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Process

plate, lines, dots, image, screen, printing and block

PROCESS (Fr., Procede en demi teintes or Simi/i ; Ger., Halbtonverfahrung or Autotypie) A process in which the half-tones of a photo graph are reproduced by breaking up the image into dots of varying size, transferring this dot image to a metal plate, and etching the dots into relief for typographic printing. The shades of colour or tone between black and white in the photographic image are termed half-tones. To reproduce these half-tones in a block for typo graphic printing, in which a uniform layer of ink must be employed, it is necessary to break up the image into fine dots of varying size— largest, and therefore printing darkest, in the shadows, and smallest, therefore printing light est, in the high lights ; the intermediate sizes printing the half tones.

The credit of the idea of breaking up the image into dots by means of a network screen is due to Fox Talbot, who proposed it in a patent dated 1852. He used crape, or gauze, but also suggested the use of a glass plate ruled with fine opaque lines, which latter is the means employed to-day for executing the process. Various experimenters carried on the process with gauze, or with lines on glass obtained by photographing lines ruled or printed on paper, and with lines ruled through a blackened ground on glass, but it was not until 1893, when Max Levy, of Philadelphia, patented and introduced commercially his process of engraving and etch ing the lines into the glass and filling in the furrows with black pigment that the process became a commercial success. The Levy screen consists of parallel lines ruled diagonally on two glasses, which are sealed together face to face with Canada balsam, thus forming a network of crossed lines. Varying degrees of ruling are used, from fifty-five lines per inch (for coarse newspaper work) up to 400 lines per inch, according to the degree of fineness required in the block. Ordinary magazine illustrations are made through a screen of about i33 to i so per inch; the half-tone plates accompanying this work were made through screens of 175 and 200.

In carrying out the process the screen is placed in a holder in front of the sensitive plate, and, by means of mechanism in the back of the camera, is moved nearer to or further from the plate according to the extension of the camera and the degree of the ruling—long extension of the camera requiring greatest distance of the screen, and fine rulings the smallest distance.

Some cameras are fitted with screen adjustment gear. Square, cross, star, and other shaped diaphragms are put in the lens, alternately with the usual round diaphragm, the object being to promote the better formation of the dots.

The negative is made either by the wet or dry plate process, or with collodion emulsion, the first-named process being generally preferred, and it needs to be developed, intensified, and cleared to sharpen up the dots. A copper or zinc plate is coated with a solution of fish-glue and ammonium bichromate (see " Enameline" and " Fish-glue "), and exposed to light under the negative. The image is developed by dye ing the plate with aniline violet and washing with water until the dots stand out clear on the bare metal. The plate after being dried is held in pincers over a gas-stove until the image is " burnt-in " or converted into an enamel which is extremely acid-resisting.

The etching is done with dilute nitric acid for zinc, or with ferric perchloride for copper, and it is carried to such a depth as will prevent the printing ink from filling up the spaces between the dots.

To increase contrast and to bring out detail, the plates are usually " fine etched." Parts that are sufficiently etched in the first or " deep " etching are stopped out with acid-resisting var nish, and the remaining parts again etched ; this may be repeated several times.

The plate is finally trimmed to size, the edges bevelled with a special plane or machine, and fastened to a wooden mount or block by nails driven through the bevelled edge ; or a metal block may be used. The mount brings the plate to type height, so that the block can be printed along with type. W. G.