In this rnanner, two parallel plains are obtained, the lower one formed by the intaglio, and the upper by the plate-glass. The next stage in printing requires that the intaglio should be slightly greased, and covered in the centre with a swam solution of semi-transparent gelatine and Indian-ink. A sheet of bard-pressed, smooth, white paper, is next laid on the ink, and the upper lid of the press is brought down, and locked. The superfluous gelatine is thus expressed, the portion left in the mould sets in 2-3 minutes, and the resulting proof is a permanent pictorial relief in Indian-ink. The high lights of the photograph have been pressed out by the projections in the intaglio, leaving the white paper exposed; while the semi-transparent jelly ascends in beautiful gradations, attaining its highest relief, and therefore its greatest opacity, in the deepest shadows. The relief is low, and when dry, the surface of the print appears to be perfectly flat. As in carbon printing, so in this process, a wide range of colour may be etnployed in preparing the printing-ink.
It is a purely mechanical process after the relief has been printed from the negative, and, on that account, is extremely useful in producing large numbers of prints without the aid of light. Thousands of copies may be pulled from Oile impression, and the number obtainable from a single gelatine relief is almost incredible.
Collotype Printing.—Collotype printing also owes its origin to the mingling of chromium salts with gelatine, but it differs from kindred processes in the principle upon which it is based. Here it is not a question of the solubility or insolubility of the gelatine film, so much as of chemical affinity. Thu collotype plate, after exposure in the printing-frame, is treated much in the same way as a lithographic stone. When damped over ssith a wet sponge, thc parts unaffected by light absorb water, while the parts affected repel water, and have an affinity for fatty ink. To enter into the process more folly, tsvo squares of plate-glass are ground together with flne emery until they are obscured. Take ono of the plates, wash, and coat with 4 oz. albumen and 20 gr. ammonium biehromate. The albumen must firet be beaten into a froth, and allowed to settle. Test the solu tion, and neutralize with dilute ammonia ; coat the plate, and dry by heat at 35° (95° F.). Expose tho plate to daylight long enough to print an ordinary silver positive. Wash the plate in water for ten minutes, and again dry. Coat with 1 oz. gelatine, 1 dr. potassium bichrornate, and 20 oz. water. Dry by heat as befnre, and apply a third coating of 1 oz. gelatine (hard), 20 oz. water, 3 oz. alcohol, 100 gr. ammonium biehromate, and 15 gr. calcined magnesium. Add to this solution, just before using„ 5 gr. chrome alum and oz. water. Coat on t/. levelling-stand, and dry by heat not exceeding 38° (100° F.).
After the plate thus prepared has been printed under the negative, sponge over with water, blot off excess of moisture, and ink with a roller as in the lithographic process.
The pigment in the ink must be in a fine state of division, and rendered fluid by the addition of oil of turpentine. If tho exposure of the plate has been properly timed, a positive impression in prioting-ink will develop under the roller, and may be pulled off on paper in a collotype press.
The plate, indeed, should yield some hundreds of proofs of uniform quality.
There are a number of methods of utilizing the properties developed by the exposure to light of bichromatized gelatine. By some methods, reliefs arc obtained, and electrotyped in meted, or cast in type-metal. Copies of line engravings and drawings are thus made on a reduced or an enlarged scale, and worked with the text in ordinary printing. Some examples of this process may be seen in Tissandier's 'History and Handbook of Photography.' Ceramic Photographs.—Eucaustio photographs may be produced in a variety of ways. A secret method is practised, and excellent results obtained, by using carbon-tissue. The tissue is prepated by a special process, which obviates any risk of the gelatine cracking up during the baking of the enamel.
Another method is the " dusting-on" process, previously alluded to. But in place of employing graphite in developing the image, s. finely-powdered encaustic colour is used. An exhaustive account of this process is given in the ' Photographic News Almanac ' for I8SI.
The substitution process is one by which the finest results are obtained. It consists in copying a negative through the camera on a wet collodion plate. But the silver of which this positive is made up, were it left in the film where burnt in, would yield a sickly-yellow picture. It is, there fore, nec,essary to treat the silver positive with a solution of salts of platinum or iridium, until the silver has been transforined into chloride and replaced by metallic platinum or iridium. Atter the chloride of silver has been removed in the fixing-butt, the picture may be transferred to its support and fired.
The collodion positive must be developed with protosulpbate of iron ; the image must be dense, approaching opacity in the shadows, and perfect transparency in the highest lights. When washed after fixing,.it is detached from the edges of the plate by a penknife, and placed in water acidulated with sulphuric acid. This hardens the film, and it may then be floated off the glass. It is next washed free from acid in several changes of water. The film is then transferred t,o a solution of 2 dr. platinum chloride, and 1 oz. water. Neutralize with soda bicarbonate, then acidulate with one or two drops of nitric acid. Here the film should repose until the image ia black, after which, i t is fixed in 3 dr. soda hyposulphite and 1 oz. water. Again wash, and transfer the film to a basin of clean water. Place the enamelled plate beneath the film, with the collodion side next the enamel ; float into position, and lay down with a fine camel-hair brush, excluding air-bubbles.
After burning in, the enamel is glazed with a flux made up of 8 parts powdered glass, 5 nitre, and 6 flint. A. thin solution of indiarubber in benzol should be used as a varnish to the enamel, and the flux dusted over this, and burned in.
Platinotype Printing.—Permanent prints are obtained on paper by this process. A sheet of paper is coated with a solution of platinum chloride and iron oxalate. After the paper has been dried and exposed under a negative, a faint image is observed. This image develops into a rich black, by immersing the print in a solution of oxalate of potash, heated to 77° (170° F.). An acid water bath is all that is required for fixing the image.