Brought indoors into heated work-shops or rooms further drying and shrinking takes place, so that furniture in Great Britain con tains about 7 to 9%, and in drier parts of South Africa and the United States only 6% or less.
Placed in a drying oven at a temperature of i oo°C, all the water, save that which is chemically combined, is regarded as having been dried out and the wood is described as absolutely dry. When dry wood is once more exposed to moister air it reab sorbs water and swells.
Different kinds of wood under go different changes of volume with the same percentage gain or loss of water: heavy or non resinous woods generally shrink or swell more than do light or resinous woods, with the conse quence that softwoods usually show smaller changes of shape and volume than do hardwoods. But shrinkage and swelling are not proportional to density, for non-resinous woods of the same density show considerable differ ences in this respect; for in stance, Central American mahog any with changed water-content undergoes comparatively slight change of size and shape.
A piece of wood tends to shrink and swell unequally in different directions. Along the grain shrink age and swelling are so slight that measuring rods made of well-seasoned wood are very re liable. Across the grain shrinkage and swelling are many times as great : in a radial direction 3o to 5o times and tangentially (or par allel to the annual rings) 6o to 100 or more times. Consequently when a piece of wood dries it undergoes greater or less change of shape. The accompanying diagrams show cross-sections of pieces of wood respectively cut into boards, joists and cylindrical rods, and the change of shape caused by drying; the warping of boards 2 to 4 contrasts with the preservation of flatness of board 1 which is cut nearly parallel to the radii (quarter-cut or rift-sawn).
A piece of wood whose fibres, like those of very knotty wood or burrs, run in various directions obviously is especially liable to split when dried. It is partly for this reason that the burr-wood used in the manufacture of beautifully figured furniture is cut into very thin sheets, termed veneers, which are glued on to straight-grained wood. Plywoods (including the commonest of these, three-plywood) consist of thin veneer-like sheets glued to gether so that the grains of the successive sheets cross one another at a given angle (usually at right angles), with the consequence that plywood warps less and shrinks more uniformly than does plain wood and is less unequal in strength in different directions.
Colour.—The different kinds of wood may be wholly white or in the heart may be black or have colours ranging from brown, yellow, red, green to violet, or mixtures of these. It is quite exceptional for one and the same wood when perfectly sound to show such a wide range of variation as does that of Liriodendron (whose English name is Canary white wood and North American name is yellow poplar) : this wood varies in colour from light yellow to green and iridescent blue. The colour thus facilitates identification and helps to decide the decorative use to which a wood shall be put. It is not true that dark coloured woods are more durable than light coloured, though this is true of the heart wood when compared with the sap-wood of the same timber. Abnormal discolouration of wood is very frequently caused by fungi attacking the wood or its contents, such as the case with the blue sap(wood) of softwoods, rusty red of woods attacked by various fungi (including some causing dry rot), black lines and white patches of many rotting timbers, and sometimes false heart wood in trees which normally lack a coloured heart-wood.