THIS process is the transfer of a dye image to a gelatine film; as the result, prints are formed consisting only of transparent aniline dyes in a single layer of gelatine. The finished pictures are very luminous and rich in coloring. There are various methods by which this result can be obtained; either by using a primary gelatine relief, staining this up, and using as the dye matrix, or by using the property that hardened gelatine possesses of not absorbing certain dyes, and the fact that these dyes will migrate or wander into another gelatine film.
Practically, one may liken the relief processes to the use of a rubber stamp, which temporarily takes up the coloring matter, only to give it up when pressed against an absorbing support.
The pinatype process was originally suggested by Edwards in 1875, and independently by Cros in 1881, and was introduced commercially by Meister, Lucius & Bruning in 1906. The original process, as suggested by the last-named firm, required the three original negatives, three transparencies from the same and three print-plates, which were prepared from the transparencies by exposure of bichromated gelatine plates; it will thus be seen that, exclusive of the negatives, six plates were required before a single print was possible. But equally good results can be obtained by using the transparencies themselves as print-plates, thus saving one step in the process; this modification was suggested by Didier, the actual inventor of the process.
The following is the method of making the bichro-mated print-plates. It is assumed that the three constituent negatives have been secured and that ordinary silver transparencies have been made from them. As regards these last, the only comment necessary is that they should not be too hard and should partake rather of the character of negatives than of brilliant lantern slides. The bichromated plates can be prepared with:
Hard gelatine 50 g.
Ammonium bichromate 20 g.
Water 1000 ccm.
The gelatine should be cut up into small pieces and soaked in the water for about thirty minutes and then melted by the aid of heat, the bichromate added and the mixture filtered, while hot, through Canton flannel or two or three thicknesses of linen. The quantity of this mixture should be 400 to 500 ccm for every square meter of glass. The preparation of the solution and the coating of the glass can be performed by weak daylight or artificial light. The glass can be old negative glasses, well cleaned and polished. They should be placed on a leveled slab and the requisite quantity of the gelatine mixture poured on them and coaxed out to the edges. It will be found that a pipette is the most handy tool for this, and the temperature of the solution may be about 550 C. (1300 F.). As soon as the gelatine has firmly set, the plates can be racked for drying, and the remarks already made as to this operation in the case of filters and plate sensitizing apply here also. The plates must be dried in the dark. A possible variation of this process, and one that may commend itself, is to omit the bichromate salt from the above formula and coat a stock of glass with plain gelatine solution; sensitize this as required by immersion in plain bichromate solution, about 5 per cent, then rinse and dry. By this method there is naturally less chance of the plates spoiling.
In either case the bichromated plates are exposed under the transparencies to daylight, and as in the carbon process, an actinometer should be used; but it will be possible to do without this, as the image is easily seen in a brown color against the bright yellow ground. The exposure should be such that the image is visible in all its details, even in the high-lights. After exposure, the plates are soaked in cold water until the drainings are colorless, or, if time is a consideration, the plates may be immersed in a 10 per cent solution of sodium bisulphite until colorless and then washed. Though not absolutely necessary, it is advisable to let them dry then, but before this they should be immersed, just for a minute or two, in their respective dye baths, as this enables one to tell which plate is which. Otherwise it is extremely difficult to distinguish them, as the image is almost invisible.
The alternative process, in which the silver transparencies are used as the print-plates, is to be preferred, as the films are actually harder. The transparencies must be somewhat denser than for the last process and have absolutely clean whites, and it is important not to use any developer that has a tanning action on the gelatine. The transparency for the yellow impression should be sensitized in:
Ammonium bichromate 12.5 g.
Ammonia 100 ccm.
Distilled water 1000 ccm.
Those for the blue and red are bathed in:
Ammonium bichromate 20 g.
Ammonia 200 ccm.
Distilled water 1000 ccm.
The plates should be immersed for five minutes and then dried. Before exposure, the backs of the plates must be thoroughly cleaned, as any dirt would show in the result as a darker patch. They are placed in an ordinary printing frame, gelatine side inside, and in contact with the gelatine should be placed a sheet of printing-out paper, which acts as an actinometer. The insolation is carried on until the details appear in the shadows, and naturally this image is a negative; the yellow plate requires about double the exposure of the others. After exposure, the plates should be well washed, or, to shorten the time, may be immersed in the bisulphite solution recommended above, or in a 5 per cent ammonia solution.
The transparencies are then ready for staining up, but as the presence of the black silver image makes it difficult to see the dye image in the subsequent superposition, it is advisable to dissolve the silver with weak hypo and ferricyanide. This process not only presents the advantage of cutting out the making of the print-plates, but the correct exposure can be easily determined. The plates are very hard, and will stand subsequent treatment without damage. In consequence of the feeble relief, the prints show a most delicate detail; and the successive prints are very regular owing to the superficial film being backed up by the insoluble film caused by the exposure through the back.