A printing defect that occurs in flexography or screen printing with nitrocellulose-based inks utilizing hygroscopic (absorbing moisture readily) solvents, in which the nitrocellulose, being water-insoluble, precipitates out of the ink when the solvent picks up water from the atmosphere. Although the addition of other substances (such as esters) will help alleviate the problem, the best defense against souring is to minimize the exposure of the ink to humid environments by using lids and fountain covers, or to use a small amount of ink in the ink fountain and refresh the ink often.

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