Printing-processes.—Printing of positives from negatives may be divided into two classes. .The first, which still finds many adherents, is based on the blackening of silver salts by the aid of light. Prints produced by this method are liable to fade. The second class includes all the modern pro cesses, which secure permanency in the finished prints.
In ordinary silver-printing, the paper required is generally albnminized and salted ready for the silver-bath. This paper is best procured from a dealer. It may even be purchased sensitized, ready for the printing-frame.
To prepare the ordinary albuminized salted paper,pour into a fiat dish a solution of 50 gr. silver nitrate, and 1 oz. distilled water. The sheet of paper to be sensitized is lifted, face downwards, by the two diagonal corners, and lowered on to the solution, beginning at the bend in the centre, and steadily lowering until the sheet lies flat on the surface. By this means, air-bubbles are expressed. About four minutes is sufficient to impregnate the salted surface, after which, lift a corner of the paper, and pass a glass rod beneath. Draw the paper over the rod, to get rid of superfluous moisture, and pin up to dry in the dark room. When dry, the paper is placed in contact with the negative in the printing-frame, film to film. Perfect contact is established by the pressure-frame, Figs. 1095, 1096. Now expose to light until the print is somewhat darker than required when finished. The print should be taken out and trimmed to size round the edges, the cuttings being preserved to recover the silver.
Toning the _Print. —Place the print in water, to wash out the free silver ; change the water several times, until no trace of milkiness is observed. Immerse the print in a bath of 1 gr. gold chlmide, 4 gr. soda bicarbonate, and 8 oz. water, to be used immediately after being made up ; or 1 gr. gold chloride 25 gr. soda acetate, and 8 oz. water, to be made 21 hours before being used. The red colour of the washed prints will speedily give place to a darker purpled hue, which must be a trifle darker than what is required for the finished print. Remove from the toning-bath, and wash ; then immerse the print in 1 oz. soda hyposulphite, and 6 oz. water. The print should remain for 20 minutes in the fixing-solution, after which, it should be washed in running water for several hours.
The positive, after washing, may be pressed between folds of blotting-paper, and, while damp, brushed over with a paste made of ordinary starch. Place the picture on its card mount, cover with a sheet of clean paper, aud press down.
Yellow stains en the prints are caused by touching the albuminized surface with fingers soiled with soda hyposulphite. When the bath contains too little silver, it will dissolve the albumen.
When the bath becomes brown, add 1 dr. powdered kaolin, shake up, aud filter. The hyposul phite bath should be used only once.
Albuminzzed ;Sensitive Paper.—After salted albuminized paper has been sensitized, it may be preserved in its sensitive state, without discolouring, for several months by rendering the paper acid with citric acid. When the silver solution has been applied, and while the paper is still damp, float on a bath composed of 6 gr. citric acid and 1 oz. water. The paper should be taken from the acid bath as soon as possible, and hung up to dry. By increasing the proportion of acid, the keeping qualities are irnproved, but the paper should be neutralized by ammonia fumes before placing in the printing-frame.
Permanent _Positive Printing.—Carbon Printing.—Theeretically, the process of carbon printing is simple ; practically, it is difficult, and demands technical knowledge, and careful treatment in all its details. A principle, extremely rudimentary in itself, underlies all printing processes in which carbon or permanent pigment is used. It is that gelatine charged with potassium bichromate, when dried and exposed to white light, becomes insoluble. In addition to the bichro mate in the photographic film of gelatine, it carries a powdered pigment in a fine state of division. This film, when exposed to light under a negative, is rendered insoluble, in a ratio corresponding with the lights and shadows of the negative. The deep shadows of the picture become quite in soluble while the high lights remain unchanged and perfectly soluble. Fig. 1097 represents a traneverse section of the carbon film. The light passing through the clear parts of the uegative has penetrated through the film at A. This point represents the deepest shadow, and moat in aoluble position of the film. At B, the light has pierced midway through the film, where the half tones of the negative have interposed. The film at C remains in its normal condition, no light having passed through. This point anawers to the high lights of the negative. By immersing the iiIM, 80 printed, in warm water, the gela tine and pigrnent will dissolve out at C, partly at 13, and remain intact at A. In this manner, a positive print is obtained in low relief, the deepest shadow being represented at A, and highest light at C. But the pig mented film when supported on a white ground, by virtue of its semi-transparenoy, discloses a visible picture, having all the gradations of light and shadow of the negative from which it has been printed. The quantity of pigment held by the gelatine is so adjusted as to yield perfect opacity in the parts only where it has been most affected by the light.