Photo-engraving, Photo-etching and Photo-lithography in Line and Half-tone / Also Collotype and Heliotype

by W. T. Wilkinson

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}