Scumming

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In offset lithography, scumming is a generic term for the presence of ink on non-image areas of the printing plate, which can be caused by a variety of press conditions, in particular the use of a fountain solution that is too acidic, where the solution begins to eat away at the surface of the metal plate and its protective coating, or by a fountain solution that is too alkaline, as the desensitizing properties of gum arabic are lost at a pH greater than 5.0. (See Fountain Solution: pH.) Scumming characterized by the dispersion of ink pigment particles in the fountain solution, which increases as temperature increases, is known as tinting or hot-weather scumming. Scumming is also called greasing. See Tinting.

In gravure printing, scumming is an alternate term for a similar problem called fogging. See Fogging.