(a) One of the German photographic papers gives the following as a good method of stripping the film from the glass plate, but it is necessary to be cautious with the hydrofluoric acid - which should preferably be kept in a very weak solution. In commencing, the negative must be freed from the varnish, which is done best with methylated alcohol. The negative is then placed on a levelling stand, and coated over its whole surface with a rather thick film of 2 per cent, plain collodion; after setting, the plate is thoroughly rinsed with water, until all greasiness of the film disappears. The plate is then placed in a solution of 10 drops of hydrofluoric acid in 100 cc. of water; the solution should not be used stronger, as otherwise the film would probably frill. In order to avoid this failure entirely, it is well to add to two parts of the above weak hydrofluoric acid solution one part of alcohol. As soon as the film begins to loosen at the edges, the plate is removed from the bath and rinsed carefully, but thoroughly. After washing, the plate with the film side up is placed on a soft support (a few sheets of moistened blotting-paper), covered with a moistened smooth piece of good letter paper, and the latter slightly pressed by means of the ball of the thumb or by a rubber squeegee.
The paper is next very carefully stripped off, together with the film; if this is done skilfully, the film will adhere well to the paper; the latter should have been cut a little larger than the film. In order to remove the paper from the . film, and at the same time to give to the latter a suitable support, a well polished glass plate, a little larger than the stripped picture, is rubbed with talc powder, then coated evenly with col-lodion, rinsed with water, and placed aside for about 5 minutes to drain. The still moist plate is then coated with a 20 per cent. gelatine solution, and completely dried on a levelling stand. Such plates keep well, so that they may be placed in stock. After drying, one of these plates is placed in a dish, with cold clean water, and at the same time the paper, together with the film, is placed in the water; the whole is then brought in contact under water without producing air-bubbles; the plate is next removed, and is once more squeegeed, and the paper is stripped off while still moist.
(b) The first matter for attention is the preparation of the gelatine solution intended to form the stripping support, and this is simple enough if a little care be devoted to it. In order to shorten as far as possible the operation of drying, it is desirable that the solution should be as thick as can be conveniently employed, that is to say, that it shall contain as small a proportion of water to gelatine as possible. The quantity of bichromate is not of much consequence, provided it be not sufficient to crystallise out on the dried film; the sensitiveness of the film, i. e. the length of exposure that will be required to render it insoluble, will depend to a slight extent upon the quantity of chromic salt used; but as this is not a matter of great importance, it is better to err on the side of too little than too much. Add a certain proportion of glycerine to the gelatine solution, for the purpose of giving flexibility to the stripped skin; for though the greater portion so added is no doubt removed in the subsequent washing operations, sufficient remains, or at least the effect, to produce a decided difference in the nature of the dried film.
The following formula is found to answer very well: -
Nelson's No. 1, "flake" gelatine 1 oz. Potassium bichromate .. .. 10 gr.
Water ........ 5to8oz.
Dissolve the bichromate and glycerine in the water, cold, and place the gelatine to soak for an hour or two, or until thoroughly swelled. The smaller quantity of water will be the better as regards rapidity of setting and drying, but the solution will be more difficult to filter, and will also require greater dexterity in applying to large surfaces, so that it will be advisable to commence with the larger quantity. Distilled or rain water should be used it convenient, if not, ordinary tap water, rendered very feebly alkaline with ammonia, boiled, allowed to cool, and filtered through fine filter paper may be substituted. The filtration should be performed at as high a temperature as possible in order to secure the greatest fluidity, and in the same manner adopted by the cook in making jelly, namely, by suspending a bag of wash-leather, felt, or other thick, close material, in front of a clear, hot fire. Through this the solution will pass with perfect freedom, • in spite of its thickness as compared with an ordinary emulsion, and it will do no harm if the operation is repeated once or twice in order to secure a thoroughly bright result.
The filtration may be performed in full daylight, as the solution and jelly are practically insensitive to light, only the dried film being rendered insoluble on exposure. The mixture need not, however, be unnecessarily exposed to strong light, as the decomposition that proceeds spontaneously is likely to be hastened in that manner, and the keeping of the jelly curtailed. If kept in a cool, dark place, it will remain perfectly soluble and good for many days, or perhaps weeks, though it is in its best condition for use when two or three days old.
To use this mixture it is only necessary to pour it warm upon the negative film to be stripped, the glass having been first accurately levelled. If any difficulty be experienced in preventing the solution overrunning the edges of the plate, especially when a very thick layer is applied, it may be desirable to apply some sort of edging to the negative. For this purpose, strips of paper may be pasted round the margin of the glass, so as to form a shallow dish, or a slight ridge may be raised by means of a mixture of paraffin wax and tallow, melted together so as to form a sufficiently plastic compound; but perhaps the simplest and easiest plan, where any such treatment is necessary, is to simply go round the edges with a piece of flannel, or a tuft of cotton wool, charged with any greasy material, in precisely the same manner as in " tipping " the edges with rubber substratum. This, by rendering the edges repellent, prevents the gelatine solution running over, and is sufficient with the thinnest gelatine solution, no matter how thickly applied.