The degree to which a paper will resist a reduction in thickness when exposed to a compressing force, as during printing, writing or typing. (The degree to which a paper will allow a reduction in thickness is called its softness.) A low degree of hardness aids in producing a good printed image, as the squeezing down of surface contours enables the printing plate or blanket to contact the paper more completely. The degree of hardness required varies according to the printing process to be used, and other paper properties involved (in particular resiliency and compressibility which, with hardness and softness, define the paper's printing cushion). Generally, easily-compressible paper is preferred for printing, although it is less of an issue in offset printing, and stronger paper may be required in the case of end uses involving folding or handling. (See also Resiliency, Compressibility, and Softness.)

The term hardness is also used to describe the surface properties of printing press rollers and cylinders (either the ink or dampening rollers, and blanket used in offset lithography, or the impression roller used in gravure). Each type of roller, printing process, and substrate require differing degrees of roller or cylinder hardness. Hardness of rollers and cylinders is measured in, and is also referred to as, durometer. (See Inking System: Offset Lithography, Dampening System, Blanket, Impression Roller, and Durometer.)

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