A printing problem found on aluminum plates used in offset lithography characterized by thousands of tiny, inked dots in non-image portions of the plate. Ink dot scum is caused by corrosion of the aluminum, which forms thousands of tiny pits that, when the film of fountain solution wears off, fill with ink. The corrosion is commonly caused by adding a layer of water to the surface of the plate and allowing it to evaporate slowly, providing enough time for oxidation of the metal surface to occur. It is also found frequently in a band corresponding to the position of a wet dampening roller. If the scumming is caught in time, and its effects are still localized within a small region of the plate, a solution of phosphoric acid and gum arabic can be used to eliminate it. If it has progressed far, the plate may be unusable.
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