A type of printing plate used in offset lithography. Diazo plates are characterized by an aluminum (or other metal) base to which a diazo coating (a light-sensitive organic compound) has been added. When making diazo plates, a film negative or positive is placed against the surface of the unexposed plate, and an intense light is shined through the negative. When using a film negative ( such a plate being called a negative-working plate), the plate is exposed in the image area of the plate only, which causes the diazo coating to harden and become insoluble, while the coating in unexposed (non-image) areas remains soluble. After exposure, a chemical treatment of the plate with an acid solution of lacquer and gum etch dissolves the coating in non-image areas, and deposits a layer of gum in those portions of the plate that need to be water-receptive and a layer of lacquer in those portions that need to be ink-receptive. Plates that require the image areas to be treated with a special reinforcing material are known as additive plates, while those that are manufactured with a layer of lacquer and do not need an additive during development are known as subtractive plates. Additive plates are capable of print runs of up to 150,000 impressions, while subtractive plates are capable of up to 250,000 impressions. (See Plate: Offset Lithography.)
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