Relief awl Photo-engra, ing Processes.—Relief processes are those which produce plates or blocks with raised lines, capable of being printed from like type in an ordinary printing-press. They are adapted only to line drawings, and are unsuited fer the reproduction of toned work. Engraved plates have the lines of the original drawing in depression, and are adapted to the same class of work as relief processes. Both are produced by the same general method and on the same prin ciple, of which, the following is an outline. The foundation of the system is the fact that asphalt or bitumen, when exposed te light, becomes insoluble in its ordinary solvents if partially saturated. In Nie-pce's process, the first based en this ground, silver plates were coated with bitumen, the unaltered portions of which were dissolved away after exposure ; iodine was applied, the remaining bitumen was removed, and the result was a metallic silver image on a ground of silver iodide• The solvent generally employed is chloroform. The coated plate is dried, and exposed beneath a subject. The portions to be protected from the influence of the light will depend upon whether the plate is to be engraved or in relief ; in the former case, the lines will need protection. Care must be taken that the opacity, where required, is perfect. For engraved plates, a reversed positive is necessary ; for relief blocks, an ordinary unreversed negative. The original picture is placed in contact with the prepared plate, and exposed as long as is considered necessary ; the,soluble por tions of the bitumen are then removed by a nearly saturated solvent, leaving the metal bare. This latter may be zinc, copper, or steel ; the first is most commonly used for relief blocks, while the two List are more convenient for engraving. The " biting-in," or development of the lines, is effected, in the case of zinc, by simple hydrochloric acid, though it is advisable to previously dip the plate in a sulphate of copper solution ; for copper and steel, a mixture of hydrochloric acid and potassium chlorate is preferred. With relief blocks, the biting-in is a tedious operation, having to he carried as deep as in a wood-block. After the first biting, which gives the clear lines, the plate is heated, dusted ever with resin, and reheated to make the bitumen quit the lines, these operations being repeated till sufficient depth is attained. In appreciably large spaces, the metal is rerneved by engravers' tools.
Ehrard's biting-in process differs sernewhat from tbe preceding. A transfer is prepared as for zincography, transferred to a copper plate, and plunged into an electre-plating bath for a fevv minutes, thus coating the copper with a thin silver film, while the lines are protected by the greasy ink; the plate is rinsed in dilute acid, and placed in a mercuric chloride bath, vvhere a double chloride is formed ; after washing, and removal of the ink, the biting-in proceeds.
Fox Talbot proposed a modification, which consisted in printing the negative on a gelatine film, washing away the unaltered gelatine, and making au electrotype. Scamoni has some plan of building up a relief on the negative itself, and taking an electrotype from it. The foregoing methods, with perhaps semo other modifications, are in extensive use on an industrial scale. Several firrns are largely engaged in making relief blocks and phote-engravings, notably Leitch, Dallas, and Cattell, London, besides many others on the Continent and in America. The illustrations in this Eucyclopmdia have been prepared by the fimt-named firm, from drawings en stone hy B. Alexander, Castle Street, Holborn.
Much has been done in the more difficult task of reproducing half-tone drawings and photo graphs fmm nature, by Woodbury, Dallas, Lenoir, and others. A manag,er of Goupil's, named
Roussillon, availing himself of the Woodbury-type process (see p. 1617), gives a grain to the picture by the avtion of light, suitably regulated, and thus obtains a mould capable of giving mezzo-tints from ordinary negatives. They require some mechanical touebiog-up, however. Leuoir has re cently made publio a new process fir producing engraved plates from negatives photographed from nature, which is substantially as follows. A metallic plate is lightly coated with a mixture of albumen, carmine, and potassium bichromate. The carmine (for which, gamboge and various resins may be substituted with almost equal sucoess) serves both as a dye and to assist in the lifting of the film, by its solubility in ammonia, drawing the albumen with it more or less in the stripping off, the exposure having taken place upon the upper surface. When the film is stripped off, an image remains formed of albumen, in itself unable to resist the action of acids. It must, there fore, be rendered insoluble. There are two ways by which this may be effected; one is to cause the albumen to absorb a solution of gum lac, dissolved in hot water.with borax ; the other, and preferable, is to plunge the plate, once stripped, in a solution of potassium bichrornate, then drying at about 49° (120° F.). The albumen by this means acquires the required resistance to the action of acids. The plate is next engraved, to give it a grain necordiog to the amount of ink it should take up. Upon the unabsorbent and. stripped plate, a film is spread, consisting of a solution of bitumen and turpentine mixed vvith carbonate of lime. When plunged in all neid bath, earbonio acid is liberated ; it forms tiny canals, through which, the acid attaeks the metal more or less quickly, by reason of the thickness of the albumen. The acid bath is composed of water acidulated with nitric and oxalic acids and alum. An oxalate of the metnl is thus formed on the sides of the canals, and causes them to adhere to tho plate. The texture of the etching is more or loss fine aceording to the length of time the albumen is allowed to absorb the acid. In this state, the plate is finished ; it requires only to be dried, and is ready to be printed from immediately. No pre liminary preparation is necessary, as the whole operation may be conducted in three hours.
Warnerke has recently published some improvements based upon the discovery that a gelatioe plate submitted to pyrogallie acid becomes insoluble in the parts exposed to light. The ordinary gelatiue process requires very accurately-timed exposure ; but with the pyrogallie acid, and using tho ernulsion on paper, no amount of over-exposure witl do harm, provi& d the developer is sufficiently restrained. The transfer of the image from the paper to glass is very simple. The former is irrunersed in water, and placed iu contnet with a glass plate; the superfluous moisture is removed by a squet gee, and the paper is stripped off, leaving the gelatine on the glass, when the application of hot water dissolves all the gelatine not acted upon by the light, and the image is left in relief on the glass. Intensification is effected by mixing with the emulsion n non-actinic colouring matter which is not affected by silver ; aniline colours answer the purpose viten. Relief is said to be obtained far more easily than by the ordinary biehromatized gelatine, and the process is therefore specially applicable to Woodbury-type. It may also be adapted to engraving, enamel ling, and eallotype purposes.