PHOTOGRAVURE (MACHINE). Photogravure (ma chine printed) is a photo-mechanical intaglio printing process. By this method prints are obtained from a plate or cylinder upon which the design is etched below the surface.
The subject to be reproduced is photographed and "reticulated" by means of a ruled screen, the "cavities" varying in depth according to the tones of the original; thus the solids are deeper and contain more ink than the middle tones, while the high lights are represented by the paper. The characteristic of this class of printing is revealed by its velvety depths and satin-like finish.
Machine photogravure was invented by Karl Klietsch, who, in conjunction with a firm of calico printers in Lancaster, pro duced prints by this method in 1895, although previous to this an employee of the firm, Samuel Fawcett, had been working an intaglio photo-engraving process of his own. The latter assisted Klietsch in perfecting the process now known as "Rembrandt" photogravure. The method of producing prints by the Rem brandt process was kept a close secret, and for many years those interested in reproductive processes had no authentic information as to how the printing plates were produced. In time, however, prints by similar processes were published, but these, at the time, were not equal to those printed by the Rembrandt process.
The principle of producing photogravure prints by machine is similar to that of printing calico and wallpaper, where engraved copper cylinders are used and printed on a rotary machine.
After the etching of the photogravure plate or cylinder has been completed the printing is of a mechanical character. The etched cylinder turns in a trough of printing ink of volatile qualities (sometimes a "water ink" is used), and as the cylinder rotates a fine steel knife, known as the "doctor," scrapes the sur plus ink from the surface, leaving it clean. The cells or cavities, however, representing the tones of the design, retain the ink, which is transferred to a sheet of paper as it passes between the printing and impression cylinders.
A carbon print is next obtained on what is known as "carbon tissue" (a sheet of paper coated with a gelatine solution to which has been added a pigment—usually red—and sensitised by im mersion in potassium bichromate, squeezed on to a sheet of glass, and dried). A piece of carbon tissue is cut slightly larger than the positives and placed along with the positives in a special pneumatic printing frame from which the air is extracted, thus bringing the positive and tissue into close contact. These are exposed to light, the positives are removed, and another exposure is given to the tissue through a specially ruled screen. The screen "breaks up" the tones of the photograph and provides the reticu lation which is necessary for all printing processes. The usual ruling of the screen is 15o or 175 lines to the inch. The carbon tissue is next soaked in water and placed on the copper plate or cylinder which has previously been made chemically clean and free from grease. The tissue is then "squeegeed" in order to remove all moisture and air from between the tissue and the printing surface, which is then dried.