The final stage of the printing ink manufacturing process in which the dispersed mixture of pigment and vehicle is passed through bag or cartridge filters to remove dirt, undispersed pigment particles, and other debris and detritus. Careful attention must be paid to the viscosity of the ink; too great a flow, or a clog in the filter, will build up excessive pressures in the filtration chamber and rip bits of fibers out of the filter into the ink, defeating the purpose of filtration. One of the "contaminants" of ink that filtration cannot remove is trapped air, bubbles of which have the capacity to promote the premature production of an ink skin, typically in inks that dry by oxidation. In lithographic ink which dries by a mechanism other than oxidation, the air bubbles are less of a problem, as the press dampening system will negate any detrimental effects likely to be caused by the bubbles.

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