The Technique of FURNITURE MAKING
Artificial defects
All woods shrink on drying, some pronouncedly so, thus creating internal strains and stresses. Normally, the natural elasticity of a healthy wood structure will distribute these stresses evenly, but if the structure is unequal or lacking in elasticity (innate defect), and if incorrect seasoning imposes too great a strain, then various forms of distortion, splitting, etc. will occur. Weighting down during seasoning helps to reduce distortion.
Cupping or rounding (17:13) The plank hollows across the width, forming a rounding on the underface, often due to incorrect piling.
Bowing (17:14) The plank is curved like a bow throughout its length. A succession of short bows is usually caused by sagging between too widely spaced stickers or by stickers which are not placed exactly over each other.
Springing (17:15) Sometimes known as 'edge bend', the wood remains flat but bends edgewise on its own plane.
Twisting (17:16) The plank twists on its longitudinal axis with the result that the long edges are straight, but the diagonals are curved. Usually known as 'in winding'.
Warping, casting Synonymous terms for distortion in one or more directions (see Twisting, above).
End splitting (17:9) The butt end of the plank splits open, usually caused by too rapid drying. but some species will always split.
Sun checking (17:10) The wood surface is covered with small splits along the grain caused by too rapid drying in hot sun. Not serious unless the splits penetrate deeply.
Flaking The surface of the wood lifts in innumerable small flakes or layers which spring under the cutting action, preventing a smooth surface. Sometimes due to structural weakness, but can also be caused by incorrect seasoning.
Diagonal grain The grain runs obliquely to the longitudinal axis, usually due to incorrect sawing, but some timbers exhibit marked deviations in grain direction which cannot be avoided. Although the condition may make surfacing more difficult it is not important, except in structural members where the impact strength loss is high, and in bending where a grain slope of 1 in 25 may mean a bending strength loss of 4 per cent, and a 1 in 5 slope a loss of 45 per cent.
Case hardening (17:11) If the wood is kiln dried too quickly then the surfaces dry out at a rate quicker than the rate of movement of moisture by capillary attraction from the centre of the plank, with the result that the dry outer layers are in tension, and the moist interior in compression. Cuts which close ahead of the saw are often due to case hardening. Provided the actual wood fibres are not ruptured the condition can be cured by steaming and redrying.
Honeycombing (17:12) If the kiln drying of case-hardened timber is continued to dryness then the natural shrinkage movement of the moist interior as it dries will be locked in by the rigid outer skin, resulting in severe internal stresses and subsequent checking or disruption of the wood fibres, not visible from the outside. There is no cure for the condition, which severely depreciates the value of the timber.
Collapse The too rapid kiln drying of green timber can result in a flattening of the wood cells, caused by vacuums created by the withdrawal of water to below fibre saturation point at a rate faster than it can be replaced by either air or live steam. This condition is known as 'collapse' and is characterized by extensive shrinking and warping, particularly in the springwood, giving a washboard effect. It can also be caused by too slow drying at too high a temperature, or too high a humidity rate, and can sometimes be remedied by steaming and reworking in the dry kiln.
