Paper characteristic describing the uniformity of its structure, especially to the extent that it relates to paper smoothness. When cellulose fibers form the paper web on the papermaking machine, non-uniform distribution generates "peaks" and "valleys" in the surface of the paper (visible as dark and light regions when the paper is held to the light). An excessive degree of non-uniformity is referred to as a wild formation, while a high degree of uniformity is referred to as a close formation. The degree of wet pressing, and such post-forming operations as calendering and supercalendering help even out these variations. A close or wild formation affects such other paper properties as opacity (a wild formation results in variable degrees of opacity across a paper's surface) and surface levelness. It can also have deleterious effects on ink absorbency and can result in ink show-through and incomplete ink transfer, especially in letterpress and gravure printing. (Wild formation doesn't typically cause problems in offset printing, due to the flexibility of the rubber blanket which can counteract the problems of inconsistent formation.) Prolonged fiber refining also helps improve formation uniformity. Close formation, however, can also compromise other desired paper properties. (See Paper and Papermaking: Paper Properties.)

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