Chapter III. Photo-Lithography by the Transfer of a Greasy Print on to Stone or Zinc
1 The Home Preparation of Bichromated Gelatine Papers
Photo-lithographic or gelatine papers are articles of commerce and entirely fulfil their purpose, and must before use be sensitized in a bichromate bath. In establishments where much photo-lithographic paper is not used, it is better to obtain it commercially from some reliable firm and not to make it oneself, as this is somewhat troublesome, and requires special appliances, some practice and experience, and with small consumption does not pay.
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.
(A.) the Gelatine
The gelatine must possess certain qualities, of which the principal are as follows. Before all things it must be pure and free from grease, or else small round pits form on the surface, which give rise to troubles in developing and printing, and only produce defective transfers. A large proportion of carbonic acid will produce the same faults, which may, however, be remedied by a small addition of ammonia shortly before use.
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.
(B.) the Chromate Salts
The sensitiveness to light of the different chromium compounds forms at present the foundation of several photographic and photo-mechanical processes.
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 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.
(C.) The Gelatinizing
The gelatinizing of the paper and the preparation of the gelatine solution is effected as follows:—
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.
(D.) Sensitizing
The first thing to be treated of is the preparation of the bichromate bath. The necessary quantity of the bichromate of potash is rubbed up as fine as possible in a porcelain mortar with a little water; it should then be transferred to a larger flask and the necessary quantity of water added. When all the bichromate is dissolved the solution should be filtered, and so much ammonia added as will convert the orange yellow colour into a straw yellow. This bath can be kept, in a dark cool room, fit for use for a long time. The proportions for the solution are—
| Potassium bichromate | 1 | part |
| Water | 15 | parts |
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.
(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 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.
(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 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.
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.