Negative-Working Plate

One of two types of photolithographic printing plates produced by bringing an unexposed plate into contact with a photographic negative of the image to be printed, and exposing it to a high-intensity light. The plate's coated metal surface is exposed in the image areas (where light passes through the negative) and unexposed in the non-image areas (where light does not pass through). The exposure to light renders the image areas hard and insoluble, and after platemaking the plate is chemically treated to dissolve the unhardened non-image areas, rendering that portion of the plate water-receptive, the exposed image areas remaining hard, durable, and ink-receptive. The non-image areas are then treated with a solution of gum arabic (or other gum, either natural or synthetic), which desensitizes the non-image areas, adding a higher degree of water-receptivity. Although negative-working plates are the most prevalent, a disadvantage of them is that they tend to reproduce halftone images with a higher degree of dot gain than plates made from positives. (See also Positive-Working Plate.) (See Plate: Offset Lithography and Platemaking.)

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