For the production of perfect impressions avoid contact of the hands with the face of the paper or stone, and care must be exercised that all matters connected with the printing are kept scrupulously clean, any contamination of a fatty or greasy nature especially being strenuously avoided. The paper used in lithographic printing is usually slightly damped by placing between sheets of damp blotting paper; such proceeding is not absolutely necessary, as dry sized paper may also be used.
The above detailed description of the necessary sequence of operations in the Autographic transfer process will assist in understanding that of photo-lithographic transfer, which requires still more care, and the use of a special transfer paper so prepared that it is sensitive to light.
THE BITUMEN METHOD.
This, the earliest photographic process, was discovered and described by Niepee even before the discovery of photography or silvered plates. It depends on the peculiar property asphalt possesses of losing its solubility in ether, benzine, or turpentine after prolonged exposure to light. If it be dissolved in turpentine in a photographically dark room and the solution poured upon a well-polished metal plate or lithographic stone, dried, and exposed under a reversed negative (otherwise the impression yielded in printing will be reversed) for two or three days[P] in sunlight, the picture may be developed with turpentine or benzine, either of which solvents will remove the layer from such portions as have been unexposed to the light and reveal a picture in asphalt that may be rolled or inked-in and capable of withstanding etching. This direct process, as applied to photo-lithography, is now of little practical value owing to the introduction and perfection of the transfer process. It is, however, most valuable in heliographic engraving or etching and for the production of zinc and copper printing plates.
[P] The bitumen most suitable is that variously known as Syrian asphalt, Jew’s pitch and bitumen of Judea. This is finely powdered and placed in a bottle with twice the bulk of ether and occasionally agitated for five hours, at the end of which time the ether may be poured off and fresh supplies added so long as any portion soluble in the ether remains. The purified residuum maybe dried and dissolved in benzole or chloroform; this will yield a solution which may be used as above, but poisoning, perhaps, twenty times its sensitiveness. The picture is developed with turpentine.
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.