9 Differentiation of the Printing Processes
Technically we differentiate the principal printing methods into three, according to the method of production. The first and oldest is the typographic or letterpress printing, which is founded on the principle that every line or every point which ought to print must stand up on the plate, and all those places which must appear white in the print must be sunken in. The artistic effect is obtained by tones which are formed of individual lines or points, which are closer together or further apart, or else print finer or deeper. In printing a forme or plate, etc., it must be “made ready,” that is to say, the pressure in printing must be so arranged for each part of the picture that on the lighter places or finer places less pressure must be exerted, whilst on the deeper tones more pressure should fall, according to their gradation. This balancing, which is technically termed “making ready,” is effected by cutting out the delicate and building up the strong or dark parts on the overlay. With a proper overlay the artistic effect of the picture can be much increased; with a faulty overlay it can be quite spoilt. In letterpress printing the artistic element lies in the making ready.
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 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.