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Summary of the More Important Printing Processes with Chromated Gelatine.

THESE processes are all based upon the change in the hygroscopic properties of chromated gelatine brought about by the action of light, which renders the parts exposed insoluble.

Carbon or Pigment Printing.—In this interesting process finely ground colouring matter is added to the solution of chromated gelatine, and spread upon paper; or the paper may be coated with the gelatine and pigment, and afterwards excited by immersion in a solution or bath of one of the bichromates. If, after drying such paper or tissue, it be exposed under a negative in an ordinary printing frame, and then immersed in warm water (90° to 130° F.), the water will dissolve only such portions as may not have been affected or hardened by the action of light through the negative, while the parts that have been exposed to its action will be insoluble, and remain attached to the paper support, forming a positive picture. Images produced in such a manner, however, are devoid of half-tone, and this procedure is only suitable for line subjects, while even for such the paper must have been very evenly coated, or portions of the picture may be washed away. For the production of half-tone subjects it is necessary that the picture be developed from the back. This may be effected by transferring the printed tissue, after first soaking in cold water, to a surface, usually either of paper or glass, and developing it in contact with, and allowing it to remain upon, such surface. This procedure is known in carbon printing as single transfer. Unless the negative be a reversed one—or a film printed from the reverse side—the resulting print will be subject to the disadvantage of being reversed as regards left and right. In cases where ordinary negatives have to be used, and this reversal cannot be tolerated, it is overcome by the “double transfer” method of printing, the print in such cases being developed upon a so-called “temporary support,” and again transferred to its “final support,” when it appears as a non-reversed image in permanent pigment.[D]

[D] For further details of this interesting process see the “Autotype Manual,” by J. R. Sawyer; or Liesegang’s “Carbon Process.”

The “Powder” Process.—Take, instead of gelatine, a solution of gum arabic and sugar, to which is added a solution of bichromate of potassium or ammonium. With this a glass plate is coated, dried, and exposed under a negative, if it be desired to produce a negative; or, if a positive be desired, the exposure must be made under a positive. After a short exposure the plate is dusted over with a powder in an extremely fine state of division, such as graphite if for negatives; if for burnt-in enamels, a specially-prepared enamel, ground extremely fine, must be used. Such powders will only adhere to the plate in exact proportion to the action of the light during the exposure in the printing frame, and a reversed copy of the original will be produced.

Collotype and Glasstype.[E]—In these processes the printing depends upon the property possessed by the bichromated gelatine film, which has been properly exposed under a negative, of absorbing water in varying proportions, in exact accord with, but in inverse ratio to, the action of the light. The application of a roller charged with fatty ink of a suitable consistency, will deposit upon the gelatine surface a layer of ink in exact proportion to the action of the light and in inverse proportion to the amount of water absorbed by the film. In this manner a picture is produced in ink which by printing is transferred to paper, and will exhibit all—even the finest—gradations of light and shade.

[E] Glasstype, more fully described later on, has never been practised commercially in England.

Woodburytype.—If an especially thick film of gelatine, containing a small proportion of pigment, be sensitised, exposed under a negative, and subjected to prolonged soaking in warm water, a picture exhibiting considerable relief will be obtained, which, after drying, may, by pressing into lead or treated by the galvanoplastic method, yield a matrix from which casts may be produced in a coloured gelatine solution, or ink, which is poured into the mould; on this is placed a waterproof paper; the press, which is of special construction, is closed, and the ink allowed to set; the paper is then removed, bearing upon its surface the printed Woodburytype, which is hardened by alum, and then scarcely distinguishable from a silver print.

Stannotype is a modification of the last-described process by the same inventor. A positive is first produced from the original negative, and from the positive a print is made on a thick gelatine tissue. This is developed by long washing in warm water, the resulting relief being a negative; this is coated with tinfoil of extreme thinness, and used as a mould for printing from with the gelatine ink, as previously described. This obviates the use of hydraulic pressure and special and expensive appliances.

Heliogravure; Photogravure.—If, instead of printing from the Woodbury mould in the gelatinous solution, a fatty ink is used, the process is known under these titles. The exact methods of roughing the depths of the plates to facilitate their retention of the ink are regarded as secrets by the most successful operators.

Photo-Lithography and Photo-Zincography.—In dealing with a subject in lines, dots, or stipple, a paper may be coated with a solution of chromated gelatine, dried, exposed under a suitable negative, coated with a film of fatty ink, and allowed to soak in cold or tepid water; this, penetrating the unhardened portions of the film, causes them readily to part with the ink, which, however, adheres tenaciously upon the hardened portions, representing the picture. This image, when properly cleared, may be transferred to stone or zinc, and printed from in the lithographic manner upon a suitable press, or if the image upon the zinc be etched into relief by the process known as Gillotage, and mounted on a wood block of suitable height, it presents a printing surface which may be used in conjunction with type.

Gelatinotype.—It has long been felt as a desideratum to make use of the photogenic properties of the gelatino-bichromate for letterpress printing, i.e., to find out a simple and satisfactory method by which collotype plates could be transformed in letterpress printing plates. There have been made many attempts in this direction, but hitherto without any practical results; the difficulties seemed to be too great. Recently, however, Professor J. Husnik, of Prague, has overcome these difficulties, his new patented process, called by him “Leimtypie,” that is to say, Gelatinotype, being one of the greatest practical value, quick, cheap, and giving most satisfactory results. The process consists in the production of high relief blocks in gelatine. Husnik exposes a gelatino-bichromate tissue under a negative, and then develops the relief picture in a peculiar manner.

The method consists in the application of a saturated solution of bichromate salts for developing, a new and previously unknown property of the chrome salts thus coming into employment. These salts, in comparison with the acids, have the advantage not only of dissolving all unexposed gelatine during development, but also in hardening the already exposed parts of the picture upon the copy to be developed, the impression received from the light being increased by contact with the chrome salt solutions. In this way the relief can be developed a longer time, and becomes deeper.

Another new phase is the manner of the second development. The first development is interrupted before the fine parts have been injured, the relief allowed to dry, the white parts (the sub-ground) covered with printing ink diluted with turpentine, using a fine brush quite near to the lines, and then the whole relief exposed once more to light. The relief, on account of its having taken up much chrome salt during the first development, is very sensitive to light, and hardens not only on the surface, but also on the sides of the lines. After removal of the black ink it can be developed to any depth, particularly if the larger white parts have before been cut out with a knife.

This process has lately been further improved by the inventor, by the use of gutta-percha films, which are placed between the gelatine film and the zinc plate on which the gelatine relief has to be fastened. This is effected as follows:—The zinc plate, after having been well cleaned and rubbed over with emery paper, is coated with a gutta-percha solution and dried. It is then heated to about 212° F., and allowed to cool. During this cooling process the bichromated gelatine film, before being developed, is laid on at a temperature of between 111° and 77° F., whereby the hardening gutta-percha film secures a perfect combination with the zinc plate. When this has been done the gelatine film is developed in the above described manner to a relief. By this improvement the gelatine blocks become so solid that they will yield more than 50,000 prints in the printing machine without being injured.