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.
CHAPTER II. PAPER TRANSFERS.
Paper for photo-lithographic transfers may be made by coating bank post paper or a good hand-made writing paper, with