Photo-Lithography

For those who are interested in the preparation of chromated gelatine paper I will now give the method, and I would remark that this paper possesses all the necessary qualities for this process.

A sheet of well-sized paper is soaked in a bath of cold water, or evenly damped with a clean pad, till it has become slimy. The water is allowed to drain off, and then the sheet of paper placed on a plate of glass which has been accurately levelled, and which rests on a levelling stand with screws, so that it can be easily levelled. By placing over it a sheet of blotting paper and squeegeeing with an india-rubber or flannel squeegee the air bells between the paper and glass are removed and any excess of water pressed out. Then the edges of the sheet are bent up to about the depth of 2 cm., and wood or iron rods laid outside, and a warm 1 : 30 gelatine solution poured on to it. A sheet 70 by 70 cm. will require 305 g., that is 7·5 g. of gelatine and 297·5 g. of water. The gelatine will set in a few minutes, when the sheet is removed from the glass and laid on a rack in a place free from dust to completely dry, and should remain from a day and a half to two days. Such gelatinized sheets can be kept in stock, and will keep fit for use in a cool, dry place for from six to eight months. For this purpose only good pure rag paper should be used, as the ordinary cheap papers contain many additions which render them unsuitable for this work.

Gelatine is very hygroscopic, and therefore attracts moisture from the room where it is kept and dissolves completely in warm water. Even in very great dilution, e.g., 1 : 100, it sets again to a jelly in the cold, but loses the setting power after being repeatedly warmed, or if the temperature was raised considerably higher than was necessary to melt it. Damp gelatine decomposes in the air very soon; the decomposition, however, can be delayed with salicylic and carbolic acids and glycerine. Greasy gelatine can be improved by the addition of 5 per cent. of alcohol. Too soft gelatine can be hardened with chrome alum, and one part of chrome alum may be used to 200 parts of gelatine, and by this addition the melting point is raised about 7° C. Too hard gelatine gives rise to the formation of bubbles. If the gelatine solution is slightly acid it is not a disadvantage.

The gelatine used in our establishment is of French make, and is known as “transparent white gelatine,” and is also very suitable for collotype. I mention this because the gelatine frequently gives rise to failure. For practical work, then, it is advisable to stick to one make when it has been proved to be absolutely satisfactory.

To those who would study more closely this very important material for photo-lithography and collotype, I would strongly recommend the brochure “Ueber die Reactionen der Chromsäuren und der Chromate auf Gelatine, Gummi, Zucker und audere Substanzen organischen Ursprunges in ihrer Beziehung zur Chromatphotographie,” by Professor Eder, in which the most exhaustive information on the application of these two materials so important in photography, gelatine and the chromate salts, is given in a concise, popular, and easily understood form.

According to Dr. Eder, Vauquelin discovered in the year 1798 chromium and chromic acid, and made at the same time the observation that chromic acid formed with silver a carmine red salt, which became purple-red by the action of light. In the year 1832 the philosopher Dr. Gustav Suckow, in his work, “Die chemischen Wirkungen des Lichtes,” stated that the chromate salts were also sensitive to light in the absence of silver if an organic substance was added, as in the light lower (green) oxidation products were formed. In 1839 Ponton added to Vauquelin’s and Suckow’s discoveries, and discovered the photographic application of chromate of silver, the light-sensitiveness of potassium bichromate on paper. The chromates are per se stable in light; in the {49} presence of organic substances, however, such as gelatine, albumen, gum, etc., a quick reduction of the same takes place in light. Talbot finally found that a mixture of gelatine and a chromate became brown in light, and lost at the same time its solubility in warm water and its power of swelling up in cold water. According to the same authority simple potassium chromate is from 20 to 25 times less sensitive to light than potassium bichromate, and the simple ammonium chromate shows the same sensitiveness to light as ammonium bichromate. For photo-lithography the latter salt, potassium bichromate, is very important, and it has the chemical formula of K₂Cr₂O₇.

The gelatine, accurately weighed out, is placed in small pieces in the proper quantity of water, either in a large measure or some other vessel, and placed in a water bath and immediately heated. It is advisable to place the vessel containing the gelatine into the water bath before the latter is heated, so that both may heat together and fracture of the glass be thus avoided. When the gelatine is completely dissolved the warm solution is filtered through a linen filter, and so that it may not be cooled the vessel into which the gelatine solution is filtered is also placed in the water bath.

Whilst still warm and liquid the gelatine is poured on to the previously prepared paper, and any bubbles of air brought to the edge of the sheet with a strip of stiff paper or a feather.

It should be especially noted the gelatine solution should be used up as quickly as possible or else it will deteriorate. Gelatine which remains for a long time in a state of solution becomes soft and fluid, and if the chromate bath is not well cooled a partial or complete dissolving of the same takes place in sensitizing or later in washing and developing.

and as much ammonia as will make it straw coloured. The bath is neutralized with ammonia or a soda or potassium salt, because solutions of the bichromate salts have the property of dissolving gelatine even in the cold.

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.

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.

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.