This process is for making photographic reproductions of a subject in black lines on a white ground, half-tones and shadings being indicated by hatching, or stipple, but no washes of color are permissible. The object sought for from such a subject is to produce a transfer which can be laid down upon stone or zinc, and proofs printed therefrom in the ordinary lithographic press or machine.
To gain this end there are three methods: the first, by printing upon paper prepared with bichromated gelatine, the lines of which, after exposure to light under a direct negative (i.e., a negative taken without a mirror behind the lens), will retain ink, whilst the gelatine protected from light absorbs water, and rejects the ink. The result is a replica of the original in fatty ink, and may be transferred to stone or zinc, and printed from by the usual lithographic method.
In the second method, a print is made upon zinc from a reversed negative (in the same manner as directed in Chapter V., Part I.) in bichromated albumen, which is rolled up in transfer ink, and from that a transfer is pulled on Scotch transfer paper, which may then be transferred to stone or zinc, and printed from at a litho. press or machine.
In the third, and by far the best method, the print is made from a reversed negative on zinc, in bitumen, as directed in Chapter V., Part I., then rolled up in transfer ink, the transfer pulled on Scotch transfer paper, then transferred to stone or zinc, and printed from at a litho. press or machine.
The defects inherent to the first method are, first of all, in the preparation of the gelatine paper; this is tedious and messy. Then there is always danger of breaking the lines in transferring, and also the difficulty of working exactly to scale. {130}
The second method is quick, easy, and simple, the only drawback being a tendency of the lines to thicken. With this and the third method there is no difficulty in getting absolute scale.
The third method is perfect, and presents no drawbacks.
The two latter methods having been amply explained in Part I., it will not be necessary to redescribe them, except to explain that when the print is made on the zinc in albumen, it is gummed in, fanned dry, then damped off with a wet sponge, followed by a damp cloth, rolled up with transfer ink, and the transfer pulled upon good Scotch transfer paper.
The same remarks apply to bitumen prints.
For a transfer printed in bitumen, the image is developed in turpentine, and when this is completed, wash the plate thoroughly under the tap, then immerse it in the graining bath (Part I., Chapter I., page [24]), and keep the tray rocking for a few minutes; the effect of this will be to destroy the polished surface of the metal, substituting a pearly matt surface, which will show up the picture properly, and allow the operator to judge whether any of the fine details are wanting; if there are, it will be necessary, if the details are dissolved out, to repolish the zinc, first with pumice-stone, then with snake-stone, then with pumice powder, finishing with fine rouge; then coat with bitumen, whirl, and again expose to light, giving this time, longer exposure under the negative.