DIRECT PHOTO-LITHOGRAPHY.
This method is little known, and it may be said that it has never been developed to a practical issue. It is here only incidentally referred to, but at the same time the fact may be pointed out that these two direct photo-lithographic processes possess a value, inasmuch as they may be utilised in a comparatively easy manner for the production of grained or half-toned pictures, presenting less difficulties than by the transfer process.
On to a well-cleaned stone, the surface of which may be either finely grained or polished, a solution of gum arabic and bichromate is spread, dried in the dark, and exposed for a sufficiently long time under a negative; if the latter be a film it will facilitate the examination from time to time of the progress of the copying. On completion of the exposure the stone should be well washed with water and rolled in with ink.
Morvan’s process is of a similar nature; a stone is coated with an admixture of albumen 200 g., water 300 g., bichromate of potassium 50 g., dried and exposed under a stripped or paper negative. The stone is then first washed with soap, and afterwards distilled water, and then rolled up, gummed, and the ink allowed to set awhile, after which printing may be at once commenced. In none of these methods is mention made of etching the stone after inking-in, as is usual in lithography; it may be here noted that photo transfers will not withstand so vigorous an etching as drawings executed upon stone in the usual manner. A photo-lithographic transfer may be quite ruined, even entirely destroyed, by careless etching; this is therefore a point of great moment, and one which has not usually been considered or dealt with in published methods of procedure. We shall have occasion to again refer to it.