The extent to which a printing ink vehicle, solvent, or varnish (or any other liquid, such as water) is absorbed into the surface of a substrate. Inks that dry by absorption of the vehicle require a paper (or other surface) that will allow a high degree of penetration (but not enough to allow the fluid to soak through to the opposite side of the sheet), while inks that dry by other means, such as evaporation or oxidation, require a low degree of penetration.
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