Photomechanical Arts

PHOTOMECHANICAL ARTS. - This name designates those processes that allow images to be printed by means of a matrix which, instead of being worked by hand (v. ENGRAVING), is obtained by means of light. The process for engraving the matrix is based on the property that gelatin treated with potassium bichromate has of becoming insoluble after exposure to light, so that a metal plate, for example, with its surface prepared for the purpose and exposed to light under a photographic negative, will after washing present an image suitable for being reproduced by a mechanical process.

There naturally exist for printing various systems that therefore make use of various forms of matrices. The most widespread reproduction, because it allows images to be printed together with the text, is the phototypographic one (also called zincotypy and zincography and photozincotypy, because it uses a zinc plate). Such a system, however, would allow only so-called line reproductions, that is, in black and white without gradations and halftones, if other expedients did not intervene, such as the halftone screen, to break up the halftones into dots. It is obvious that the fidelity of the reproduction is thus tied to the quality of the screen, which can be more or less dense, and therefore more or less visible. The images in this process are therefore constituted by projecting reliefs on which the ink takes hold; in other processes, by contrast, the image on the matrix is not in relief but recessed and receives the ink while the background is not inked. This is the so-called intaglio printing used by rotary presses and therefore also called rotogravure: it makes use of a copper cylinder on which, besides the images, the text too can be transferred by means of the screen, and it is therefore much used today for printing illustrated newspapers. Reproductions that do not present, on the other hand, the characteristic screen peculiar to the cliché, used by the typographic process, are those obtained by means of planographic processes, comprising phototype, photolithography and the Offset process, which make use of the processes proper to lithography for transferring the image onto a flat surface, so that the matrix is neither in relief nor recessed but smooth, with parts that, suitably prepared, take up the ink and others that repel it. This is the system that allows the best reproductions as regards fidelity of halftones. For reproduction in colors, mention is made of trichromy, based on the superimposition of the fundamental colors (yellow, red and blue), and of four-color printing, in which black is added for intense shadows (phototypographic process); chromophotolithography (planographic process) and rotogravure color printing (intaglio process).

Gilberto Ronci