The Technique of FURNITURE MAKING

Natural defects

Knots These are in effect the basal stumps of incipient or cast-off branches in the living tree. Where the tree itself naturally prunes its branches owing to lack of light caused by overcrowding, or where such branches are artificially pruned in controlled forestry and cleanly sawn, then the cambium layers will heal over the wound and the knot is then live or embedded (17:1 A). Where, however, a mature branch is broken off, leaving a long ragged stump, then the cambium layer cannot heal the wound and the stump dies, forming a deal or loose knot, often rot affected (17:2A).

All knots whether live or dead affect the mechanical strength of the timber, owing to the abrupt change in the direction of the fibres, and constitute blemishes which detract from the value. They are, therefore, graded as follows:

Pin knots Small knots 1/2 in (12.5 mm) or under, often caused by the shedding of early branches. Usually allowable in prime timber.

Spike or splay knots (17:3) Knots sliced through their length during sawing, and commonly known as 'slash' knots. They are difficult to plane up, especially in softwood, while large specimens are not permissible in hardwood unless allowed for in the measurement.

Encased knots Dead knots which are still sound and difficult to dislodge, and often ringed with resin in softwood.

Branched knots Two or three knots springing from a common centre.

Knots are classified as small, medium and large, the latter usually 11/2 in (38 mm) in diameter and over; but gradings are not precise and vary according to the country of origin.

Shakes Both the medullary ray and springwood cells of ring-porous hardwoods are weaker than the remainder, and built-in tensions are created which tend to level out. either in the growing tree under certain adverse conditions or in the felled log during seasoning. Thus extensive splitting may occur in the weakest links, i. e. radially along the medullary rays, and tangentially at the junction of springwood and summerwood. Various forms of shake are common, as follows:

Radial shakes The log splits from the pith or heart radially along the medullary rays, usually indicating that the tree has passed its prime. Sawing losses can be minimized by placing the cuts either side of the shake, always provided the growth of the tree does not twist upon its axis, in which case the shakes become spiral, rendering the log useless for long lengths. Where only one shake is present it is known as a 'simple heart shake', while two shakes in line compose a 'double heart shake' (17:4), and several a 'star shake' (17:5).

Frost shakes (17:7) project inwardly from a definite frost rib on the cambium and are. as their name implies, the result of severe weather.

Tangential shakes The soft springwood of the log splits away from the harder summerwood, either during seasoning or through shearing stresses in the growing tree caused by old age, excessive bending under strong winds, intense heat, etc. A frequent cause in oak is the depradations of the tortrix viridana moth, whose caterpillars strip the young leaves in early summer, with the result that growth is checked and the wood rings fail to cohere. Where such shakes run along part of the annual ring only, then they are known as "cup shakes' (17:6A); but where the log is completely encircled then they become 'ringshakes' (17:6B). Usually such shakes seriously detract from the value of the timber. English walnuts are particularly prone to cup and ring shakes, as the trees are rarely felled until they are long past maturity.

Cross shakes {thunder shakes) These failures are caused by compression and not by splitting or shearing, while the actual rupture is across the grain and not with it as with all other shakes. The probable cause is not thunder, as the name suggests, but either felling shatter (the sudden impact as the felled log hits either hard ground or another fallen log), or mechanical strain in the living tree. Chiefly confined to the softer varieties of tropical hardwoods, and appearing either as a definite fracture or an overriding of the fibres, showing only as a faint raised line across the width of the wood, which will snap like a carrot under strain. This particular type of shake often occurs with a soft condition in the heartwood, known as 'brittle heart', 'carrot heart", etc.. and agba is particularly liable to this defect. End splitting and sun checking (see below) are usually regarded as artificial defects due to errors, in seasoning, but a marked propensity to split and check may be inherent in some species and such defects may be part natural and part artificial.

Pitch veins, pitch pockets, etc. Sometimes known as resin pockets, they can appear either as thin veins or shallow cavities filled with resin. Usually caused by damage to the cambium layer in resinous woods, they may remain hidden and thus constitute a serious danger if the wood is used structurally.

Pith flecks Repeated damage to the cambium layer by small insects is often healed over with bark, and may show as small dots or patches of brown cork deeply buried in some woods, notably birch, alder and sycamore. They have no effect other than that of unsightliness.

Rind galls, etc. Patches of ingrowing bark, probably caused by exterior damage to the

growing tree. Other natural defects include 'callus', or tissue formed over a wound in a tree resulting in unnatural growth incorporated in the normal wood growth; 'canker', caused by fungoid disease; and 'cat face", a partially healed fire scar.

Internal sapwood Normally, the sapwood dies ring by ring, forming heartwood. but on occasions patches of sapwood survive within the heartwood, and show as lighter patches as sometimes seen in Rio rosewood. It is not known how the condition arises. Sapwood also can be prematurely killed by frost or other agents, while the cambium is repaired and continues to grow, forming new sapwood over the dead patches which appear in later years as a dark ring. The wood usually separates and breaks away along the ring during conversion.

Burrs/Burls These are not usually classed as defects as they may enhance the value of the timber considerably; they are, however, true defects. They can be caused either by fungal or insect attack irritating the cambium layers, and resulting in large rapid growths, usually at the base of the tree, or by numbers of small twigs which fail to develop owing to insufficient nourishment, forming a dense mass. The knobs in severely lopped or pollarded trees, and the witch's broom in birch trees, are typical examples.