PART IV.PHOTO-LITHOGRAPHY IN LINE.CHAPTER I.
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., page24), 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.
But if the fine details are buried in undissolved bitumen, a further immersion in the turpentine bath will soon clear them; then again wash under the tap, immerse in the graining bath, again wash, then, with a sheet of clean blotting paper, remove the surplus water, and allow the surface to dry in a cold current; but beware of using heat, as it has a tendency to make the bitumen image refuse ink in rolling up.
The zinc being dry, it is gummed in, fanned dry, then the gum is washed off, and the image rolled up with a leather roller charged with transfer ink. As soon as the bitumen has taken the transfer ink all over, a transfer is pulled upon Scotch transfer paper, which in turn is transferred to stone for printing from.
In rolling up these prints upon zinc, care must be taken to keep the surface (whether the image is in albumen or bitumen) properly damp.
The preparation of paper transfers will be treated upon in the next chapter.
The negative from which these transfers are made, must have all the characteristics of a good negative for printing on zinc for etched blocks, viz., the lines{131}must be represented by clear glass, and those portions of the negative representing the whites of the original, must be as near opaque as possible.
The subjects to which photo-lithography is applicable, are those that are strictly in line, or stipple, or dots, or cross hatched, or chalk drawings; in fact, any subject that may be done by lithography, or in black and white; but no shading by color washes is permissible.
Transferring to stone or zinc is done as follows:
First of all, the transfer is trimmed to proper size; it is then placed in a damping book, made by placing wet sheets of brown or good printing paper alternately with dry sheets, then placing all under a weight for a few hours, until the whole of the book is evenly damp; the trimmed transfer is allowed to remain here until limp; it is then sponged on the back with a very little of oxalic acid (half an ounce), water eighty ounces; now lay the transfer carefully down on a perfectly clean and polished stone, backing the transfer with a piece of smooth paper. Having adjusted the pressure of the scraper on the stone, pull through twice, then reverse the stone in the press, and pull through twice more. Then remove the backing and lift the transfer from the stone, when the whole of the ink should be upon the stone, leaving the paper quite clean. Now, if the picture is quite satisfactory, wipe the stone first with a wet sponge, then with a damp cloth, then gum in and allow to dry, and, if possible, to stand with the gum on all night.