The technique of gluing together thin sheets of wood in a balanced construction with each layer crossed at right angles so that the wood grains are locked in position, effectively preventing all shrinkages across the width, dates from the seventeenth century, but it was only in 1896 that plywood was commercially produced for cheap tea-chests. Since then ceaseless development has produced a bonded sheet which is completely free from lamination, and which is generally available in thicknesses from 1/32 in (0.8 mm) to 1 in (25 mm), in a range of qualities for every purpose.
The original tea-chest ply was composed of three layers—a central core and two balancing veneers—thence the familiar term 'three-ply'
which is still used for three-layer construction. Plywoods with more than three layers are usually classed as multiply, although the term 'five-ply' is sometimes used for five-layer construction. Obviously, the layers or veneers of which the ply is composed must be relatively thin or the strength of the timber and its tendency to shrink and distort will be greater than the strength of the bonding adhesive; therefore for strong plywood additional layers must be used, one on either side for each increase in thickness, so that the total number will always be odd for a balanced construction, as distinct from an unbalanced assembly of an even number of veneers which will have a greater tendency to pull or distort. In practice, the central core can be relatively thick (1/8 in [3 mm] or 1/6 in [4 mm]), and of lower density wood to reduce the overall weight; but wherever possible the combination of veneer thickness and wood species should be chosen to give the finished sheet equal stiffness both parallel and perpendicular to the grain of the face veneers. For this reason each pair of balancing veneers, including the face and backing veneers, is usually of the same thickness and of the same or comparable species. The number of glue-lines also affects the overall stiffness, and the thinner the veneers, with the consequent increase in the number of glue-lines, the greater the rigidity of the finished sheet will be.
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