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 {48} 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.