[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.