CHAPTER IV.APPARATUS FOR PRINTING UPON ZINC.
A suitable negative (the subject being in line,not half tone) having been obtained, the next stage toward producing a relief block is to make from that negative a print in ink upon a sheet of zinc. To do this a sheet of polished zinc is first of all subjected to the action of a solution of nitric acid, alum, and water. It is then thoroughly washed, placed in a machine called a whirler, then coated with a solution of albumen (white of egg) and bichromate of potash; this coat or film of bichromated albumen should be dried over a spirit lamp.
The negative is then placed, face uppermost, in a printing-frame, the coated side of the zinc in contact with the face of the negative. The negative is now exposed to the action of light.
The exposure being complete, the frame is taken into a room lighted by yellow light, gas, or lamp-light, the zinc plate is removed, and its surface covered with a thin film of transfer ink, by means of a type-printer’s roller; then the zinc plate is immersed in clean cold water and the image developed by gently rubbing it with a pledget of cotton-wool.
POLISHING THE ZINC.Zinc plates suitable for photo-engraving can be obtained either in large sheets, or cut to size as ordered, and, when new, require polishing with a damp rag dipped in levigated pumice powder—i. e., pumice powder washed in water, so as to get rid of all grit.
Zinc plates suitable for photo-engraving can be obtained either in large sheets, or cut to size as ordered, and, when new, require polishing with a damp rag dipped in levigated pumice powder—i. e., pumice powder washed in water, so as to get rid of all grit.
Lay the plate upon a board covered with muslin and polish with a “from-and-to” the body motion, not circular; do not use a great deal of pressure, the object being to get the plate highly polished with a perfectly smooth surface.
Plates that have been printed upon, and are not satisfactory, may be cleaned and repolished.
Plates that are scratched require polishing with emery cloth. Scratches interfere with the picture.{40}
To remove scratches, sprinkle the face of the plate with turpentine, then rub with a piece of FF emery cloth stretched over a carpenter’s cork-covered rubber. Use the same motion, to-and-fro, until all the turpentine has evaporated and the surface of the zinc is polished. If the scratches are removed, the plate is ready for use, but if not, then the operation must be repeated. If the scratches are too deep for emery cloth alone, first use a piece of fine pumice stone, followed by snake stone, pumice powder, rotten-stone, and rouge. Too much care cannot be taken with the zinc plate at this stage, for, after the block is etched, if it is discovered that the plate has not been properly polished, all will have to be done over again, entailing a loss of time, labor, and materials.
Remember, that no matter what polishing agent is used, the plate must be polished with a to-and-fro from the body motion, not a circular motion such as is usual in polishing metal.
GRAINING THE PLATE.The next operation is to give the plate a slight tooth so that the sensitive solution will flow evenly over the surface. Do this by removing all traces of grease.
The next operation is to give the plate a slight tooth so that the sensitive solution will flow evenly over the surface. Do this by removing all traces of grease.
This graining is done in a wooden tray, 24 inches by 18 by 6, lined with asphaltum or gutta percha and mounted upon rockers; it is quite necessary that the graining tray be of large size, as if only a little larger than the plate the returning wave of acid water will mark the edges of the zinc.
Into this tray pour a quart of clean water, and add one drachm of nitric acid and one ounce of a saturated solution of common alum. Place the plate in this, face up, and commence rocking it at once or else the acid will mark the surface, and it will require repolishing; rock slowly for five minutes, during which time the polished surface will give way to a fine matt, like fine frosted silver. Now remove the plate and rinse it under the tap, rubbing it gently with a fine sponge, or a pledget of cotton to remove the scum or deposit formed by the acid. The surface at this stage should be quite smooth; if it is at all rough, the acid is too strong, and the solution must be diluted with water; if the action of the acid is very slow, then a little more must be added; but it will only be necessary to add more alum when the plate is a bluish color instead of being a pearly gray.
A drop or two of acid will be required each time plates are to be grained.
