A perfect print on the zinc must contain all the grain intact, for if there is any break in its continuity, it is of no use to proceed further with it.
The printing in bitumen requires no more comment than to say care is necessary to avoid dust. The smallest speck will be likely to spoil an otherwise perfect print.
CHAPTER III. ETCHING IN HALF-TONE.
WHEN THE PRINT IS IN ALBUMEN.
The zinc plate with the image upon it in ink, is dried, then warmed slightly on the hot plate, cooled, and coated with thick gum* and fanned dry (not dried by heat, or the gum is certain to crack and bring away the image or part of it); then wash it under the tap, and rub it gently with a fine sponge so as to remove all gum from the ink, but not from the zinc. Next proceed to roll up as directed in Chapter V., using the India-rubber roller charged with stiff ink. The surface of the zinc should be kept well covered with water. Use as little pressure as possible, but let the motion be fairly rapid, and if any portions of the image should smear, roll rapidly with slightly increased pressure.
* See page [55,] Part I., Gum and Nut-galls.
This operation of rolling up the image is the crucial one, for unless it is done properly, when it is finished, the result is a strong black and perfectly even image; it is of no use proceeding further with it, for in fact the picture must be perfect.
The rolling up being done, any touching up that may be required can now be done, and any fine details clouded over, or made indistinct by the grain, strengthened or opened out. Additions should be made with transfer ink thinned with turpentine, laid on with a very fine sable brush. The whites are put in by scratching through the ink with a fine etching needle.
To rest the hand upon in doing this touching up, so as to avoid touching the image, a bridge is used made of a strip of smooth wood, one inch wide, quarter of an inch thick, and sufficiently long to well span the plate; at each end of this strip nail narrow strips a quarter of an inch thick to serve as the feet for the bridge to stand upon. Then the zinc may be slid under it without touching the surface against the under part of the bridge.
The zinc is now ready for the first etching in the rocking tray. The nitric acid solution must be very weak indeed to the taste, the presence of acid being just perceptible. The time of immersion in the first bath must not be longer {70} than thirty seconds; then the zinc is removed, washed under the tap, and gently rubbed with a sponge; dry it gently on the hot plate, allow the zinc to cool, coat it with gum, fan it dry, rinse it under the tap and rub it with a sponge; then again roll up, when it is ready for the second etching; that is done in the same bath as the first, without any addition of acid. Keep the tray gently rocking for three minutes, then again withdraw, wash it under the tap, rubbing with sponge, coat with gum, fan dry, wash under the tap rubbing gently with sponge, and again roll up, using the ink on the roller a little thicker, but stiff. When the image has taken the ink equally, place it on the hot plate and allow the zinc to get hot enough to melt the ink slightly; then allow it to cool, and place it in the etching bath, slightly reinforced with a few drops of nitric acid. Immerse it this time for five minutes.