Photo-Lithography

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.

The paper more recently produced by Husnik is a pure gelatine paper, which is rendered sensitive to light in—

As much ammonia is added to the bichromate solution as will convert the reddish colour into yellow, and the bath smells of ammonia. The bathing of the paper must be done as quickly as possible, and the sheet should only be drawn through the solution. The exposure for this paper is from one to three minutes in the sun, or from ten to thirty minutes in diffused light.

The inking up of the print should be done with a greasy transfer ink, to which one-sixth part of wax has been added, and the whole dissolved in turpentine to the consistence of oil. The print is inked up with this thin ink, and then gone over with a pad of cotton wool till it has taken an even gray tint.

When the turpentine has evaporated the print is laid in cold water, and, after about ten minutes, developed with a soft thick pad with a continuous circular movement and quite light pressure. After development all adhering water is removed from the print with damp blotting-paper, and the transfer can be effected after from fifteen to twenty minutes.

Husnik, however, did not rest, and soon prepared a still better paper, which is known at the present time commercially as “Husnik’s Autotypic Paper.” He produces this paper in large quantities, the coating of the gelatine being effected by machines, which certainly tends to ensure equality.

Husnik found that by the use of less concentrated bichromate baths the fine lines were stronger after printing than from the nature of the negative they should be. From one and the same negative there was obtained by the use of a bichromate bath of 1 : 100 a relief five times as broad as when a bath of 1 : 15 was used to sensitize the paper. This he ascribes to the colour of the gelatine being less, so that more rays of light penetrated to the surface of the paper, from which it would be again reflected, and every point or line again reproduced, which must obviously lead to an increase of the size of the same; on the other hand, with strongly chromated paper, the rays of light would not only be absorbed by the intense yellow colour of the chromium salt, but still more by the brown tone formed immediately after printing, which makes a reflection impossible. With autotype negatives this is specially striking, as with weakly-chromated paper black shadows without details occur, because here the quantity of light is distributed by reflection over the fine points, and they will {55} therefore become thicker. When using strongly-chromated papers the details in the shadows appear open, clear, and sharp.

The concentration of the bichromate bath cannot be increased at will, because other disadvantages are produced, especially such a firm adherence of the greasy ink to the print that it transfers badly and a part of it always remains behind. The bichromate salt, when as strong as 1 : 18, dissolves the gelatine at ordinary temperatures, and the baths, neutralized with ammonia, have the disadvantage that the greasy ink adheres too little to the print and will be rubbed away in developing the drawing. In order to entirely obviate the reflection of the rays of light Husnik uses an addition to the gelatine besides a 1 : 22 acid bichromate bath, which produces a pure yellow tone in sensitizing the papers, is insoluble, and completely penetrates the film of gelatine, so that a reflection of the chemically active rays is entirely avoided.

This paper, which is also darker coloured, does not allow the light rays to pass through, and is specially suitable for the transfer of autotypes to stone or zinc.

For sensitizing this paper bichromate of potassium, sodium, or ammonium is used, but the sodium salt has acted the best.

In winter a 1 to 20 bath is best; in summer, 1 : 22 or 1 : 24. The treatment of the paper, as well as of the print, is exactly the same as with other transfer papers. The bath will keep for fourteen days. Caustic ammonia ought not to be used.

This is also a pure gelatine paper. But Franz adds to the gelatine solution manganese sulphate (this must not be confounded with manganese borate, siccative powder, which makes the transfer ink dry so quickly that a transfer to the stone or zinc is impossible).

Through the researches of MM. Lumière the manganese salts have gained considerably in importance, and they have been introduced into photo-mechanical processes. When an addition of about 10% of manganese sulphate is made to the acid bichromate baths for sensitizing photo-lithographic papers, the printed places assume a very much darker colour, which is an advantage in judging the print, in developing the finest lines, and points are retained much more easily than on paper sensitized with the addition of ammonia.

This paper possesses the advantage, which is also more important, that the—especially in summer—unpleasant ammonia bath need not be used. The paper is sensitized in a solution of—

Ammonium bichromate ought not to be used. The bath would then become cloudy immediately, and both bath and paper made useless.

When sensitized the paper can be kept from eight to ten days without losing its good qualities.

With this a transfer ink is supplied in tubes, which must be mixed with an equal volume of turpentine and benzine, so that it becomes quite fluid. The print, when it is taken from the frame, is fastened on a board with drawing pins, and with a fine camel’s hair brush coated with this fluid ink as evenly as possible, and just so much that a light grey tone is formed on the print. This paper must not be inked up with a roller. As soon as the turpentine and benzine have evaporated the print is laid in cold water, and after about ten minutes developed with a clean tuft of cotton wool. The print does not show a strong relief; the development takes place quickly and regularly. The ink, in spite of its being so thin, is very greasy, so that only a very light skin is necessary for the subsequent transfer. The tint produced by a negative slightly too thin cannot be easily removed.

The other operations are the same as for the other photo-lithographic papers.

Franz’s photo-lithographic paper is characterized, as already mentioned, by giving extraordinarily sharp clean transfers, which, indeed, maybe due to the use of the very thin liquid greasy ink. It is, therefore, especially to be commended for very fine work.

The sensitizing bath consists of—

and to this so much ammonia is added as will convert the reddish colour into a light yellow; a small excess of ammonia does no harm. The bichromate bath should not have a lower temperature than +15° R, or else the solution cannot penetrate the film sufficiently, which results in a defective development. The duration of sensitizing is from four to eight minutes, till the paper is quite soft and damped through. It is preferable to sensitize too long rather than too short. {57}

The paper should be squeegeed on to plate-glass, as already described, and allowed to dry spontaneously. The paper when stripped from the plate-glass shows a beautiful high glaze, and can be kept fit for use for twenty-four hours. When the paper is older it can still be used, but the development is more difficult, and it is better to avoid this for fine drawings or autotypes.

Albert does not recommend a photometer to be used when printing this paper, but by examination of the print to judge when this is correct. The image should have a brown tone, and be easily distinguished by the eye from the ground. Over-printing is disadvantageous, as the lines and points become broader, the prints develop badly, the tint adheres firmly to the ground, and the details in the deepest shadows are lost.

The inking and developing can be done by any of the known processes which is thought desirable. The chief thing is that too much ink must not be used in the first inking up; any intensification required may be effected with a second coating. This is, especially for fine drawings and autotypes, very important, whilst for coarser work also once inking is sufficient. The prints are then placed in cold water, and after sufficient washing should be developed with a soft pad or pad of wool, with which any tint can be very easily removed from any parts which should remain white.