In rubbing with sponge, or a pledget of cotton when under the tap, care{41}must be taken not to scratch the surface of the zinc, or else it will need repolishing.
COATING THE PLATE WITH SENSITIVE ALBUMEN.If the solution of albumen was poured over the plate and drained and dried, the film would be too thick at the bottom and too thin at the top; it is, therefore, necessary, to insure an even film of sensitive albumen, to subject the plate to a quick circular motion, so as to spread the film of albumen evenly, and to get rid of the surplus solution. This is effected by placing the grained zinc in the jaws of an instrument called a whirler and made as follows:
If the solution of albumen was poured over the plate and drained and dried, the film would be too thick at the bottom and too thin at the top; it is, therefore, necessary, to insure an even film of sensitive albumen, to subject the plate to a quick circular motion, so as to spread the film of albumen evenly, and to get rid of the surplus solution. This is effected by placing the grained zinc in the jaws of an instrument called a whirler and made as follows:
Two pieces of wood, each half an inch thick, twelve inches long, nine inches wide at one end, and six inches at the other; upon the narrow end of one piece fix four screws, a piece of iron the shape of a┻, the top end of which is shaped to fit into the socket of a carpenter’s brace (which must have the centre handle revolving, not fixed); the cross piece must be flat, and pierced with four holes, countersunk to admit of stout screws.
Place the two pieces of wood together, the┻piece between the two, and with a piece of stout leather hinge the two together, nailing the leather not only on the outside, but upon the top also; now screw a narrow batten on each of the pieces, about an inch from the widest end, to keep the wood from warping.
Four inches from the top (the hinge being the top) bore a hole through both pieces, and pass through a couple of strong leather laces; by one end fix these on the outside of the cheek to which the iron┻is screwed, and on the outside of the other cheek, seven inches from the top, hinge a piece of wood (so as to fall toward the bottom) 5 inches by 2 by1⁄2; and in the centre of the batten screwed on to prevent warping, fix a broad-headed nail or a turn-button, over which the free end of the leather laces can be passed.
Now about half an inch from each end drive through the wood, six or eight wire nails one inch long in a line, and so that the points project on the inside of each cheek. Next fix the top of the iron into the socket of the brace and the whirler is ready for use.
To use this instrument, lay it upon the bench, raise the upper jaw, place the grained zinc behind the teeth (face out) close the jaw, and after seeing that the face of the zinc is close up to the teeth, tighten the shoe lace, pass it over the hinged piece (which is lying down close to the cheek), and around the batten with two or three turns, then pass it back and tie the loose ends around the iron shank at the top; then raise the hinged piece, which by straining on the{42}leather laces will cause the jaws to grip the zinc plate tight and hold it in position. (See Fig. 4.)
Fig. 4. The Whirler.
Fig. 4. The Whirler.
Now lift up the whirler from the bench, grasping the handle of the brace firmly in the right hand. Hold it at arm’s length, and by a series of quick jerks set the plate revolving. This will require a little patience to do evenly and rapidly, but with practice it will become quite easy.
The plate is coated twice, whirling it after each application of the sensitive mixture. The whirler is then laid upon the bench, and the hinge piece let down. This relaxes the strain upon the leather laces, and allows the jaw to be lifted and the plate removed.
An inverted revolving egg-beater may be made to serve for the same purpose.
DRYING THE COATED ZINC.The film of albumen now requires drying, which should be done over a spirit lamp. The best form of lamp is one of the small pocket spirit stoves sold by ironmongers, and having the three supports for a kettle cut away. The zinc is held over the flame and kept in constant motion, so as to dry the film as quickly and evenly as possible; no fear need be felt at applying too great a heat so long as the metal can be held comfortably in the fingers. When dry, the plate is ready for printing upon.
The film of albumen now requires drying, which should be done over a spirit lamp. The best form of lamp is one of the small pocket spirit stoves sold by ironmongers, and having the three supports for a kettle cut away. The zinc is held over the flame and kept in constant motion, so as to dry the film as quickly and evenly as possible; no fear need be felt at applying too great a heat so long as the metal can be held comfortably in the fingers. When dry, the plate is ready for printing upon.
