The Technique of FURNITURE MAKING
Stability
As wood is a hygroscopic material, shrinking and swelling under varying temperature/ humidity conditions, plywood, which is composed entirely of wood, must also shrink and swell, but the total movement is very small owing to the strength of the glue-lines which lock the fibres in all directions. For instance, the Forest Products Research Laboratory has established in tests on 22 different species of 3/16 in (5 mm) thick three-ply that the mean swelling is only 0.18 per cent along the grain, and 0.27 per cent across the grain when subjected to a humidity range of 30 to 90 per cent, equivalent to raising the moisture content from 7 to 20 per cent. For all practical purposes in furniture the movement can be disregarded, although it will show on the edges of untreated ply as a fractional creep of long-grain veneers. The thicker the ply the less the movement is and the greater the measure of stability, while synthetic resin glues give greater measures of stability over the other glues. An additional advantage is that plywood has no line of cleavage and therefore will not split under adverse atmospheric conditions. This absence of lines of cleavage also affects the impact strength of plywood which is greater than that of solid wood subj ected to a similar loading; and in all practical applications thicknesses over solid timber can be reduced, with a 3/4 in (19 mm) thickness of ply roughly equivalent to a 1 in (25 mm) thickness of solid wood.
