A property of printing inks that describes the cohesion that exists between particles of the ink film, the force required to split an ink film or, in other words, its stickiness. An ink with a high degree of tack (or a tacky ink) requires more force to split than does a less tacky film. Offset lithographic inks require tackier inks than do other printing processes so as to avoid excessive emulsification of the ink by the fountain solution, and to print sharp halftone images. Tacky inks, however, do not print solids very well. The tack of the ink should not exceed the surface strength of the paper, or picking, splitting, and tearing of the paper will occur. In multi-color printing, the first ink printed must have greater tack than the ink that will be printed on top of it, or the latter will not trap on the first ink. The tack of an ink can be measured with an Inkometer or a Tackoscope.
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