Positive-Working Plate

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One of two basic types of photolithographic printing plates produced by bringing an unexposed plate into contact with a photographic positive of the image to be printed, and exposing it to a high-intensity light. The plate's coated metal surface is exposed in the non-image areas—where light passes through the positive—and unexposed in the image areas—where light does not pass through. The exposure to light renders the non-image areas soluble, and after platemaking the plate is chemically treated to dissolve the non-image areas, rendering that portion of the plate water-receptive, the unexposed image areas remaining hard, durable, and ink-receptive. Some positive-working plates can be thermally cured, which further hardens the image areas, making them suitable for longer than usual press runs. 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. The advantage of positive-working plates is that they tend to reproduce halftone images with less dot gain than plates made from negatives. (See also Negative-Working Plate.) (See Plate: Offset Lithography and Platemaking.)