A printing defect symptomatic of poor trapping, the ability to print one ink on top of another previously-printed dry ink, in which the dry ink film—which has been allowed to dry too hard—repels the wet ink, causing crawling, mottle, or rub-off. Crystallization occurs with inks that dry by oxidation and when the second ink is printed at a much later time than the first ink. Crystallization can commonly be avoided by reducing the amount of drier in the ink. (See Trapping.) As used in printing terminology, crystallization has nothing to do with the chemical term crystallization, which describes an entirely different phenomenon.