PHOTO-MECHANICAL PROCESsEs. by which photographic impressions could he repro duced by mechanical means. and so used directly on the printing press. have naturally been sought for, and have been invented. Such processes de pend for the most part upon the action of light on a gelatin film impregnated with a biehromate, generally of potassium or ammonium. The pro cesses in question may he divided into two groups.
In processes of the first group, the picture is molded in gelatin and colored by a pigment. The group includes the Woodburytype or Photoglyph and the :stanuotype. Lr the a sheet of bichromatized gelatin, a highly sensitized mix ture of gelatin and potassium bichromate. is pre pal( a a • • I and thin film exposed under a negative to a strong light and then washed to remove the un changed gelatin that was protected from the light by the negative, and dried. This film is placed upon a sheet of soft metal, usually lead, and forced into it by hydraulic pressure, producing a mold of the picture. in which the dark ports are in intaglio and the light ones in relief. This mold is then inked with a solution of warm gelatin colored with pigment, and a sheet of paper laid upon it. a strong pressure ex erted in a horizontal press, the excess of ink being forced out by piessure. The print is then im mersed in a solution of alum. which renders the picture insoluble. The Stannotype, which is a modification of the foregoing process, consists in substituting for the lead plate a thin sheet of tin-foil properly strengthened in the back by elec trotyping material. In the photo-mechanical processes of the second group. the Octi ore is printed with ordinary printing ink from a gelatin surface, from stone. from metallic relief surface, or from an intaglio copper plate. The `collotypc' or 'phototype' processes of this group, in which the picture is printed from a gelatin surface, are represented by the .t/bertype, invented by Joseph Albert, of Alunich, in 1S69. This process consists of coating it sheet of plate glass with a thin film of chromatized albumen and gelatin, which is then laid face down on black cloth and exposed to light. The insoluble gelatin next to the plate adheres firmly to the glass and serves as a foun dation for a second film. which consists of chro matized gelatin. The dry film is placed un der the negative in a printing frame and exposed to the light until the shades of the iinage,...are visible through the glass. Next the plate is washed in water to remove the soluble hichro mate. and the film hardened with chrome alum and dried. The plate is fastened to the bed of a printing press by means of plaster of paris. The irtotype, which is similar to the foregoing, was invented in ISTS. A mixture of albumen and soluble glass is used for the foundation on which the sensitive film is afterwards placed. At such
it film does not require to he hardened by light. opaque metallic plates may be used instead of a glass plate, as in the previous process. indo hats or on foglyphs are produced by processes similar to the foregoing, except that the support for the gelatin film is usually of copper slightly roughened in order to cause the sensitive film adhere firmly. The addition of alcohol to the chromatized gelatin is said to increase the tough ness and tenacity of the film. After exposure un der the negative the hiehromate is washed out and the plate is dried. Prints. from such plates can then he taken on a power press. The heliot3.Te process differs from the foregoing by the hardening of the gelatin film with chrome alum and the detaching of it from the support upon which it was first prepared; thus yielding, when completed, a thin sheet or skin of gelatin that is both tough and flexible, and which may he placed on a plate of zine or attached to a cyl inder when used for printing.
Excellent results in color have been obtained in the foregoing processes by preparing several gelatin plates, each of which corresponds to a different color in the original, and these are then used for printing with colored inks. Prints have been produced in which as many as seven successive distinct colors have been used. In those processes in which the picture is printed from stone, a sensitive mixture of albumen and ammonium bichromate in water is passed over a lithographic stone which has been thoroughly polished and cleaned. The excess is carefully rubbed off and the surface of the stone dried so as to remove all traces of moisture. it is then ex posed under a negative, and wherever the light penetrates to the ehromatized albumen it becomes insoluble. After sufficient exposure the surface of the stone is covered with lithographic ink and washed with water so as to remove the soluble parts of the film, and then treated with the acid and gummed and printed as in ordinary lithog raphy. The diflieulty in handling the heavy stones soon led to the invention of an improved process, in which a sheet of paper, sensitized with a solution of albumen, gelatin, and potas sium biehromate, was placed face down on a sheet of smooth copper, and then exposed under a nega tive, after which it was coated uniformly with the usual lithographic transfer ink. The paper was then floated on boiling water in order to coagu late the albumen on the film, and the unaltered gelatin which was protected by the opaque por tions of the negative absorbs moisture and swells, leaving the unaltered gelatin depressed. This print, again washed and then dried, is transferred to stone by simply placing it upon that material face downward and passing it through the press.