CHAPTER III.ETCHING IN HALF-TONE.
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 page55,Part I., Gum and Nut-galls.
* See page55,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.
The operations of washing, gumming, rolling up, etc., are to be repeated until it is judged that the etching has been carried far enough. The time of immersion in the acid solution with each etching may be lengthened, but take special care to keep the bath weak until near the last etching, when the strength may be slightly increased.
The resist is cleaned off with turpentine and a stiff brush, and then a proof is pulled. If further etching will improve it wholly or in part, the plate is inked up and again subjected to the action of the acid bath.
If only portions require reëtching, cover the rest with bitumen varnish; but it will be scarcely safe to give more than one etching after cleaning off the original resist.
WHEN THE IMAGE IS IN BITUMEN.A bitumen image is etched in the first bath in which the acid is just perceptible to the taste, without any preliminary rolling up. The duration of the first etching may be prolonged for three or five minutes; then the plate is withdrawn and well washed. Remove any scum with a soft sponge; then dry it in a cold current, and smear it with gum, fan it dry, and roll up with a leather roller charged with stiff ink, until all portions of the image have taken a film of ink. Now reinforce the nitrate bath with a few drops of nitric acid, and again immerse the plate, rocking the bath for ten minutes; watch the picture carefully so that in case of undercutting the plate may be removed before any mischief is done.
A bitumen image is etched in the first bath in which the acid is just perceptible to the taste, without any preliminary rolling up. The duration of the first etching may be prolonged for three or five minutes; then the plate is withdrawn and well washed. Remove any scum with a soft sponge; then dry it in a cold current, and smear it with gum, fan it dry, and roll up with a leather roller charged with stiff ink, until all portions of the image have taken a film of ink. Now reinforce the nitrate bath with a few drops of nitric acid, and again immerse the plate, rocking the bath for ten minutes; watch the picture carefully so that in case of undercutting the plate may be removed before any mischief is done.
The plate is again removed, washed, rubbed with a sponge, and dried; it is{71}then warmed on the hot plate, so as to slightly melt the ink, then allowed to cool, smeared with gum, fanned dry, damped over with a wet sponge, and rolled up, the roller being charged with stiff ink. The plate is now ready for another etching, for which the bath must be strengthened by the addition of more acid. The immersion (rocking all the time) may be fifteen or twenty minutes, after which the plate is withdrawn, washed, rubbed with a sponge, and if it is thought another etching would improve it, the cycle of operations is repeated, beginning with warming the zinc on the hot plate.
After the resist is cleaned off (which with bitumen must be done first with turpentine and then with benzoline) further etching may be given, if deemed necessary, first inking up well with the hard ink mentioned in Chapter VIII., and then reimmersing in a weak bath of acid and water. If only a portion requires reëtching, those parts that are not to be touched, are protected by coating with bitumen varnish.
The plate being etched, now requires finishing.
The outside zinc is removed with a circular saw and then the edges are filed and holes drilled at an angle, so that the metal can be fixed upon the block.