On examining the zinc plate after removing it from the whirler, it will be difficult to realize that there is really a sufficient film of bitumen on the zinc, but there will be quite sufficient.
In printing upon bitumen from negatives that have been varnished, it will be as well to rub the varnished surface of the negative with a pledget of cotton dipped in dry French chalk.
The same care in putting into the printing frame, as enjoined in the chapter on zinc printing with bichromated albumen, is requisite with these plates, as is also the applying the pressure by means of the screws. The exposure to light will vary from ten minutes in the sun, to two or three hours in the shade.
DEVELOPING THE IMAGE.
To develop, place the exposed plate, face up, in a shallow tin tray, and pour into the tray sufficient turpentine to cover the plate; rock the tray so as to keep the solution in motion, and (if the exposure has been right) the image will gradually make its appearance; the bitumen protected from the action of light by the dense portions of the negative gradually dissolves away. As the development approaches completion, great care must be used, and when the last detail is visible, remove the plate, and rinse it with water from a rose top; now, if the image is quite perfect, immerse the plate at once in a mixture of nitric acid and water, the acid being just strong enough to taste. Keep the tray containing this acid solution rocking for about a minute, then remove the plate, and wash it thoroughly, rubbing it very gently with cotton-wool. If the exposure under the negative has been barely sufficient, the image will be too delicate for touching, so before rubbing it with the cotton-wool, try some portion of the margin, and if that stands the friction, it is all right; if not, be content with rinsing with water.
The immersion in the acid water will have removed the shiny appearance of the zinc, and the matt surface will enable the operator to see if all the requisite details of the image are visible. If not, and they are covered with bitumen, another immersion in the turpentine will complete the development; but if any of the lines are rotten or the details are dissolved away, the exposure has been too short, and the plate will require repolishing and another exposure.
The image being satisfactory, the water is blotted off with clean blotting paper and allowed to dry. Don’t use heat for drying. It may then be exposed to light for a few minutes. This is not absolutely necessary, although it tends to make the image finer. The plate is now ready for etching in relief.
CHAPTER VII. DIRECT TRANSFERS TO ZINC.
We have now treated of the various stages necessary to obtain a photographic transfer upon zinc, from a drawing or engraving, in line, in dot, or in stipple, first by making therefrom, in the camera, a negative—same size, reduced, or enlarged—and from that negative a print on zinc, either in ink on an albumen basis, or in bitumen.
These transfers can be used, either for yielding re-transfers for lithographic press, or (as is at present our intention) for etching into relief, for use as blocks for printing from letter-press.