The print is developed by being floated back downwards on water at a temperature of 100° to 122° F. (38° to 50° C), till the lines appear as depressions. It is then washed with water at about 158° F. (70° C.) on an inclined slab, by which the soluble gelatine is removed with the ink that coated it, and the image remains as ink lines on ridges of insoluble gelatine. The developed print is washed in cold water, and hung up to dry; it is then ready for transferring to stone or zinc, being first damped till it becomes limp. The subsequent manipulation is a mere repetition of lithographic printing.
(20) Niepce's Process
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 for 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 principles, of which the following is an outline. The foundation of the system is the fact that asphalt or bitumen, when exposed to light, becomes insoluble in its ordinary solvents if partially saturated. In Niepce's process, the first based on 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 portions 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 last 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 be carried as deep as in a wood-block. After the first biting, which gives the clear lines, the plate is heated, dusted over 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 removed by engravers' tools.
(21) Ehrard's biting-in process differs somewhat from the preceding. A transfer is prepared as for zincography, transferred to a copper plate, and plunged into an electro-plating bath for a few 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, where a double chloride is formed; after washing, and removal of the ink, the biting-in proceeds.
(22) Fox Talbot proposed a modification, which consisted in printing the negative on a gelatine film, washing away the unaltered gelatine, and making an electrotype.
(23) Scamoni's Process
The originals which have to be reproduced are carefully touched up, so that the whites are as pure, and the blacks as intense, as possible, and then the negative is taken in the ordinary way, the plate being backed in the camera with damp red blotting-paper, to prevent reflection from the camera, or back of the plate. The negative is developed in the ordinary manner, intensified by mercuric chloride, and varnished. A positive picture is taken in the camera, the negative being carefully screened from any light coming between it and the lens. This is intensified by pyrogallic acid, and afterwards washed with pure water to which a little ammonia has been added. It is then immersed in mercuric chloride for 1/2 an hour, and again intensified with pyrogallic acid. This is repeated several times. When the intensity of the lines is considerable, the plate is well washed, treated with potassium iodide, and finally with ammonia, the image successively appearing yellow, green, brown, and then violet brown. The plate is then thoroughly drained, and the image is treated successively with a solution of platinic chloride, auric chloride, ferrous sulphate, and finally by pyrogallic acid, which has the property of solidifying the metallic deposits.
The metallic relief thus obtained is dried over a spirit lamp, and covered with an excessively thin varnish. This varnish, which is evidently a special preparation, retains sufficient tackiness to hold powdered graphite on its surface (bronze powder may be employed instead), which is dusted on in the usual manner. After giving the plate a border of wax, it is placed in an electrotyping bath, and, after a few days, a perfect facsimile in intaglio is obtained.
(24) The editor of the British Journal of Photography writes: - The great secret in obtaining a successful negative from an oil painting, is to have it illuminated entirely by diffused light. Some Continental artists who are very successful in this branch of photographic work, lay the painting flat on the ground, and point the camera downwards.
(25) Zincography is thus described by Nuth: - The zinc. plate with the transfer laid on is first gently warmed over a gas or spirit lamp; when cooled, finely-powdered resin is dusted over it out of a linen bag; this is brushed over the plate by means of a round camel-hair-brush, an even circular motion being maintained. When sufficient resin has adhered to the plate, it is again warmed, to ensure its firm connection with the fatty design; the plate is then cooled in cold water, and allowed to remain a few minutes in a 3 per cent-solution of commercial nitric acid, until a depth of about '005 in. is* obtained. This depth has to be determined by frequent experiment, as it is very important it should not be exceeded. The plate is now bitten to such a trivial degree that the lines are not under-bitten to any appreciable extent, and it may be mentioned that this first etching is the keystone to the rest. If it "goes wrong" you cannot doctor up your plate, and you may save further trouble by beginning afresh. When dry, it is again dusted with resin, and warmed, the fringe of resin thus adhering to the sides of the lines, serving to protect the etched part from any further action of the acid; the plate is then rubbed over lightly with strong gum mucilage, which is immediately washed off.