Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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A term which has virtually the same meaning asSEASONING , but, unlike that term, is generally considered to refer to the exposure of paper to an accurately controlled and specified environment for the purpose of bringing the moisture content of the paper into equilibrium with the surrounding atmosphere. Standard atmospheric conditions in North America are considered to be 50% relative humidity and 73° F. In other countries the standard calls for a relative humidity of 65% (+2%) and a temperature of 68° F. +3° F.). Conditioning is of considerable importance for papers which must lie flat in sheet form, or which must give good register when printed. Unequal internal strains are set up when paper is dried on the papermaking machine, because the tension in the direction of the web travel. i.e., the MACHINE DIRECTION , produced by drying, is greater in the cross direction. See: ANISOTROPIC BEHAVIOR .

Since most papers are dried to a moisture content of about 3% (by weight of the paper), they tend subsequently to absorb moisture until the moisture content amounts to approximately 5 to 9% (depending upon the humidity of the atmosphere in which they are stored). This tendency is increased once the paper is cut, because cut edges are able to absorb atmospheric moisture faster than the surface of the sheet itself, especially when the paper is stacked in piles. When a stack of paper absorbs moisture, the edges, particularly those corresponding to the cross direction, will expand more rapidly than the center, which causes cockling. In like manner, a "spongy" effect may occur, due to absorption of moisture by the top surface of the upper sheets in the pile. This expansion may take place between the printings of a multi-color printing sequence, or when the paper is dampened as in the offset-litho printing process, with the result that the colors used will overlap in some places and leave gaps in others. Moisture absorption, particularly when the grain direction of the paper does not run parallel to the spine of the book, can cause cockling along the binding edge and waviness in the leaves, resulting in a book that will not close properly. The latter problem can sometimes be overcome, at least to a degree, by pressing the book in an atmosphere of low relative humidity, but the problem of cockling along the binding edge can only be overcome by a method ofIMPOSITION which results in the grain direction of the paper running parallel to the binding edge. (17 , 42 , 144 , 156 , 276 )

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