Oil and water will never agree; they repel each other. Consequently, if a sheet of paper or a glass plate is coated with a bichromated gelatine, exposed under a negative and then soaked until the unchanged gelatine is heavily charged with water, any pigment of greasy nature will adhere only to those parts of the print which, being insoluble, have absorbed no water. Here we have the foundation of another method of printing photographs which admits of all the variations and individual treatment characteristic of gum-bichromate.
Several ready-prepared papers may be had specially suited for oil pigmenting, and these ready-made papers are far preferable for the beginner unversed in the many difficulties and uncertainties of gelatine. We ourselves often revert to the lithographic transfer papers of Albert & Husnik, obtainable from Messrs. Winstone, of Shoe Lane, E.C. Or a good cartridge or drawing paper may be coated with this solution:
Amber Gelatine (Nelson's or Coignet's) . . . 1 oz.
Soak the gelatine for about twelve hours, add the sugar, melt in a jar over a hot plate or in a bain-marie, pour in a few drops of chrome alum, and strain through muslin. When the paper is dry float a second time, taking care that the coating is as evenly applied as possible. Some prefer to lay the paper on a perfectly level glass plate, and pour on the melted gelatine, using about 2 1/2 ounces of solution for a sheet 16 by 13 inches at a temperature of 750 Fahr. The paper may be made wet with warm water before coating, and in about an hour should have set sufficiently to be hung up to dry. If floated dry on the gelatine, the coating will set immediately. Under either condition bubbles or bare places must be corrected.
Tea Time. By. P. Maskell.
When required for use the prepared paper is immersed for three minutes in:
Potassium Bichromate . . . . . . . 1 oz.
Potassium Ferricyanide.......120 gr.
Methylated Spirit........ 5 „
Ammonia . . . . . . . . . 10 min.
And then drained and squeegeed on a ferrotype under blotting-paper to secure an even surface, and put away to dry on its support. Or, for rough work, the sensitised sheet may be simply hung up to dry in the dark room. The sensitised paper will keep for about a week, or longer if under pressure and protected from changes of temperature. M. Demachy sensitises by laying the gelatined paper on a sheet of thick blotting, and brushes it over with a two-inch flat hog-hair brush dipped in the solution, this device enabling the paper to be dried within about half an hour. The other method, however, secures an image penetrating far more deeply into the film.
The image prints out quickly under the negative, and in summer all details will be visible in about three minutes. Under-exposure gives too great contrast; over-exposure will mean the veiling of the high lights. When printing appears to be of sufficient depth, the paper is removed from the frame and washed in cold water till all traces of bichromate are removed. About twenty minutes in three changes of water will accomplish this. It can then be placed in warm water, say at 900, to develop the grain. Lift at intervals, blot off a portion, and see if the picture is well in relief. If not, warmer water must be applied. The exact temperature at which sufficient water is absorbed will vary with the kind of paper and the time of the year. With Albert paper in winter we have sometimes used water at 2000 Fahr., to which a little ammonia has been added. But, as a rule, water at 100° is sufficient. In summer even at this heat the gelatine will be rendered too brittle. Caution is necessary until experience can adjust these little difficulties.
Lay the fully-soaked print, protected by a piece of hard blotting, on a glass plate or slab, blot off superfluous moisture from the surface, and apply the pigment. A very solid hog-hair brush, with the ends of the bristles cut and ground level, is the favourite tool, but some use a pledget of cotton wool for dabbing the colour on, while one enthusiast of our acquaintance declares that the ideal "dabber" is the ball of the human forefinger. A dabbing motion is the best for getting the pigment well on and cleaning it off the high lights. Apply very little pigment at first, increasing the thickness by degrees especially where greater contrasts are aimed at. The actual pigment may be composed of lithographic ink thinned with turpentine, or ordinary oil-tube colours, such as are used for oil-painting, mixed with a little copal oil and then thinned with turpentine. As the turpentine evaporates so will the power of strengthening the shadows and clearing the high lights increase. A harder ink for greater contrasts is obtained by adding a drop or two of some oil varnish. If the high lights refuse to clear, sponge the picture gently with warm water, or dip it once more into warm water. But this must not be done until the best part of the turpentine is evaporated from the ink, or it will run. In order to secure this steady evaporation, oil prints should only be pigmented at a temperature of 650; in winter there should always be a fire.
When the whole image has taken the ink, the picture may be touched up with a finer brush, removing excess of ink, clearing high lights, and adding more colour in places that seem to require such treatment. Lastly it is dipped once more into cold water, and clipped to a board to dry. If allowed to dry in a loose condition it will roll up and become unmanageable, probably smeary. When the gelatine surface is dry the ink is still capable of absorbing any metallic dusting powder, gold, silver, or bronze, if such things appeal to the taste of the producer. They are laid on usually from a pepper castor, and the excess brushed away with a soft camel-hair brush.