CHAPTER XV
Photo-Lithography
Early Experiments—An Analysis—The Direct Process—Transfer Process—Line and Half-tone—Some Difficulties—A Natural Grain—Ink Photo-screen Effects—Essential Features.
One of the most promising features of lithography is its co-partnership with photography as a rapid and accurate method of reproduction. The resources offered by this combination are very extensive. For facsimile copying and proportionate enlargement or reduction photography stands unrivalled, and, although in certain phases it may be somewhat mechanical in its effects, its relation to lithography as a reproductive art is nevertheless of an intensely practical nature, and far from inartistic in character.
The first idea of inking up a photographic print so that it might be transferred to the lithographic stone was suggested in the simplest possible manner. A brief account of its inception will be instructive as well as interesting, inasmuch as it will lead to a clearer conception of the elementary principles involved.
During the early experiments in carbon printing it was discovered that a gelatinous film sensitised with certain bichromates could be charged with a coloured pigment, and a picture developed thereon. At first it was not realised that images produced by the action of the light on such a surface could be inked up with a greasy composition and afterwards transferred to the lithographic stone, but it was not long before this important point became apparent. It was found that after exposure under the negative the transfer ink would only adhere to such portions of the gelatinous surface as had been acted upon by the light.
Photo-lithography will best be considered under two sections, namely:—
1. The direct process, in which the actual printing surface is prepared and exposed under the negative.
2. The transfer process, in which a gelatine-coated paper is sensitised in a solution of bichromate of potassium and the photographic print made upon it.
The direct process in its application to the lithographic stone is uncertain in its results. It is impossible to secure sufficiently close contact between the negative and the stone, particularly when large surfaces are under operation, and consequently the print is rarely if ever an unqualified success. The erasure of defective work is also a serious matter, and can only be effected by polishing and preparing the stone again.