Collotype and Photo-lithography. Julius Schnauss

Collotype and Photo-lithography - Julius Schnauss


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a peculiar manner.

      The method consists in the application of a saturated solution of bichromate salts for developing, a new and previously unknown property of the chrome salts thus coming into employment. These salts, in comparison with the acids, have the advantage not only of dissolving all unexposed gelatine during development, but also in hardening the already exposed parts of the picture upon the copy to be developed, the impression received from the light being increased by contact with the chrome salt solutions. In this way the relief can be developed a longer time, and becomes deeper.

      Another new phase is the manner of the second development. The first development is interrupted before the fine parts have been injured, the relief allowed to dry, the white parts (the sub-ground) covered with printing ink diluted with turpentine, using a fine brush quite near to the lines, and then the whole relief exposed once more to light. The relief, on account of its having taken up much chrome salt during the first development, is very sensitive to light, and hardens not only on the surface, but also on the sides of the lines. After removal of the black ink it can be developed to any depth, particularly if the larger white parts have before been cut out with a knife.

      This process has lately been further improved by the inventor, by the use of gutta-percha films, which are placed between the gelatine film and the zinc plate on which the gelatine relief has to be fastened. This is effected as follows:—The zinc plate, after having been well cleaned and rubbed over with emery paper, is coated with a gutta-percha solution and dried. It is then heated to about 212° F., and allowed to cool. During this cooling process the bichromated gelatine film, before being developed, is laid on at a temperature of between 111° and 77° F., whereby the hardening gutta-percha film secures a perfect combination with the zinc plate. When this has been done the gelatine film is developed in the above described manner to a relief. By this improvement the gelatine blocks become so solid that they will yield more than 50,000 prints in the printing machine without being injured.

       Table of Contents

      Collotype.—Apparatus.

      BEFORE commencing any practical work it will, of course, be necessary to procure various utensils and material not usually found in the photographic studio. All these should be procured from reliable dealers and of the best quality, for the evil consequences of false economy will make themselves felt in endless failures. The best to be obtained are none too good for collotype. In the practice of photography the whole success depends on a series of apparent trifles, and the same may be said to hold good, but in a far greater degree, in this process, which is one in which the difficulties can scarcely be overestimated.

      The photographer will most likely be already in possession of many pieces of apparatus he may utilise—for instance, dishes. The best and dearest are those of china; but for many—in fact, most—collotype purposes, those of tin or zinc may be used.

      For warming or cooking the gelatine solutions tin vessels are the handiest, as they easily conduct the heat and are unbreakable. Although the chromated gelatine may remain in them for a short period without harm, it is not advisable to allow it to do so for any length of time, but to remove the solution and wash the vessel thoroughly with hot water, and at once carefully dry, otherwise they will soon corrode, and contaminate the gelatine solutions. The best utensils to use are wide-mouthed shallow jugs, as they are easily kept clean, and in them the chromated gelatine solution keeps well, and with their use no fear of decomposition need be entertained.

       Fig. 1.

      Printing Frames of different sizes will be found to hand in the photographic studio, and may be utilised without alteration for printing the collotype plates, if they are deep and strong enough to bear the necessary pressure, which is usually applied through the medium of springs; these are better removed, and wooden wedges inserted in their stead between the cross-bars and the loose wooden back of the frames, as by these means far more pressure may be applied. By lifting the one half of the hinged back of the printing frame an examination by transmitted light of the collotype plate may be made and an experienced operator will in this manner judge the exposure of the plate.

      The Actinometer is, however, recommended, particularly for a beginner, as it greatly aids in forming a correct idea of the exposure.

      The Drying Box is of great importance to the successful working of the process. The opinions of the various practitioners with regard to the temperature at which the drying of the plates should be effected differ as widely as upon the advisability or otherwise of admitting a current of air through the box during the operation. The drying should be completed as rapidly as possible from the commencement of the operation, care being taken that the heat never exceeds 50°C. Many plate-makers simply dry the plates in an open apartment—of course, only illuminated by a non-actinic light—simply placing the plates on a horizontal surface, which may be maintained at the temperature indicated by a water bath, a lithographic stone, or merely a cast-iron plate arranged in a suitable manner for heating from below. This method of drying is open to many objections: the surface of the plate is seldom free from dust, and the gelatine coating is too liable to irregularities from draughts admitted to


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