the cut-out parts, and is wiped off from the surface of the plate. By stronger or weaker wiping the print receives a stronger or weaker local tone, which is of great advantage from an artistic point of view, but which forms no small obstacle to the reproduction.
The impression is represented in relief on the paper. According to the depth of the lines they take more or less ink, and appear therefore raised up in the impression according to the amount of their depth in the plate. Reproduction photography has to reckon on this disadvantage, for with a side illumination the raised up lines cast shadows, and a correct negative does not follow.
By the third method, lithography, of which we have already spoken, the impression lies generally flat on the paper like a drawing, if a very strong deep etching was not used, in which case it is also slightly raised. In printing from the graver or from stone etching a faint relief is generally noticeable, which is never so high as with copperplate printing, and which is no important obstacle to making a photographic negative.
CHAPTER I.
1. GENERAL NOTES ON PHOTO-LITHOGRAPHY.
BY photo-lithography we generally understand that process by which it is possible to prepare a printing plate or a stone by the aid of photography from any original, whether it be a drawing, a print, or an oil painting, either of the same size or smaller or larger than the original, which can be reproduced in the lithographic press.
The main characteristic of this process is that either the original from which the reproduction is to be made must be prepared in distinct lines, strokes or points, or that any half-tone which may be present in the photographic negative must be broken up on the stone or plate into lines or points.
The basis for the preparation of a photo-litho is as a rule a photographic negative on glass or gelatine, which, as the word negative implies, when examined by transmitted light, must have all its tones reversed, that is to say, the drawing or the lines, strokes or points which must in the print be black, must look transparent, whilst the other part of the negative which forms the groundwork must be covered or opaque.