A paper characteristic that determines the quantity of ink that will penetrate its surface, or the rate at which ink will penetrate a paper's surface. The desired amount of ink absorbency (or its converse, ink holdout) is dictated by the type of ink used and the nature of the printing job itself. In some high-quality sheetfed offset presswork, the paper must be absorbent enough to prevent ink setoff, yet not so absorbent that ink that primarily dries by exposure to air is absorbed before it has a chance to dry. Ink holdout is primarily responsible for printed gloss. Lack of printed gloss and even ink chalking can result if the ink vehicle is drained from the ink film by absorption. On the other hand, in high-speed web printing, fast ink vehicle absorption is necessary to prevent smudging. In newspaper printing, rapid absorption of the ink vehicle traps the pigment on the surface of the paper, which is why newspaper ink never dries enough to keep from blackening the reader's hands.
A variety of interrelated paper properties affect ink absorbency and ink holdout. Primary among them is porosity, or the size and quantity of tiny interfiber air spaces in paper. It is by capillary action that inks and other fluids are absorbed into paper. Various procedures, such as the refining of paper fibers, the addition of fillers, the degree of calendering or supercalendering, and the application of various types of coatings affect ink absorbency. (See Porosity and Paper and Papermaking: Paper Properties.)
Various tests can determine the ink absorbency of a paper. (See Vanceometer Test and K and N Ink Absorbency Test.)