CHAPTER VI.PRINTING ON ZINC IN BITUMEN.
The process described in the previous chapter is most suitable for printing on zinc, for blocks that are not overburthened with fine lines, and is intended for rough printing; and also, when from want of sunlight, and the exigencies of business, the exposure has to be made by electric light; but when the subjects are very fine, and the blocks must be of the very best, it will be better to print the image in bitumen; then the first etching can be done without having to ink up previous to immersing in the acid bath (which must be done after developing the inked albumen image), so getting the sharpest possible results, and also securing better depth to close work than when the inking up has to be done before biting in.
Now the generally accepted idea of bitumen is, that it requires such a long exposure to light as to be practically useless, except for direct sun printing; but that only stands good where the bitumen is not prepared properly and where too thick a film is used upon the plate.
By preparing the bitumen as described below and taking care to have a very thin film of the sensitive bitumen upon the zinc plate, then the exposure either to sunlight or electric light is not more than double that of albumen.
There is one especial virtue in a bitumen film. If by chance too much exposure to light has been given, a prolonged immersion in the developing liquid (turpentine) will remedy the defect, whereas if an albumen image be over-exposed it is quite spoiled, and the work must be done again.
We are still upon the subject of line work, and the same class of negative desirable for printing on an albumen film is necessary for printing upon bitumen, viz., the lines must be clear glass without veil, and the rest of the negative must be sufficiently opaque to stop the passage of light.
The sensitive solution of bitumen is made by procuring a small quantity of photographic bitumen or asphaltum, and powdering it in a clean, dry mortar. Next take a clean and dry glass beaker, half filled with methylated ether, and pour into it the powdered bitumen, stirring with a glass rod for a few minutes; now cover the beaker with a piece of thin sheet India-rubber, and upon that{49}place a piece of plate glass, so as to make the vessel as nearly air-tight as possible. Allow to stand a few hours, then stir up again, and again allow to stand, this time all night. Then pour away the ether, draining the liquid as closely as possible, so as to get rid of all those constituents which are soluble in the ether. Now add some more fresh ether, stir thoroughly, and again allow to stand and settle, putting on the cover as before, so as to prevent evaporation. After again settling, the ether is poured off, and all liquid and semi-liquid matter carefully drained away. More ether is added, and the operation is repeated until the added ether, after standing all night, is nearly or quite colorless. The residue in the beaker is removed to a glass plate and spread out over its surface, so that any ether remaining may quickly evaporate. During this evaporation of the ether, the bitumen must be protected from the action of white light.
By thus purifying with ether we get rid of those constituents of the bitumen which are not sensitive to light, and thus we secure a film of bitumen sensitive to the action of light.
When the ether has evaporated, take of the residue half an ounce and dissolve it in pure benzole (free from water) fifteen ounces, allow it to stand all night, then filter it through filtering paper (placing a sheet of glass over the funnel to prevent evaporation). It will keep six months.
New zinc plates will require polishing first with rotten stone, and finishing with rouge, but plates that have been used should be first polished with fine emery cloth, followed by pumice powder, then by rotten stone, and finished with rouge, remembering that a circular motion must not be used.
If, after polishing, the surface is at all greasy, immerse it in the graining bath used for the albumenized plates, wash well and rub away the scum; then with a perfectly clean piece of blotting paper, remove the water from the surface and dry it over the spirit flame.
To coat a piece of zinc with bitumen, place it in the whirler, and when fixed in position, dust the surface with a clean camel’s-hair brush; then flow over it sufficient of the filtered bitumen solution in benzole, then lower the whirler and set it in motion; whirl rapidly for about a minute. This will equalize the coat or film of bitumen and get rid of superfluous solution. When taken from the whirler the plate will be ready at once for the printing frame. These plates do not deteriorate, and a stock of them may be prepared and stored away in the dark till wanted. Great care must be taken not to allow the slightest ray of daylight to fall upon them, else the action once set up, will continue, and spoil the plate.{50}
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.
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.