The combination of the paper qualities of compressibility, resiliency, and hardness or softness, which determines the extent to which a paper's surface will deform under a compressing force to allow complete ink transfer from a printing plate or blanket and be able to return to its original dimensions when the compressing force is removed. They are all determined by such factors in the papermaking process as the degree of refining of the paper fibers, and the degree of calendering or supercalendering. Generally, a soft, easily-compressible paper is desirable, but different degrees of each of these characteristics may be required, depending on the printing process to be used, and the end-use characteristics of the paper. Letterpress and gravure require more compressible and resilient papers than offset lithography or flexography, while offset lithography needs harder papers to withstand the stress of being peeled off an inked blanket. The degree of strength required—such as folding endurance, or the ability to withstand other types of handling—may also be the determining factor in the combination of printing cushion qualities.
The softness or compressibility of a paper can be quantified with an air-leak method, in which a paper sample is inserted into a device which presses a metal plate on top of it and measures the rate of air leakage between the surface of the paper and the metal plate. As the metal plate will press more deeply into softer paper, the rate of air leakage will be greater for harder paper.