For drying large plates, two or more of these stoves would be required, but it is very easy to extemporize a suitable arrangement, by laying a large pad of cotton saturated with spirits of wine in a tin dish, and setting it on fire; by this means a stove of any desired dimensions can be prepared quickly. To extinguish such a flame, cover it with a large sheet of zinc.
It must be borne in mind that the film on the zinc is now very sensitive to{43}light, therefore the preparation of the film must be done in a yellow light, as must also the operation of placing it in the printing-frame, inking up, and development; gas or lamp light may be used, as the film is not sensitive to artificial light of a low actinic power.
THE PRINTING FRAME.The printing-frames used for this purpose must be of the box pattern, fitted with stout plate-glass fronts, the cross bars behind being fitted with wooden screws, instead of springs, as absolute contact can only be obtained between the surfaces of the glass negative and zinc plate by means of screw-pressure.
The printing-frames used for this purpose must be of the box pattern, fitted with stout plate-glass fronts, the cross bars behind being fitted with wooden screws, instead of springs, as absolute contact can only be obtained between the surfaces of the glass negative and zinc plate by means of screw-pressure.
The front glass of the printing-frame must be kept perfectly clean, and especial care must be taken that no grit be upon either glass, inside the frame, or on the back of the negative, else the negative will be broken.
TIMING AND EXPOSURE.The time of exposure to light is measured by means of an actinometer, which is simply an instrument in which is a strip of sensitive paper, exposed under a graduated series of different thicknesses of translucent paper, each division or gradation being distinguished by a number, thus: No. 1 has only one thickness of paper over it; No. 2, two thicknesses, and so on. But on account of the ever varying intensity of the light, a screen actinometer that is reliable is almost an impossibility. The best must be closely watched in a bright light, so that the tints are changed evenly.
The time of exposure to light is measured by means of an actinometer, which is simply an instrument in which is a strip of sensitive paper, exposed under a graduated series of different thicknesses of translucent paper, each division or gradation being distinguished by a number, thus: No. 1 has only one thickness of paper over it; No. 2, two thicknesses, and so on. But on account of the ever varying intensity of the light, a screen actinometer that is reliable is almost an impossibility. The best must be closely watched in a bright light, so that the tints are changed evenly.
Johnson’s actinometer is very simple and handy; it consists of a cubical box with two lids, the inner one serving to press the paper in contact with the glass of the outer lid; the outer lid has a circular opening with a narrow rectangular strip in the centre transparent, the rest of the glass being covered with pigment the color assumed by the sensitive paper after exposure to the light. When the strip of sensitive paper inside has assumed this color, it is called one tint; the strip of paper is then pulled forward, and another portion, quite white is brought under the transparent portion of glass, and when the light has turned that the color of the pigment, that counts two tints, and so on. The paper used in the actinometer is ordinary albumenized paper.
The exposure of zinc in the printing-frame under a good line negative, will be about six or eight tints in diffused light, but in direct sunlight the exposure may be timed by the watch, a suitable line negative never requiring more than from three to five minutes.{44}
INKING THE EXPOSED ZINC.The sensitive zinc having been exposed to light under the negative, next cover the surface with a thin coating of litho-transfer ink, stone to stone.
The sensitive zinc having been exposed to light under the negative, next cover the surface with a thin coating of litho-transfer ink, stone to stone.
For this purpose we require a type-printer’s composition roller, mounted upon a litho stock. This roller must be perfectly smooth and cast in a solid mould, as the line along the side, formed by a split mould would render it useless for inking up a zinc plate. A roller eight inches long by five or six inches in diameter is a useful size. We also require an inking slab, a palette knife, a bottle of turpentine, with the cork cut so that the turpentine can be sprinkled out without having to remove the cork, a tin of transfer ink, and a linen cloth or two.
The inking slab may be made of smooth iron, or a suitable lithographic stone is good.