Wood, Defects of

flaws in wood that impair its properties and limit its possible use. Defects develop in growing trees (knots, warping) and in timber (blue stain, browning). Some defects are characteristic of both living and felled trees (cracks, rot, wormholes). Woodworking defects are produced during the procurement, transport, and mechanical working of the wood. The seriousness of a defect is determined by its type, size, and location, as well as by the purpose for which the wood is to be used. Thus, defects undesirable in some types of timber may be disregarded or even valued in others. For example, cross grain is unacceptable in resonant wood, acceptable in commercial lumber, and highly valued in plywood, to which it imparts a decorative quality.

The main defects of wood include knots, cracks, fungal damage, warping, slanting of grain, and wormholes. A knot is a part of a branch embedded in wood. Knots mar the appearance of wood and disturb its uniform structure. They twist the grain and the annual rings and weaken the wood when it is pulled with the grain and when bent. On the other hand, knots increase the strength of wood that is compressed transversely or sheared longitudinally.

Cracks are separations in the grain that result from internal stresses; they radiate out or occur between the annual rings. Cracks are caused by tree growth, by low winter temperatures, and, in timber, by desiccation. They destroy the integrity of wood, thereby weakening it.

The most serious fungal defects of wood are rots and stains. Some forms of rot affect only living trees; for example, heart-wood rot is not commonly observed in felled trees. Other forms of rot, such as sapwood rot, destroy moist timber. Dry rot is particularly dangerous, because it arises not only in green lumber, but also in comparatively dry lumber. All forms of rot severely weaken wood.

Fungal stains affect living trees (heartwood spots and bands) and cut wood (blue stain). Heartwood spots and bands, which are produced by the causative agents of heartwood rot, are the first stage in the development of the rot. Sapwood fungal stains are produced by tree-staining fungi that do not cause rot. Fungal stains have little effect on the mechanical properties of the wood, but they alter the wood’s appearance and permeability to gases and water. For example, blue stain increases permeability, whereas browning diminishes it. Impregnating wood with antiseptics prevents fungal defects.

Slanting grain is a deviation in the direction of the grain, making it nonparallel to the longitudinal axis. Grain deviations make shaving and splitting of wood difficult. They lessen the wood’s ability to bend, increase longitudinal shrinkage and warping, and reduce tensile strength with the grain and in bending.

Wormholes are passageways and openings made in the wood by insects. The activity of insects generally ceases after the bark is removed and after the wood is dried or treated with antiseptics. Superficial wormholes (about 3 mm deep) do not affect the mechanical properties of the wood. Deeper holes disturb the integrity of wood and may decrease its strength. Wormholes usually promote the development of sapwood fungal stains and sapwood rot.