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PHOTO-MECHANICAL PROCESSES.—The term photo-mechanical is applied to all pro cesses in which the action of light upon chemical substances becomes the means of preparing printing surfaces from which many impressions can be made without any further assistance of light.

The first photo-mechanical process known was that invented by Nicephore Niepce about 1827, when he discovered the peculiar sensitiveness of asphaltum to light. He coated metallic plates with a solution of this substance, and exposed them in the camera to the image. Those parts acted upon by the light became insoluble, while the remaining portions were dissolved away with oil of lavender. By this means a reversed picture of bitumen was obtained on the metal plate. By next applying an etching acid the parts of the metal unprotected by the bitumen image were eaten into, and after clearing away the image a printing plate was obtained.

Many photo-mechanical processes are based upon this property of asphaltum, but many more upon the sensibility of dichromatized gelatine, albumen, and gum arabic, and,their peculiar properties. These substances are used to produce the design, either flat for etching or direct printing, or in relief for moulding electrotypes, stereotypes, etc.

For some time the chief difficulty that was encountered was in the correct rendering of the half tones. This was eventually overcome by breaking up the image into lines, dots, or stipples.

Photo-mechanical processes may be divided under two different heads, i.e., those in which the image is moulded in pigmented. gelatine, and those in which the picture is printed with the ordinary fatty printing inks. To the first class belong the Woodburytype and Stanno type processes.

The Woodburytype invention of W. B. Woodbury. In this a sheet of bichromatized gelatine is first exposed under the negative, and afterwards washed in warm water, which removes the soluble parts, leaving the image in relief. When the gelatine relief is dry it is exceedingly hard, and is pressed by means of hydraulic pressure into a sheet of lead. By this means an intaglio mould is formed. This is placed in a specially constructed press, having a heavy and perfectly true lid. A little warm gelatine solution, containing any desired pigment, is poured on to the intaglio mould, a piece of prepared paper laid on top of it, and the heavy lid brought down.

This squeezes the excess of colored gelatine, allowing only that to remain which lies in the mould. This sets, and at the same time adheres to the paper support, which, when removed, has attached to it the gelatinous image. This is dried and hardened with chrome alum. If glass be used instead of the paper very beautiful transparencies can be made.

The Stannotype process is a modification of the Woodburytype by the same inventor. In this tinfoil, properly backed by electrotyping or by other means, is substituted for the lead plate, thus doing away with the necessity of employing expensive machinery.

A very large number of processes come under the second classification, i. e., those which are printed in ordinary printer's ink. These, however, may be again divided into the following : ) Processes in which the image is printed from a gelatine surface. (2) Processes in which the picture is printed from stone. (3) Processes in which the image is printed from a metallic surface in relief. And (4) processes in which the image is printed from an intaglio metal plate.

Among the processes in which the picture is printed from a gelatine surface the chief are : Collotype or Albertype, Artotype, Chromo-collotype, Indotint, Leimtypie, and Heliotype.

In the Collotype process a sheet of thick plate-glass is first coated with a film of albumen and gelatine, to which a dichromate has been added. This is then laid on a piece of black cloth exposed to light, washed and dried, and the plate is then coated with dichromatized gelatine, exposed under a reversed negative, soaked in water to remove all soluble chromium salt, hardened with alum, and finally dried. By this means a scarcely visible image in gelatine is the result, those parts which have been exposed to the light being insoluble and repellant for water, the remaining parts retaining their absorbing properties. The plate is fastened to the bed of an ordi nary lithographic press, the printing being very similar. A wet sponge is used to moisten the absorbing parts of the gelatine (the whites in the picture), and an ink roller inks the image part. A sheet of paper is laid over it, and, after pressure is applied, the ink is transferred to the paper. Prints may also be made upon cotton and silk fabrics.

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