As has already been mentioned, glue or gelatine is insoluble, and only swells up in cold water. In warm water, however, the gelatinous substance dissolves completely, and sets on cooling to a jelly. If a suitable sheet of paper is coated with the warm gelatine solution, either by floating or pouring it over it, and it be allowed to set, and if this film is now made light sensitive in a solution of potassium bichromate and dried in the dark, we shall have a photo-lithographic paper which, according to the greater or less quantity of the chromium salt used, has a more or less intense yellow colour. If the paper thus prepared is exposed to light under a negative, or in another way if some places before exposure are protected by black ink or strips of paper, the places affected by light become dark coloured and brown, and at the same time they have lost their power of swelling up in cold water. The whole surface of the paper, that is to say the exposed and unexposed films before being laid in water, are flat or in one plane; if, however, the print is laid in cold water the exposed parts are not affected by the water, that is to say they have lost the property of swelling and remain in their original plane. The unexposed parts swell up and appear raised up on the print, and thus make the drawing sunken in.
The exposed places have, however, now received the property of taking up and holding greasy ink, whilst the unexposed have taken up water and repel greasy printing ink.
If the paper be exposed under a negative the drawing appears sunken in after development with water; the other parts, which must in printing appear white, are raised up; by exposure under a positive the reverse is the case.
If these prints are in any way, either by rolling up or brushing over, given a coating of greasy ink, the ink only adheres to the exposed places and a print in greasy ink is obtained, which, like any other greasy impression, can be transferred to a stone or a metal {19} plate, which can be printed from direct, or the transfer may be made on to a metal plate for relief etching, that is to say for the preparation of a typographic block.
In Poitevin’s process the stone itself was coated with a light sensitive chromated film, and exposed under a reversed negative. After developing and careful preliminary preparation of the stone the ink only adheres then to the places affected by light.
This is also the case if the stone is coated with light sensitive asphalt, and the same printed on direct. After exposure the parts not affected by light can be washed off with turpentine, benzine, or linseed oil, so that the stone is laid bare; the places, the parts of the drawing affected by light, however, are not dissolved. If the stone is now prepared with the solution of gum as has already been described, and then rolled up with an ink roller, these places will take the ink, and by etching, etc., the stone may be so prepared that the same may be printed from like any drawing or engraving.
By photo-lithography only line or grained drawings can be reproduced, and half-tones, as is possible with collotype and photogravure, cannot be obtained.
Now, with the aid of autotypic transfers and the asphalt process on grained stones, we have a perfectly satisfactory method of reproducing in an excellent manner half-tone drawings by the aid of photo-lithography.
Photo-lithography in all its various branches of application is at the present time so perfected, and rests on so comparatively a simple principle, that a technical printer, with very little practice, experience, and observation of the formulæ given, can attain in very short time absolutely good results. At the same time it must be said that frequently very great difficulties have to be contended with; especially as regards the quality of the materials and negatives great care must be used.