Photo-Lithography

by Georg Fritz

(F.) THE INKING UP AND DEVELOPING.

As soon as one is satisfied that the exposure is sufficient, the print should be taken from the frame in the dark room. The next process is inking up and developing. This is effected in various ways; the ink is either spread on the dry print with a velvet roller or a thin developing ink is used with a brush, or the bichromate is first washed out and then the ink spread on. I prefer to ink up the print with a velvet roller, on which is the developing ink, before washing. By this method I obtain between ink and gelatine the soluble chromium film. The print develops then very quickly and well. For this purpose it is laid on a glass plate, a litho stone or any other flat surface, a small quantity of good litho transfer ink is distributed on an inking stone with an ordinary litho roller, and the velvet roller charged with the ink {52} from the stone by rolling it several times over the stone. The print is now rolled up with the velvet roller till it appears of a general grey tone; the details of the picture, however, are still visible through the ink. With this first inking up, too little rather than too much ink should be used. When the inking up is finished, the print should be laid in cold water, care being taken that no air bells form on the surface, and it is also advisable to frequently turn the paper over once or twice in the water. After ten or fifteen minutes a change will be seen on the print, the exposed parts will be somewhat more depressed than the unexposed, which will have swollen to some extent in the water. The print is now taken out of the water and spread out exposed side up on a sheet of glass, very carefully avoiding folds, and dried with blotting paper, and the water which is on the back of the print is removed by strong pressure on the print from the front.

The print whilst still damp is again for a second time rolled up with the velvet roller, by which means the details of the drawing again take the ink. From the unexposed parts the ink now goes on to the roller, in exactly the same way as in cleaning the litho stone, and they appear covered now with a very faint tone. If the first rolling up was done with too much ink, it will be accompanied by two disadvantages. Firstly, that too much ink will adhere to the details of the drawing, and there will be the danger that in transferring the same will appear thick, and secondly the tint will not be so well removed from the unprinted parts, which will produce a difficult development. It is therefore advantageous only to give in the second inking the printed parts the quantity of ink requisite for transfer.

After being inked up a second time the print is again placed for a short time in the water, then taken out, spread out on the sheet of glass, and now completely developed with a pad or a tuft of cotton wool. This is effected by going over all the print with light pressure and not too quickly with a circular movement, till the drawing appears quite clear and clean on a white ground. In this operation it is advisable to use as much water as possible, in order to remove all the bichromate from the print, which might be troublesome in the subsequent operations. The washing should not therefore be stopped till the unexposed parts have lost all yellow tinge and appear nearly white. If the bichromate remains in the unexposed parts, the process continues during the drying of the print; these places then become hardened, and are frequently the cause of an unsatisfactory transfer. One of the chief requisites is, however, that too much ink should not be used in inking up. The developed drawing should not appear deep black, but grey, yet covered in all places. The developing ink contains so large a quantity of grease that the least quantity is sufficient to form the combination previously described on the stone. {53}

The developed print is freed from all adhering water by pressure with half-damp blotting-paper, and then fastened on to a board with drawing pins to completely dry.

The room where the development is carried on should be well ventilated, and not be heated too much, as the latter makes the work more difficult. The drying ought not to be forced and should be allowed to take place spontaneously at the ordinary temperature of the room.

For transfer to zinc and for sending photo-lithographic prints it is advisable to bathe them in a 10 per cent. solution of alum for about five minutes, so that the gelatine film is hardened. When the prints are to be used or to be transferred to stone immediately this treatment may be omitted without harm.

2. VARIOUS OTHER PHOTO-LITHOGRAPHIC PAPERS.

The paper described above is a pure gelatine paper, and although this has proved in practice to be satisfactory for all work that may arise, still I will describe some other papers, which have been also used in practice with good results, and for those who do not care to prepare their own paper they have the advantage that they may be obtained commercially.

Albumen was used first instead of gelatine and rendered sensitive by the addition of a chromium salt. This paper had the disadvantage that it could not be kept, and only sufficient paper and albumen solution for one day’s work could be prepared. A much greater disadvantage, however, was that in developing, the albumen film was rubbed off with the pad, and the paper was more or less rubbed up. A tint was produced through this which transferred to the stone, and thus gave rise to considerable troublesome and tedious retouching. This is not the case, however, with the gelatine coating, which is comparatively more resistant.