Photo-Lithography

by Georg Fritz

The day before being used the sheet of gelatinized paper is placed in this bichromate bath, which should be as cold as possible; in summer it should be cooled with ice. It should be completely immersed in this bath, so that the solution covers every part of the sheet to the depth of 1–2 cm., and should be left in it about three minutes; care must be taken that no air bells form on the gelatine. If this should happen they should be removed with a soft brush, which ought not to be used for any other purpose. On the places not covered by the bichromate solution, which has been kept off by the air bells, the paper would not be sensitive to light.

For ordinary printing the paper may be dried in the air by placing it on a board, or better on a rack; for better class work, and actually for all work, it is better to squeegee the paper as soon as it is removed from the bichromate bath on a sheet of plate glass which has been well cleaned and polished with talc, the excess of solution or any air bubbles being removed by squeegeeing thoroughly under blotting-paper. Many operators slightly grease the glass plate. I have found that this is not necessary if a sufficiently cold sensitizing bath is used, the plate glass thoroughly cleaned and well rubbed with talc, and it is dried in an airy room. It is advisable to see that there is always ventilation in the drying room, and the more this is done the better the paper will behave in the subsequent operations.

To completely dry, the paper requires when drying by itself from three to four hours; from ten to twelve hours when squeegeed to glass. The best way is to sensitize the paper in the afternoon or the evening, so that it may be used the next day in the morning. The paper when sensitized will, when properly kept in a dark cool room, be in a fit state to use for several days. By squeegeeing on to the plate glass the paper takes a very high glossy surface, and thus in printing comes into intimate close contact with the negative. In exposing, therefore, every fine line appears with greater precision than on paper which has become wrinkled and uneven, and which has not been previously glazed. It should also be mentioned that this gloss is frequently obtained by burnishing the sensitized paper when dry.

(E.) THE PRINTING.

After the paper has been sensitized and well dried it is printed.

The negative is first laid in the printing frame film side up. Then the paper is laid with the sensitive yellow side on the negative, and the frame closed. Before it is placed in the light, care {51} should be taken to see that the paper lies closely everywhere on the negative, otherwise it will in parts be indistinct, and the print be useless. For estimating the correct exposure a photometer should be used, and I take as an example Vogel’s Fig. 6.

Fig. 6.

When beautiful clear negatives are used they should be printed to 14 to 16 degrees Vogel. Obviously no strict instructions can be given for every case, and it will be dependent on the quality of the negative and the object.

If the negative has to be covered, or any other dodge used, the time of exposure must of course be arranged accordingly. As a general guide it may be stated that the details of the picture should appear brown and distinct from the ground when the print has been correctly exposed. This examination of the picture must be done in a dark room, and only one side of the printing frame should be opened, so that the print does not shift. If it is over-printed, the picture develops badly, the lines become broader, the details disappear in the deep shadows, and the ink adheres to the unexposed parts. If, on the contrary, it is under-printed, the ink does not adhere well to the exposed places, and washes off in developing from the fine parts. Practice, as with all manipulations, is necessary, and therefore one must not be disheartened by the first failure.