The thickness of a sheet of paper, commonly called caliper. In book production, bulk is a factor in determining book size. (See Bulking Number and Bulking Index.)

Bulk is determined by many different variables in the papermaking process, including the amount of refining, the rate at which wet pulp is deposited on the forming wire, the degree of wet pressing, the extent of calendering and supercalendering, as well as the fibrous and non-fibrous content of the pulp itself. Different types of paper require different degrees of bulk, but the most important consideration is consistency of bulk along a particular paper web. Not only is consistent thickness required in web offset lithography, where varying bulk within a single roll can cause the roll to unwind with uneven tension, which is responsible for various printing defects as well as web breaks, but it is also required in the manufacture of business forms, envelopes, and other such products where it is important that a set quantity will fit into the container or packaging designed to hold it. (See Caliper, High Bulk, and Low Bulk.)

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