METROPOSTCARD.COM Rotogravure


Rotogravure


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The rotogravure process grew out of a variety of technical advances made to photogravure. It had been impossible to photographically expose an image onto the cylinder of a rotary press, but in 1879 a method was found by which a flat sheet of photosensitive gelatin tissue infused with a random dot screen could be exposed to a transparency and then adhered to a cylinder. The gelatin exposed to light is hardened into an acid resist, and the remaining gelatin is wash away in proportion to the density of the transparency that covered it, either nearly or completely exposing the metal surface of the plate. When rotated through successive acid baths of decreasing strength, the metal will dissolve in the exposed areas between the dots and the thinner areas of gelatin will eventually wear away in proportion to their exposure to light and reveal more bare metal of the plate underneath. Because these areas have less contact time with the acid they will not etch as deeply, and these shallow wells will hold less ink producing lighter tones when printed. By 1904 rotogravure was in widespread use.

In 1908 a new method was introduced by which gelatin tissue is double exposed, first to create a solid crossline screen pattern across the entire surface of the cylinder in the form of an acid resist. This screen will not print but remain white while allowing the square cells between them to be further exposed to the transparency that holds the image. This causes the square cells to harden in proportion to the amount of light filtering through the film. The tissue is then adhered to a cylinder and the soft areas are washed out with water leaving a hard acid resist behind. The acid bath will incise a continuous toned image into its surface between the screened lines by creating small ink cells of substantial but varying depth. The deeper depressions will transfer more ink to the printing surface creating darker areas while the shallow inkwells will print lighter. The areas of the cylinder that are not etched at all become the non-image areas where the screen pattern completely disappears. In dark areas the fine lines of the screen pattern that were not etched will leave behind a faint white grid around the wells. In 1910 hand wiping was replaced by a mechanical method in which the etched screen cylinder revolves in an ink fountain where it is coated with fluid ink and then wiped with a blade that clears all excess ink away from the surface areas while leaving ink safely in the depressions of the cylinder protected by the crossline grid. Rotogravure continues to be used but images are usually now transferred by digitally guided lasers.


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