Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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A change in the dimensions of a material, such as paper, because of a change in the ambient relative humidity of the atmosphere surrounding the material. The phenomenon is usually expressed as a percentage, and is generally several times greater in the cross direction of a paper than in the MACHINE DIRECTION , because paper fibers expand much more in diameter than in length when wetted. Hygroexpansivity is of considerable importance where the dimension of paper is critical, such as in multi-color offset printing. See also: ANISOTROPIC BEHAVIOR ;DRIED-IN STRAIN . (17 , 72 )

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