The composition of the ink, especially if it was too hard, may give rise to bad transfers.
Photo-lithographic papers, which are prepared with very hard gelatine, must be laid before transferring in lukewarm water. By this means the hard gelatine becomes rather sticky, and the greasy ink transfers much better. Even those points which in developing have taken up little ink, and appear grey, transfer completely to the stone and adhere firmly.
CHAPTER IV. PHOTO-LITHOGRAPHY BY DIRECT PRINTING ON TO STONE OR ZINC.
The processes of photo-lithography previously described are based upon the printing on to bichromated gelatine or albumen paper development with a greasy ink and subsequent transference of the greasy print to plate or stone. A process may also be used in which the stone or plate is printed on direct. There are in this various methods, of which the principal are printing on to light-sensitive asphalt, or an organic substance, gelatine or albumen, in combination with a bichromate salt. The reason is the high sensitiveness, which is possible, of the substances used.
1. THE ASPHALT PROCESS.
(A.) THE LIGHT-SENSITIVE ASPHALT.
Light-sensitive, or so-called Syrian, asphalt, which is prepared for photo-lithographic and photo-zincographic purposes, can be obtained from various firms who deal in photographic goods, and one has only to dissolve the same according to the prescribed directions. This kind of asphalt is, however, not very sensitive, and in use must be printed for some considerable time. If, therefore, an asphalt of greater sensitiveness is desired it is not sufficient to merely dissolve Syrian asphalt, but it must undergo a preliminary preparation.
Only that part of the asphalt is sensitive or useable for photo-lithographic work which does not dissolve in ether. The insensitive part must, therefore, be separated out, which is done by finely powdering the asphalt, sifting it through a fine sieve, and then treating it with ether. All that is soluble in ether goes into solution; the marc which is not soluble in ether is then dissolved in benzole and used as wanted.
(a.) HUSNIK’S PROCESS.
Professor Husnik has improved this process in so far that he does not dissolve the asphalt powder in ether, as, according to his view, a complete solution of all the insensitive portion is not attained by this process, but he dissolves coarse asphalt powder completely in rectified spirit of turpentine to the consistence of a moderately-thick syrupy mass, until no hard particles or a residue are contained in the solution. To this viscous asphalt solution a plentiful quantity of ether is added gradually, and with constant stirring, and a big flask should be used for this purpose. After a sufficient quantity of ether has been added a pitch-like deposit {61} separates out at the bottom. In order to test whether there is still any light-sensitive asphalt in the supernatant liquor a small quantity is poured into a glass and mixed with ether, when if a precipitate ensues more ether must be added to the big flask.